Technical Report for the Design, Construction and Commissioning of

the DESPEC MOdular Neutron time of flight SpectromeTER (MONSTER)

Technical Design Report

1

FAIR PAC NUSTAR

FAIR-TAC HISPEC/DESPEC

Date: 05 December 2013

Technical Report for the Design, Construction and Commissioning of

the DESPEC MOdular Neutron time of flight SpectromeTER

(MONSTER)

Abstract

This document presents the design details of the MOdular Neutron time-of-flight SpectromeTER

(MONSTER) to be employed in the determination of the energy spectra of -delayed neutrons

emitted from exotic nuclear species implanted at the focal plane of the NUSTAR Super-FRS. The

spectrometer is a key instrument of the DESPEC experiment and the proposed implementation

follows extensive design studies and prototype tests.

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Members of the DESPEC MONSTER Collaboration

CIEMAT, Madrid, Spain

D. Cano-Ott, J. Castilla, A.R. García-Rios, J. Marín, T. Martínez, G. Martínez, E. Mendoza, M.C.

Ovejero, E. Reillo, C. Santos, F.J. Tera, D. Villamarín

IFIC, Instituto de Física Corpuscular, CSIC-Univ. Valencia, Valencia, Spain

J. Agramunt, A. Algora,, J.L. Tain, M.D. Jordan, B. Rubio, C. Domingo-Pardo

VECC, India

C. Bhattacharya, K. Banerjee, S.Bhattacharya, P.Roy, J.K.Meena, S.Kundu, G.Mukherjee,

T.K.Ghosh, T.K.Rana, R.Pandey

BARC, India

A.Saxena

Panjab University, India

B.Behera

JYFL, University of Jyväskylä, Finland

H. Pentillä, A. Jokinen

and the DESPEC neutron detector working group

We would like to acknowledge the collaboration and support from the scientists from LPC-CAEN

L. Achouri, F. Delaunay, N. Orr, M. Parlog and M. Senoville.

Project Leader/spokesperson: Name: Daniel Cano-Ott E-Mail: daniel.cano@ciemat.es

Technical Coordinator: Name: Trino Martínez E-Mail: trino.martinez@ciemat.es

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Contents

1. Introduction and overview ............................................................................................................. 4 2. Physics requirements and design considerations ......................................................................... 5

2.1. Choice of materials ................................................................................................................... 6 2.2. General design criteria on the detector geometry ................................................................. 6 2.3. The cylindrical cell concept ..................................................................................................... 6 2.4. The rectangular bar concept ................................................................................................... 8 2.5 Optimisation of ancillary detector setups: AIDA ................................................................. 11

3. The design of the MONSTER cell prototype .............................................................................. 13 3.1. Optical design ......................................................................................................................... 13 3.2 The MONSTER cell prototype .............................................................................................. 15

4. Performance of the MONSTER cell prototype .......................................................................... 16 4.1. Light collection ....................................................................................................................... 17 4.2. Single photoelectron measurement ....................................................................................... 18 4.3. Energy threshold .................................................................................................................... 19 4.4. Timing resolution ................................................................................................................... 21 4.5. Pulse shape neutron/ discrimination ................................................................................... 22 4.6. Prototype detector development at VECC (India). ............................................................. 24

5. Characterization of the detector response function ................................................................... 26 5.1. Experimental results .............................................................................................................. 26 5.2. Monte Carlo simulations ....................................................................................................... 27

6. The MOdular Neutron SpectromeTER ...................................................................................... 28 6.1. General characteristics of MONSTER ................................................................................. 28 6.2. High voltage supply and gain monitoring system ............................................................... 30 6.3. Mechanical support and shielding ........................................................................................ 31 6.4. Possible upgrades of MONSTER .......................................................................................... 32

7. The digital data acquisition system for MONSTER .................................................................. 33 7.1. The DAQ concept ................................................................................................................... 34 7.2. DAQ Level 1 (local level) ....................................................................................................... 35 7.3. DAQ Level 2............................................................................................................................ 39 7.4. DAQ Control and operation modes ...................................................................................... 39 7.5. Pulse Shape Analysis developed for liquid scintillators...................................................... 41

8. Radiation and safety ..................................................................................................................... 42 9. Production, quality assurance and acceptance test .................................................................... 43 10. Test beams and commissioning .................................................................................................. 44 11. Installation and logistics ............................................................................................................. 45 12. Acknowledgments ....................................................................................................................... 45

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1. Introduction and overview

Beta-decay studies of exotic nuclei are one of the main goals of the DEcay SPECtroscopy

(DESPEC) setup [Des] to be installed at the Low Energy Branch (LEB) after the Super FRagment

Separator (Super-FRS) of the FAIR facility. The Super-FRS is the central instrument of the NUclear

STructure and Astrophysics Research (NUSTAR) collaboration, where beams of different exotic

species are separated and finally stopped for the measurement of their -decay properties with the

different devices of the DESPEC collaboration.

The knowledge of the β-decay strength function Sβ(E) and the properties of nuclei lying far from

stability contributes decisively to our understanding of nuclear phenomena in the nuclear structure,

astrophysics and nuclear technology fields. Special interest have received the exotic nuclei in the

neutron rich side where the nucleo-synthesis r-process take place or where the evolution of the shell

structure is unknown as it approaches the neutron drip line. The accurate measurement of the half-

lives, magnetic moments, masses, distribution of decay probabilities and particle emission

probabilities provides essential data for the determination of the full β-strength distribution in exotic

nuclei.

One of these data is the β-delayed neutron properties. In the very neutron-rich nuclei, the small

neutron separation energy (Sn) and the large Qβ decay energy often leads to the population of

unbound states in the daughter nuclei. As a result β-delayed neutron (βdn) emission becomes more

important as the neutron drip line is approached. The decay features, energy and branching ratios of

βdn are essential to the mapping of the β-strength function Sβ(E) of nuclei far from stability.

Moreover, the delayed neutron emission of the neutron rich nuclei plays an important role in the

nucleosynthesis r-process as well as in the kinetic control of advance reactors, in particular the

determination of the energy spectra contributes to determine the neutron capture cross section rate in

nuclei of difficult access by other techniques. Concerning the nuclear technology field the accurate

knowledge of the delayed neutron spectra from precursors are essential for satisfactory static and

dynamic reactor calculations, being of great importance the comparison between summation

calculations and macroscopic data [Kra, Das].

Another important area is the isospin dependence of nuclear level density (NLD). NLD is one of the

important ingredients of all statistical model calculation. The excitation energy and angular

momentum dependence of nuclear level density is well explored; however very little is known to the

isospin dependence of NLD [Cha1, Cha2]. The set-up proposed for DESPEC will give suitable

opportunity to study NLD by detecting the evaporated neutrons of neutron rich nuclei in-

coincidence with daughter nuclei in the DSSD.

In order to determine the neutron emission features, two complementary neutron detectors have

been proposed, a high efficiency moderator based 4π neutron counter and a time-of-flight

spectrometer based on scintillation detectors. This report is devoted to the design and technical

description of a Time Of Flight (TOF) MOdular Neutron SpectromeTER (MONSTER) for

performing neutron spectroscopy at DESPEC.

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Figure 1. Typical decay scheme of a neutron emitter precursor. The delayed neutron are

emitted from unbound levels in the daughter nucleus.

MOSTER will allow to measure the energy spectra of β-delayed neutrons and their partial branching

ratios to the excited states in the final nucleus by applying --n coincidences.

The actual physics requirements and design considerations leading to concrete design options are

detailed in Section 2. The design of a MONSTER module by means of Monte Carlo simulations and

the description of the first prototype are presented in Section 3. The results of all the test

measurements with the prototype performed at the laboratory and its characterization with reference

neutron beams are summarized in Section 4 and 5 respectively. Detailed specifications of the

spectrometer and associated equipment are given in Section 6. The digital data acquisition system

(DAQ) electronics specially developed for MONSTER is described in section 7.

2. Physics requirements and design considerations

At FAIR exotic nuclei will be produced by the interaction of intense high energy ion beams on thick

targets. The Super-FRS located after the production target serves primarily to select the specie(s) of

interest and drastically reduce the amount of accompanying ions which are a source of unwanted

signals in the detectors. At DESPEC the beams will be stopped at the implantation device, typically

a silicon strip detectors. Each implanted ion will be identified using the information gained from the

Super-FRS ancillary detectors and the implantation setup. The -decay spectroscopy will be

performed by measuring the -delayed radiation (mainly γ-rays and neutrons) correlated to the -

particles detected at the implantation setup. .

