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Considerations for external targets in an intense intermediate energy proton beam
NUCLEAR INSTRUMENTS AND METHODS IOO (I972) 361-364; © NORTH-HOLLAND P U B L I S H I N G CO.
C O N S I D E R A T I O N S F O R E X T E R N A L T A R G E T S I N AN I N T E N S E I N T E R M E D I A T E E N E R G Y P R O T O N BEAM S. WYNCHANK Department of Physics, Brooklyn College, C.U.N.Y., New York, U.S.A.
Received 6 December 1971 Problems of shielding requirements and of minimisation of induced activity are considered for external Be targets in a proton beam of intensity 50 #A and energy about 600 MeV.
1. Introduction Intense external proton beams of intermediate energy are now available from new and modified accelerators. Here considerations are given for shielding needs in regions round external targets. The beam parameters used below are similar to those of the Nevis Cyclotron of Columbia University which is shortly expected to operate with an external proton beam of ~mensity between 20 p A and 50 p A and energy 565 MeV. This beam traverses in succession two Be targets, each 3 cm thick and each transmitting about 95% of the incident protons. The proton beam is finally led to a beam dump structure where it strikes a large mass of a light element. Here " b e a m stop" describes the material which the proton beam strikes directly and " b e a m d u m p " describes the total structure including beam stop and its associated cooling equipment and protective shielding. After determining the primary and secondary radiations resulting from interactions of protons with materials in their paths, shielding requirements can be described. Details of ejected particles and residual nuclei after energetic protons strike a variety of materials have been calculated, considering intranuclear cascades, by Bertini 1-4) and othersS). These calculations employ the postulate of Serber 6) which assumes that reactions between energetic incident particles and nuclei can be considered as taking place in two distinct stages. Firstly the energetic projectile interacts with effectively free particles in the nucleus. During this stage fast cascade particles are emitted. The residual nucleus is left in a highly excited state after this type of emission. In the second stage the nucleus de-excites by emitting evaporation particles of much lower energy than those from the nucleonic cascade. The number of evaporation particles emitted from a nucleus, for a given projectile, depends strongly on the nucleus' 361
position in the periodic table as is shown in table 1.
2. External target shielding Energies of cascade neutrons and protons so produced at 0 °, and 90 ° to incident 660 MeV protons on Be are shown in figs. 1 and 2. This energy distribution differs markedly from this reaction's evaporation particle distribution (figs. 3a and 3b) both in form and range of energy. All angles referred to below are with respect to the incident proton beam. Secondary cascade nucleon fluxes at angles between 0 ° and 127 ° produced from a Be target 3 cm thick have been considered. This target is presumed to be in the path of a proton beam of energy 660 MeV at a distance 25 cm from an Fe shielding wail which is parallel to the incident proton beam. The production of secondary neutron fluxes emitted in slices at a variety of angles
TABLE 1
The measured mean number (Nm) of evaporation neutrons resulting from 660 MeV protons interacting with various nuclei, per inelastic collision, from ref. 7. Nucleus
Nm
Be C At Cu Cd Sn Pb Bi U
1.54-0.2 1.5_+0.2 2.8 + 0.3 4.4 + 0.4 7.4 + 0.7 7.8 ___0.7 11.9+1.0 11.9+1.0 16.8+1.2
362
s. WYNCHANK r FRACTION OF
FRACTION OF CASCADE PROTONS
CASCADE NEUTRONS
(x2) AT 0° I 1
90 °
0.8 . . . .
0'6 i
0.6
0.4
0.4
0.2
0.2 ---]
0
I L------I.~
200
AT 900
....
'"1 I
400
I
I
600
0
AT 0° (x2)
I
i
.....
L___ I
I
]
200
I i
400
J
I
i
600
PROTON ENERGY (MeV)
NEUTRON ENERGY (MeV)
Fig. l. Energy distribution of cascade neutrons from 660 MeV protons on Be at angles 0° and 90° to the incident protons, adapted from ref. 4.
