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Gamma flash suppression for the ORELA pulsed neutron source
14UCLEAR INSTRUMENTS AND METHODS 91 (1971) 79- 84 ; (0, NORTH-HOLLAND PUBLISHING CO .
GAMMA FLASH SUPPRESSION FOR THE ORELA PULSED NEUTRON SOURC!']* R. L. MACKLIN Oak Ridge National Laboratory, Oak
Tennessve, U.S .A .
Received 3 August 1970 A Siumm my flash suppression system for neutron time-of-flight experintents at the Oak Ridge Electron Linear Accelerator (ORELA) has been found very effective for partial cross section section measi#ements. For capture cross measurements at 40 m
the residual pileup flash pulse from a typical sampl~ - is well *ithin the neutron binding energy range, where the detector respon-V is linenr. This has allowed two parameter (time and pulse height) analysis down to 0.5 psec after the flash.
. iinuodwflon In using a pulsed electron beam for neutron time-ofRight partial cross section measurementzi. a chain of particle reactions is involved. First, the eneFgefic electrons an stopped in a target producing plama. rays (bremssftMung). These separate neutrons from the nuclei of the same target and/or secondary photoneutron tupt. At a considerable distance (the flight path), typically 25-40 m, the neutrons are absorbed by nuclei of the sample being studied and the reaction products detected as a function of time after the initiating electron burst. For neutron radiative capture, particularly, the detectors must accurately measure the prompt capture gamma iays from the sample, yet not be overloaded too severely by the several million times more intense gamma pileup flash from the primary target. The gamma rays emerging from the target are somewhat degraded from the typical (thin target) bremsstrahlung spectrum. The energy spectrum is still approximately hyperbolic from the electron energy down to a few MeV, and pronounced forward peaking is still evident. Typical high-Z beam filters such as 2 "U and Pb can reduce the gamma intensity from the target by a few orders of magnitude at the expense of appreciable neutron attenuation and degradation of the time and energy resolution. Also, electronic techniques have been developed for detector recovery, including fast photomultiplier electrode gating") but these compromise the stability and linearity of the detection system. The essentially geometric "shadow bar" system to be described has achieved the full Jlash reduction required to bring the flash pulse in capture cross section detectors down to the linear range, Z 10 1,AcV, without using these techniques . In addition to gamma flash suppression a number of other requirements had to be considered, such as good
time, resolution, high energy neutron suppression, adequate shielding outside but close to the beam and maximum moderated neutron flux. Neutrons from a few MeV to 140 MeV present in the beam could gi ,%v "time-dependent background" effects similar to garrima flash recovery problems and these two mechanisms had not been experimentally separated.
I
Research sponsored by the U. S. Atomic EnerW1 Commission under contract with the Union Carbide Corporation .
2. Electron source characteristics The ORELA beam used for high-resolution neutron time-of-flight experiments has a nominal 15 A electron current with 140 MeV energy. per electron in bur-sts as short as 2 x 10 - ' sec. Fig. I shows a time spectrum of the photons produced by stopping the beam). The beam size is about 1 .5 x 2.0 cin at the target as determined by foil activation') . 5
CI, F C 0 ~,, % 7 S WITHIN F\N~- 10 1
-
1%
WITHN TWICE F~"Ai
I "WE ( n S eC
1
Fig. 1. Time distribution of the shvrlesE ekx%ixvn bi-cnixstrah pulses from ORELA . The nc-gligible 1-Wing, :md dark, cuvrei'll enhinec the achievable dynamic rangr fov hi_gh- ,vso1kst,,,-n neuiron time of-file,ht expenmetnts-
R . L . MACKLIN
VARIABLE H 2 0 THICKNESS
20
'10-1100
0.8 To + 0.2 H20 BY VOLUME
kov
I
1-10 k9V FIGURE OF MERIT 1- 10 k!V~p
MeV - i keV
IMIZ H 20
NEUTRON SOURCE POINT
10- 100 av
M 0.5
3.0 3.5 1.5 2 .0 2 .5 HpO THICKNESS (Cin) Fig . 2. 12-Ompuler fluxes and figure of merit for a ring (or halo) moderator as a function of its thickness .
