- Email: [email protected]
650-MeV photofission yields for uranium
Notes
679
J. lnorg. Nucl. Chem., 1966, Vol.28. pp. 679 to 681. PergamonPress Ltd. Printedin Northern Ireland 6 5 0 - M e V photofission yields for u r a n i u m
(Received 21 April 1965; in revised form 20 August 1965) A NUMaERof studies have been made on the pattern of mass number dependence of fission yields in the photofission of uranium. Scrt~nr'r and SUGARMANtt} showed that the peak-to-trough ratio decreases from 300 to 4 as the maximum X-ray energy increased from 7 MeV to 300 McV. RICHTER and CORYELLez) reported a "spike" or fine structure near mass number 133 in their study on low energy photofission yields for U ~Ss. An excellent summary report by DUFFIELD,SCH1MITrand SHARP(a~ gives a complete tabulation of the results, as well as an interpretation of the data reported prior to 1958. The uranium photofission yields have never before been measured beyond the X-ray energy of 300-MeV, however, and it was felt worthwhile to study the mass-yield curve at a higher energy in some detail. A 200-g sample of uranyl acetate (Yokozawa Kagaku-K6gy5 Co.) was irradiated for about 1 hr with 650-MeV bremsstrahlung from the Institute for Nuclear Studies, University of Tokyo, I'3-BeV electron synchrotron, t4~ The uranium salt was dissolved in 1 1. of 3N HNOa immediately after the irradiation. Aliquots were taken from the stock solution at various time intervals, and the fission yields of bromine, iodine, molybdenum, strontium, silver and barium isotopes were measured. Radiocbemical procedures used were essentially the same as those employed by BROOM~5~ at the University of Arkansas in the studies of fast neutron-induced fission of uranium and thorium. Prior to the irradiation of the 200-g sample of uranyl acetate, a 20-g sample was also irradiated for about 1 hr with 700-MeV bremsstrahlung and radiochemical measurements of a number of fission products were carried out. However, the activities obtained were too low in the latter case and the data presented in this report were obtained after the sample size was increased from 20 g to 200 g. Although the irradiation of the 200-g sample was done only once, radiochemical measurements of the fission yields were carried out in duplicate, except in the case of silver and molybdenum isotopes. A control sample (50 g of uranyl acetate) was placed near the bath of bremsstrahlung during the irradiation period. Iodine isotopes were isolated from the control sample and counted. An activity of less than one count per minute was observed in the iodine fraction isolated from the control sample. The contribution of the fast-neutron induced fission resulting from (y, n) reactions under similar experimental conditions was reported to be about 1 per cent by SCHMrrT and SUGARMAN.I1) It is possible that the contribution from secondary fission is substantially larger at 650-MeV because of the increase in neutron multiplicity at this energy. However, our data on the yields on iodine isotope are quite different from the yields of iodine isotopes from the 14"5 MeV neutron-induced fission of U ~38 reported by BROOM.~5~ A Geiger-Miiller counter (PC-50 type, Kobe K6gby Corp., with SA-250 scaler) was used at the Institute for Nuclear Study for the radioactivity measurements. This counter was calibrated against the CE-14SL Low Background Beta Counter of the University of Arkansas, by counting a number of reference standards in both counters. The M e 99 yield was assumed to be 6'6 per cent and the yields for other mass chains were calculated relative to the Me 99 yield. From a knowledge of the absolute activity at the end of the irradiation after correcting for parent-daughter growths and decays, branching decays and counting efficiency, the total number of atoms of a particular species produced during the irradiation was determined by means of the following equation: AtotaI = [A2a/(1 -- e-~at)] t, where Atotal is the total number of atoms of species A produced during the irradiation, A2a is the activity of species A at the end of the irradiation, and t is the length of the bombardment. Then, 6'6 Yield of A (per cent) - (Atotal) MOgatota1 ") R. A. SCHMITTand N. SUGARMAN)Phys. Rev. 95, 1260 (1954). c9) H. G. RICHTERand C. D. CORYELL,Phys. Rev. 95, 1550 (1954). (a) R. B. DUFFIELD, R. A. SCHMITI"and R. A, SHARP, Paper P/678, P. 202 Prec. 2nd U.N. Conf. Peaceful Uses of Atomic Energy 15, Geneva, 1958. ") Institute for Nuclear Study, University of Tokyo, INSJ Annual Report, April 1960-March 1962, p. 22 (March 31, 1962). (5) K. M. BROOM,Phys. Rev. 126, 627 (1962).
680
Notes TABLE 1.---650-MeV PHOTOFISSION YIELDS FOR URANIUM
Mass
Isotope
83 89 99 111 131 132 133 134 135 140
Br Sr Mo Ag I I I I I Ba
Fission yield (~o) 1"8 ± 0"6 3"7 -4- 1-1 6"6 ( = standard) 1"8 ± 0"4 1"5 -4- 0"5 2"6 4- 0-5 2"0 4- 0-4 3"9 4- 0"8 2'5 -4- 0"5 5.7 :E 1.7
Since the number of neutrons emitted per fission was unknown, the iodine yields were calculated from the counting data assuming the ~H = 3. In calculating the mirror points, a value of P = 5 was assumed. The values of ~ arc reported to be 3 at 10- and 16-MeV and 4 at 48-MeV. ~8~ Table 1 and Fig. 1 show the experimental results. The area under the mass-yield curve shown in Fig. 1 is approximately 200 per cent. It has to be noted here that the molybdenum-99 yield was determined only once and it was taken to be 6.6 per cent. The error limit indicated is a combination
T
T Z t,l.I C, IE i.i 0,.
