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On the neutronics of SINQ, a continuous spallation neutron source
116
Nuclear Instruments and Methods in Physics Research,A249 (1986) 116-122 North-Holland, Amsterdam
ON THE NEUTRONICS OF SINQ, A CONTINUOUS SPALLATION NEUTRON SOURCE W.E . FISCHER SIN, Swiss Institute for Nuclear Research, CH-5234 Villigen, Switzerland
The paper gives a short technical description of the principles of SINQ, the reference design of which is based on neutronics investigations at various mock-up systems. We present some of these experimental results and the corresponding performance which can be expected from this neutron source . 1. Introduction The Swiss Institute for Nuclear Research (SIN) operates a cyclotron system that produces a high-intensity proton beam of 592 MeV. The beam intensity available at present is about 150 jtA, and mainly used for meson production at two special target stations . The secondary meson beams are used for nuclear and particle physics. This research forms the main activity for SIN, although some condensed matter physics is done at a muon beam. After having passed through the meson targets, about 70% of the power of the original primary proton beam is still left . This beam is at present stopped in a beam dump . A new injector system is now being commissioned to allow routine operation of the SIN experimental program with a proton current in excess of 1 mA. With such a beam current, the construction of a spallation neutron source with a performance comparable to that of a medium-flux reactor becomes feasible. Owing to the absence of any macropulse structure in the extracted proton beam this neutron source will be of the continuous type. The layout of the accelerator system and experimental facilities, following completion of the upgrade program and including SINQ, is shown in fig. 1. The major components of SINQ are (1) an extension of the present proton channel, (2) the neutron source itself, (3) the building for the source and neutron beam-tube experimental area, (4) the neutron guide hall, and (5) the scattering instruments. 2. The spallation source - SINQ 2 .1 . Principle
The source is to be mounted on an extension of the present proton channel beyond target "E" in a new 0168-9002/86/$03 .50 C Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
building on the east side of the experimental hall. This building is to contain the source itself and the experimental area for instruments requiring beam tubes. The noninteracting part of the proton beam will be collected beyond target "E", deflected donwnwards to pass under the beam-tube area, thence to the centre of the source and finally pitched vertically upwards to the spallation target . The proton energy at the spallation target will range from 560 to 590 MeV (depending on the thickness of target "E") with an initial intensity of 1 mA . A vertical section through the central region of the source is shown in fig. 2. The target consists of a "pot" of Pb/Bi eutectic mixture (molten) of inner diameter about 15 cm and 2-4 m high . Most of the beam power is deposited in the bottom 30 cm of the target . Natural convection is to be used to transport this heat to an exchanger mounted at the top of the target and hence away from the source [1]. The "active" region of the target (the bottom 30 cm) is located at the centre of a tank of Dz0 with a diameter and height of about 2 m. The Dz0 thermalizes the fast neutrons resulting from high-energy proton interactions with the Pb/Bi in the target. Design work is presently active for : (1) The target plug including the heat exchanger and the cold traps for volatile spallation products [2]. This whole system is being designed as an entity and to have a lifetime of more than one operational year . (2) The beam entrance collimator system in front of the spallation target . Flasks for the exchange and disposal of the target plug and the collimator system . (4) The D20 system . (5) The proton beam extension to the spallation target . 2 .2. Beam tube layout and instrumentation
A horizontal cut through the source, showing the layout for the beam tubes, is shown in fig. 3 . The
W.E. Fischer / The neutronics of SLNQ
117 v
L
ô on v a a L ro L b oq L c ro
ä z a C O U cC L
c 0 c
L O C O U L
d
U . 0 x v L
w c E v 0
u 00 ro z L
ô
. U
u..
â
W. E. Fischer/ Th e neutronics of SINQ
truncated heptagon is the fixed part of the source but with the shielding around the beam tubes mounted in boxes to allow these to be rearranged as the source develops during operation. External to the source will be mounted monochromators ; these will require shielding which is more easily constructed onto flat faces (hence the heptagon). 3. Neutronics experiments An experimental program for the investigation of the neutronics of spallation neutron sources has been realized at a secondary proton beam at SIN in collaboration with KFA-Jülich and KfK-Karlsruhe. A rather large variety of mockup systems has been examined [3]. In the following we present, however, only those that turned out to be relevant for the reference design of the SIN neutron source. 3.1 . Bare target
Fig. 2. Schematic view of the vertical section through the central region of the source .