0
1 .0
I
3. Neutron target And moderator veral model 'targets and moderators were studied"). To handle the designed ~O kW electron beam, water cooled tantalum (rather than tungsten or ura nium) was chosen to stop the electrons on the basis of contemporary favorable experience with it at Rensselaer Polytechnic Institute) . The first figure in that ref. 7 shows the geometric arrangement used there to suppress the gamma Bash. The tantalum target was placed just outside the collimated flight path line-oft. The neutron moderator in that arrangement at 135~~ t,j the electron beam direction intercepts only ,out 71% of the flux from the tantalum target, bw .avoids the forward intensity neak of the ga.,:nma rays. ring go.-oinetry with a central coniczl plUg 8) 01 , ow bar") would allow the moderator to intercept nearly haic, the. flux from the tantalum while still provid2ng shielding against the direct flux'es of gammas w-id high energy neutrons from the tantalum. The gamma rays and high energy neutrons scattered by the moderater ring (or halo) lose considerable energy in ger.-ral, and those of the highest energies
have an appreciable probability of passing straight through the moderator to the walls of the target cell. In the Monte Carlo model studies intensity and resolution were investigated for 1-10' eV neutrons as a function ofinoderator configuration and dimensions . A moderator slab beside the target or a pair on each side with B4C decoupling layers did not appear advantageous compared to the halo design chosen. This last was a disc with the target centered in a . hole at its center. A (radial) slot was cut for the electron beam. The recommended disc radius of 7.5 cm (14,0) was calculated to give 90% of the infinite radius intensity, calculated for a point isotropic source of I MeV neutrons at the center of the target. Fig..,2 indicates calculated quantities for several thicknesses of moderator. The total intensity appears to rise somewhat even beyond the 3.2 cm moderator thickness finally adopted and this is an advantage in some experiments at the lower energies . The target and moderator assembly is shown in fig. 3. For high resolution work a figure of merit is more nearly the ratio of the intensity o the square of the time spread (or the equivalent flight path
GAMMA FLASH SUPPRESSION FOR THE ORELA PULSED NEUTRON SOURCE
length uncertainty) introduced by the moderator. The figure of merit (dashed line, -fig. 2) is seen to peak near 2.3 cm but not to fall off rapidly at a greater moderator thickness. In addition to intensity, resolution and flux calculations for a flight path normal to the moderator face (i.e. perpendicular to the electron beam), the same quantities were tabulated for Right paths 15' and 30* from the normal . A particular flight path, for example, located at about 13' to the normal gives a computed resolution due to the 3 .2 cm thick moderator, equivalent to a spread of 3.2 cm (fwhm) in flight path. The intensity from the moderator is calculated to be 6% less than for the flight paths normal to the moderator . The energy spectrum below a few tens of keV is roughly of the form E-0 * 8-2 (per eV), as also calculated by Michaudon") and found experimentally") . This corresponds to an average leakage probability per collision in the moderator of ~k-. 8%. The fraction of neutrons reaching thermal energies in the moderator is estimated to be 3 x 10` and 12% of these should be captured by hydrogen to give 2 .2 MeV gamma rays. This thermal capture should show an exponential period of about 24 psec in such a small high leakage system. Thus, for a flight path of
40 m, the ratio of 2.2 MeV gamma flux to neutron nux should peak for neutrons of about 10 keV energ), w 5 x 10-4. 4. CoHimation and beam dependent background Copper was chosen for the collimator jaws and the shadow bar for several! reasons . The elastic scattering angular distributions are not too strongly for%vard peaked and the inelasticz- scattering is strong. The de~islty is fairly high leading to neutron mean free paths near 4 cm even at many tens of IvIeV . With two siable isotopes, interference minima in the ross section :!, present little problem, the worst is at 7 .2 keV %vhere the mean free path rises to about 6.5 cm. The cross section is much smaller than for the aigh Z materials, yet the energy loss on elastic scattering is mw:h less than fz)r the low Z materials . A total of 57 cm of copper was used for neutron attenuation, backed LJJ) with 15 cm of lead for additional gamma ray ;.Itenuation. The scatter and trap design is indicated in fig . 4 as ii was used for Monte! Carlo calculations'- 12) . The borated concrete regions are kept out of the direct beam after the first collimator and out of the line of' sight from the target through the last collimator.
