Long pulse spallation sources

ELSEVIER Physica B 234-236 (1997) 1227-1232

Long pulse spallation sources

F. Mezei Hahn-Meitner-lnstitut, Glienicker str. 1000, D-14109 Berlin, Germany

Abstract

Design considerations for new spallation source projects have to involve a global optimisation of the whole accelerator-target stations-instrument suite ensemble. One particular aspect of this process is concerned with the fact that compared to a linac plus storage ring or synchrotron configuration, a given proton beam power can be achieved at substantially lower costs by using a linear accelerator only with long pulses in the ms range. Long pulses also make it possible to deliver higher proton beam energies per pulse, which primarily determines instrument performance in low wavelength resolution applications such as small-angle scattering. Monte-Carlo simulation results also show that the long pulse approach is also superior to the short pulse option in a category of thermal neutron experiments, such as medium-resolution powder diffraction. Short pulses are only required for best use of the epithermal neutron spectrum and for the highest wavelength resolution thermal neutron work. Long pulse sources can be either conceived as stand-alone facilities or as one of the target stations at a combined short pulse long pulse installation, and have the potential to offer higher neutron intensities by more than an order of magnitude in comparable experiments than the best existing high-flux reactors.

Keywords: Neutron sources; Spallation; Long pulse sources; Instrument simulation

1. Introduction

The existing spallation sources are based on accelerators originally conceived and built for another use. Thus, the source design involved an optimisation of the ensemble target station plus instruments. In contrast, planning a green field facility calls for a global optimisation of the whole set-up, i.e. of accelerator-target station-instruments ensemble. This is a rather formidable task since it involves far fetched fields of physics and engineering in an intri- cate web. The aim of this paper is to contribute to these optimisation considerations by showing that, on the basis of present-day accelerator technology, the use of long proton pulses in the ms range is more favourable in a number of neutron scattering applications than the conventional approach of delivering ~ts length short pulses on the target.

To start with we have to clarify the meaning of the word "favourable" in the present context. It is

rather obvious that assuming the same proton beam power and the same pulse repetition rate, a shorter proton pulse can offer better time-of-flight (TOF) resolution for the neutron instruments if needed. Thus, if the accelerator would come free of costs and technical limitations, one would never come to the idea of long-pulse spaUation sources (LPSS) by only considering the neutron scattering side of the problem. In particular, if the most cost effective way of producing the high-energy proton beam was the use of an accelerator that inherently produces short pulses (e.g. rapid cycling synchrotron), the pulse length would not become a parameter in the optimisation. Since, at least in the proton-beam power range relevant for advanced spallation sources, the proton-beam power can be more cost efficiently achieved by linear accelerators (linacs), the short pulse advantage on the neutron scattering end has to be weighted against the additional costs of making the proton pulses short

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1228 F. Mezei / Physica B 234-236 (1997) 1227 1232

compared to the pulse length naturally offered by high-power proton linacs, i.e. not less than half a ms. (The technically most demanding parameter of a proton linac is the quasi-continuous peak power during the pulse, which is actually limited to some 150-200 MW. Therefore, a source with e.g. 5 MW average power has to operate with a duty factor of at least 2.5-3.3% which implies at a repetition rate of 25 Hz, for instance, minimal pulse lengths of 1-1.3 ms. This limit could in principle be stretched by making more than one linac simultaneously shoot on the same target, but at the present stage of technical progress we will not consider such extreme solutions, even if the use of several parallel ring accelerators to deliver beam quasi- simultaneously on the same target has become a standard design feature for the next generation of short-pulse spallation sources (SPSS) in order to overcome the severe space charge limitation in the rings.) Thus our central concern in this paper will be to evaluate the relative merits of delivering short proton pulses at substantially higher costs and with more seriously limited energy per pulse compared to adapting the neutron instrumentation to the long linac pulses and if necessary producing short neutron pulses by using the kind of highly flexible chopper technology well established in TOF spec- troscopy on reactor sources. We will consider kind of a cost/benefit ratio. Here the cost side character- ises expenses and technical difficulties and the benefit side stands for the information obtained in the neutron-scattering experiment. In a convenient first approximation we can take as a measure of this information the number of neutrons delivered to the sample at a given desired instrumental resolution. It is, however, much more relevant to compare expected experimental results for identical samples with the help of detailed simulation calculations. In what follows, such sample results will be pre- sented and interpreted in the light of general ana- lytic arguments.