HETC calculations for a 10 cm radius target give an escape evaporation neutron intensity of 10.4/proton, with an average energy of 1.7 MeV . The calculated distribution of surface brightness is shown in fig. 4 together with the measured values . The neutrons leaving the target were detected by
O N
W
0
Fig. 3. Horizontal cut through the source, showing the layout for beam tubes and instruments .
WE. Fischer / The neutronics of SINQ NEUTRONS/ Cm 7 P 10 -3
20
10
DISTANCE
30 FROM
FRONT
40 OF
TARGET
50
ICm1
Fig. 4. Surface brightness for fast neutron escapes from a 10 cm radius Pb/Bi target . Histogram - calculation, solid line experiment . activation of rhodium mounted onto the surface of the target cylinders. The high energy particle escapes per incident proton calculated are: neutrons 0.59 of mean energy 71 MeV, protons 0.008 of mean energy 100 MeV, P1 + 0.002 of mean energy 54 MeV, Pi0.0006 of mean energy 46 MeV,
Both measurements were performed with a primary proton beam intensity of 1 nA and gave consistent results. We present here results for the following configurations: A) Pb/Bi target+ D20 moderator. B) Pb/Bi target + 3 cm air gap (void) + D20 moderator. C) Pb/Bi target + 5 cm Be + D20 moderator. D) Dep.U target + 6 cm Be + D20 moderator. For the purpose of discussion, (A) is taken as a reference system. The thermal flux as a function of radius for two axial distances along the target is shown in fig. 6 for systems (A), (B) and (C). For further comparison, the flux maps for cases (A) and (C) are shown in fig. 7 . (The flux in the figures below is normalized to a proton beam intensity of 1 mA). The 3 cm void of case (B) leads to a peak thermal flux depression of 15%, with an outward shift of approximately 5 cm. At larger radii, the flux penalty is of the order of 8% . The effect of the void is to create an additional leakage path for neutrons .
4 n
SK
/
0
3.2. Moderated system
A general layout of the target-moderator system examined is shown in fig. 5. A cylindrical target of 15 cm diameter and 60 cm length is surrounded by a D20 moderator in a moderator tank. The size of the moderator corresponds to a D20 cube with a side length of 170 cm . Flux measurements for thermal neutrons in the D20 have been made by means of two methods : a) with a net of Dy foils in the D20 a flux map was established ; b) the outflux of thermal neutrons through a beam tube was measured with a BF3 counter.
t
\
v
v
s
09
,\
0. 5
\
Z=45cm
Al-Tube 200 0
I AI-Tube 1500
~m
02 fl-Level
I" ---<~I~rrlrç f~lm .%' -l Pb-B1 Target I 1500
0.2
\
02 0
ur Ph/B,
AI
Fig. 5. General arrangement for thermal neutron flux measurements.
Z=12cm
OZ
0
Be
10
R 10
30
40
50
60
70 [cm]
Fig. 6. Measured variation of thermal neutron flux in a D2 0 moderator as a function of radial distance from the target axis . Z is measured from the front surface of the target. Full line : case (A), dash-dotted line : case (B), dashed line : case (C) .
WE. Fischer / The neutronus of SINQ
120
50
40
30
20
10
-20
-10
0
10
20
30
40
50 Zfcml
Fig. 7. Measured thermal-neutron flux map in a Dz0 moderator. The curves are marked with intensity as a fraction of peak flux. Full line: case (A), dashed line : case (C).