The estimated production cross sections of very neutron rich nuclei at FAIR are relatively low

compared to those of stable nuclei. For this reasons, efficient and selective techniques are essential

in the determination of decay properties of -delayed neutron emitters. High efficiency Double

Sided Silicon Strip Detectors (DSSSD) and HpGe germanium detectors have been proposed for the

β and -ray detection. The MOdular Neutron SpectromeTER described in this report is proposed for

performing the -delayed neutron spectroscopy.

The main physics requirements that should be accomplished by a MONSTER are:

• High detection efficiency, which allows detecting neutrons emitted by exotic nuclei produced at low yields.

• High energy resolution, in order to reveal fine structure of neutron emission in nuclei with high level density.

• Low energy detection threshold, which allows for improved detection efficiency at the

energy range of -delayed neutrons, from a few tens of keV up to tens of MeV.

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• Neutron-gamma discrimination, because of neutron emission competes with gamma de-excitation and signal to room background ratios will be low when measuring very exotic

species.

Modularity, in order to distinguish between single and multiple neutron emission events (n,

2n, 3n,…), by applying cross-talk rejection and allow an optimal geometric configuration

depending on which complementary detectors are used.

All these requirements, starting from the appropriate detection material, the geometry of the

detectors as well as the response function have been considered for reaching an optimal design.

2.1. Choice of materials

Neutrons can be detected only by nuclear reactions. The typical ones are elastic scattering, typically

(n,p),nuclear reactions resulting in secondary charged particles and γ-rays.

Among different types of materials used for neutron detection, organic scintillation materials (high

content of hydrogen) are preferred for fast neutron spectrometry due to the fast response and high

intrinsic detection efficiency. Several plastic, liquid and crystalline organic scintillators have been

widely used in time-of-flight detection technique. Furthermore, some organic liquids (NE213-type,

NE321A-type, etc.) and crystals (Stilbene) allow discriminating between neutron and γ-ray incident

particles based on the shape of the electronic pulse produced by the interaction.

After an careful review of the existing literature and the performance of tests at the laboratory with a

large variety of materials (BC501A, EJ301, BC400....), it has been concluded that the equivalent

BC501A and EJ301 liquid scintillators offer at present time the best performance in terms of

efficiency, time response and neutron/ discrimination.

2.2. General design criteria on the detector geometry

The conceptual design of the TOF spectrometer should fulfil the requirements listed above. The

target requirements have been established according to the measurement conditions for very neutron

rich nuclei: moderate production rates (≥10 at/s), large Qβ and large level densities of the daughter

nuclei. Therefore, having the largest possible efficiency over a broad energy range and the best

energy resolution are crucial detector requirements. The modularity is also required for the detection

of multiple neutron emission events in β2n and β3n decays. Furthermore, the flexibility in the

geometric configuration is also demanded for making the spectrometer compatible with

complementary detection setups and for reducing the cross-talk. The neutron/ separation is crucial

for suppressing the coincident -ray background at typically unfavourable signal to background

ratios

Following the design of neutron spectrometers like EDEN [Lau], DEMON [Til] and TONNERRE

[But] among others, two designs have been proposed for the MONSTER spectrometer: one

consisting of an array of cylindrical cells, each coupled to a single photomultiplier and an array of

rectangular section long bars readout by two photomultipliers placed at the extremes. The

geometrical parameters have been optimised [Rei] by Monte Carlo simulations with GEANT4

[Ago] for both detector geometries taking into account the efficiency (detector surface and

thickness) and the energy resolution (time resolution and flight path). The intrinsic efficiency, time

and energy resolutions and light collection efficiencies have calculated for the two geometric

concepts and the results obtained are reported in the following subsections.

2.3. The cylindrical cell concept

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The intrinsic efficiency of a cylindrical cell increases with its thickness. However, the uncertainty in

the neutron interaction point along the thickness of a neutron detector leads to a degradation of the

energy resolution, which depends both on the uncertainty in the reconstruction of the flight path and

the time resolution.

The contribution of the thickness to the efficiency and the energy resolution has been evaluated for a

2 m flight path and 1 ns detector time resolution by Monte Carlo simulations. The neutron response

calculations has been performed with the light output functions obtained by Dekempeneeer et al.

[Dek] The intrinsic efficiency of a 20 cm diameter cell as a function of the thickness is shown in the

left panel in Figure 2. On the other hand, the effect in the energy resolution is shown in the right

panel. It has been found that a 5 cm thickness represents a reasonable trade-off between intrinsic

efficiency and energy resolution.

Figure 2. Optimization of cylindrical cell detector. Left panel: Variation of intrinsic efficiency with the thickness of a cell diameter 20cm. Right panel: Variation of energy resolution of a

cylindrical cell with the thickness for intrinsic detector time resolution of 1+1 ns at flight

path of 2m.

Figure 3. Monte Carlo simulation of the light collection for a detector cell of 20 cm diameter

and 5 cm thickness coupled to a light guide of 31 mm thickness. The internal walls of the cell

and the external surface of the light guide have been coated with diffuse painting. The

photon source is placed inside the cell at 2.5 cm height and at different radius position.

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The size of the cell in the transversal direction of the incident neutron has a significant effect in the

total efficiency (geometric efficiency) and in the light collection efficiency. The effect of the

detector size in the light collection has been calculated by Monte Carlo simulation with the

GEANT4 optical physics package for various cylindrical cell diameters. As expected, it was found

that a larger cell diameter increased the geometrical efficiency but worsens the light collection, due

to the limited photomultiplier tube (PMT) sizes available. It was found that a cell diameter of 20 cm

has a good collection efficiency (~40%) by using a diffuse paint with an acceptable uniformity with

variation below 10%. The results of the optical photon simulations are shown in Figure 3.

Thus, the design study favours a cylindrical cell of 20 cm diameter and 5 cm thickness. Such a

design is very similar to the EDEN module.

2.4. The rectangular bar concept

The second geometric concept considered for MONSTER is an array of rectangular position

sensitive bars (similar to the TONNERRE detector). The effect of the dimensions of the bar such as

the length, width and thickness on the detection efficiency, energy resolution and light collection

have been investigated by Monte Carlo simulation and verified with tests at the laboratory. As in the

previous case, the thickness that optimizes both the energy resolution and intrinsic efficiency has

been estimated to be around 5 cm. The results obtained for a 1 m long bar at a flight path of 2 m are

shown in Figure 4. In the left panel, it can be seen that a 5 cm thickness increases significantly the

intrinsic efficiency in comparison to the 3 cm case, while it has a very reduced effect on the

reconstructed neutron energy resolution. In this case, the position of the neutron interaction was

reconstructed by requiring a delayed coincidence between the two PMT tubes at the bar end and a

1ns time resolution was adopted for each PMT.

Figure 4. Optimization of bar cell dimensions. Left panel: Variation of intrinsic efficiency with

the thickness for a bar with 100cm length and 10cm width. Right panel: Variation of energy

resolution of a long bar cell with the thickness for intrinsic detector time resolution of 1+1

ns at flight path of 2m.

The optical transport simulations performed for the long bars has revealed that the light collection

depends largely on the interaction point of the neutron. Simulations have been performed for 2

different bar lengths (50 cm and 100 cm) and two types of reflective coating along the bar: a diffuse

reflector and a polished reflector. The results are shown in Figure 5, where the left column

corresponds to the 50 cm case and the right column to the 100 cm case. The conclusion of the study

is that a polished reflector coating shows a much better performance than a diffuse reflector.

Furthermore, even for the 50 cm bar, the variations in the light collection efficiency as a function of

the interaction point have been found twice as large as for the cylindrical effect.

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Figure 5. Light collection for squared section bars. Left panels: Collection efficiency (single end and total) for bars with dimension 50cmx5cmx5cm, with diffuse reflector (up) and

polished reflector (down). Right panels: Similar light collection figures for bars of dimension

100cmx5cmx5cm.

Such a result becomes more important when looking at the light collection efficiency of each

individual PMT. Even for the best possible case (a 50 cm bar, polished reflector), the PMT closest to

the interaction point collects about twice as much light as the opposite PMT. Such difference must

affect severely the effective threshold in the lowest neutron energy that can be detected when a

coincidence is required between the 2 PMTs. In addition, a lower light collection will also affect a

the neutron/ discrimination quality at low energies.

For this reason, a series of tests were made at the laboratory with a bar consisting of a rectangular

aluminium container of 5x5 cm2 section and 50 cm length, filled with EJ301 liquid scintillator

(almost equivalent to BC501A scintillator). Two Phillips XP2262B with VD122K voltage dividers

were coupled to the bar at its extremes. The internal walls of the container were polished, in order to

improve the position dependence in the light collection.

