P. Schotanus SCIONIX Holland B.V. Dedicated Scintilllation Detectors scionix.nl

P. Schotanus

SCIONIX Holland B.V.

Dedicated Scintilllation Detectors

www.scionix.nl

Recent developments in neutron detection with scintillators : some new data on

liquid scintillators for neutron / gamma discrimination

IAEA neutron workshop March 21-24 2011

Detection of Neutrons :

- Physics (e.g. particle Physics, HEP)

- Security (SNM e.g. Pu, U)

- Health Physics (dosimetry, often non spectrometric)

Neutron energy : Thermalised 0.025 eV – MeVs

(fast neutrons > 50 keV)

Interaction with Nucleus of absorber : A. Scattering or

B. Nuclear reactions

- Elastic scattering (protons)

- Inelastic scattering + prompt gammas

Nuclear Reactions e.g. : 10B(n α) 7Li , 3He(n p)3H, 6Li(n γ) 3H,

157Gd(n α) 158Gd


Most common detection method for neutrons :

1. 3He- tubes ( pressurised)

2. BF3 counters

Most unproblematic detector is the He-3 tube :

- Easy to operate (no special electronic needed)

- gamma / neutron rejection > 106

- No serious safety issues

- Large sizes possible ( meters long)

World wide structural He-3 shortage availability / cost problems

Possible alternative : Detection of neutrons with scintillators :

A. Thermal neutrons via nuclear reactions on Li, B or Gdin the material.

B. Fast neutrons via elastic (recoil) scattering in proton containingmaterials (organic scintillators)


Usually , neutrons associated with gammas, both will

interact with most scintillators but :

Neutron + 6Li alpha + Triton (4.78 MeV total)

(particles !) Peak at > 4 MeV

LiI(Eu) mixed Cs-137 / thermalised Cf-252 spectrum

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10

100

1000

1 101 201 301 401 501 601 701 801 901 1001

Pulseheight (arb. units)

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Neutron / gamma separation possible via Pulse height

LiI(Eu) scintillator

However LiI(Eu) 96 % enriched is a relative expensive material that cannot be made in large sizes.

The 6LiI(Eu) alternative is an excellent solution for hand held instruments and dosimeters but is not an option for e.g. radiation portals


Alternative 6-Li containing scintillators

6Li Loaded glass scintillator (Ce doped)

- Also expensive

- Low neutron peak location ( approx. 1.8 MeV) implying more problem with gamma rejection

- no large lengths possible

6-Li glass Cs-137/Cf252 spectrum

1

10

100

1000

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1 101 201 301 401 501 601 701 801 901 1001

Pulse height (arb. units)

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An alternative if gamma flux is low and of low energy and if time resolution is an issue

( scintillation is fast, 60 ns decay time)

New materials like CLYC:Ce adressed in other presentation.


Neutron / gamma spectrum 6-Li glass

Boron Loaded scintillators : No inorganics known except for Li Borate glasses

with low light output

The other alternative : Liquid scintillators

Some activated organics liquids show different pulse shape for neutrons and gammas.

Liquid scintllillators known since 1960’s

Most well known NE213( = EJ301 = BC501A)

Xylene based.

Real measured pulse shape difference


EJ 301 neuton / Gamma response

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1 101 201 301 401

Time ( 355 ps /channel)

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neutrons+gammas

gammas

EJ301 = NE213 EJ309

EJ309 Neutrons/ Gammas

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TIme ( 355 ps / channel)

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gammas+neutrons

gammas

Different ways to do neutron/gamma separation with PSD

1. QDC with 20 ns and 1 microseconds gate (or different)

2. Converts signals to time spectrum using a double delay line amplifier and CFDs (time spectrum shows two peaks)

3. Digitize the signal using wave form analyzer (500 Mhz to 1 GHz flash ADC)

Traditionally NIM electronics were used.

For many real field applications outside Physics this is very unpractical

Other disadvantages of liquid scintillators :

- Expand with increasing temperature (expansion volume is needed)

- e.g. EJ301 is a low flash point (flammable) toxic material (transport + safety issued)

New developments : - High flash point non toxic liquids

- PFGA based pulse digitising techniques


Neutron PSD almost just as good as for old fashioned liquids

These new liquids open up applications

EJ301

EJ309 (NEW) EJ301 is a scintillating liquid equivalent to NE213

EJ309 is a scintillating liquid especially designed for neutron BC501A specially designed for neutron / gamma

discrimination It has a high flash point, low vapour pressure discrimination.

and low toxidity.