1.iK-in .-wide, 2.000--in .-high' TANTALUM PLATES THICKNE-SSES FROM .060 TO .310 in .
--- ALUMINUM HOUSING
WATER OUT
81
WATER IN
Fig. 3. The high power (50 kW) water cooled tintalurn neutron target and moderator assembly for ORFLA.
R . L. MACKLIN CONCRETE
Cu SOURCE
Cu
C
-20 -40
CURVED TARGET CELL WALL APPROXIMATED BY A PLANE SLAB
0
0 .5
1 .0
.
I
1 40 m
201 m 1
1 .5 2 .0 2 .5 HORIZONTAL DISTANCE (cm)
3 .0
3.5
(X 10 3 )
4 Simplified collimation geometry used for Monte Carlo studies of neutron beam degradation.
-~,~itrons scattered bythecoppercanbemoderated in the borated concrete with little chance n.- The Monte Carlo results indicate only lux at 40 m has lost > 0. 1% of its energy > 0.05% late compared to the unscattered %lost neutrons suffaing less than five collisions , :311imGon system and then reaching the dein within these resolution limits. Indeed, all such scatter4d neutrons remain "good"'. ewSy loss in the degraded tail ranged CV to W-201% at a féw kéV. rgle~; up to 35 keV, overall neutron backeasumd with a "'BF3 chamber and "black fifters are less than 0.5%"). At 105 keV cm sulfur filter and "'B(may) detector the ~ jjjjtt i S 1 %. 1rhe Steep rise of the 27AI (n,n ,' 1050 keV (fig. 5) indicates a similar limit V.
(0.5 mm. Ta) the residual gamma flux in the beam induces a 500 keV pulse height in each (of two) detectors for 2.5 ns pulses . Using fast tunnel diode dis-
vi-f4) 1rumflee
instrumented with the r and collimation system describe& The -riment utilizes non-hydrogenous liquid just outside the beam to detect neutron anma rays from stable isotope samples") . I , ~ai gamma flash scattered into the detectors outputs are summed) is much less than ~-i ;) binding energy for 0.1 gram mole samples cniolution conditions (electron pulses 2-5 curftnt). W`ith a typical high Z sample iHustrates, a ffight path
I i
2.0
j 1-8
I
1 .6
I 1 .0 iA 1 .2 NEUTRON ENERGY WeV)
0.8
Fig. 5. Neutron induced gamma ray production from aluminum, principally the yield of the 1013 keV inelastic scattering level, as a function of neutron time-of-flight (hence, energy).
83
GAMMA FLASH SUPPRESSION FOR THE ORELA PULSED NEUTRON SOURCE
HYDRAULICALLY OPERATED BEAM STOP SAMPLE CHANGERS (4)
SHADOW BAR CENTERED IN FIRST COLUAPATOR
RESONANCE BLACKOUT FILTERS (AJ.CF2 , $)
GAMMA RAY SCINTILLATION DETECTORS(2)
PLASTIC SCINTILLATOR FLASH DETECTOR
INDICAT1,1RS - AND COffrROLS
Sm
.1
PULSED ELECTRON BEAM (140MeV 15 A)
TARGET CELL
FUTURE EXPERIMENT PROFILE OF ORELA FLIGHT PATH 7 FINAL COLLIMATOR (DOUBLE TAPERED)
TYPICAL INTERMEDIATE COLLIMATOR
Fill. 6. Stable isotope neutron capture cross section experiment and flight path layout at ORELA . The shadow bar is located near 3 m in the wall of the target cell.