The light attenuation curve has been measured by integrating the anode signal produced by a

collimated 60Co γ-source placed in steps of 10 cm along the bar. A lead block of 10 cm x 10 cm with

a 5 mm aperture and 23 mm thickness was used as a collimator. The attenuation has been measured

for both PMTs independently and by adding both signals in a Fan-In NIM module. The variation of

the Compton edge at 1.3 MeV -ray as a function of the interaction point is shown in Figure 6. The

values obtained at 25 cm from the centre are affected of the perpendicular position relative to the

quartz window at the entrance of the photocathode. Attenuation values of more than 35% are

obtained at the center of the scintillator bar from the light collection values obtained at the end of the

bar.

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Figure 6. Light attenuation measured in the rectangular bar.

The number of photoelectrons per deposited energy has also been determined at different positions

along the bar, leading to values of 130±44 phe/MeV at the center and 211±85/MeV phe at the

extremes (20 cm from the center), showing also a large position dependence.

The time resolution has been measured with a collimated 22Na source. The bar was scanned with the

source in steps of 5 cm from one extreme to the other. The distributions of the time-difference

between the two PMTs for position separated by 10 cm are shown in Figure 7. From the width and

the position of these peaks we have obtained an averaged time resolution of 2.3 ns and a position

resolution of 14.7 cm with a threshold in the discriminator of 200 keV.

Figure 7. Timing spectrum between both PMT measured by moving a 22Na source a long the

bar scintillator. Position resolution is derived from these measurements.

Finally, the neutron/ separation quality of the bar scintillator was measured by using a 252Cf source.

The bar was irradiated both at the center and at one extreme, close to the PMT, by using the neutron

source collimated with lead and blocks of polyethylene. The anode signals from the PMTs were

added together and digitized with an 8 bit and 1 Gsample/s commercial Acqiris digitizer [Agi]. A

pulse shape routine and a digital charge integration method were used to determine the figure of

merit as a function of the energy following a methodology which is described in section 4.4. The

results obtained for the bar at both positions are given in Table 1. It can be observed that the

separation values (FOM) improve (i.e. increase) at the extreme of the bar due to the better light

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collection. However, the absolute values are worse than the ones obtained for a cylindrical cell (see

section 4.4 for details). Figure 8 shows the neutron/γ separation distribution at 200 keVee (left panel)

and 500 keVee (right panel) at the centre of the bar (black curve) and one extreme (red curve). It can

be observed that the distribuitions are broader at the centre, thus resulting into a sizeable position

dependence in the neutron/ γ separation efficiencies.

Energy

(keVee)

Bar cell Cylindrical

Centre End Centre

200 0.41 (2) 0.60 (1) 1.23 (2)

300 0.52 (2) 0.85 (1) 1.53 (3)

500 0.67 (2) 0.98 (2) 1.76 (4)

Table 1. Figure of merit of the neutron/ separation obtained for the long bar detector. The 252Cf source was placed in the centre and at the one end. The FOM values for the cylindrical

cell have been included for comparison.

Figure 8. Neutron/ separation obtained with source placed in the centre of the bar (black histogram) and at 20 cm from the centre, close to the PMT(red histogram). Better separation

is clearly obtained at the ends of the bar due to lower light attenuation.

It can be concluded that both geometric designs, the cylindrical cell and bar-like module, can be

used for reaching a similar ovearall performance in terms of efficiency and energy resolution at high

neutron energies (Eneutron > 1 MeV). However, the long rectangular bars show a clear disadvantage

due to the lower light collection efficiency and its large dependence on the interaction position. For

this reason, the cylindrical cell concept was favoured for the design of MONSTER.

2.5 Optimisation of ancillary detector setups: AIDA

The spectroscopy of the -delayed neutrons at DESPEC will require the use of MONSTER arranged

in a compact configuration around the Advanced Implantation Detector Array (AIDA) [Aid] for the

detection of -particle and a gigh resolution γ-ray detector array for detection of γ-rays emitted in

coincidence with delayed neutrons. Therefore, the design and configuration of the other detection

systems will have an important influence on the performance of the neutron spectrometer and it has

been considered in the design of MONSTER.

γ-rays

neutrons γ-rays

neutrons

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The basic unit of AIDA is an 8cm8cm DSSSD with 1 mm thickness and 128 strips in both

horizontal and vertical directions. The beam coming from the Super-FRS has a wide spatial

distribution, in particular in the horizontal direction for some of the spectrometer optical modes. In

one of the AIDA configurations several stacks of 3 DSSSD in a row will be mounted in order to

maximize the coverage of the focal plane. The enclosure of AIDA in this configuration has a

transversal dimension of 10cm28cm (seeFigure 9) and it consist of two frames, a thin cooling

frame and a support frame.

Figure 9. (Left panel) View of AIDA implantation detector showing the triple stack of DSSSD at

the front. (Right panel) Schematic view of a possible DESPEC Ge array surrounding the

AIDA implantation detector

The -ray detector array for DESPEC has not yet been decided, at present various options have been

considered. An ambitious proposal will consist of a series of modules each consisting of three stacks

of planar double sided Ge strip detectors sharing the same cryostat. A more realistic option could be

to use several segmented CLOVERS detectors. These modules can be arranged in different

geometries optimized to the different types of experiments envisaged at DESPEC. The high

granularity of the Ge detectors is important in order to assure high efficiency of the array during the

"prompt flash" of radiation associated with the implantation of high energy ions into the focal plane

catcher and thus to allow the study of decays with very short lifetimes. The right panel in Figure

9shows a schematic drawing of the DESPEC Ge planar detectors array.

An important constraint in the design of the spectrometer is the effect of the interaction of delayed

neutron with both the implantation and gamma detection arrays. In order to evaluate the effects of

the detector components and materials of the AIDA implantation detector, extensive MC

simulations have been performed [Rei]. For the structural materials employed in the vacuum

chamber, aluminium, carbon fibre and stainless steel of different thicknesses have been considered.

The neutron transmission probability and neutron induced γ-ray background have been computed

for neutron energies ranging from thermal (0.025 meV) up to 10 MeV. The results of the

simulations have shown that aluminium is the best material due to the higher transmission in the

whole energy range although an increase of the interaction probability is produced at energies below

1 MeV. The induced γ-rayradiation by inelastic and capture reactions is below 1% in the whole

energy range of interest.

Then, the neutron interaction in the several components of the AIDA detector has been analysed by

simulating an isotropic and monoenergetic point-like neutron source placed in the central DSSSD

stack. The effect of AIDA was estimated by looking at the angular distribution, energy spectra and

integrated fluence of the neutrons crossing a spherical surface with an inner radius of 10 m and

surrounding the AIDA setup. The neutron interaction probability has been determined, at different

neutron energies, for different separations distance (i.e pitch) between the DSSSD stacks and for the

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different components of the AIDA detector: DSSSDs, vacuum chamber, support frame. The results

obtained have shown that the main effect in the neutron fluence comes from the silicon detectors.

The DSSSD stack affects significantly the energy spectrum and the angular distribution of detected

neutrons (see Figure 10). The strongest effect appears at neutron energies around 1MeV, due to a

resonance in the capture cross section in 28Si. The scattering effect is smaller in the aluminium

components but not negligible. The angular distribution of neutrons is minimized at 90º (in the plane

of the DSSSD stack) due to large amount of silicon that the neutron needs to cross. This effect is

also largest at neutron energies around 1MeV and for smaller pitch values, or closely packed

DSSSD stacks.

Figure 10. (Left) Interaction probability as a function of neutron energy for the DSSSD stack

with a distance of 0.5cm and for different components of the AIDA array. (Right) Angular

distribution of the neutron flux.

Therefore, the design of the AIDA implantation detector should consider the spacing between

DSSSD stacks as large as possible according to design specification, in order to reduce the

scattering effects on delayed neutrons. On the other hand, position at 90 degree will be avoided for

the MONSTER cells.

3. The design of the MONSTER cell prototype

3.1. Optical design

The optical design of the prototype cell for MONSTER i.e. all aspects concerning the transportation

of the scintillation light, has been performed by means of Monte Carlo simulations with the

GEANT4 package. This study has been focused on the cylindrical cell. In order to validate the

performance of GEANT4, the work performed by Klein and Schölermann [Kle1] was taken as a

reference. The influence of the geometrical shape, the type of reflective surface (polishing), the

presence of a light guide and the optical properties of the materials on the light collection efficiency

and uniformity have been evaluated.