Fig. 2. Energy distribution of cascade protons produced under the same conditions as for fig. 1.
a n d their f r a c t i o n c o m p a r e d with an i s o t r o p i c p r o d u c tion are given in t a b l e 2. I t is seen t h a t the fastest cascade neutrons, w h i c h a~e the m o s t p e n e t r a t i n g , are e m i t t e d preferentially in the f o r w a r d direction. A l t h o u g h a m i n i m m n thickness o f F e is in the direct i o n p e r p e n d i c u l a r to the b e a m , the emission o f p e n e t r a t ing cascade particles is sufficiently low in this direction for there to be a m a x i m u m t r a n s m i t t e d flux at 90 ° since calculations show t h a t greater n u m b e r s o f fast n e u t r o n s e m i t t e d at small angles have p r o p o r t i o n a l l y m o r e shielding to penetrate. I f the thickness o f Fe is 4 m (fig. 4) a c a l c u l a t i o n o f t r a n s m i t t e d n e u t r o n fluxes in the various a n g u l a r ranges o f t a b l e 2 shows the greatest t r a n s m i t t e d flux is 0.01 fast n e u t r o n s / c m 2-sec a t 90 ° to t h e 50/~A p r o t o n b e a m . A thickness o f 4 m o f F e shielding is therefore adeq u a t e for all needs except those very small ranges o f n e u t r o n energies in the r e g i o n o f several h u n d r e d kiloelectron volts where interference effects cause a very small t r a n s m i s s i o n cross section. The use o f concretes l o a d e d with n e u t r o n a b s o r b e r s such as b o r o n has been previously described s) to a u g m e n t the p r o t e c t i v e effects o f Fe shielding in this case. R e c e n t i m p r o v e d m e a s u r e m e n t s o f transmission m a x i m a for n a t u r a l i r o n 9) have shown t h a t t o t a l n e u t r o n cross sections at these energies are lower t h a n h a d f o r m e r l y been believedt °).
Fluxes o f s e c o n d a r y particles a n d their a b s o r p t i o n were considered at a variety o f e l e m e n t a l areas, on the F e wall, which s u b t e n d different angles at the Be target a n d lie in directions between 0 ° and 127 °. I f a p r o t o n b e a m o f energy 660 M e V a n d intensity 5 0 / t A is a p p l i e d to a Be target o f thickness 3 cm for a y e a r the resulting m a x i m u m activity on a bare Fe w a l l i s a b o u t 4900 r a d / h , cm 2 at a distance o f 1 cm. F o r p r o t o n i r r a d i a t i o n the resulting activity is due m a i n l y to S~Mn, w i t h h a l f life 314 d, a n d some other isotopes o f M n , Cr a n d V. This is true for i r r a d i a t i n g p r o t o n energies f r o m 200 M e V to 3 G e V ~t). Also the g a m m a decay o f Fe after i r r a d i a t i o n by n e u t r o n s p r o d u c e d by 600 M e V p r o t o n s on Be is described by Barbier 12) a n d this is similar to the case for direct p r o t o n i r r a d i a tion. The i n d u c e d activity on the inner surface o f the F e can be r e d u c e d if it is lined w i t h m a r b l e (fig. 4). By
3. Induced activity The m a x i m u m i n d u c e d activity in the F e shielding due to s e c o n d a r y p a r t i c l e emission f r o m the Be is at the p o i n t m a r k e d X in fig. 4, nearest to the target. This d e d u c t i o n arose from calculations similar to those o f the previous section which d e d u c e d the direction o f the m a x i m u m n u m b e r o f t r a n s m i t t e d s e c o n d a r y particles.
TABLE 2
The percentages of cascade neutrons emitted in various angular ranges and the numbers of neutrons emitted into these angular slices with energy above 132 MeV. The isotropic distribution is also given. It is assumed that 50 FA of protons of energy 660 MeV are incident on a target of Be of thickness 3 cm. The data in column 2 are adapted from ref. 4.
0 0 ° - 18012' 18°12' - 25051, 53° 8'- 66025' 84°16' - 95°44' 113035'-126054 '
Percentage of particles emitted in range 0 All cascade Isotropic neutrons distribution 18.7 10.4 12.6 5.3 2.2
10.1 4.3 7.4 6.4 7.4
Neutrons (E=~>150 MeV) 1.09x 10"2 5.3 x 1012 6.1 × 101° 3.4 x 109 6.6 x 108
EXTERNAL
TARGETS
IN
AN
INTENSE
PROTON
363
BEAM
presuming marble to be CaCO 3 the m a x i m u m induced activity in the Fe has been calculated to be reduced by a factor of 0.45. K n o w n cross sections of Ca, C and O
VACUUM
[ ] Be PROTON BEAM
161 - 25cm
~-
Fig. 4. Schematic shielding beside an external Be target.