crimination, the random background level is observed in an interval from 0.3 to 0.4,usec after each linac pulse. As pulse height analysis earlier than 0.5 psee has not been achieved, shorter recovery times than 0.3 psec for the timing pulses have not been needed in the two parameter experiment. The neutron flux has been detected above random background beyond 0.4 ptsec after the gamma flash, corresponding to an upper neutron energy limit near 30 MeV. A few millimeters of lead as a fliter in the beam reduces the gamma flash nearly another order of magnitude and this is necessary for other experiments where a detector must be placed directly in the beam. More c"reful alignment of the shadow bar and target may also further reduce the gamma flash as moderate misplacement has been observed to increase the flash pulse three orders of magnitude' 1). At some sacrifice of neutron intensity, the gamma flash intensity can be reduced a further order of magnitude by also shadowing the region of the moderator ring forward of the electron beam. Since the gamma
my flux is peaked in that direction and the neutron distribution is nearly isotropic this appro,%ch may be valuable in some experiments . J. H. Gibbons contributed many helpful discussions to the design of this equipment . B. J. Allen has underexperimental work with the flight taken much of theThe path illustrated. ORELA co-directors, J. A. Harvey and F. C. Maienschein, have contributed advice, criticism, and support of the design and construction. Referenccs
R. A. Schrack, H. T. Heaton 11 and R. B. Schwartz, Nucl. Instr. and Meth. 77 (1970) 17S ; and earlier references therein . 2) Gulf General Atomic ieports a recent major improvement in GA-9751 (1969) . L. W. Weston and J. 11. Todd, Oak Ridge National Laboratory Technical Memo 2833 (1970) unpublished . 4) J. L. Rodda 11, Nucl. 1nstr. and Meth. 80 (1970) 333 . 5) R. L. Macklin, G. de Saussure, R . C. Block and later L. W. Weston served on the ORELA Target and Cell Planning Committee . 1)
I
R. L. MACKLIN tan, A. M . Craig and J. G. Sullivan adapted the Carlo code to the model studies . For relevant cascade calculations sec: G. R. Alsmiller, Jr. ,1'oran, Nucl. Instr. and Meth. 48 (1967) 109 . ; nbujy et al., ftys. Rev. 178 (1969) 1746. Jr., in Fast neutron physics (Interscience, New ,~I
IWJ) ch. IVE.
9) 10) 11) 12) 13)
P. H. StOson et al ., Nucl . Phys. 68 (1965) 97. A. Michaudon, J. Nucl . Energy A/B 17 (1963) 165. G. de Saussure, private communication . R. L. Macklin, L. W. Weston, and G. de Saussure served on the ORELA Flight Path Committee . R. L. Macklin and J. H. Gibbons, Phys. Rev. 159 (1967) 1007.
GAMMA FLASH SUPPRESSION FOR THE ORELA PULSED NEUTRON SOURC!']* R. L. MACKLIN Oak Ridge National Laboratory, Oak
Tennessve, U.S .A .
Received 3 August 1970 A Siumm my flash suppression system for neutron time-of-flight experintents at the Oak Ridge Electron Linear Accelerator (ORELA) has been found very effective for partial cross section section measi#ements. For capture cross measurements at 40 m
the residual pileup flash pulse from a typical sampl~ - is well *ithin the neutron binding energy range, where the detector respon-V is linenr. This has allowed two parameter (time and pulse height) analysis down to 0.5 psec after the flash.
. iinuodwflon In using a pulsed electron beam for neutron time-ofRight partial cross section measurementzi. a chain of particle reactions is involved. First, the eneFgefic electrons an stopped in a target producing plama. rays (bremssftMung). These separate neutrons from the nuclei of the same target and/or secondary photoneutron tupt. At a considerable distance (the flight path), typically 25-40 m, the neutrons are absorbed by nuclei of the sample being studied and the reaction products detected as a function of time after the initiating electron burst. For neutron radiative capture, particularly, the detectors must accurately measure the prompt capture gamma iays from the sample, yet not be overloaded too severely by the several million times more intense gamma pileup flash from the primary target. The gamma rays emerging from the target are somewhat degraded from the typical (thin target) bremsstrahlung spectrum. The energy spectrum is still approximately hyperbolic from the electron energy down to a few MeV, and pronounced forward peaking is still evident. Typical high-Z beam filters such as 2 "U and Pb can reduce the gamma intensity from the target by a few orders of magnitude at the expense of appreciable neutron attenuation and degradation of the time and energy resolution. Also, electronic techniques have been developed for detector recovery, including fast photomultiplier electrode gating") but these compromise the stability and linearity of the detection system. The essentially geometric "shadow bar" system to be described has achieved the full Jlash reduction required to bring the flash pulse in capture cross section detectors down to the linear range, Z 10 1,AcV, without using these techniques . In addition to gamma flash suppression a number of other requirements had to be considered, such as good
time, resolution, high energy neutron suppression, adequate shielding outside but close to the beam and maximum moderated neutron flux. Neutrons from a few MeV to 140 MeV present in the beam could gi ,%v "time-dependent background" effects similar to garrima flash recovery problems and these two mechanisms had not been experimentally separated.