The light collection efficiency has been studied for a simplified cylindrical detector geometry: a

BC501A active volume enclosed in an aluminium container with the internal walls coated with

diffuse reflector or a polished surface and coupled to a quartz window. It was found that for this

geometry, the use of diffuse reflector paint improves the collection efficiency with respect to a

polished internal wall of the aluminium container. The effect of the detector cross section on the

light collection efficiency at the optical window was investigated, since polygonal shapes can lead to

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a more compact arrangement of the array than circular cross sections. Three container shapes with

squared, hexagonal and circular transversal sections have been evaluated. All cells were defined

with the same transversal section and thickness. The results showed that the collection efficiency is

largest for the cylindrical container. This is due to the lower number of Lambertian collisions

experienced by the photons before reaching the window.

Therefore, the cylindrical cell was chosen due to its larger collection efficiency and, as described in

section 2.3, a cell with a 20 cm diameter and a 5 cm thickness is the best suited for MONSTER.

Figure 11. Radial dependence of the light collection efficiency for a cylindrical detector of 20 cm diameter and 5 cm height, coupled with a light guide of 3cm (upper panels), 5cm (middle

panels) and 12 cm thickness (bottom panels). In central panels, the light collection curves

for polished surface are shown. Right panels show the light collection for diffuse coated

surface. Lines of different thickness represent radial profiles at different transversal planes

in steps of 10 mm being the thickest line the closest to the light guide.

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The optical design of the cell has included a light guide due to the large diameter of the cell and the

limited sizes of commercial photomultipliers. Phototubes with a 5” diameter were considered as the

largest acceptable ones in terms of cost and performance. The effect of the light guide on the light

collection efficiency has been investigated carefully by Monte Carlo simulations.

The results of the simulation are shown in Figure 11. Various conical light guides of different sizes

with polished and totally coated surfaces have been simulated. The optical properties of the light

guide were obtained for a poly-methyl methacrylate (PMMA) material, which is adequate for this

purpose since its refractive index is very close to that of BC501A, thus guaranteeing a good

transmission at the interfaces. More details about the description of the simulations can be found in

[Gar1].

The conclusion is that the use of a diffusive coating around the light guide improves the light

collection effciciency A good trade-off between the light collection efficiency and its uniformity has

been achieved for a light guide of 3 cm thickness, which shows the largest overall efficiency and the

in-homogeneity is below 10%.

3.2 The MONSTER cell prototype

Two companies, St. Gobain Crystals and SCIONIX, were contacted for manufacturing the

MONSTER cell prototype according to the design specifications. The proposal made by SCIONIX

was not accepted since its current bubble free container design does not allow the use of a light

guide between the scintillator container and the PMT. Thus, the construction was assigned to St.

Gobain Crystals.

The characteristics of the MONSTER cell prototype shown in Figure 12 are;

• BC501A organic liquid scintillator which light yield is around 78% of Anthracene’s yield. Features three main decay components at 3.2, 32.3 and 270 ns, a H/C atom content of 1.21

and exhibits excellent PSD properties.

• Mechanical structure: Cylindrical cell dimension of 20cm diameter and 5 cm thickness made of 1.6 mm thick aluminium. The internal faces are coated with diffusive reflector paint

BC622A (based on TiO2).

• An expansion reservoir with Teflon capillary tubing or equivalent system for liquid expansion.

• Optical window: a 9 mm thick and 206mm diameter quartz window.

• Light guide with a tronco-conical shape of 31 mm thick, 206mm and 128 mm diameters made of UVT graded PMMA material (BC-800). External surface coated with reflector paint

BC620.

• Photomultiplier tube: Photonis 5” XP 4512. After the disappearance of Photonis, the tube was replaced by a Hamamatsu 5” diameter R4144 model. Fast response 1.8 ns SPE signal

rise time, eight stages, linear focused SbCs dynode structure, bialkali photocathode. The

PMT is easily removable.

• Voltage divider for the R4144 (see Figure 13).: tesistor chain in tapered configuration with damping resistors in the latter stages. Wired for negative high voltage (SHV connector).

Output signals: anode and last dynode (BNC connector).

• Magnetic shield: 0.64 mm thick µ-metal.

• Light pulser port: Connector type SMA 905 for optical fibre coupling on the light guide. The port will be used for stability monitoring purposes by using an external light source.

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Figure 12. MONSTER cell prototype.

Figure 13. Scheme of the Hamamatsu PMT R4144 model and VD circuitry.

The final spectrometer will consist of 100 of such cells, combining a number of cells manufactured

by St. Gobain Crystals with other cells built at VECC (India) and/or other manufacturers following

the same design specifications.

4. Performance of the MONSTER cell prototype

A large number of test measurements have been performed for characterising the performance of the

MONSTER cell prototype: the light collection, photoelectron yield, the energy threshold, timing

resolution and neutron/γ discrimination.

Unfortunately the XP4512B PMT by Photonis requested for the prototype was discontinued during

the test phase. A big effort was made in finding an alternative PMT model which could replace the

XP4512B, which has been a standard choice for many applications since 90’s. The other models

been investigated are the Hamamatsu R877-100 super bialkali photocathode, the R877-01MOD for

17

intermediate fast response, the R4144 very fast and low gain model, the R1250 fast and high gain

model and the E9823B high gain and fast response from ET Enterprises LTD.

4.1. Light collection

The energy resolution of a large scintillator is affected by the unequal light transmission inside the

cell, from the different points where the primary light is produced. In addition, other components as

the optical coupling elements (light guide) and the PMT determines the efficiency and uniformity in

the light collection. As it is mentioned in the previous section, a detailed design study has been made

by Monte Carlo simulation for improving all the optical elements.

The optical performance of the cell prototype (scintillator container, diffusive coating, quartz

window, light guide and PMT) has been determined with a dedicated X-Y γ-ray scanner (see Figure

14) built for this purpose. The cell has been mapped by moving on its front surface a collimated 22Na γ-ray source in steps of 2 mm. A lead collimator block of area 5cm x 5cm section and 2.5 cm

thickness (shown in Figure 14) with a hole of 5 mm diameter, has been placed between the source

and the detector surface.

Figure 14. Setup for the light collection measurements (Left) Fully automatic scanner built for

the 2D mapping of the light collection efficiency of the MONSTER cells. (Right) Scheme of

the detector arrangement with a Pb collimator.

The anode signal was preamplified and fed to an ORTEC 570 amplifier. The signal from the

amplifier was converted in a PalmtopMCA type MCA8k-01 multi channel analyzer (MCA) to

generate the pulse height spectrum. The shifts in the 1274 keV γ-ray Compton edge at each source

position (see Figure 15) been used for quantifying the in-homogeneity in the light collection. Figure

16 shows the detailed 2D mapping performed for the R4144 PMT. Table 2 contains the relative light

output data (with respect to the central position) as a function of the distance to the centre obtained

for the XP4512B and R4144 PMTs. The measured values are in excellent agreement with the

simulations.

PMT

Relative Light output (%) at distance (cm)

-10 -8 -6 -4 -2 0 2 4 6 8 10

XP4512 94 95 97 99 102 100 101 100 98 97 96

R4144 92 93 95 97 99 100 99 97 94 92 91

Table 2. Relative light collection values obtained for the detector at different point of irradiation in the surface.

Pb

Collimator

18

Figure 15. Pulse height spectra obtained with 22Na source placed at different positions on the

front surface of the BC501A detector. The shift in the response shape is outlined in the right

panel, where the spectra have been renormalized.

Figure 16. Spatial uniformity in the light collection obtained for the prototype detector coupled

to R4144 PMT.

4.2. Single photoelectron measurement

The measurement of the PMT photoelectron yield for a given scintillator depends on the quantum

efficiency and homogeneity of the photocathode, the photoelectron focussing, Any reduction in the

number of photoelectrons due to a lower light collection from the light guide and the edges of the

photocathode can be troublesome.

The measurements of the photoelectron yield were made using the method described by Bertolaccini

et al. [Ber]. The number of photoelectrons Nphe per energy unit lost by -rays in the BC501A

scintillator is measured directly by comparing the relation existing with the amplitude of the

Compton edge visible in the scintillator spectrum CCE (pulse height spectrum due to several

photoelectrons), the amplitude of the single photoelectron peak CSPE (pulse height spectrum of a

19

single photoelectron), the gain applied in each measurement GCE and GSPE and the energy of the

Compton edge ECE.