Case (C) has produced the most surprising result ; although the peak thermal neutron flux is reduced by approximately 2%, at large radii the fluxes are identical. There seems to be no overall neutron gain with a Be sleeve . In the axial direction the flux decrease with the Be sleeve is somewhat faster, as may be seen in fig. 7. The gain factor, as estimated from the measured spectra of Cierjacks et al . [4] and published Be(n, 2n) cross section values [5], was approximately 14%. The measurements indicate that the increased absorption by the Be should reduce this gain factor to about 9% . This is a significant overestimate ; of the several possibilities, we believe the most probable cause to be that the neutron spectrum used in the calculation is too hard . Further examination of this question is in progress . Case (D) has been included to give an evaluation of depleted uranium as a target material. The thermal neutron flux at the peak was increased by 70%, which should be compared to the source strength gain of 2.8 compared to Pb/Bi as measured by Bauer et al . [3] . The flux depression of 40% is caused by the absorption of neutrons in the uranium. The peak flux position is shifted outwards by approximately 4 cm, a definite advantage for installation of beam tubes. The model target, being solid, was highly unrealistic, lacking for instance any cooling medium and cladding . The considerable uranium density decrease in a technologically feasible target will lead to a further reduction of flux, which has been estimated to be at least 20% . The flux increase using depleted uranium would not
seem to justify overcoming the prodigious technological problems its use would require. 4. Neutron flux and source performance Design and performance of SINQ are based on the results from these mockup experiments . One of the flux maps, relevant to the planned SIN source, is shown together with its comparison with a Monte Carlo simulation in fig. 8. By means of flux distributions such as these, the optimal position of the cold source as well as the position and size of the beam tubes may be determined . The last experiments at the mockup model examined several configurations of cold hydrogen sources and their spectra [6]. From these data we can estimate the spectral fluxes at monochromator (or velocity selector) positions of beam tubes and neutron guides . The spectral fluxes shown in fig. 9 are based on the following tube and guide cross sections and a primary beam current of 1 .5 mA (1 MW beam power) : height h = 10 cm, width d = 5 cm. Furthermore the guides are assumed to be straight and plated with "Ni. For background [9] suppression, we will need to instal benders into the guides ; as a result, the flux at the guides of the actual source will be somewhat lower at long wavelength and the spectrum will cut off at short wavelength. It is evident from fig. 9 that experiments using a wavelength region of 2-4 f1
W. E. Fischer / The neutronrcs of SINQ
60 50 X
a 40 30
vZ 20 Q H O
a 10
AXIAL
DISTANCE
FROM
FRONT
FACE
OF TARGET
[CM)
Fig. 8. Flux map for thermal neutrons in the Dz0 moderator for 1 mA proton beam current. Dashed lines: Mock-up experiment, solid lines : Monte Carlo simulation . should be installed at beam tubes viewing one of the cold sources; for wavelengths above 4 t1 the guides give a higher flux . The flux at a thermal guide is given for reference only; it is not planned to instal these at SINQ . 5. Heat deposition by radiation fields All material in the vicinity of a spallation neutron target is heated by the neutron- and gamma-field .
Knowledge of this heat deposition in the target containment, in the moderator - particularly the cold neutron source - and in the shielding structure is essential for the design of a high-intensity source . Calculated energy depositions will be subject to many uncertainties, principally: (1) There is some discrepancy between the calculated Monte Carlo neutron spectra (HETC) and the spectrum measurements at bare spallation targets, especially in the region of high energy cascade neutrons .
Fig. 9. Spectral fluxes at monochromator positions of SINQ for tubes and guides . The primary proton beam current is 1 .5 mA .
122
W E. Fischer/ The neutronics of SINQ
Q [ mwlgr 10y A 1 50 40 30
M\
20
10
Be \ 5
\'
Zr
AI
10
50
100
ipb, Bi 200
A
Fig. 10 . Heat deposition per gram in the various samples by fast neutrons and prompt gammas . The data have been normalized to a proton current of 10 pA . The (n, y) contribution is subtracted . The dashed line and the dot-dashed line indicate the A-dependence of the contribution of the elastic and the inelastic neutron scattering contribution as obtained by a Monte Carlo calculation [9]. (2) The transfer of kinetic energy of neutrons into heat for the energy region
E > 20 MeV is, for many
materials, only marginally known.
(3) The calculations of the gamma-spectra and -inten-
heating. The enhanced heat-up due to capture of cold
neutrons - which is a considerable part of heat produc-
tion in a cold neutron source - is therefore not included .
sities for the spallation target contain uncertainties .