I
Research sponsored by the U. S. Atomic EnerW1 Commission under contract with the Union Carbide Corporation .
2. Electron source characteristics The ORELA beam used for high-resolution neutron time-of-flight experiments has a nominal 15 A electron current with 140 MeV energy. per electron in bur-sts as short as 2 x 10 - ' sec. Fig. I shows a time spectrum of the photons produced by stopping the beam). The beam size is about 1 .5 x 2.0 cin at the target as determined by foil activation') . 5
CI, F C 0 ~,, % 7 S WITHIN F\N~- 10 1
-
1%
WITHN TWICE F~"Ai
I "WE ( n S eC
1
Fig. 1. Time distribution of the shvrlesE ekx%ixvn bi-cnixstrah pulses from ORELA . The nc-gligible 1-Wing, :md dark, cuvrei'll enhinec the achievable dynamic rangr fov hi_gh- ,vso1kst,,,-n neuiron time of-file,ht expenmetnts-
R . L . MACKLIN
VARIABLE H 2 0 THICKNESS
20
'10-1100
0.8 To + 0.2 H20 BY VOLUME
kov
I
1-10 k9V FIGURE OF MERIT 1- 10 k!V~p
MeV - i keV
IMIZ H 20
NEUTRON SOURCE POINT
10- 100 av
M 0.5
3.0 3.5 1.5 2 .0 2 .5 HpO THICKNESS (Cin) Fig . 2. 12-Ompuler fluxes and figure of merit for a ring (or halo) moderator as a function of its thickness .
0
1 .0
I
3. Neutron target And moderator veral model 'targets and moderators were studied"). To handle the designed ~O kW electron beam, water cooled tantalum (rather than tungsten or ura nium) was chosen to stop the electrons on the basis of contemporary favorable experience with it at Rensselaer Polytechnic Institute) . The first figure in that ref. 7 shows the geometric arrangement used there to suppress the gamma Bash. The tantalum target was placed just outside the collimated flight path line-oft. The neutron moderator in that arrangement at 135~~ t,j the electron beam direction intercepts only ,out 71% of the flux from the tantalum target, bw .avoids the forward intensity neak of the ga.,:nma rays. ring go.-oinetry with a central coniczl plUg 8) 01 , ow bar") would allow the moderator to intercept nearly haic, the. flux from the tantalum while still provid2ng shielding against the direct flux'es of gammas w-id high energy neutrons from the tantalum. The gamma rays and high energy neutrons scattered by the moderater ring (or halo) lose considerable energy in ger.-ral, and those of the highest energies
have an appreciable probability of passing straight through the moderator to the walls of the target cell. In the Monte Carlo model studies intensity and resolution were investigated for 1-10' eV neutrons as a function ofinoderator configuration and dimensions . A moderator slab beside the target or a pair on each side with B4C decoupling layers did not appear advantageous compared to the halo design chosen. This last was a disc with the target centered in a . hole at its center. A (radial) slot was cut for the electron beam. The recommended disc radius of 7.5 cm (14,0) was calculated to give 90% of the infinite radius intensity, calculated for a point isotropic source of I MeV neutrons at the center of the target. Fig..,2 indicates calculated quantities for several thicknesses of moderator. The total intensity appears to rise somewhat even beyond the 3.2 cm moderator thickness finally adopted and this is an advantage in some experiments at the lower energies . The target and moderator assembly is shown in fig. 3. For high resolution work a figure of merit is more nearly the ratio of the intensity o the square of the time spread (or the equivalent flight path
GAMMA FLASH SUPPRESSION FOR THE ORELA PULSED NEUTRON SOURCE