CE

SPE

SPE

CE

CE G

G

C

C

E

1/MeVN phe

A 662 keV -ray from a 137Cs source was used for this purpose. The anode signal from the PMT was

fed to a Canberra preamplifier and then to the spectroscopy amplifier. The amplification gain GSPE

used to resolve the single photoelectron peak in the spectrum was set to high values, in contrast to

low gain GCE used to record the γ-ray pulse height spectrum corresponding to the 137Cs source. Both

pulse height spectra are shown in Figure 17.

Figure 17. (Upper panel) Single photoelectron pulse height spectra measured with the XP54512B photomultiplier at a HV of -1500V. In red a threshold has been applied to avoid

electronic noise. (Lower panel) Spectrum of 137Cs -ray source. The gain of the amplifier was lowered by factor of 50 to observe the γ spectrum.

The value of Nphe/MeV determined for the PMTs tested is given in Table 3.

PMT model Q.E.(%)/Gain Nphe/MeV

XP4512B 24/ 5x106 1175 60 R877-MOD 24 / 2x106 1580 50

R4144 22 / 1.4x106 1050 50

R1250 22 / 1.4x107 920 30

E9823KB 27 / 8x107 < 800

Table 3. Values of the Nphe for different PMTs.

4.3. Energy threshold

20

The detection threshold limits the lowest possible energy deposition detectable and thus modifies the

detection efficiency. At very low deposited energies the amount of scintillation photons is extremely

low and several difficulties arise to distinguish such small signals from noise.

The minimum threshold achievable has been investigated for two dynamic ranges with an 8-bit

digitizer, in order to take into account the effect of using a digital data acquisition system and its

effective number of bits:. a dynamic range up to neutron energies of 2 MeV (corresponding to light

equivalent of 500 keVee) and an energy range up to neutron energies of 5 MeV (corresponding to 1.8

MeVee). The threshold has been determined by comparing the spectrum of γ-ray sources obtained

with a MCA analyzer with the spectrum obtained by digitizing the signals when a CFD with a level

threshold of 1mV and 6 mV were triggered. A light output calibration (in energy equivalent units)

was obtained by using the Compton edges of the -rays emitted by 22Na and 137Cs sources and the

photopeak of the 59.6 keV -ray from an 241Am source. The empirical relation found by Dietze et

al. [Die1] for the electron light output emitted by the Compton electron energy was applied:

keVEEEkL cce 50 ,0

In this relation, an offset of E0=5 keV is used to compensate the non-linearity of the light output due

to quenching effects at low electron energies (for the slope k set to 1 MeV-1). The position of the

Compton edge has been estimated to be located where the 75% of the maximum of the peak

distribution is reached.

With this ight (energy) calibration, the threshold values obtained for the 2 MeV and 5 MeV neutron

energy ranges are of 20 keVee and 30 keVee, respectively. The 241Am source spectrum is shown in

Figure 18 for the case of 2 MeV energy range.

Figure 18. Pulse height spectrum for an 241Am source obtained by digitizing signals and setting

different threshold values in the CFD (black and red lines) and compared with a MCA

system with a threshold just above the electronic noise (blue line).

These data serve to envisage the limits in the threshold when a 12 bit digitizer will be used. Thus,

the digitized spectra obtained for an 8-bits digitizer and a 2 MeV neutron energy range would

correspond to the first part of the spectra obtained with a virtual digitizer of 12 bits resolution for an

21

energy range up to 20 MeV neutron energies. Therefore, for lower dynamic energy range and

assuming good integral linearity similar or even lower threshold values could be obtained with a 12

bit digital acquisition system.

4.4. Timing resolution

The timing resolution of the BC501A prototype has been measured with respect to a fast EJ200

plastic scintillation detector with a 500 ps resolution (measured for a 60Co source) used as reference

detector.

The time resolution of the BC501A cell has been measured with both standard analogue nuclear

electronics and Acquiris digitisers. For the analogue case, the detector anode signal was shaped with

an ORTEC 474 Timing Filter Amplifier and fed to an ORTEC 583 Constant Fraction Discriminator

(CFD). The combined time resolution obtained with a 60Co source and a CFD threshold equivalent

to 500 keV was of 0.9 ns FWHM.

Figure 19 shows the Gaussian fit (black line) of the measured time spectrum (red histogram).

Figure 19. Timing spectrum of the BC501A prototype measured with the coincident 60Co source

events against a fast EJ200 plastic scintillation detector.

For the digital method, the anode signals from both detectors were digitized when a coincidence

were detected in the Logic module (ORTEC CO4020) module used as a trigger for the digitizer.

Then a digital leading edge (DLE) and digital constant fraction discriminator (DCFD) algorithms

were implemented in order to determine the time distribution of the signals. A threshold value of

30% has been selected in the LE algorithm as a time reference for the signals. In the case of the

DCFD method the optimum fraction value (~ 30%) and delay value (10 ns) provided a combined

timing resolution of 0.9 ns FWHM for the EJ200 and BC501A.

PMT model Rise time (ns) Intrinsic time resolution of the

BC501A (ns)

XP4512B 2.5 0.7 (1)

R877-100 10 1.5 (1)

R877-MOD 4.3 1.2 (1)

R4144 1.5 0,9 (1)

R1250 2.5 1.0 (1)

22

E9823KB 2.7 1.1 (1)

Table 4. Values of the intrinsic time resolution achieved with different PMTs.

4.5. Pulse shape neutron/ discrimination

The neutron/ discrimination capability in BC501A is based in the relative proportion of fast and

slow fluorescence components in the light yield produced in a -ray (i.e. electron) or a neutron

interaction. Several methods have been widely applied in order to exploit such a feature, based on

time characteristics of the pulses (zero crossing method) or by integrating the charge in the pulse

over different time gates (charge integration method) [Rou, Bro].

The charge integration method has been adopted in this work. The charge of a pulse has been

integrated in two time gates: a full gate covering the total charge of the pulse and a shorter and

delayed gate, covering the last part of the pulse (see Figure 20). The figure of merit (FOM) defined

in [Re66] has been applied to the distribution of the delayed area/total area ratios for quantifying the

quality of the separation between neutrons and γ-rays.

Figure 20. Average pulse shape for neutron and -ray for XP4512B PMT. The optimized time

gates (240 vs 180 ns) that provide the maximum separation are shown.

The figure of merit is defined upon the distribution of the separation parameter (Adelayed/Atotal) as:

n

PFOM

where P is the distance between peaks, and and n are the FWHM for both peaks.

23

Figure 21. Neutron-gamma separation obtained with the optimized gate widths. Left, 2-dim plot

of Delayed/Total parameter vs total light and (right) delayed area vs total area (light).

The detector was irradiated with a low intensity 252Cf source (~1000 neutrons/s). Two dimensional

histograms (see Figure 21) were built for the FOM and the delayed/total ratios as function of the

light expressed in keVee.

The FOM was computed at each energy interval by fitting two Gaussians to the experimental

delayed/total area ratio distributions. Figure 22 shows several projections at 100, 200, 300 and 500

keVee, which correspond to incident neutron energies of 1 MeV, 1.3 MeV, 1.6 MeV and 2.2 MeV.

Figure 22. Quality of neutron/γ separation obtained at different light equivalent values. By

setting cuts on total energy equivalent, the corresponding projection of ratio delayed area

24

over total area has been fit with two Gaussians in order to determine the FOM parameter at

each energy.

Table 5 shows the values obtained for the FOM as a function of the energy (keVee). As an

indication, a good separation is obtained when the FOM is greater than 1. It can be observed that the

values found for the MONSTER cell are similar to those reported for other detectors like EDEN and

DEMON ([Lau, Til]), obtained in different experimental conditions (with neutrons produced in

reactions). It should be said that such values will depend from cell to cell and variations up to 5%

have been found for identical cells.

Energy

Threshold

(keVee)

MONSTER EDEN DEMON

220/180 400 300/270

60 0.50 (4) - -

80 0.70 (4) - -

100 1.00 (4) 1.00 (3) 1.09

200 1.30 (4) 1.46 (3) -

300 1.60 (4) 1.72 (4) 1.73 (9)

500 1.76 (4) - -

Table 5. FOM values obtained for neutron-gamma separation for different neutron energies.

4.6. Prototype detector development at VECC (India).

The Indian part of the MONSTER collaboration has developed a first proto-type of a MONSTER

cell. The actual photograph of the MONSTER cell is shown in Figure 23. The cell was initially

coupled with Photonics PMT XP4512B and tested. It will be replaced by the Hamamatsu PMT

R4144 (decided by the collaboration), which has been ordered already.

Figure 23. Proto-type MONSTER cell developed at VECC.

The proto-type cell was tested with analogue electronics. The pulse shape discrimination property

was measured with 241Am-9Be neutron source.