162
/ 0
I
I
5 10 15 20 EVAPORATION NEUTRON ENERGY (MeV)
I
25
Fig. 3a. Evaporation neutron energy distribution resulting from 660 MeV protons on Be adapted from ref. 4. The fraction of evaporation neutrons in a given energy range is plotted against energy.
0.03
0.02
0.01 0
0.5
1.0
1.5
2.0
2.5
EVAPORATION NEUTRON ENERGY (MeV)
Fig. 3b. As for fig. 3a, but over a smaller energy range where the the distribution of evaporation neutrons peaks and is shown in greater detail.
were used and the transport of neutrons through the marble was considered. The induced activity for marble has been studied by Barbier t2) in the presence of cascade neutrons resulting from 600 MeV protons interacting with Be. Absolute induced activity and decay times of marble thus irradiated are both smaller than that of Fe by factors of 102 and 103 respectively. Hence the use of marble to reduce induced activity in iron does not cause an increase in total activity. The induced activity in the Be target is not high. This element is not suitable for examination by Rudstam's theory 13) which can give spallation cross sections for production of radioactive residual nuclei only for target nuclei with A > 2 0 . However, Bertini 4) has determined from the residues after an intranuclear cascade calculation for 660 M e ¥ protons on Be, that the principal radioactive product in the Be is 7Be. It has a half life of 53.4 d. The production rates for all other conceivable long lived products, 3H and 1°Be, are negligible. Hence the calculated induced activity in a natural Be target of thickness 3 cm penetrated by 50 #A of 660 MeV protons for 1 y is ~ 1 0 mCi. The slow neutrons arising mainly as evaporation products can easily be captured by a mixture of moderator and capturer, say in a boron loaded concrete, in addition to the Fe. A further pattern of radiation leakage which has recently been recognized is earthshine under massive shielding walls. I f the calculations of Alsmiller et al. 14) are applied to a case where 2 m of concrete lie between proton beam and earth it is readily shown this leakage is negligible. This is true both for the regions around the external target and the beam dump. The author wishes to express his thanks to Dr. H. W. Bertini for communicating unpublished material and to Mr. P. M. Lorio and Dr. M. H. Holland for valuable discussions.
364
s. WYNCHANK
References 1) H. W. Bertini, Phys. Rev. 188 (1969) 1711. 2) H. W. Bertini, ORNL TM-1996 (1967). 3) H. W. Bertini, Proc. 2nd Intern. Conf. Accelerator dosimetry and experience (S.L.A.C., Stanford, Calif., 1969) p. 42. 4) H. W. Bertini, private communication (1971). a) V. Barashenkov, K. Gudima and T. Toneev, Acta Phys. Polon. 36 (1969) 887. 6) R. Serber, Phys. Rev. 72 (1947) 1114. 7) e. Vasilkov, B. Govorkov, V. Goldanskii, V. Konshin, O. Lupadin, E. Matusevich, B. Pimenov, S. Prokhorov and S. Tsypin, Soviet J. Nucl. Phys. 7 (1968) 64.
8) S. Wynchank, Nncl. Lnstr. and Meth. 93 (1971) 85. 9) F. Rahn, H. Camarda, G. Hacken, W. W. Havens, Jr., H. Liou, J. Rainwater, M. Slagowitz and S. Wynchank, Nucl. Sci. Eng. 47 (1972) 372. 10) M. Goldberg, S. Mughabghab, B. Magumo and V. May, Brookhaven National Laboratory Report no. 325, suppl. 2, 2A (1966). 11) T. Armstrong and J. Barish, Nucl. Sci. Eng. 38 (1969) 271. 12) M. Barbier, Induced radioactivity (North-Holland PuN. Co., Amsterdam, 1969). 13) G. Rudstam, Z. Naturforsch. 21A (1966) 1027. z4) R. G. A1smiller, F. Mynatt, M. Gritzner, J. Pace and J. Barish, Nucl. Instr. and Meth. 89 (1970) 53.
