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Photoneutron production from 34- and 100-MeV electrons in thick uranium targets
NUCLEAR INSTRUMENTS AND METHODS 51 (I967) 339-340; © NORTH-HOLLAND PUBLISHING CO. LETTERS TO THE E D I T O R
P H O T O N E U T R O N PRODUCTION FROM 34- AND 100-MeV ELECTRONS IN T H I C K U R A N I U M TARGETS* R. G. ALSMILLER, Jr. and H. S. MORAN Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.
Received 6 February 1967 The number of photoneutrons produced by 34- and 100-MeV electrons in targets of natural uranium and of 2asU are calculated as a function of target thickness. For '2zsUthe number of neutrons produced by photofission is calculated separately. In the calcula-
tions only photoproduction is considered; that is, no attempt is made to correct or account for neutron-induced fission in the target.
Photoneutrons produced by high-energy electrons in thick targets are often used as an experimental source of low-energy neutrons, and it is therefore of interest to study this production as a function of electron energy, target thickness and material. In our previous paperl), electron-photon cascade calculations and neutron yields from high-energy (30-200 MeV) electrons in a variety of thick, nonfissionable targets were presented. In this paper, similar calculations are carried out, and results on photoneutron production, including production from photofission, in thick targets of natural uranium and 235U are given. Calculated results of the photoproduction in natural
uranium, which are in substantial agreement with those obtained here but which were based on very approximate electron-photon cascade calculations, have previously been given by MacGregor2). The electron-photon cascade calculations are carried out as in 1), using the IBM-7090 code written by Zerby and Moran3-5). From the calculated photon track length and measured photoneutron-production cross sections, the photoneutron production is calculated as before. In 1) it was assumed that all produced neutronescape from the target so the photoneutron production in the target could be equated with the photoneutron yield from the target. In uranium this is not a valid assumption because of the possibility of neutrons induced fission in the target. In the results presented
* Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. (x,IO - 4 )
'
1
NATURAL URANIUM • 235 u I o BARBER AND GEORGE ( E X P E R I M E N T ) ---PHOTOFISSION PRODUCTION IN 235U _ __
Z 0 r~
-F-0 = t 0 0 M e V - ~ . ~ o ~
600
<. 0 z z 0 F-
o 400 Q
g
/
Z 0
!
I
!
M,V'
eo=,OO M~v~
.
__'3----
F-
200 o I0 'lQ_
4
"-'~--'--'~--t
Eo
34 MeV
~.
j,
2
4
6
TARGET THICKNESS ( R A D L E N G T H S } ( t R A D
~0 LENGTH= 0.288cm)
Fig. 1. Photoneutron production in uranium vs target thickness. 339
340
R . G . A L S M I L L E R , JR. A N D H. S. M O R A N
here, only production is considered, i.e., no attempt is made to correct or account for neutron-induced fission in the target. The targets considered are cylindrical in shape with a radius of 1 cm. In carrying out the calculations, it was assumed that the photon track length calculated for a target of Z38U could be used for targets of natural uranium and of 235U. The photoneutron-production cross section for natural uranium was taken from the measurements of Gavrilov and Lazareva6), Nathans and HalpernV), and Jones and TerwilligerS). The production cross section for 235U for photon energies of less than 22 MeV was taken from the work of Bowman et al. 9) and above 22 MeV was assumed to be constant. This latter assumption is reasonable since the measurements of Jones and Terwilliger s) on a variety of elements, including natural uranium, show the cross section to be slowly varying at the higher photon energies and since there are few high-energy photons produced in the target. In fig. 1 the photoneutron production from 34- and 100-MeV electrons in targets of natural uranium and of 235U is shown as a function of target thickness. The solid curves give the results for natural uranium while the plotted points give the results for 235U. Also shown in the figure are the experimental neutron yields from 34-MeV electrons in natural uranium obtained by Barber and George1°). The production in natural uranium and in 235U is, for all practical purposes, the same for both incident energies considered. This does not mean that the yields from the two materials would be the same, but it does indicate that any difference in the yields is due to neutron-induced fission and not to photoproduction. This conclusion is obvious since it seems reasonable to assume that the photon track length is the same in the two materials, as we have done, and since there is not much difference in the measured production cross sections. The neutron-yield measurements of Barber and George 1°) are in rather good agreement with the calculated photoproduction. This result, however, is somewhat inexplicable because, in the large target used in the experiment (a cross-sectional area of approximately 4½" on a side), one would expect neutron-
induced fission to lead to significant neutron multiplication in the target2). The production shown in fig. 1 includes a contribution from photofission. In the case of 235U, the production cross section from photofission has been measured by Bowman et al. 9) so this contribution can be calculated explicitly. The measured cross section extends only to photon energies of 19 MeV and was assumed to be zero above this energy. The dashed curves in fig. 1 show the calculated neutron production from photofission for incident electron energies of 34 and 100 MeV. Because the cross section has been taken to be zero above 19 MeV, the values shown must be considered to be lower limits. This assumption of a zero cross section at the higher energies is rather arbitrary but is justified to some extent by the fact that there are few high-energy photons. For example, if the cross section is assumed to be constant above 19 MeV, the values given by the dashed curve with E o = 100 MeV will be increased by less than 30% at all thicknesses and by only about 15% at the large thicknesses. The photoneutron production from a 100-MeV electron in a cylindrical target of uranium with a radius of 1 cm and a thickness of 10 radiation lengths is approximately 2 times that in a tantalum target of the same dimensions. Since 40% of the production in 23sU is due to photofission, this increased production, at least in 135U, is due to a large extent to photofission.