25

Figure 24. Pulse shape discrimination spectra.

Figure 25. Figure of merit vs. pulse height threshold.

Figure 26. Intrinsic time resolution.

The pulse shape discrimination property has been tested with zero cross- over technique. The pulse

shape discrimination spectrum was extracted at a threshold of 150keVee, shown in Figure 24. Figure

25 shows the figure of merit vs. pulse height threshold applied. The FOM of merit initially increases

and then it saturates at a threshold of 150 keVee. The intrinsic time resolution of the detector has

26

been measured using a 60Co source with a 1" thick BaF2 detector. The intrinsic time resolution is

shown in Figure 26. The intrinsic time resolution of the MONSTER cell was found to be 700 psec.

More detailed tests, such as the efficiency measurements and pulse height resolution are in progress.

5. Characterization of the detector response function

The response function BC501A detector module has been characterized both experimentally and by

Monte Carlo simulation. The BC501A cell has been irradiated with mono-energetic neutron

reference fields in the energy range between 0.2 and 14 MeV at the PIAF facility of Physikalische

Technische Bundesanstalt (Germany) with the support of the EFNUDAT transnational access

program of the European Commission and at the Van de Graaff 4MV facility of the CEA/DAM

research centre (France). Neutron beams of energies above 5 MeV were produced at PTB through

the D(d,n)3He reaction in the TCC CV-28 cyclotron. The neutron energies below that energy were

produced through the D(d,n)3He, T(p,n)3He and 7Li(p,n)7Be reactions at the Van de Graaff

accelerators both at PTB and CEA/DAM. The response function and the intrinsic efficiency have

been determined experimentally at several energies by the time of flight technique.

In addition, an modified version of the GEANT4 simulation toolkit was used for the calculation of

the response function at the energies measured experimentally. The comparison of the experimental

data with the Monte Carlo simulations has allowed to determine the light output function for protons

at different energies and to compare with the equivalent light yields for electrons (gamma

interaction) and the uncertainty in the determination of the detector efficiency as a function of

incident neutron energy and detection threshold. The Monte Carlo technique will be essential for the

calibration of the 100 different modules, since it will be impossible to perform metrologic

calibrations for each individual cell.

5.1. Experimental results

Two different setups were used for the characterisation of the detector with monochromatic neutron

beams. At the PTB cyclotron, the detector was placed at a distance of 10.5 m from the target and at

the Van de Graaff hall the detector was placed at flight path distance between 1 and 3 m, depending

on the energy of the neutron beam. Absolutely calibrated BF3 long counters with a 3% uncertainty

were used to monitor the beam and determine the neutron fluence for each measurement.

At the CEA/DAM Van de Graaff accelerator, the detector was placed between 2 and 3 m from the

production target. An absolutely calibrated and large volume BF3 neutron detector was used for

monitoring the neutron beam with a 6% uncertainty.

The MONSTER cell prototype calibrated at PTB was using a a Photonis XP4512B PMT Data were

collected at neutron energies of 0.565, 8.1 and 10.1 MeV. The MONSTER cell calibrated at

CEA/DAM was using a Hamamatsu R4144 PMT and data were collected at neutron energies of

0.93, 1.96 and 4.34.

In both cases, a digital data acquisition system based on commercial Acqiris (now Agilent) digitisers

DC271 (8 bits) and DC282 (10-bits). The anode signal from the detector and the signal from the

accelerator were digitised at 1 GSample/s of sampling rate. The event trigger was built from the

detector signal in coincidence with the accelerator signal in order to optimize the recording of data.

The digitised waveforms were stored into raw data files for further off line pulse shape analysis. A

pulse shape analysis (PSA) software was developed to determine the parameters needed for the

analysis: the baseline, time, amplitude, total charge and delayed charge of the signals. the

27

multivariate analysis of the different parameters has been performed with a dedicated application

developed for the ROOT toolkit [Bru]. The total charge parameter has been calibrated in terms of

energy equivalent light units (MeVee) by using standard γ-ray sources. The Time-of-Flight spectrum

(TOF) was obtained from the time difference between the accelerator signal originated when the

projectile hits the target and the detector signal.

The different response functions obtained are shown in Figure 27 (black curves). All were

normalised to the number of incident neutrons.

5.2. Monte Carlo simulations

The response functions at each experimental energy were simulated with a modified version of the

GEANT4 code specifically developed for such neutron detector [Gar2, Gar3]. The new version of

GEANT4 uses a modified physics package which includes the 12C(n,n’)12C* and 12C(n,)9Be*

reaction channels, not present in the standard G4NeutronHP physics package [Goh]. In addition, the

standard ENDF/B-VII data library was used instead of the G4NDL library distributed with

GEANT4 [Men]. The performance of this code has been compared with the well validated NRESP7

code [Die2] by simulating the same response functions for the neutron energies investigated. An

excellent agreement was achieved between the two simulation codes (overall differences below 1%),

providing the necessary thus guaranteeing the reliability of the results.

The simulated response functions at all energies are compared to the experimental data in Figure 27.

A general good agreement with the experimental responses has been achieved in general and some

of the remaining differences are due to a neutron background which was neither subtracted nor

included in the simulations.

28

Figure 27. Detector response functions obtained from the irradiation with mono-energetic neutron beams of 0.565, 0.93, 1.96, 4.34, 8.1 and 10.1 MeV energies (in black). The neutron

energies of 0.93, 1.96 and 4.34 MeV have been measured at CEA/DAM. The neutron

energies of 0.565, 8.1 and 10.1 MeV were measured at PTB. The experimental response

functions are compared with the Monte Carlo simulated response (in red).

6. The MOdular Neutron SpectromeTER

This section provides a summary of the main characteristics of MONSTER, its mechanical structure

and complementary equipment.

6.1. General characteristics of MONSTER

A summary of the general characteristics relevant to the performance of MONSTER follows.

6.1.1. Intrinsic efficiency curve as a function of the neutron energy

The intrinsic efficiency of a MONSTER module (see Figure 28) ranges from 60% at 1 MeV to 25%

at 14 MeV. The values have determined experimentally and by Monte Carlo simulations.

29

Figure 28. Intrinsic efficiency of a 20 cm diameter and 5 cm thickness BC501A cell obtained by

Monte Carlo simulation including the calibrated light output functions from PTB [Die2].

6.1.2. Intrinsic time resolution

The time resolution of the detector MONSTER module has been measured taking a 1x1x10 cm

EJ200 plastic scintillator as a reference. The intrinsic time resolution of the MONSTER module with

a XP4512 PMT is 750 ps and with a Hamamatsu R4144 is 900 ps.

6.1.3. Energy threshold

The lowest energy threshold measured with standard -ray sources was 20 keVee, which correspond

to a neutron energy of ~250 keV.

6.1.4. Light collection efficiency

The light collection efficiency of the MONSTER module varies between 50(7)% at the centre and

45(7)% at the border. The in-homogeneity in the light collection is below 10%.

6.1.5. Total detection efficiency and energy resolution.

Figure 29 shows the total detection efficiency of the 100-module configuration of MONSTER for

three different flight paths.

30

Figure 29. Total detection efficiency of MONSTER (100 modules) at different flight paths.

Figure 30 shows the energy resolution achieved with MONSTER for three different flight paths.

Figure 30. Energy resolution (constructed from the time of flight) of MONSTER for different

flight paths, assuming an overall time resolution of 1.5 ns and an uncertainty in the flight

path reconstruction of 2.5 cm.

6.2. High voltage supply and gain monitoring system

31

The performance of the spectrometer and the accuracy of the results are related to a stable operation

of the different photomultiplier tubes. In particular, the detection threshold and the intrinsic

efficiency require to have stable gain in each module over a long period.

The stability in the voltages applied to the PMTs will be provided by a SY2527 remotely

controllable power supply system from CAEN, equipped with A1733 and A1535D HV cards with

common floating return for improved noise performance.

In addition, a laser based gain monitoring system has been designed for providing a stable light

reference to each individual MONSTER module. The proposed light monitoring source has not yet

been tested, but it will consist of a laser diode light source emitting at around 405 nm with pulses of

32

The detectors are mounted in the structure by means of several mechanical pieces, two lateral

supports allow for polar angle rotation and fine detector position adjustment, a cylindrical case

which holds the detector and serves as union between detector and lateral supports, bolts and fixing

pieces (Figure 32). These pieces have been manufactured at CIEMAT’s workshops in aluminium

and small amounts of stainless steel when additional stability was required.

Figure 32. (Left) Exploded drawing view of the detector full assembly. (Right) Pictures of some of the supporting pieces and cylindrical case.