C O N S I D E R A T I O N S F O R E X T E R N A L T A R G E T S I N AN I N T E N S E I N T E R M E D I A T E E N E R G Y P R O T O N BEAM S. WYNCHANK Department of Physics, Brooklyn College, C.U.N.Y., New York, U.S.A.
Received 6 December 1971 Problems of shielding requirements and of minimisation of induced activity are considered for external Be targets in a proton beam of intensity 50 #A and energy about 600 MeV.
1. Introduction Intense external proton beams of intermediate energy are now available from new and modified accelerators. Here considerations are given for shielding needs in regions round external targets. The beam parameters used below are similar to those of the Nevis Cyclotron of Columbia University which is shortly expected to operate with an external proton beam of ~mensity between 20 p A and 50 p A and energy 565 MeV. This beam traverses in succession two Be targets, each 3 cm thick and each transmitting about 95% of the incident protons. The proton beam is finally led to a beam dump structure where it strikes a large mass of a light element. Here " b e a m stop" describes the material which the proton beam strikes directly and " b e a m d u m p " describes the total structure including beam stop and its associated cooling equipment and protective shielding. After determining the primary and secondary radiations resulting from interactions of protons with materials in their paths, shielding requirements can be described. Details of ejected particles and residual nuclei after energetic protons strike a variety of materials have been calculated, considering intranuclear cascades, by Bertini 1-4) and othersS). These calculations employ the postulate of Serber 6) which assumes that reactions between energetic incident particles and nuclei can be considered as taking place in two distinct stages. Firstly the energetic projectile interacts with effectively free particles in the nucleus. During this stage fast cascade particles are emitted. The residual nucleus is left in a highly excited state after this type of emission. In the second stage the nucleus de-excites by emitting evaporation particles of much lower energy than those from the nucleonic cascade. The number of evaporation particles emitted from a nucleus, for a given projectile, depends strongly on the nucleus' 361
position in the periodic table as is shown in table 1.
2. External target shielding Energies of cascade neutrons and protons so produced at 0 °, and 90 ° to incident 660 MeV protons on Be are shown in figs. 1 and 2. This energy distribution differs markedly from this reaction's evaporation particle distribution (figs. 3a and 3b) both in form and range of energy. All angles referred to below are with respect to the incident proton beam. Secondary cascade nucleon fluxes at angles between 0 ° and 127 ° produced from a Be target 3 cm thick have been considered. This target is presumed to be in the path of a proton beam of energy 660 MeV at a distance 25 cm from an Fe shielding wail which is parallel to the incident proton beam. The production of secondary neutron fluxes emitted in slices at a variety of angles
TABLE 1
The measured mean number (Nm) of evaporation neutrons resulting from 660 MeV protons interacting with various nuclei, per inelastic collision, from ref. 7. Nucleus
Nm
Be C At Cu Cd Sn Pb Bi U
1.54-0.2 1.5_+0.2 2.8 + 0.3 4.4 + 0.4 7.4 + 0.7 7.8 ___0.7 11.9+1.0 11.9+1.0 16.8+1.2
362
s. WYNCHANK r FRACTION OF
FRACTION OF CASCADE PROTONS
CASCADE NEUTRONS
(x2) AT 0° I 1
90 °
0.8 . . . .
0'6 i
0.6
0.4
0.4
0.2
0.2 ---]
0
I L------I.~
200
AT 900
....
'"1 I
400
I
I
600
0
AT 0° (x2)
I
i
.....
L___ I
I
]
200
I i
400
J
I
i
600
PROTON ENERGY (MeV)
NEUTRON ENERGY (MeV)
Fig. l. Energy distribution of cascade neutrons from 660 MeV protons on Be at angles 0° and 90° to the incident protons, adapted from ref. 4.
