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The SNQ thermal neutron source diane and its experimental options
Physica 136B (1986) 114-116 North-Holland, Amsterdam
THE SNQ THERMAL NEUTRON SOURCE DIANE AND ITS EXPERIMENTAL OPTIONS
B. A L E F E L D , G.S. B A U E R , H. C O N R A D and H.H. S T I L L E R Projekt Spallatins-Neutronenquelle, Kernforschungsanlage Jiilich GmbH, Postfach 19 13, D-5170 Jiilich, Fed. Rep. Germany The neutron source of the SNQ project is designed to combine a high time average level (HFR-regime, around 1015 cm 2 s - l ) and a pronounced time structure of the thermal neutron flux. Pulse duration and repetition rate have been chosen according to experimental needs and existing technical and physical limitations as 250/xs and 100 Hz respectively, resulting in an expected peak flux of about 4 × 10 TM cm 2 s 1 and a peak-to-average ratio of 35. Instruments have been conceived which make optimal use of this situation, leading to considerable gains for almost all types of instruments, with the actual figures depending on the particular instrument. Even "notorious cw-spectrometers" will profit from the time structure in practical operation because of short data collection periods and efficient suppression of background and spurious effects.
1. Introduction
Steady state high flux reactors and pulsed spallation neutron sources constitute different approaches to the solution of the problem of obtaining a high signal rate at the detector in a neutron scattering experiment. They are essentially linked to energy selection techniques using crystals on the one hand and time of flight on the other. While crystal methods are ideally suited for continuous sources, time-of-flight techniques require pulsed beams which, on a steady state reactor imply exorbitant losses and hence are competitive only on intrinsically pulsed sources. The SNQ neutron target station D I A N E is designed to have the best of both worlds: With its high time average thermal neutron flux of 1 . 2 x 1015 cm 2 S-1 it will be competitive with a high flux reactor even for the very extreme continuousbeam techniques such as conventional triple axes spectroscopy, but its neutrons will be delivered in pulses whose peak value is about 35 times more intense than the average flux value. This enhances chopper-based TOF-techniques by a factor 35 relative to a comparable high flux reactor and thus will inevitably shift the figure of merit in the direction of time-of-flight techniques. 2. The neutron source DIANE
The SNQ facility is designed to be driven by a
linear accelerator which can supply pulses of varible duration and repetition rate but is limited in its pulse current and time average power. Thus a trade-off between pulse length and repetition rate is possible in such a way as to suit the experimental needs best. The reference combination selected is: proton pulse duration tp = 250/xs; repetition rate u = 100Hz. This choice was based on the following arguments: The decay time of a neutron field in a HzOmoderator as planned for D I A N E is about z 120/xs according to experimental results [1]. If fast neutrons are injected into such a moderator at a constant rate and during a time period tp, the time dependence of the resulting thermal neutron field is roughly given by • (t) = ~as(1 -- exp(-t/~-)),
for O~ t----
@(t) = q)as(1 -- exp(-tp/Z)) × exp(-(t -
tp)/r),
for t -> tp ,
where qbas is the neutron flux in the moderator that would be obtained, if the fast neutrons would be injected for very long time_(but at the same rate). The time average flux is @ = @astpu with the duty factor tp u = 2.5 × 10 -2 for the SNQ. The shape of the thermal neutron pulse for various pulse durations (and repetition rates) is shown in fig. la. Fig. lb shows the product ~ u = ( ~ / t p ) ( 1 -- exp(-tp/t)). Obviously, with the
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
B. Alefeld et al. / The S N Q thermal neutron source D I A N E 200 100 i
fp 250 500 750 ps v 100 /50 /33...... s 1 ¢
'I
50
25 v[s1]
10 \\ \
i
(a) O0
500
tips]
(b)
I
soo
i
k
tp[l~S]
Fig. 1. a) Time-dependent thermal neutron flux at a constant rate of fast neutron injection into a moderator with a decay time of the thermal neutron field of 120/xs and for different fast neutron pulse length which, at q~ = const., correspond to different pulse repetition rates u. b) The product ~v of peak flux and repetition rate resulting from the various cases of fig. la.
time average flux remaining constant, the peak flux does not grow in inverse proportion to the repetition rate with longer pulses: Hence, for TOF-instruments, for which a chopper will be synchronized to the peak of the pulse, a higher repetition rate is to be preferred. (This is in compliance with certain operating considerations of the linac which also call for short pulses.) For neutrons of less than 4 ,~ wavelength a practical upper limit of 100 Hz results from the fact that there exists a time dependent background correlated with the proton pulses and an average flight path around 10 m.