length uncertainty) introduced by the moderator. The figure of merit (dashed line, -fig. 2) is seen to peak near 2.3 cm but not to fall off rapidly at a greater moderator thickness. In addition to intensity, resolution and flux calculations for a flight path normal to the moderator face (i.e. perpendicular to the electron beam), the same quantities were tabulated for Right paths 15' and 30* from the normal . A particular flight path, for example, located at about 13' to the normal gives a computed resolution due to the 3 .2 cm thick moderator, equivalent to a spread of 3.2 cm (fwhm) in flight path. The intensity from the moderator is calculated to be 6% less than for the flight paths normal to the moderator . The energy spectrum below a few tens of keV is roughly of the form E-0 * 8-2 (per eV), as also calculated by Michaudon") and found experimentally") . This corresponds to an average leakage probability per collision in the moderator of ~k-. 8%. The fraction of neutrons reaching thermal energies in the moderator is estimated to be 3 x 10` and 12% of these should be captured by hydrogen to give 2 .2 MeV gamma rays. This thermal capture should show an exponential period of about 24 psec in such a small high leakage system. Thus, for a flight path of
40 m, the ratio of 2.2 MeV gamma flux to neutron nux should peak for neutrons of about 10 keV energ), w 5 x 10-4. 4. CoHimation and beam dependent background Copper was chosen for the collimator jaws and the shadow bar for several! reasons . The elastic scattering angular distributions are not too strongly for%vard peaked and the inelasticz- scattering is strong. The de~islty is fairly high leading to neutron mean free paths near 4 cm even at many tens of IvIeV . With two siable isotopes, interference minima in the ross section :!, present little problem, the worst is at 7 .2 keV %vhere the mean free path rises to about 6.5 cm. The cross section is much smaller than for the aigh Z materials, yet the energy loss on elastic scattering is mw:h less than fz)r the low Z materials . A total of 57 cm of copper was used for neutron attenuation, backed LJJ) with 15 cm of lead for additional gamma ray ;.Itenuation. The scatter and trap design is indicated in fig . 4 as ii was used for Monte! Carlo calculations'- 12) . The borated concrete regions are kept out of the direct beam after the first collimator and out of the line of' sight from the target through the last collimator.
1.iK-in .-wide, 2.000--in .-high' TANTALUM PLATES THICKNE-SSES FROM .060 TO .310 in .
--- ALUMINUM HOUSING
WATER OUT
81
WATER IN
Fig. 3. The high power (50 kW) water cooled tintalurn neutron target and moderator assembly for ORFLA.
R . L. MACKLIN CONCRETE
Cu SOURCE
Cu
C
-20 -40
CURVED TARGET CELL WALL APPROXIMATED BY A PLANE SLAB
0
0 .5
1 .0
.
I
1 40 m
201 m 1
1 .5 2 .0 2 .5 HORIZONTAL DISTANCE (cm)
3 .0
3.5
(X 10 3 )
4 Simplified collimation geometry used for Monte Carlo studies of neutron beam degradation.
-~,~itrons scattered bythecoppercanbemoderated in the borated concrete with little chance n.- The Monte Carlo results indicate only lux at 40 m has lost > 0. 1% of its energy > 0.05% late compared to the unscattered %lost neutrons suffaing less than five collisions , :311imGon system and then reaching the dein within these resolution limits. Indeed, all such scatter4d neutrons remain "good"'. ewSy loss in the degraded tail ranged CV to W-201% at a féw kéV. rgle~; up to 35 keV, overall neutron backeasumd with a "'BF3 chamber and "black fifters are less than 0.5%"). At 105 keV cm sulfur filter and "'B(may) detector the ~ jjjjtt i S 1 %. 1rhe Steep rise of the 27AI (n,n ,' 1050 keV (fig. 5) indicates a similar limit V.
(0.5 mm. Ta) the residual gamma flux in the beam induces a 500 keV pulse height in each (of two) detectors for 2.5 ns pulses . Using fast tunnel diode dis-
vi-f4) 1rumflee
instrumented with the r and collimation system describe& The -riment utilizes non-hydrogenous liquid just outside the beam to detect neutron anma rays from stable isotope samples") . I , ~ai gamma flash scattered into the detectors outputs are summed) is much less than ~-i ;) binding energy for 0.1 gram mole samples cniolution conditions (electron pulses 2-5 curftnt). W`ith a typical high Z sample iHustrates, a ffight path
I i
2.0
j 1-8
I
1 .6
I 1 .0 iA 1 .2 NEUTRON ENERGY WeV)
0.8
Fig. 5. Neutron induced gamma ray production from aluminum, principally the yield of the 1013 keV inelastic scattering level, as a function of neutron time-of-flight (hence, energy).