2. General considerations

We will first derive an estimate of the neutron flux delivered to the sample by a pulsed source. The peak flux of the source will be denoted by ~bp(2),

where 2 is the wavelength. The pulse length 8t is defined in such a manner that the time average flux ~bAV is given by the relation

~bAV(/~ ) = Y S t (Dp(/~), (1)

where v is the pulse repetition rate and thus c. = v 8t is the neutron duty factor. Note that, since the neutron pulse length depends on the wavelength and on eventual pulse shaping choppers, the neutron duty factor is not a unique characteristics of the source, in contrast to the analogously defined duty factor of the proton beam c. For a usual (not too sharp edged) line shape the effective wavelength resolution 52 at a detector/sample distance L from the source can be given, together with the available wavelength band A),, as

52 = A 8t/L and A2 = A/vL, (2)

where A = 3956 ,~ m/s. In the narrow wavelength band limit A2 ~ 2, the neutron flux on the sample (per definition understood as time average) for a given beam-collimation solid angle 8g2 is

~s = A2 50 St v q~p(2), (3)

if we ignore neutron losses such as due to absorb- tion or due to data collection dead-time at the limit of two subsequent pulse frames. (For more details see Ref. [1].) There is, however, one important efficiency factor inherent to pulsed-source instruments which in general cannot be ignored: wavelength resolution better than required. This applies particularly for small-angle scattering (SANS), neutron-spin echo, diffuse scattering, etc. In these cases, the minimal distance between source and detector imposed by the thickness of the bulk shielding and also by the natural length/collimation length of the instrument leads to 52 values much below the ~ 10% required and available on CW reactor sources. This effect will be taken into account by a factor g = 8.~/8,~.desire d.

In many practical cases the narrow band condi- tion is not or cannot be met on pulsed source instruments. For example, a usual high-intensity powder diffractometer on a SPSS will have about 10 m source to detector distance, giving a wavelength band of A2 = 8 ~ at 50 Hz repetition rate, while the most interesting range of data collection is around I ~. So data are not collected with equal

F. Mezei / Physica B 234-236 (1997) 1227-1232 1229

efficiency over the whole wavelength (time) frame• Ideal data-collection efficiency would correspond to the division of the wavelength band into n equal subbands and delivering wavelengths within the ith subband to the sample for an optimised fraction ti of the total measuring time. The factor f = 1/n max(q) is a convenient measure of the efficiency of the equal time information collection in the total band A2. Thus, in view of Eqs. (1)-(3), the useful flux on the sample becomes in the general case

~s = .q f 82desired q~p(2). (4)

Here )~ is identified with the wavelength which would be optimally chosen for the experiment on a CW source (i.e. the one corresponding to max(q) above) and, thus, a CW source which delivers the same neutron intensity on the sample has to have the flux

q~cw(2) = gf~bp(2). (5)

This relation (referred to as mean-flux theorem in Ref. [1]) implies that for an ideally adapted instrument a pulsed source delivers a useful neutron flux on the sample equivalent to that provided by a CW source with a flux equal to the peak flux of the pulsed source.

3. Examples

The following two examples of simulated experiments illustrate the relative performance of three types of neutron sources: a CW reactor of the power of ILL, a 5 MW, 50 Hz repetition rate SPSS and a 5 MW, 25 Hz LPSS with 2 ms pulse length• The construction costs of the LPSS as a stand- alone facility would amount to about half of the SPSS, since it requires a linac with less peak-beam power (due to the absence of chopping) and a simpler ion source (H ÷ instead of H-), no injec- tion into storage rings as there are no storage rings and only one target station for some 50 beam positions instead of two for the SPSS. The latter difference is due to the fact that the optimal LPSS instruments tend to require long neutron guides between source and sample, thus offering more room for the instruments. The feasibility of using long guides both for thermal and cold neutrons will be discussed elsewhere, although one of the examples below gives an illustration•

Fig. 1 shows the simulation results obtained for a SANS experiment on a nondispersive ensemble of hard spheres. All the three instruments were assumed to have the same overall lengths from first

-'1

__>, r -

e..