3. Three classes of instruments
115
and thus reduce considerably all sorts of background and "spurious" effects. It is estimated that for standard triple axes spectrometers and similar instruments this results in a figure of merit of about two due to improved significance of the data collected.
Instruments using the peak flux Instruments using time-of-flight techniques to define one (or more) of the resolution elements will be designed such as to take advantage of the peak flux of the source, e.g. by proper phasing of the choppers. Relative to similar instruments on a continuous source of the same time average flux the figure of merit will be up to 35. This substantial enhancement may lead to a shift of emphasis towards this class of instruments if the same information can be obtained by different techniques. Instruments especially adapted to the D I A N E time structure The option of exploiting the DIANE time structure by designing instruments which, while using the peak flux, at the same time use the full time period between two pulses, has triggered an avalanche of ideas [2]. Although it is true that many of the proposed spectrometers are based on older ideas so far not capitalized up on, the fact that considerable additional gains can be achieved makes it worthwhile to consider more sophisticated experimental techniques than presently in use.
With its unique combination of high time average flux and a pronounced time structure, it can be forseen, that a source like DIANE will host three classes of instruments for neutron scattering:
Spectrometers which essentially use the time average flux These instruments will collect all relevant information in very short time intervals at an instantaneous rate about 35 times higher than at an equivalent continuous source. While eventually requiring fast data acquisition techniques, this allows one to gate the detector in such a way, that it is open only while "relevant" neutrons arrive
4. A true step forward
The gains to be expected for various types of instruments at a source like SNQ-DIANE relative to an HFR of similar average flux were analyzed in detail [3] and are summarized in table I. Some of the figures may become significantly higher, if new techniques that have been proposed mature (see also [4]). These figures are exclusively based on the exploitation of the time structure and refer to instruments of "best design" at the HFR, not necessarily to existing ones.
B. Alefeld et al. / The S N Q thermal neutron source D I A N E
116.
Table I Summary on an intensity comparison for instruments at SNQ and an HFR of similar ~th Type of instrument
Gain
TOF powder diffractometer (200 m) res. 2
× 10 -4
TOF single crystal diffractometer LiSo4 cell = 6/~ protein cell = 50/~ protein cell = 300/~ Small angle scattering
beam O 10 mm, 10mm, 1 mm,
res. 10% 1% 10%
35 10-35 9-30 3 2 20 12
Diffuse scattering
7
TAS conventional tripple axes spectrometer
1
Voxp = 250 Hz Vexp= 100 Hz
14 17
MAX, inverted TOF with multi arm analyzer
10
TOF inelastic thermal neutrons cold neutrons
IRIS* inverted TOF with backscattering resol.: 10-50/zeV analyzer analyzer MUSIC multi-crystal backscattering, res. 0.1 to 1/zeV Spin echo
res. 20 neV
12-15 12 3-6
* Designed (for higher resolution) and under construction at SNS.
References [1] G.S. Bauer, H. Conrad, H. Spitzer, K. Friedrich and G. Milleret J/il-Conf-45 (181) 475-488. [2] B. Alefeld, in: Realisierungsstudie zur Spallations-Neutronenquelle, Tell 1 KFA-Report J/il-Spez-ll3/KfK 3175 (1981).
[3] R.Scherm and H. Stiller, eds. Proceedings of the Workshop on Neutron Scattering Instrumentation for SNQ KFA-Report J/i1-1954 (1984). [4] G.S. Bauer and R. Scherm, Physica 136B (1986) 80 (these proceedings).