Although the uncertainties as individual items may be tolerable, taken together they constitute an inaccuracy that could be embarrassing. In order to obviate this problem we have chosen a pragmatic approach and measured the heat deposition in samples of various materials [8]. The thermal facility (TNF) at TRIUMF consists of a 15 .4 cm diameter cylindrical lead
target fed by a
- 500 MeV proton
beam . The target is surrounded by a D20 reflector (lower part) and a HZO reflector (upper part).
A cylindrical, evacuated aluminium tube mounted
vertically into the system could be used to mount the calorimetric samples at 10 cm distance from the surface
of the target . The experiment was run with a primary proton beam intensity of 10 ftA.
The final results are presented in fig. 10 . For the samples Fe, Cu and particularly W, a considerable heat deposition due to capture gammas (n, y) has been subtracted - hence the data correspond to the heat deposition by the fast neutron field and by gamma radiation.
The data for H and D have been extracted from measurements
at
Hz0 and 13,0, respectively
using the
stoichiometric ratios and assuming that oxygen heating is given by the average of the carbon- and aluminium
References Y. Takeda, Nucl. Instr. and Meth . A237 (1985) 455; Y. Takeda and Ch . Gerber, Proc. ICANS-VIII, Oxford (1985) . [2] F. Atchison and W.E . Fischer, Proc. ICANS-VII, ChalkRiver, AECL-8488 (1983) . G.S . Bauer, W.E . Fischer, F. Gompf, M. Küchle, W. Reichhardt, H. Spitzer, Proc. ICANS-V, Jülich (1981) ; G.S. Bauer (ed.), SNQ - Realisierungsstudie zur Spallationsquelle, Teil III; Annex A, Band 2, Jülich/Karlsruhe ; V. Drücke, N. Paul and W. Litzow, Proc . ICANS-V, Jülich (1981) .
[4] S. Cierjacks, Y. Hino, S .D . Howe, F. Raupp and L. Buth, Proc . Conf. on Nuclear Data for Science and Technology Antwerpen (Reidel, Dordrecht, 1983). B.R . Bergelson, A.P. Suvorov and B.S . Torlin, Multigroup methods of neutron shielding calculations (Atomizdat, Moscow, 1970) in Russian. [6] G.S . Bauer, H. Conrad, W.E . Fischer, K. Grünhagen and H. Spitzer, Proc . ICANS-VIII, Oxford (1985) F. Atchison, ibid . W.E . Fischer, L. Moritz, H. Spitzer and I.M . Thorson, SIN-preprint PR-83-09 (1983) to be published in Nucl . Sci. and Eng. M. Pepin, Proc . ICANS-VI, ANL-Argonne (1982) .
Nuclear Instruments and Methods in Physics Research,A249 (1986) 116-122 North-Holland, Amsterdam
ON THE NEUTRONICS OF SINQ, A CONTINUOUS SPALLATION NEUTRON SOURCE W.E . FISCHER SIN, Swiss Institute for Nuclear Research, CH-5234 Villigen, Switzerland
The paper gives a short technical description of the principles of SINQ, the reference design of which is based on neutronics investigations at various mock-up systems. We present some of these experimental results and the corresponding performance which can be expected from this neutron source . 1. Introduction The Swiss Institute for Nuclear Research (SIN) operates a cyclotron system that produces a high-intensity proton beam of 592 MeV. The beam intensity available at present is about 150 jtA, and mainly used for meson production at two special target stations . The secondary meson beams are used for nuclear and particle physics. This research forms the main activity for SIN, although some condensed matter physics is done at a muon beam. After having passed through the meson targets, about 70% of the power of the original primary proton beam is still left . This beam is at present stopped in a beam dump . A new injector system is now being commissioned to allow routine operation of the SIN experimental program with a proton current in excess of 1 mA. With such a beam current, the construction of a spallation neutron source with a performance comparable to that of a medium-flux reactor becomes feasible. Owing to the absence of any macropulse structure in the extracted proton beam this neutron source will be of the continuous type. The layout of the accelerator system and experimental facilities, following completion of the upgrade program and including SINQ, is shown in fig. 1. The major components of SINQ are (1) an extension of the present proton channel, (2) the neutron source itself, (3) the building for the source and neutron beam-tube experimental area, (4) the neutron guide hall, and (5) the scattering instruments. 2. The spallation source - SINQ 2 .1 . Principle
The source is to be mounted on an extension of the present proton channel beyond target "E" in a new 0168-9002/86/$03 .50 C Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