References 1) R. G. Alsmiller, Jr. and H. S. Moran, O R N L TM-1502 (1966)
and Nucl. Instr. and Meth. 48 (1967) 109. 2) M. H. MacGregor, Neutron production by electron bombarding of uranium, General Atomics GAMD-6805 (1965). 3) C. D. Zerby and H. S. Moran, Studies of the longitudinal development of high-energyelectron-photon cascade showers in copper, ORNL-3329 (1962). 4) C. D. Zerby and H. S. Moran, A Monte Carlo calculation of the three-dimensional development of high-energy electronphoton cascade showers, ORNL TM-422 (1962). 5) C. D. Zerby and H. S. Moran, Neutron Phys. Div. Ann. Progr. Rept. (Aug., 1963) ORNL-3499, Vol. If, p. 3. 6) B. J. Gavrilov and L. E. Lazareva, Soviet Phys. JETP 3 (1957) 871. 7) R. Nathans and J. Halpern, Phys. Rev. 93 (1954) 437. s) L. W. Jones and K. M. Terwilliger, Phys. Rev. 91 (1953) 699. 9) C. D. Bowman, G. F. Auchampaugh and S. C. Fultz, Phys. Rev. 133 (1964) B676. 10) W. C. Barber and N. D. George, Phys. Rev. 116 (1959) 1551.
P H O T O N E U T R O N PRODUCTION FROM 34- AND 100-MeV ELECTRONS IN T H I C K U R A N I U M TARGETS* R. G. ALSMILLER, Jr. and H. S. MORAN Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.
Received 6 February 1967 The number of photoneutrons produced by 34- and 100-MeV electrons in targets of natural uranium and of 2asU are calculated as a function of target thickness. For '2zsUthe number of neutrons produced by photofission is calculated separately. In the calcula-
tions only photoproduction is considered; that is, no attempt is made to correct or account for neutron-induced fission in the target.
Photoneutrons produced by high-energy electrons in thick targets are often used as an experimental source of low-energy neutrons, and it is therefore of interest to study this production as a function of electron energy, target thickness and material. In our previous paperl), electron-photon cascade calculations and neutron yields from high-energy (30-200 MeV) electrons in a variety of thick, nonfissionable targets were presented. In this paper, similar calculations are carried out, and results on photoneutron production, including production from photofission, in thick targets of natural uranium and 235U are given. Calculated results of the photoproduction in natural
uranium, which are in substantial agreement with those obtained here but which were based on very approximate electron-photon cascade calculations, have previously been given by MacGregor2). The electron-photon cascade calculations are carried out as in 1), using the IBM-7090 code written by Zerby and Moran3-5). From the calculated photon track length and measured photoneutron-production cross sections, the photoneutron production is calculated as before. In 1) it was assumed that all produced neutronescape from the target so the photoneutron production in the target could be equated with the photoneutron yield from the target. In uranium this is not a valid assumption because of the possibility of neutrons induced fission in the target. In the results presented
* Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. (x,IO - 4 )
'
1
NATURAL URANIUM • 235 u I o BARBER AND GEORGE ( E X P E R I M E N T ) ---PHOTOFISSION PRODUCTION IN 235U _ __
Z 0 r~
-F-0 = t 0 0 M e V - ~ . ~ o ~
600
<. 0 z z 0 F-
o 400 Q
g
/
Z 0
!
I
!
M,V'
eo=,OO M~v~
.