1E-6

1E-7

1E-8

1 E-9

I

0000 O0

~ 0 0

0 0

0 0

Y~ 0

O g ~ O 0

NO

0

1E-10 , I , I = I ,

0,00 001 0,02 003

Q [All

I I

o LPSS 5 MW25 Hz SPSS 5 MW 50 Hz

<> ILL

0 0 o o 0 o o 0

y~ Y~ ~ ~ o

O~ 0 0 0 o o ~ , 0 0 ~ o o

0 oy~ ~ o o ~ 0 0 0 0

I J I i

0,04 0,05 0,06

Fig. 1. Simulated SANS scattering intensity versus wave number q histograms for a hard sphere sample normalised to simulated H20 calibration spectra. The intensity relations reflect the integrated count rates in the H20 spectra.

1230 F. Mezei / Physica B 234-236 (1997) 1227-1232

diaphragm (just outside the bulk shielding for the spallation sources) to detector (18 m) in order to obtain equal angular resolution at equal beam cross-section. For the LPSS and the SPSS instruments two slow choppers are assumed to eliminate frame overlap (cf. Ref. [1]). This makes 33 ms overlap-free data-collection time available within the 40 ms repetition frame of the LPSS, which efficiency is about equal to the transmission of velocity selectors on reactors. The curves show the normalised count rates after adding all data channels (TOF and multidetector) belonging to the same nominal q value. Since the resolution on both LPSS and SPSS is better than required and assumed for the ILL experiment (10% in 62/2), the neutron count rate finally depends only on the proton-beam power per pulse, which is two times higher for the lower repetition rate LPSS. The 5 MW power of the SPSS was assumed to be shared in the ratio 2 : 1 between the two necessary target stations and the SPSS performance is roughly independent of the choice of the target station (since on the higher frequency target station the wavelength band is narrower.) With respect to Eq. (5), we have here g about 0.5 for the LPSS and 0.08 for the SPSS, for the latter the peak flux is about 3 times higher on the coupled cold neutron moderator than that of the LPSS [2].

The second example concerns medium-resolution powder diffraction, comparing a standard, 9 m moderator to sample and 1 m sample to detector distance high-intensity diffractometer on a decoupled, high-intensity water moderator on the SPSS target station which receives 2 of the power. The LPSS instrument is designed to give comparable resolution. The neutron spectra of the SPSS moderator and the coupled moderator used in a LPSS (Fig. 2) were taken from Ref. [3] together with the temporal lineshapes. For the Maxwellian part of the spectra these latter correspond to a time constant of 26 gs for the high-intensity and to 110 and 900 gs, respectively, for the two roughly equal intensity components for the coupled moderator [3]. The decay constant of the epithermal slowing down component was supposed to follow the well- known ~ oc 2 law with z = 7 ps at l A. The LPSS instrument consisted of a pair of parallel rotating choppers with 200 Hz rotation frequency at 4 and

==

o

I 011

1 0 TM

1 0 °

1 0 e

1 0 7

1 0 "s

i

o

, , . . . . . . I . . . . , , , , I . . . . . . . I • f I l i H

1 0 .2 10 - 1 1 0 ° 101

N e u t r o n e n e r g y (eV)

Fig. 2. Time averaged brightness of two room temperature H20 moderators for 1 gA, 800 MeV proton beam (after Ref. I-21): the decoupled "high intensity" one is appropriate for a medium resolution SPSS diffractometer and the "coupled" one is adequate for all thermal neutron LPSS beams.