building on the east side of the experimental hall. This building is to contain the source itself and the experimental area for instruments requiring beam tubes. The noninteracting part of the proton beam will be collected beyond target "E", deflected donwnwards to pass under the beam-tube area, thence to the centre of the source and finally pitched vertically upwards to the spallation target . The proton energy at the spallation target will range from 560 to 590 MeV (depending on the thickness of target "E") with an initial intensity of 1 mA . A vertical section through the central region of the source is shown in fig. 2. The target consists of a "pot" of Pb/Bi eutectic mixture (molten) of inner diameter about 15 cm and 2-4 m high . Most of the beam power is deposited in the bottom 30 cm of the target . Natural convection is to be used to transport this heat to an exchanger mounted at the top of the target and hence away from the source [1]. The "active" region of the target (the bottom 30 cm) is located at the centre of a tank of Dz0 with a diameter and height of about 2 m. The Dz0 thermalizes the fast neutrons resulting from high-energy proton interactions with the Pb/Bi in the target. Design work is presently active for : (1) The target plug including the heat exchanger and the cold traps for volatile spallation products [2]. This whole system is being designed as an entity and to have a lifetime of more than one operational year . (2) The beam entrance collimator system in front of the spallation target . Flasks for the exchange and disposal of the target plug and the collimator system . (4) The D20 system . (5) The proton beam extension to the spallation target . 2 .2. Beam tube layout and instrumentation
A horizontal cut through the source, showing the layout for the beam tubes, is shown in fig. 3 . The
W.E. Fischer / The neutronics of SLNQ
117 v
L
ô on v a a L ro L b oq L c ro
ä z a C O U cC L
c 0 c
L O C O U L
d
U . 0 x v L
w c E v 0
u 00 ro z L
ô
. U
u..
â
W. E. Fischer/ Th e neutronics of SINQ
truncated heptagon is the fixed part of the source but with the shielding around the beam tubes mounted in boxes to allow these to be rearranged as the source develops during operation. External to the source will be mounted monochromators ; these will require shielding which is more easily constructed onto flat faces (hence the heptagon). 3. Neutronics experiments An experimental program for the investigation of the neutronics of spallation neutron sources has been realized at a secondary proton beam at SIN in collaboration with KFA-Jülich and KfK-Karlsruhe. A rather large variety of mockup systems has been examined [3]. In the following we present, however, only those that turned out to be relevant for the reference design of the SIN neutron source. 3.1 . Bare target
Fig. 2. Schematic view of the vertical section through the central region of the source .
HETC calculations for a 10 cm radius target give an escape evaporation neutron intensity of 10.4/proton, with an average energy of 1.7 MeV . The calculated distribution of surface brightness is shown in fig. 4 together with the measured values . The neutrons leaving the target were detected by
O N
W
0
Fig. 3. Horizontal cut through the source, showing the layout for beam tubes and instruments .
WE. Fischer / The neutronics of SINQ NEUTRONS/ Cm 7 P 10 -3
20
10
DISTANCE
30 FROM
FRONT
40 OF
TARGET
50
ICm1
Fig. 4. Surface brightness for fast neutron escapes from a 10 cm radius Pb/Bi target . Histogram - calculation, solid line experiment . activation of rhodium mounted onto the surface of the target cylinders. The high energy particle escapes per incident proton calculated are: neutrons 0.59 of mean energy 71 MeV, protons 0.008 of mean energy 100 MeV, P1 + 0.002 of mean energy 54 MeV, Pi0.0006 of mean energy 46 MeV,
Both measurements were performed with a primary proton beam intensity of 1 nA and gave consistent results. We present here results for the following configurations: A) Pb/Bi target+ D20 moderator. B) Pb/Bi target + 3 cm air gap (void) + D20 moderator. C) Pb/Bi target + 5 cm Be + D20 moderator. D) Dep.U target + 6 cm Be + D20 moderator. For the purpose of discussion, (A) is taken as a reference system. The thermal flux as a function of radius for two axial distances along the target is shown in fig. 6 for systems (A), (B) and (C). For further comparison, the flux maps for cases (A) and (C) are shown in fig. 7 . (The flux in the figures below is normalized to a proton beam intensity of 1 mA). The 3 cm void of case (B) leads to a peak thermal flux depression of 15%, with an outward shift of approximately 5 cm. At larger radii, the flux penalty is of the order of 8% . The effect of the void is to create an additional leakage path for neutrons .