Fig. 2. Energy distribution of cascade protons produced under the same conditions as for fig. 1.
a n d their f r a c t i o n c o m p a r e d with an i s o t r o p i c p r o d u c tion are given in t a b l e 2. I t is seen t h a t the fastest cascade neutrons, w h i c h a~e the m o s t p e n e t r a t i n g , are e m i t t e d preferentially in the f o r w a r d direction. A l t h o u g h a m i n i m m n thickness o f F e is in the direct i o n p e r p e n d i c u l a r to the b e a m , the emission o f p e n e t r a t ing cascade particles is sufficiently low in this direction for there to be a m a x i m u m t r a n s m i t t e d flux at 90 ° since calculations show t h a t greater n u m b e r s o f fast n e u t r o n s e m i t t e d at small angles have p r o p o r t i o n a l l y m o r e shielding to penetrate. I f the thickness o f Fe is 4 m (fig. 4) a c a l c u l a t i o n o f t r a n s m i t t e d n e u t r o n fluxes in the various a n g u l a r ranges o f t a b l e 2 shows the greatest t r a n s m i t t e d flux is 0.01 fast n e u t r o n s / c m 2-sec a t 90 ° to t h e 50/~A p r o t o n b e a m . A thickness o f 4 m o f F e shielding is therefore adeq u a t e for all needs except those very small ranges o f n e u t r o n energies in the r e g i o n o f several h u n d r e d kiloelectron volts where interference effects cause a very small t r a n s m i s s i o n cross section. The use o f concretes l o a d e d with n e u t r o n a b s o r b e r s such as b o r o n has been previously described s) to a u g m e n t the p r o t e c t i v e effects o f Fe shielding in this case. R e c e n t i m p r o v e d m e a s u r e m e n t s o f transmission m a x i m a for n a t u r a l i r o n 9) have shown t h a t t o t a l n e u t r o n cross sections at these energies are lower t h a n h a d f o r m e r l y been believedt °).
Fluxes o f s e c o n d a r y particles a n d their a b s o r p t i o n were considered at a variety o f e l e m e n t a l areas, on the F e wall, which s u b t e n d different angles at the Be target a n d lie in directions between 0 ° and 127 °. I f a p r o t o n b e a m o f energy 660 M e V a n d intensity 5 0 / t A is a p p l i e d to a Be target o f thickness 3 cm for a y e a r the resulting m a x i m u m activity on a bare Fe w a l l i s a b o u t 4900 r a d / h , cm 2 at a distance o f 1 cm. F o r p r o t o n i r r a d i a t i o n the resulting activity is due m a i n l y to S~Mn, w i t h h a l f life 314 d, a n d some other isotopes o f M n , Cr a n d V. This is true for i r r a d i a t i n g p r o t o n energies f r o m 200 M e V to 3 G e V ~t). Also the g a m m a decay o f Fe after i r r a d i a t i o n by n e u t r o n s p r o d u c e d by 600 M e V p r o t o n s on Be is described by Barbier 12) a n d this is similar to the case for direct p r o t o n i r r a d i a tion. The i n d u c e d activity on the inner surface o f the F e can be r e d u c e d if it is lined w i t h m a r b l e (fig. 4). By
3. Induced activity The m a x i m u m i n d u c e d activity in the F e shielding due to s e c o n d a r y p a r t i c l e emission f r o m the Be is at the p o i n t m a r k e d X in fig. 4, nearest to the target. This d e d u c t i o n arose from calculations similar to those o f the previous section which d e d u c e d the direction o f the m a x i m u m n u m b e r o f t r a n s m i t t e d s e c o n d a r y particles.
TABLE 2
The percentages of cascade neutrons emitted in various angular ranges and the numbers of neutrons emitted into these angular slices with energy above 132 MeV. The isotropic distribution is also given. It is assumed that 50 FA of protons of energy 660 MeV are incident on a target of Be of thickness 3 cm. The data in column 2 are adapted from ref. 4.
0 0 ° - 18012' 18°12' - 25051, 53° 8'- 66025' 84°16' - 95°44' 113035'-126054 '
Percentage of particles emitted in range 0 All cascade Isotropic neutrons distribution 18.7 10.4 12.6 5.3 2.2
10.1 4.3 7.4 6.4 7.4
Neutrons (E=~>150 MeV) 1.09x 10"2 5.3 x 1012 6.1 × 101° 3.4 x 109 6.6 x 108
EXTERNAL
TARGETS
IN
AN
INTENSE
PROTON
363
BEAM
presuming marble to be CaCO 3 the m a x i m u m induced activity in the Fe has been calculated to be reduced by a factor of 0.45. K n o w n cross sections of Ca, C and O
VACUUM
[ ] Be PROTON BEAM
161 - 25cm
~-
Fig. 4. Schematic shielding beside an external Be target.