83
GAMMA FLASH SUPPRESSION FOR THE ORELA PULSED NEUTRON SOURCE
HYDRAULICALLY OPERATED BEAM STOP SAMPLE CHANGERS (4)
SHADOW BAR CENTERED IN FIRST COLUAPATOR
RESONANCE BLACKOUT FILTERS (AJ.CF2 , $)
GAMMA RAY SCINTILLATION DETECTORS(2)
PLASTIC SCINTILLATOR FLASH DETECTOR
INDICAT1,1RS - AND COffrROLS
Sm
.1
PULSED ELECTRON BEAM (140MeV 15 A)
TARGET CELL
FUTURE EXPERIMENT PROFILE OF ORELA FLIGHT PATH 7 FINAL COLLIMATOR (DOUBLE TAPERED)
TYPICAL INTERMEDIATE COLLIMATOR
Fill. 6. Stable isotope neutron capture cross section experiment and flight path layout at ORELA . The shadow bar is located near 3 m in the wall of the target cell.
crimination, the random background level is observed in an interval from 0.3 to 0.4,usec after each linac pulse. As pulse height analysis earlier than 0.5 psee has not been achieved, shorter recovery times than 0.3 psec for the timing pulses have not been needed in the two parameter experiment. The neutron flux has been detected above random background beyond 0.4 ptsec after the gamma flash, corresponding to an upper neutron energy limit near 30 MeV. A few millimeters of lead as a fliter in the beam reduces the gamma flash nearly another order of magnitude and this is necessary for other experiments where a detector must be placed directly in the beam. More c"reful alignment of the shadow bar and target may also further reduce the gamma flash as moderate misplacement has been observed to increase the flash pulse three orders of magnitude' 1). At some sacrifice of neutron intensity, the gamma flash intensity can be reduced a further order of magnitude by also shadowing the region of the moderator ring forward of the electron beam. Since the gamma
my flux is peaked in that direction and the neutron distribution is nearly isotropic this appro,%ch may be valuable in some experiments . J. H. Gibbons contributed many helpful discussions to the design of this equipment . B. J. Allen has underexperimental work with the flight taken much of theThe path illustrated. ORELA co-directors, J. A. Harvey and F. C. Maienschein, have contributed advice, criticism, and support of the design and construction. Referenccs
R. A. Schrack, H. T. Heaton 11 and R. B. Schwartz, Nucl. Instr. and Meth. 77 (1970) 17S ; and earlier references therein . 2) Gulf General Atomic ieports a recent major improvement in GA-9751 (1969) . L. W. Weston and J. 11. Todd, Oak Ridge National Laboratory Technical Memo 2833 (1970) unpublished . 4) J. L. Rodda 11, Nucl. 1nstr. and Meth. 80 (1970) 333 . 5) R. L. Macklin, G. de Saussure, R . C. Block and later L. W. Weston served on the ORELA Target and Cell Planning Committee . 1)
I
R. L. MACKLIN tan, A. M . Craig and J. G. Sullivan adapted the Carlo code to the model studies . For relevant cascade calculations sec: G. R. Alsmiller, Jr. ,1'oran, Nucl. Instr. and Meth. 48 (1967) 109 . ; nbujy et al., ftys. Rev. 178 (1969) 1746. Jr., in Fast neutron physics (Interscience, New ,~I
IWJ) ch. IVE.
9) 10) 11) 12) 13)
P. H. StOson et al ., Nucl . Phys. 68 (1965) 97. A. Michaudon, J. Nucl . Energy A/B 17 (1963) 165. G. de Saussure, private communication . R. L. Macklin, L. W. Weston, and G. de Saussure served on the ORELA Flight Path Committee . R. L. Macklin and J. H. Gibbons, Phys. Rev. 159 (1967) 1007.