4 n
SK
/
0
3.2. Moderated system
A general layout of the target-moderator system examined is shown in fig. 5. A cylindrical target of 15 cm diameter and 60 cm length is surrounded by a D20 moderator in a moderator tank. The size of the moderator corresponds to a D20 cube with a side length of 170 cm . Flux measurements for thermal neutrons in the D20 have been made by means of two methods : a) with a net of Dy foils in the D20 a flux map was established ; b) the outflux of thermal neutrons through a beam tube was measured with a BF3 counter.
t
\
v
v
s
09
,\
0. 5
\
Z=45cm
Al-Tube 200 0
I AI-Tube 1500
~m
02 fl-Level
I" ---<~I~rrlrç f~lm .%' -l Pb-B1 Target I 1500
0.2
\
02 0
ur Ph/B,
AI
Fig. 5. General arrangement for thermal neutron flux measurements.
Z=12cm
OZ
0
Be
10
R 10
30
40
50
60
70 [cm]
Fig. 6. Measured variation of thermal neutron flux in a D2 0 moderator as a function of radial distance from the target axis . Z is measured from the front surface of the target. Full line : case (A), dash-dotted line : case (B), dashed line : case (C) .
WE. Fischer / The neutronus of SINQ
120
50
40
30
20
10
-20
-10
0
10
20
30
40
50 Zfcml
Fig. 7. Measured thermal-neutron flux map in a Dz0 moderator. The curves are marked with intensity as a fraction of peak flux. Full line: case (A), dashed line : case (C).
Case (C) has produced the most surprising result ; although the peak thermal neutron flux is reduced by approximately 2%, at large radii the fluxes are identical. There seems to be no overall neutron gain with a Be sleeve . In the axial direction the flux decrease with the Be sleeve is somewhat faster, as may be seen in fig. 7. The gain factor, as estimated from the measured spectra of Cierjacks et al . [4] and published Be(n, 2n) cross section values [5], was approximately 14%. The measurements indicate that the increased absorption by the Be should reduce this gain factor to about 9% . This is a significant overestimate ; of the several possibilities, we believe the most probable cause to be that the neutron spectrum used in the calculation is too hard . Further examination of this question is in progress . Case (D) has been included to give an evaluation of depleted uranium as a target material. The thermal neutron flux at the peak was increased by 70%, which should be compared to the source strength gain of 2.8 compared to Pb/Bi as measured by Bauer et al . [3] . The flux depression of 40% is caused by the absorption of neutrons in the uranium. The peak flux position is shifted outwards by approximately 4 cm, a definite advantage for installation of beam tubes. The model target, being solid, was highly unrealistic, lacking for instance any cooling medium and cladding . The considerable uranium density decrease in a technologically feasible target will lead to a further reduction of flux, which has been estimated to be at least 20% . The flux increase using depleted uranium would not
seem to justify overcoming the prodigious technological problems its use would require. 4. Neutron flux and source performance Design and performance of SINQ are based on the results from these mockup experiments . One of the flux maps, relevant to the planned SIN source, is shown together with its comparison with a Monte Carlo simulation in fig. 8. By means of flux distributions such as these, the optimal position of the cold source as well as the position and size of the beam tubes may be determined . The last experiments at the mockup model examined several configurations of cold hydrogen sources and their spectra [6]. From these data we can estimate the spectral fluxes at monochromator (or velocity selector) positions of beam tubes and neutron guides . The spectral fluxes shown in fig. 9 are based on the following tube and guide cross sections and a primary beam current of 1 .5 mA (1 MW beam power) : height h = 10 cm, width d = 5 cm. Furthermore the guides are assumed to be straight and plated with "Ni. For background [9] suppression, we will need to instal benders into the guides ; as a result, the flux at the guides of the actual source will be somewhat lower at long wavelength and the spectrum will cut off at short wavelength. It is evident from fig. 9 that experiments using a wavelength region of 2-4 f1
W. E. Fischer / The neutronrcs of SINQ
60 50 X
a 40 30
vZ 20 Q H O
a 10
AXIAL
DISTANCE
FROM
FRONT
FACE
OF TARGET
[CM)
Fig. 8. Flux map for thermal neutrons in the Dz0 moderator for 1 mA proton beam current. Dashed lines: Mock-up experiment, solid lines : Monte Carlo simulation . should be installed at beam tubes viewing one of the cold sources; for wavelengths above 4 t1 the guides give a higher flux . The flux at a thermal guide is given for reference only; it is not planned to instal these at SINQ . 5. Heat deposition by radiation fields All material in the vicinity of a spallation neutron target is heated by the neutron- and gamma-field .