162
/ 0
I
I
5 10 15 20 EVAPORATION NEUTRON ENERGY (MeV)
I
25
Fig. 3a. Evaporation neutron energy distribution resulting from 660 MeV protons on Be adapted from ref. 4. The fraction of evaporation neutrons in a given energy range is plotted against energy.
0.03
0.02
0.01 0
0.5
1.0
1.5
2.0
2.5
EVAPORATION NEUTRON ENERGY (MeV)
Fig. 3b. As for fig. 3a, but over a smaller energy range where the the distribution of evaporation neutrons peaks and is shown in greater detail.
were used and the transport of neutrons through the marble was considered. The induced activity for marble has been studied by Barbier t2) in the presence of cascade neutrons resulting from 600 MeV protons interacting with Be. Absolute induced activity and decay times of marble thus irradiated are both smaller than that of Fe by factors of 102 and 103 respectively. Hence the use of marble to reduce induced activity in iron does not cause an increase in total activity. The induced activity in the Be target is not high. This element is not suitable for examination by Rudstam's theory 13) which can give spallation cross sections for production of radioactive residual nuclei only for target nuclei with A > 2 0 . However, Bertini 4) has determined from the residues after an intranuclear cascade calculation for 660 M e ¥ protons on Be, that the principal radioactive product in the Be is 7Be. It has a half life of 53.4 d. The production rates for all other conceivable long lived products, 3H and 1°Be, are negligible. Hence the calculated induced activity in a natural Be target of thickness 3 cm penetrated by 50 #A of 660 MeV protons for 1 y is ~ 1 0 mCi. The slow neutrons arising mainly as evaporation products can easily be captured by a mixture of moderator and capturer, say in a boron loaded concrete, in addition to the Fe. A further pattern of radiation leakage which has recently been recognized is earthshine under massive shielding walls. I f the calculations of Alsmiller et al. 14) are applied to a case where 2 m of concrete lie between proton beam and earth it is readily shown this leakage is negligible. This is true both for the regions around the external target and the beam dump. The author wishes to express his thanks to Dr. H. W. Bertini for communicating unpublished material and to Mr. P. M. Lorio and Dr. M. H. Holland for valuable discussions.
364
s. WYNCHANK
References 1) H. W. Bertini, Phys. Rev. 188 (1969) 1711. 2) H. W. Bertini, ORNL TM-1996 (1967). 3) H. W. Bertini, Proc. 2nd Intern. Conf. Accelerator dosimetry and experience (S.L.A.C., Stanford, Calif., 1969) p. 42. 4) H. W. Bertini, private communication (1971). a) V. Barashenkov, K. Gudima and T. Toneev, Acta Phys. Polon. 36 (1969) 887. 6) R. Serber, Phys. Rev. 72 (1947) 1114. 7) e. Vasilkov, B. Govorkov, V. Goldanskii, V. Konshin, O. Lupadin, E. Matusevich, B. Pimenov, S. Prokhorov and S. Tsypin, Soviet J. Nucl. Phys. 7 (1968) 64.
8) S. Wynchank, Nncl. Lnstr. and Meth. 93 (1971) 85. 9) F. Rahn, H. Camarda, G. Hacken, W. W. Havens, Jr., H. Liou, J. Rainwater, M. Slagowitz and S. Wynchank, Nucl. Sci. Eng. 47 (1972) 372. 10) M. Goldberg, S. Mughabghab, B. Magumo and V. May, Brookhaven National Laboratory Report no. 325, suppl. 2, 2A (1966). 11) T. Armstrong and J. Barish, Nucl. Sci. Eng. 38 (1969) 271. 12) M. Barbier, Induced radioactivity (North-Holland PuN. Co., Amsterdam, 1969). 13) G. Rudstam, Z. Naturforsch. 21A (1966) 1027. z4) R. G. A1smiller, F. Mynatt, M. Gritzner, J. Pace and J. Barish, Nucl. Instr. and Meth. 89 (1970) 53.