Properties :

Light output (rel. to antracene) :

78 %

75 %

Photon yield / MeV electrons :

12.000

11.500

Maximum of emission wavelength :

425 nm

424 nm

Density:

0.874 g/ cc

0.964 g/ cc

H:C ratio:

1.21

1.25

No. C atoms per cc :

4.0 1022

5.46 1022

No. H atoms per cc :

4.81022

4.37 1022

No. electrons per cc :

2.3 1023

3.17 1023

Flash point :

26 oC

144 oC

Refractive index :

1.50

1.57

Decay time short component

3.2 ns

approx. 3.5 ns

Decay time long components

32.3, 270 ns

----


When Liquid scintillators are Boron doped,

the neutron sensitivity is increased

Boron peak at higher energies than with EJ301

(100 keV versus 60 keV)

Gammas, fast neutrons and slow neutrons can be separated by PSD

All these scatter plots measured by Swiderski et al. Soltan Institute fro Nuclear studies, Poland using the setup shown (analog technique).

Boron doped

EJ301


HOWEVER : For good gamma rejection the gamma interaction with the liquidimposes a problem. (important for detecting neutrons with

Low gamma rate

High Gamma rate

Swiderski et al. NIM In press.

It has been demonstrated that using electronic techniques the gamma pile-up can be reduced. With additional shielding the neutron / gamma separation can be improved from 3 . 104 to 5 . 106

This is still an order of magnitude + away from He-3 tubes but may be sufficient for some applications.

ELECTRONIC DEVELOPMENTS

Advanced pulse sampling techniques using FPGAs allow on line 250 kHz throughput neutron / Gamma separation

Instruments presented elsewhere in the workshop like the Mixed Field Analyser developed by Hybrid instruments (UK)


Design of Liquid cells for timing and PSD

General : 1. Liquids expand (T) and expansion space has to be available ( 3-5 % for temp. range –20 – +50 oC)With properly designed windows and hardware, “bubble free” cells are possible (all orientations).

2. Optical windows of UV glass or quartz providebest optical transmission. (Boron Free

requirement sometimes).

Readout : Classical readout with large PMTs is often NOT needed for good timing and/or PSD

Readout with smaller PMTs provides good PSD

n-g spectrum EJ 301 127 x 76 mm readout 3" fast PMT

0200400600800

100012001400

0 100 200 300 400 500

Time 358 ps / channel)

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Larger sizes cells of different geometries also provide a good neutron / gamma PSD

SOLUTIONS THAT WORK


HOWEVER :

Neutron / Gamma PSD can be spoilt by a geometry where the light paths to the PMT are much different !

a 1:1 plug-in replacement of He-3 tubes by long liquid cells is not possible !


The above geometries do not show a good neutron / gamma discrimination via PSD.

There is the widespread belief that for adequate PSD in liquid scintillators a fast PMT is needed. New data show the contrary

FOM is the same for a “ fast” XP4512 as a “ slow” ETL 9390

(data by Swiderski et al.)

HOWEVER for some applications (neutron / Gamma coincidences) a optimum time resolution is needed ( < 1 ns FWHM)

Note that Fast PMTs suffer from a non-ideal photoelectron collection efficiency


Time resolution at FWHM, t [ps] for Co-60 gamma rays

Time resolution ( ps) [phe/MeV]

XP4512-thr. 50keV 550 ± 20 1520 ± 50

ETL9390-thr. 50keV 1420 ± 40 3200 ± 100

Using a less fast PMT, a significant gain in photoelectron yield can be obtained at cost of the time resolution

Fast 127 mm diameter PMTs are very costly and bulky.

XP4512 is obsolete, fast Hamamatsy R1250 is nowadays used

Suitable PMT should have a low time jitter though…


Conclusions :

1. The current availability of high point non dangerous goods liquid scintillators opens up possibilities to use these detectors where was was prohibitive in the past.

2. The current availability of digital techniques allows the construction of novel instruments for neutron / gamma discrimination in mixed neutron / gamma fields

3. Liquid scintilltors can be a replacement for He-3 tubes in some applications

4. The size He-3 detectors are often used in ( 1 m long 50 mm diameter) is not a viable geometry for liquid scintillators where PSD is needed

5. For adequate neutron / Gamma PSD fast PMTs are not needed but to achive the best possible time resolution they are.

Thanks to : Marek Moszynski and Lukasz Swiderksi (Soltan Institute for Nuclear studies, Otwock, Swierk, Poland)

Chuck Hurlbut (ELJEN Technologies)


AND : A proper design of the electronics (CW High V supply) allows to run liquid scintillators at high rates but there are limits,

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P. Schotanus SCIONIX Holland B.V. Dedicated Scintilllation Detectors scionix.nl