Knowledge of this heat deposition in the target containment, in the moderator - particularly the cold neutron source - and in the shielding structure is essential for the design of a high-intensity source . Calculated energy depositions will be subject to many uncertainties, principally: (1) There is some discrepancy between the calculated Monte Carlo neutron spectra (HETC) and the spectrum measurements at bare spallation targets, especially in the region of high energy cascade neutrons .
Fig. 9. Spectral fluxes at monochromator positions of SINQ for tubes and guides . The primary proton beam current is 1 .5 mA .
122
W E. Fischer/ The neutronics of SINQ
Q [ mwlgr 10y A 1 50 40 30
M\
20
10
Be \ 5
\'
Zr
AI
10
50
100
ipb, Bi 200
A
Fig. 10 . Heat deposition per gram in the various samples by fast neutrons and prompt gammas . The data have been normalized to a proton current of 10 pA . The (n, y) contribution is subtracted . The dashed line and the dot-dashed line indicate the A-dependence of the contribution of the elastic and the inelastic neutron scattering contribution as obtained by a Monte Carlo calculation [9]. (2) The transfer of kinetic energy of neutrons into heat for the energy region
E > 20 MeV is, for many
materials, only marginally known.
(3) The calculations of the gamma-spectra and -inten-
heating. The enhanced heat-up due to capture of cold
neutrons - which is a considerable part of heat produc-
tion in a cold neutron source - is therefore not included .
sities for the spallation target contain uncertainties .
Although the uncertainties as individual items may be tolerable, taken together they constitute an inaccuracy that could be embarrassing. In order to obviate this problem we have chosen a pragmatic approach and measured the heat deposition in samples of various materials [8]. The thermal facility (TNF) at TRIUMF consists of a 15 .4 cm diameter cylindrical lead
target fed by a
- 500 MeV proton
beam . The target is surrounded by a D20 reflector (lower part) and a HZO reflector (upper part).
A cylindrical, evacuated aluminium tube mounted
vertically into the system could be used to mount the calorimetric samples at 10 cm distance from the surface
of the target . The experiment was run with a primary proton beam intensity of 10 ftA.
The final results are presented in fig. 10 . For the samples Fe, Cu and particularly W, a considerable heat deposition due to capture gammas (n, y) has been subtracted - hence the data correspond to the heat deposition by the fast neutron field and by gamma radiation.
The data for H and D have been extracted from measurements
at
Hz0 and 13,0, respectively
using the
stoichiometric ratios and assuming that oxygen heating is given by the average of the carbon- and aluminium
References Y. Takeda, Nucl. Instr. and Meth . A237 (1985) 455; Y. Takeda and Ch . Gerber, Proc. ICANS-VIII, Oxford (1985) . [2] F. Atchison and W.E . Fischer, Proc. ICANS-VII, ChalkRiver, AECL-8488 (1983) . G.S . Bauer, W.E . Fischer, F. Gompf, M. Küchle, W. Reichhardt, H. Spitzer, Proc. ICANS-V, Jülich (1981) ; G.S. Bauer (ed.), SNQ - Realisierungsstudie zur Spallationsquelle, Teil III; Annex A, Band 2, Jülich/Karlsruhe ; V. Drücke, N. Paul and W. Litzow, Proc . ICANS-V, Jülich (1981) .
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