The final structure will have to hold a total weight of about 850 kg (calculated for 100 detectors).

6.4. Possible upgrades of MONSTER

A stable design for MONSTER has been reached. However, such a design is flexible enough for

accommodating future developments until the first beams will be available at DESPEC. In this

regard, CIEMAT has established a collaboration with the Spanish company Scientifica International

S.L. for the development and construction of improved detector modules. Two prototypes have ben

built by Scientifica according to CIEMAT’s specification: one with low material aluminium housing

and one with a carbon fibre housing (see Figure 33 and Figure 34). Both prototypes make use of a

new expansion reservoir which simplifies the detector filling procedure, guarantees a bubble free

liquid and does not add dead material close to the front face of the detector.

33

Figure 33. CONFIDENTIAL. Prototype MONSTER cells developed by Scientifica International and CIEMAT. In the left panel, an aluminium cell has been manufactured. In the right panel,

a similar version of the cell has been manufactured in carbon fibre.

The detector prototypes have been assembled in early 2013 and will be tested at CIEMAT in 2013.

Figure 34. CONFIDENTIAL. Prototype cells coupled to the µ-metal shield.

In addition, an R&D program on new materials and new detector concepts is being carried out as

part of the NUPNET project NEDENSAA (European Commision 7th Framework Programme). In

this context, a new plastic material with discrimination capabilities and developed at LBNL has been

purchased and will be tested in 2013. It is commercialised by Eljen Technologies under the model

number EJ299-033. The collaboration has acquired as well a variety of new liquid scintillators such

as EJ309 and EJ309-B (doped with Boron), which offer similar characteristics to the BC501A but

with reduced safety requirements. Finally, a Stilbene crystal will be tested to verify if it will be

possible to extend the neutron detection efficiencies of MONSTER at lower neutron energies by the

use of a complementary neutron detector.

From the point of view of the electronics, Hamamatsu has been contacted for the possibility of

manufacturing a PMT similar to the R4144 with a superbialkali photocathode. The proposal was

welcome and work is in progress. The first tubes could be delivered by the end of 2014.

7. The digital data acquisition system for MONSTER

The recent advances in digital electronics and digital signal processing software enable radiation

detectors to be utilized in a high efficient way. The improvements accomplished in most recent

ADCs, state of the art high level FPGAs and digital signal processors (DSPs) during the last decade

has made possible to digitize detector pulses, processing them in real time, bringing the full

processing chain to the digital domain.

It has been found empirically (in tests with both inorganic and organic scintillators at the laboratory)

that a 12-bit 1 Gsample/s fast digitiser offers an equivalent or improved performance to analogue

hardware for virtually any scintillation detection application. In addition, the benefits of using digital

electronics are apparent regarding the increased counting rate management, the high number of

channels attainable (hundreds and even thousands) as well as in what concerns the ease of use

(software set-up of slow control) and the flexibility of the scientist in testing innovative analysis

34

methods. This experience has confirmed the equal or better performance of digital systems over

analogue systems in terms of stability, resolution, differential non-linearity and throughput.

The experience acquired by the CIEMAT team during the past decade in the design and operation of

a fully digital data acquisition system (DAQ) with 8 bits resolution, 1 Gsample/s sampling rate and

up 60 channels at the CERN n_TOF facility [Abb] has served to design a new fully digital DAQ for

the MONSTER spectrometer. The DAQ will consist of a scalable number of 12 bit and 1 Gsample/s

digitisers, which could be considered as a universal flash ADCs for the sampling of any detector

(except HPGe).

7.1. The DAQ concept

Digital data acquisition systems based on PC based commercial high-speed digitiser cards have

medium deep memory resources for data storage. This data is then transferred to PC memory and

analyzed by software. While the data is being transferred, the system is not ready to acquire new

data. Also, this approach results in huge volume of data to be handled by PC. The acquisition

duration is limited by the depth of on-board memory provided on the card. As the processing is done

by CPU of the PC, the throughput is affected when the number of channels increases.

In order to overcome these limitations, a fully digital DAQ architecture has been designed and

developed at CIEMAT, based on a new fast transient recorder unit. It has been conceived for

operating the 100 channels of MONSTER and the compatibility with virtually any other type of

detector. High count rate operation, pulse shape analysis, post-experiment data re-processing, pile-

up identification and treatment are provided by this digital approach.

Table 6 provides an estimation of the data rates of the DAQ during a normal operation of

MONSTER.

Counting rate per detector ~ 500 c/s

Number of channels 100

Size of one BC501A signal 2000 samples (2 µs)

Bytes/sample 2

Digital data rate per detector (digital

waveforms stored on disk) ~ 2 Mbytes/s

Processed data rate per detector (after

pulse shape analysis) ~40 kbytes/s

Table 6. Estimated counting and data rates during a regular operation of MONSTER.

The principal features of the electronic DAQ system are: a common architecture for all detector

electronic front-ends; common trigger, timing, and control interfaces; a common software data

acquisition interface and a common on-line firmware environment from the lowest level within the

front-end signal processors to the highest level user interfaces and event processing farms. By

generalizing the architecture for all systems down to the point of contact with signal digitization, the

overall maintenance, software, and training needed to operate the system are minimized, and

engineering can properly be focused on detector specific requirements.

1. The electrical signals are digitized immediately at the detector output, not requiring any analogue component. The thresholds will be adjustable both by hardware (discriminator) and

software (digital pulse processing).

2. The system has to be able to process the digitised signals in real time. The pulse shape analysis routines will have to produce the time and energy of the event, provide information

35

on the particle type (γ-ray or neutron) and reconstruct pileup events. This is granted by the

computing power of the FPGA+DSP available in each digitiser board.

3. The system should be reconfigured easily. The upload of pulse shape analysis software and the set-up of parameters that might be different channel by channel should be performed

without any major intervention.

4. The DAQ electronics should incorporate dedicated timing measurement resources for accurate time stamping of physical events.

5. The DAQ will incorporate self-calibrating capabilities for the compensation of nonlinearities in the ADCs, temperature variations, incorporating a number of hardware and software

diagnostics to identify the nature of any fault in the operation of the instrument.

Figure 35. DAQ architecture proposed for the MONSTER demonstrator.

The DAQ electronics system encompasses all analogue front-end, trigger stage, and data acquisition

and processing digital electronics. A schematic view of DAQ architecture is shown in Figure 35.

The whole DAQ is divided in digitizing units. At present, each unit deals with one electronic

channel and comprises all necessary logic within one single card. This forms the level 1 DAQ. A set

of eight digitizing modules form a module. Modules, which are mounted in a crate, incorporate

synchronizing capabilities and allow data concentration, as well as second level trigger and control

logic. This control logic is embedded in a level 1 concentrator unit, which typically performs event

filtering and data assembly. This forms level 2 DAQ. Following the same scheme, a level 2

concentrator unit synchronizes and concentrates data from level 1 hardware, forming a full

pyramidal architecture. Modularity and flexibility brings a more affective resource use to the DAQ,

which is not limited to a fixed number of channels.

7.2. DAQ Level 1 (local level)

36

Each digitiser achieves real-time pulse processing with the use of an FPGA and a DSP. The pulse

processing system is proposed to be entirely in digital domain.

Following the signal path, the output of each PMT coupled to a scintillation detector is fed to the DC

coupled, 50 ohms terminated input of the digitiser. The analog front-end has been carefully

conceived in order to obtain the lowest possible noise level within an extended input bandwidth. An

analogue antialiasing pre-filter limits the input range of frequencies to an appropriate band to avoid

higher frequency noise. Following the detector to be used, this will be typically the band where most

information is contained.

Figure 36. 2nd version of the CIEMAT digitiser board.

The PMT signal is filtered by an antialiasing 300 MHz, -3dB cut-off frequency, second order Bessel

filter and continuously sampled by two 12 bit, 500 MHz, 65 dB dynamic range ADS5463 fast ADCs

from Texas Instruments operating in interleaved mode. Five software selectable signal amplitude

input scales are possible (250 mV, 500 mV, 1 V, 2V and 5V) for accommodating the dynamic range

to the optimal detector voltages. A dedicated circuitry allows to compensate the offset for each

scale. This is necessary for accommodating different energy ranges to the ADC coding range

following the kind of detector to be used at a given experiment. The sampled data are further

processed on-line by a combination of FPGA + DSP.

The FPGA is a high speed, high-density Virtex IV field programmable gate array device from

Xilinx. This device incorporates dedicated processing resources as well as general programmable

logic for implementing custom functionality, as for example slow control, readout interfaces and fast

data capture.