__'3----
F-
200 o I0 'lQ_
4
"-'~--'--'~--t
Eo
34 MeV
~.
j,
2
4
6
TARGET THICKNESS ( R A D L E N G T H S } ( t R A D
~0 LENGTH= 0.288cm)
Fig. 1. Photoneutron production in uranium vs target thickness. 339
340
R . G . A L S M I L L E R , JR. A N D H. S. M O R A N
here, only production is considered, i.e., no attempt is made to correct or account for neutron-induced fission in the target. The targets considered are cylindrical in shape with a radius of 1 cm. In carrying out the calculations, it was assumed that the photon track length calculated for a target of Z38U could be used for targets of natural uranium and of 235U. The photoneutron-production cross section for natural uranium was taken from the measurements of Gavrilov and Lazareva6), Nathans and HalpernV), and Jones and TerwilligerS). The production cross section for 235U for photon energies of less than 22 MeV was taken from the work of Bowman et al. 9) and above 22 MeV was assumed to be constant. This latter assumption is reasonable since the measurements of Jones and Terwilliger s) on a variety of elements, including natural uranium, show the cross section to be slowly varying at the higher photon energies and since there are few high-energy photons produced in the target. In fig. 1 the photoneutron production from 34- and 100-MeV electrons in targets of natural uranium and of 235U is shown as a function of target thickness. The solid curves give the results for natural uranium while the plotted points give the results for 235U. Also shown in the figure are the experimental neutron yields from 34-MeV electrons in natural uranium obtained by Barber and George1°). The production in natural uranium and in 235U is, for all practical purposes, the same for both incident energies considered. This does not mean that the yields from the two materials would be the same, but it does indicate that any difference in the yields is due to neutron-induced fission and not to photoproduction. This conclusion is obvious since it seems reasonable to assume that the photon track length is the same in the two materials, as we have done, and since there is not much difference in the measured production cross sections. The neutron-yield measurements of Barber and George 1°) are in rather good agreement with the calculated photoproduction. This result, however, is somewhat inexplicable because, in the large target used in the experiment (a cross-sectional area of approximately 4½" on a side), one would expect neutron-
induced fission to lead to significant neutron multiplication in the target2). The production shown in fig. 1 includes a contribution from photofission. In the case of 235U, the production cross section from photofission has been measured by Bowman et al. 9) so this contribution can be calculated explicitly. The measured cross section extends only to photon energies of 19 MeV and was assumed to be zero above this energy. The dashed curves in fig. 1 show the calculated neutron production from photofission for incident electron energies of 34 and 100 MeV. Because the cross section has been taken to be zero above 19 MeV, the values shown must be considered to be lower limits. This assumption of a zero cross section at the higher energies is rather arbitrary but is justified to some extent by the fact that there are few high-energy photons. For example, if the cross section is assumed to be constant above 19 MeV, the values given by the dashed curve with E o = 100 MeV will be increased by less than 30% at all thicknesses and by only about 15% at the large thicknesses. The photoneutron production from a 100-MeV electron in a cylindrical target of uranium with a radius of 1 cm and a thickness of 10 radiation lengths is approximately 2 times that in a tantalum target of the same dimensions. Since 40% of the production in 23sU is due to photofission, this increased production, at least in 135U, is due to a large extent to photofission.
References 1) R. G. Alsmiller, Jr. and H. S. Moran, O R N L TM-1502 (1966)
and Nucl. Instr. and Meth. 48 (1967) 109. 2) M. H. MacGregor, Neutron production by electron bombarding of uranium, General Atomics GAMD-6805 (1965). 3) C. D. Zerby and H. S. Moran, Studies of the longitudinal development of high-energyelectron-photon cascade showers in copper, ORNL-3329 (1962). 4) C. D. Zerby and H. S. Moran, A Monte Carlo calculation of the three-dimensional development of high-energy electronphoton cascade showers, ORNL TM-422 (1962). 5) C. D. Zerby and H. S. Moran, Neutron Phys. Div. Ann. Progr. Rept. (Aug., 1963) ORNL-3499, Vol. If, p. 3. 6) B. J. Gavrilov and L. E. Lazareva, Soviet Phys. JETP 3 (1957) 871. 7) R. Nathans and J. Halpern, Phys. Rev. 93 (1954) 437. s) L. W. Jones and K. M. Terwilliger, Phys. Rev. 91 (1953) 699. 9) C. D. Bowman, G. F. Auchampaugh and S. C. Fultz, Phys. Rev. 133 (1964) B676. 10) W. C. Barber and N. D. George, Phys. Rev. 116 (1959) 1551.