THE SNQ THERMAL NEUTRON SOURCE DIANE AND ITS EXPERIMENTAL OPTIONS
B. A L E F E L D , G.S. B A U E R , H. C O N R A D and H.H. S T I L L E R Projekt Spallatins-Neutronenquelle, Kernforschungsanlage Jiilich GmbH, Postfach 19 13, D-5170 Jiilich, Fed. Rep. Germany The neutron source of the SNQ project is designed to combine a high time average level (HFR-regime, around 1015 cm 2 s - l ) and a pronounced time structure of the thermal neutron flux. Pulse duration and repetition rate have been chosen according to experimental needs and existing technical and physical limitations as 250/xs and 100 Hz respectively, resulting in an expected peak flux of about 4 × 10 TM cm 2 s 1 and a peak-to-average ratio of 35. Instruments have been conceived which make optimal use of this situation, leading to considerable gains for almost all types of instruments, with the actual figures depending on the particular instrument. Even "notorious cw-spectrometers" will profit from the time structure in practical operation because of short data collection periods and efficient suppression of background and spurious effects.
1. Introduction
Steady state high flux reactors and pulsed spallation neutron sources constitute different approaches to the solution of the problem of obtaining a high signal rate at the detector in a neutron scattering experiment. They are essentially linked to energy selection techniques using crystals on the one hand and time of flight on the other. While crystal methods are ideally suited for continuous sources, time-of-flight techniques require pulsed beams which, on a steady state reactor imply exorbitant losses and hence are competitive only on intrinsically pulsed sources. The SNQ neutron target station D I A N E is designed to have the best of both worlds: With its high time average thermal neutron flux of 1 . 2 x 1015 cm 2 S-1 it will be competitive with a high flux reactor even for the very extreme continuousbeam techniques such as conventional triple axes spectroscopy, but its neutrons will be delivered in pulses whose peak value is about 35 times more intense than the average flux value. This enhances chopper-based TOF-techniques by a factor 35 relative to a comparable high flux reactor and thus will inevitably shift the figure of merit in the direction of time-of-flight techniques. 2. The neutron source DIANE
The SNQ facility is designed to be driven by a
linear accelerator which can supply pulses of varible duration and repetition rate but is limited in its pulse current and time average power. Thus a trade-off between pulse length and repetition rate is possible in such a way as to suit the experimental needs best. The reference combination selected is: proton pulse duration tp = 250/xs; repetition rate u = 100Hz. This choice was based on the following arguments: The decay time of a neutron field in a HzOmoderator as planned for D I A N E is about z 120/xs according to experimental results [1]. If fast neutrons are injected into such a moderator at a constant rate and during a time period tp, the time dependence of the resulting thermal neutron field is roughly given by • (t) = ~as(1 -- exp(-t/~-)),
for O~ t----
@(t) = q)as(1 -- exp(-tp/Z)) × exp(-(t -
tp)/r),
for t -> tp ,
where qbas is the neutron flux in the moderator that would be obtained, if the fast neutrons would be injected for very long time_(but at the same rate). The time average flux is @ = @astpu with the duty factor tp u = 2.5 × 10 -2 for the SNQ. The shape of the thermal neutron pulse for various pulse durations (and repetition rates) is shown in fig. la. Fig. lb shows the product ~ u = ( ~ / t p ) ( 1 -- exp(-tp/t)). Obviously, with the
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
B. Alefeld et al. / The S N Q thermal neutron source D I A N E 200 100 i
fp 250 500 750 ps v 100 /50 /33...... s 1 ¢
'I
50
25 v[s1]
10 \\ \
i
(a) O0
500
tips]
(b)
I
soo
i
k
tp[l~S]
Fig. 1. a) Time-dependent thermal neutron flux at a constant rate of fast neutron injection into a moderator with a decay time of the thermal neutron field of 120/xs and for different fast neutron pulse length which, at q~ = const., correspond to different pulse repetition rates u. b) The product ~v of peak flux and repetition rate resulting from the various cases of fig. la.
time average flux remaining constant, the peak flux does not grow in inverse proportion to the repetition rate with longer pulses: Hence, for TOF-instruments, for which a chopper will be synchronized to the peak of the pulse, a higher repetition rate is to be preferred. (This is in compliance with certain operating considerations of the linac which also call for short pulses.) For neutrons of less than 4 ,~ wavelength a practical upper limit of 100 Hz results from the fact that there exists a time dependent background correlated with the proton pulses and an average flight path around 10 m.