4.3 m from the moderator, respectively, which de- termined the pulse length to vary between 250 gs at 0.6 A and 500 ps at 4 A wavelength [4], a 25 Hz frame-overlap chopper at 9 m, a 6 cm x 6 cm cross-section, 53 m long Ni coated guide starting at 27 m from the source and a converging guide section of 8 m length coated with 30Ni supermirrors compressing the beam to a 2 cm x 2 cm sample dimension, giving a total moderator to sample distance of 88 m. This guide system provides above 1 ~, identical beam divergence on the sample with the SPSS device. The sample to detector distance is 2 m, and there is an additional 25 Hz chopper, the wavelength band definition chopper with 20 ms opening time within a slit in the guide at 45 m from the source. The detector banks for both instruments are placed in the high-resolution position covering scattering angles from 140 ° to 170 ° . The detectors are 3 cm thick 10 bar 3He gas detectors, position sensitive with 1-2 cm resolution. The neutron counts registered as a function of TOF and position are converted into lattice spacing histograms. The sample is polycrystalline Ni with a strong incoherent scattering contribution, which reflects the incoming beam spectrum. The LPSS data were taken in two frames, one from 0.6 to

o

2.4 A and the other one for half the time from 2.4 to o

4.2 A. Assuming that this division of measuring time is about optimal ( f = 1), the SPSS instrument

F. Mezei / Physica B 234-236 (1997) 1227-1232 1231

I I

I LPSS 5 MW25 Hz 1,4x107 =

1,2x107

1,0x107

8,0x106

6,0x106

4'0x106 ~ 2,0x106

0,0 1,4x107 i

1,2x107

1,0xl 0 7 1

8,0X106

6,0X106

4,0xl 06

2,0X106 ~

i 0,0 ___._L_ 0,5

, t A IA E ~.~ I I I

¢" SPSS 5 M W 50 Hz

1,0

d[A] 1,5 2,0

Fig. 3. Simulated powder diffraction counting rate versus lattice spacing histograms for a Ni sample, including incoherent scattering. The vertical scales are identical in the two plots for equal total measuring times. In the LPSS experiment (top) this time is shared between two wavelength ranges, as described in the text.

displays a n f f a c t o r of 0.25 for its average repetition rate of 33 Hz; cf. Eq. (5). The simulated spectra in Fig. 3 clearly show that the SPSS instruments is only really superior at wavelengths below 1 A, in particular, in view of the fact that a cost equivalent LPSS should provide 3 -4 times higher flux than the equal power i.e. half price version, considered here, which already offers a peak flux of more

than 10 times that of ILL (100 M W beam power during the pulse).

4. Conclusions The two examples of detailed neutron-scattering

instrument performance simulations illustrate the

1232 F. Mezei / Physica B 234-236 (1997) 1227-1232

following general conclusions [1]: The LPSS approach provides for superior performance (cost/ benefit ratio) compared to the SPSS option in the cold-neutron range and in the Maxwellian (ther- malized) thermal neutron regime, while the SPSS approach is superior in the epithermal range (and also in the slowing down regime of thermal neutrons from high-resolution cold moderators, which case is not illustrated here.) Therefore, the LPSS option has to be considered in optimising the global performance of an advanced spallation source by taking advantage of the strength of both the short and long pulse approaches while sharing the same linac. As stand-alone facilities LPSS promises a cost effective way of increasing neutron intensities by over an order of magnitude in neutron scattering experiments currently best performed on high- flux reactors, within the complementarity of

reactors and SPSS (e.g. ILL and ISIS). It has to be emphasised, however, that the feasibility of building spallation targets for average proton beam powers in the 5 MW range (a question we could not discuss within the limits of this contribution) has not yet been definitively established either for LPSS or SPSS.

5. References

[1] F. Mezei, in: Proc. ICANS XIII, eds. G.S. Bauer and R. Bercher (PSI, Villingen, 1995) p. 400.

[2] E. Pitcher, GJ. Russell, P.A. Seeger and P.D. Ferguson, ibid., p. 323.

[3] P.D. Ferguson, G.J. Russell and E.J. Pitcher, ibid., p. 510.

[4] F. Mezei, in: Proc. ICANS XII, eds. U. Steigenberger et al. (RAL, Didcot, 1994) p. 1-377.

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Long pulse spallation sources