Additionally, each digitiser incorporates a high performance, 32 bit DSP floating point, ADSP-

21369 SHARC processor from Analog Devices, intended to be used as a co-processing unit in

general purpose applications. The inherent parallel architecture of a FPGA implementation results in

better performance under some circumstances, and the DSP floating point architecture allows the

execution of algorithms written in a standard high-level programming language.

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The logic in the FPGA implements data flow management and also latches the accurate time of

arrival provided by a dedicated timing unit. Each pulse acquired is then buffered using the on-board

SDRAM implemented as a FIFO which is then used for further analysis. The buffered data are sent to the DSP during idle periods, where it is processed and once the pulse shape analysis is completed, a

reduced amount of data will be returned to the SDRAM, The net effect is that the downstream data load

to the PC is reduced so that throughput can be increased by orders of magnitude.

Several ways of processing the incoming data flow will be possible.

• An event by event processing is available. During the experiment, a trigger is generated at the start the neutron emission (i.e. from the β-detector). This will reset the Time to Digital

Converter (TDC) timing counter. The output of the fast sampling ADC is continuously

written in to a level 0 FIFO within the FPGA. Upon the arrival of a relevant event, this level

0 FIFO is read and its output is fed into a main pulse buffer. The size of this pulse buffer

must be larger than the inspection window of input pulse. Simultaneously, it is subject to

analogue or digital peak detection, which is a combination of a threshold crossing and slope

change. Once a pulse is detected, the set of data points, before and after the threshold

crossing, will be collected from the FPGA circular buffer. At this instant, the output of the

TDC counter is also latched and attached to the set of data and the associated pulse

processing channel is signalled to start processing. The circular buffer is then made available

to receive the next pulse from the input FIFO. Since in the entire chain of logic the pulse

processing takes more time, having multiple pulse processing channels will result in

increasing event rate and close to negligible dead time.

• A second strategy consists in the buffering the incoming data flow continuously during a large period of time. This is useful for trigerless data taking experiments involving long time

reactions (of the order of 1s). For this, deep and fast memory resources have been made

available at level 1 DAQ.

With the aim of recording very long traces, the digitiser integrates a huge amount of on board

buffering resources. A double data rate (DDR2) memory module allows up to 4 GB of data storage.

This memory may be implemented as a general-purpose FIFO for event storage as well as a random

access memory for faster event location and processing. Depending of the access mode, the

measured cumulative bandwidth for data access in the memory module can range up to 2 GB/s. This

is higher than the 1.5 GB/s input data flow coming from the 12 bits, 1 GS ADC. Thus, the main

purpose of including a big data buffer is that of eliminating any dead time during raw data recording

from the fast ADC. Additionally, it provides buffering capabilities to the data processing units.

Finally, in the context of periodic pulsed driven experiments, it incorporates the flexibility of

continuously recording more than 1 second of raw data without any dead time.

The DAQ electronics integrate the possibility of obtaining the neutron energy from an accurate

measurement of time of flight. A trigger input to the system indicates the beginning of the neutron

emission process which starts a fast and accurate counter. The time of arrival (TOA) for each

neutron/γ-ray generates a pulse from a detector. Each pulse is digitized and processed in two parallel

paths, one for counting and one for pulse shape discrimination. The counting channel latches the

output of the fast counter and transfers it to a TOA FIFO. This strategy allows for an extended time

range measurement. More accurate time measurements are obtained from a dedicated time to digital

converter, the TDC-GPX component from ACAM, which provides intrinsic resolutions in the order

of tens of picoseconds over up to four parallel channels. The TOA FIFO fills then with timing labels

of successive arriving events. Such an approach results in higher count rates compared to multi-

channel scaling or time-to-analog conversion followed by MCA approach. The counting channel is

much faster than the shape-processing channel, thus limiting the event rate. Implementing a number

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of signal processing channels in parallel improves the event rate. Additionally, an auto repeating

start scheme starting with an asynchronous start time reference provides an unlimited time

measurement range of hit times. This is illustrated in Figure 37, where for each hit a start number

and a time data label are provided.

Figure 37. Hit time measurement scheme with the TDC-GPX

The acquisition system is completed by digital logic inputs/outputs whose purpose is to make

coincidences, generate triggers, vetoes and other signals that take into account the correlation

between different channels and may give further timing information. The digitizing unit is

(LV)TTL/NIM compatible, accepting input timing input signals in both level standards. These

inputs are measured and time stamped along with the remaining information. Additionally, these

may be used as global purpose input/output signals intended for implementation of custom functions

in hardware within the FPGA, as veto, coincidence and data integration or windowed data

processing.

Figure 38. Windowed data processing

Finally, the unit comprises intelligent trigger modules with the capability to trigger the input channel

in conventional external or internal post trigger modes. Additionally, advanced on-board signal

processing techniques permit more sophisticated triggering modes such as pre-trigger, which

captures events that occur prior to the trigger signal, and input trigger, which captures events based

on a threshold criteria for the event. Edge trigger is a simple trigger mode whereby an externally-

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supplied positive or negative signal edge to the trigger module starts the event acquisition process. This

has been implemented using the analogue signal prior to sampling in order to increase time accuracy. Further on, fully digital triggering is also available where the constant fraction discriminator classic

analogue strategy for accurate timing is replicated digitally.

7.3. DAQ Level 2

Level 2 DAQ comprises a module of eight digitizing units along with a level 1 concentrator unit.

They have been designed to work all together operating eight electronic channels, each of which

may be considered as level 1 hardware. Level 2 DAQ is closely coupled to the control and

monitoring software, as it decides the way the units interact.

Different clocking schemes are available at this level. Among them, the basic unit includes a low

jitter, very accurate 500 MHz clock for data sampling. Additionally, it offers the possibility to inject

an external 10 MHz clock signal to be locked to the internal fast clock. This functionality allows for

improved synchronizing features between several units within one module, effectively implementing

multichannel operation. Even more advanced schemes as daisy chain and star topology clock

distribution have been included.

7.4. DAQ Control and operation modes

Based on synchronism between units at level 1, level 2 allows considering a module as a single unit

with the concentrator unit as an interface between software and hardware. Based on this, several

acquisition modes have been implemented.

• Oscilloscope mode, which will be the “test” acquisition mode in which the digitiser will not perform any online processing. For each trigger, either external or internal, the digitizer

saves the sequence of samples (waveform) that belongs to the acquisition window into one

local memory (DDR2) buffer. This mode will be used used to monitor the signals (including

the internal signals at the output of the digital filter stages), adjust the necessary parameters

(thresholds, time windows, delays). In addition, it will be possible to store the raw data from

the digitiser and apply the off-line pulse shape analysis algorithms. This mode is intended for

algorithm development and testing and for the data taking during short periods.

• In list mode or “standard” operation mode. The processing algorithms are applied runtime by the FPGA and DSP, operating on a continuous data stream. Whenever a pulse is found,

the relevant energy, timing or other quantities are calculated and written in the memory

buffers, thus providing list mode data for one single channel. As soon as the list reaches a

certain size, the data buffer is made available for the readout while the acquisition continues

in another buffer without any dead-time. Thanks to the highly reduced number of data to

save and transfer, this mode will be able to sustain a continuous acquisition at counting rates

much higher than 5000 kcounts/s without any dead time.

Regarding oscilloscope mode of acquisition, two modes of operation have been developed at level 2

a) continuous acquisition up to a maximum duration of 2.5 s corresponding to the full on-

board memory (4 GB) of the digitizing unit: in this mode, all events are recorded

b) non-continuous acquisition in which digitization is triggered by those events whose

amplitude is above a threshold value, maybe given by an external event: the digitizing interval after

each trigger can be dynamically set according to the typical decay time of the scintillation events,

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for example, effectively suppression zeroed samples. In this mode longer acquisition durations (>2.5

s) can be reached, without any dead time needed for data storage, which makes it independent of the

incoming event rate

In both modes of operation, a common start signal is distributed among the level 1 unit to set the

acquisition start time. This initial event is recorded and time stamped by all digitizing modules.

After a start trigger is given, the input signal is sampled continuously until the card memory is

completely full. All pulses are recorded regardless of their amplitude during 2.5 s, corresponding to

more than 2 GSamples with 12-bit resolution. Afterwards, data transfer/storage/ or data processing

takes place within the DSP+FPGA. The continuous mode is suitable for high count rate operation

(MHz level) with no loss of data. Full pulse processing is then performed via embedded software.

For acquisitions lasting over 2.5 seconds an alternative scheme has been developed in which

digitization is triggered only by those events whose amplitude is above a pres