3. Three classes of instruments
115
and thus reduce considerably all sorts of background and "spurious" effects. It is estimated that for standard triple axes spectrometers and similar instruments this results in a figure of merit of about two due to improved significance of the data collected.
Instruments using the peak flux Instruments using time-of-flight techniques to define one (or more) of the resolution elements will be designed such as to take advantage of the peak flux of the source, e.g. by proper phasing of the choppers. Relative to similar instruments on a continuous source of the same time average flux the figure of merit will be up to 35. This substantial enhancement may lead to a shift of emphasis towards this class of instruments if the same information can be obtained by different techniques. Instruments especially adapted to the D I A N E time structure The option of exploiting the DIANE time structure by designing instruments which, while using the peak flux, at the same time use the full time period between two pulses, has triggered an avalanche of ideas [2]. Although it is true that many of the proposed spectrometers are based on older ideas so far not capitalized up on, the fact that considerable additional gains can be achieved makes it worthwhile to consider more sophisticated experimental techniques than presently in use.
With its unique combination of high time average flux and a pronounced time structure, it can be forseen, that a source like DIANE will host three classes of instruments for neutron scattering:
Spectrometers which essentially use the time average flux These instruments will collect all relevant information in very short time intervals at an instantaneous rate about 35 times higher than at an equivalent continuous source. While eventually requiring fast data acquisition techniques, this allows one to gate the detector in such a way, that it is open only while "relevant" neutrons arrive
4. A true step forward
The gains to be expected for various types of instruments at a source like SNQ-DIANE relative to an HFR of similar average flux were analyzed in detail [3] and are summarized in table I. Some of the figures may become significantly higher, if new techniques that have been proposed mature (see also [4]). These figures are exclusively based on the exploitation of the time structure and refer to instruments of "best design" at the HFR, not necessarily to existing ones.
B. Alefeld et al. / The S N Q thermal neutron source D I A N E
116.
Table I Summary on an intensity comparison for instruments at SNQ and an HFR of similar ~th Type of instrument
Gain
TOF powder diffractometer (200 m) res. 2
× 10 -4
TOF single crystal diffractometer LiSo4 cell = 6/~ protein cell = 50/~ protein cell = 300/~ Small angle scattering
beam O 10 mm, 10mm, 1 mm,
res. 10% 1% 10%
35 10-35 9-30 3 2 20 12
Diffuse scattering
7
TAS conventional tripple axes spectrometer
1
Voxp = 250 Hz Vexp= 100 Hz
14 17
MAX, inverted TOF with multi arm analyzer
10
TOF inelastic thermal neutrons cold neutrons
IRIS* inverted TOF with backscattering resol.: 10-50/zeV analyzer analyzer MUSIC multi-crystal backscattering, res. 0.1 to 1/zeV Spin echo
res. 20 neV
12-15 12 3-6
* Designed (for higher resolution) and under construction at SNS.
References [1] G.S. Bauer, H. Conrad, H. Spitzer, K. Friedrich and G. Milleret J/il-Conf-45 (181) 475-488. [2] B. Alefeld, in: Realisierungsstudie zur Spallations-Neutronenquelle, Tell 1 KFA-Report J/il-Spez-ll3/KfK 3175 (1981).
[3] R.Scherm and H. Stiller, eds. Proceedings of the Workshop on Neutron Scattering Instrumentation for SNQ KFA-Report J/i1-1954 (1984). [4] G.S. Bauer and R. Scherm, Physica 136B (1986) 80 (these proceedings).