.I.
l/ P1
a l hi
Z
1.0
o
(/)
_.m l,
0
E X P '.'RIME ~ITAL
X
MIRF OR (~'-5
°'~o
80
~o
~oo , o MASS
12o 13o
POIN
JOINT
ASSUME( )
I,o
zo
16o 17o
NUMBER
FIG. 1.--Mass-yield curve for 650-MeV photofission of U m .
Notes
681
of the counting error and the errors introduced in the calculations of the fission yields. The peak-tovalley yield ratio for the 650-MeV photofission of uranium, taken to be equal to the Mo'e/Agm ratio, is 3"7 + 0"8. This value is in line with the general trend of the decrease of the peak-to-valley ratio reported by previous investigators: >300 at 7-MeV, 200 at 10-MeV, 38 at 16-MeV, 23 at 21-MeV, 11 at 48-MeV, 8 at 100-MeV, and 4 at 300-MeV. clj The peak-to-valley ratio for the fission spectrum neutron-induced fission of U ~38 is 200, and the ratio for the 14-MeV neutron-induced fission of U ~38 is about 10. ~5~ The resemblance between the shapes of mass-yield curves for the photofission and fast neutron-induced fission of uranium is remarkable in the energy range mentioned above. HYD~~6~ has pointed out that, because of the dominance of the giant resonance in the excitation function for photofission, the majority of the fissioning nuclei are probably excited to about 14-MeV even when the maximum energy of the bremsstrahlung beam is considerably greater. Due to the low activity of iodine isotopes produced, and also due to the uncertainty on the average number of neutrons emitted per fission, it was impossible to show unequivocally whether or not fine structure exists in the mass 131 to 135 region. Our data indicate, however, that the mass 133 yield is not unusually high, as compared with the yields for the neighboring masses at 132 and 134.
Acknowledgement--We are grateful to Professor ITARU NONAKA, Director of the Institute for Nuclear Study, University of Tokyo, for allowing one of us (P. K. K.) to spend the summer of 1964 at the Institute to carry out this work. Our special thanks are due to Professor SEIXAROYAMA~UCI-U, Dr. t-hROSaI SASAKI,and the members of the High Energy Physics Division for their kind assistance in the irradiation experiments; to Professor SHIOEO TANAKA,Dr. MICHIKO TSUCHIMOTO,Mr. T. INOUE of the Chemistry Division for valuable suggestions and assistances during the course of this work; to Mr. B. D. PALMERof the University of Arkansas for his assistance in radiochemical work. This work was supported in part by the U.S. Atomic Energy Commission Contract No. AT-(40-1)3235.
Institute for Nuclear Study University of Tokyo
K. SAKAMTOTO
Tokyo, Japan Department of Chemistry P . K . KURODA University of Arkansas Fayetteville, Arkansas c6~E. K. HYDE, The Nuclear Properties of the Heavy Elements. III. Fission Phenomena, p. 487. Prentice-Hall, Englewood Cliffs, New Jersey (1964).
J. Inorg. Nucl. Chem.. 1966. Vol. 28. pp. 681 to 682. PergamonPress Ltd. Printedin Northern Ireland
Two Y rays in the decay of 3.0-min 86Br
(Received 10 August 1965) THE DECAYof 3"0-min SSBr has been reported ~1~to proceed solely by a/~ transition (of 2"5 MeV endpoint energy) to the ground state of SSKr. In a study ~ of the short-lived bromine fission products 54-see SeBr and 55-sec STBr, ~, rays of 0.81 and 0.92 MeV were observed to decay with a half-life of approximately 3 min, on taking successive ~, spectra of the chemically pure bromine samples. From consideration of half-life alone, we could not rule out the possibility that the 7 rays arise in the decay ~3~ of 6-rain s4'~Br although the 55-sec and 30-min bromine activities were definitely excluded. A chemical separation procedure c2~ was used in which all bromine formed independently and by the decay of short-lived precursors was removed, and then bromine was allowed to grow in from the selenium ~I~ Nuclear Data Sheets, compiled by K. WA~ et aL (Printing and Publishing Office, National Academy of Sciences--National Research Council, Washington 25, D.C.), NRC 60-3-39. ~2~E. T. WILLIAMSand C. D. COR'~LL, To be published. ~3~j. E. SATnZAt-rN, J. D. KNxorrr and M. KAHN,J. lnorg. NucL Chem. 12, 206 (1960).
679
J. lnorg. Nucl. Chem., 1966, Vol.28. pp. 679 to 681. PergamonPress Ltd. Printedin Northern Ireland 6 5 0 - M e V photofission yields for u r a n i u m
(Received 21 April 1965; in revised form 20 August 1965) A NUMaERof studies have been made on the pattern of mass number dependence of fission yields in the photofission of uranium. Scrt~nr'r and SUGARMANtt} showed that the peak-to-trough ratio decreases from 300 to 4 as the maximum X-ray energy increased from 7 MeV to 300 McV. RICHTER and CORYELLez) reported a "spike" or fine structure near mass number 133 in their study on low energy photofission yields for U ~Ss. An excellent summary report by DUFFIELD,SCH1MITrand SHARP(a~ gives a complete tabulation of the results, as well as an interpretation of the data reported prior to 1958. The uranium photofission yields have never before been measured beyond the X-ray energy of 300-MeV, however, and it was felt worthwhile to study the mass-yield curve at a higher energy in some detail. A 200-g sample of uranyl acetate (Yokozawa Kagaku-K6gy5 Co.) was irradiated for about 1 hr with 650-MeV bremsstrahlung from the Institute for Nuclear Studies, University of Tokyo, I'3-BeV electron synchrotron, t4~ The uranium salt was dissolved in 1 1. of 3N HNOa immediately after the irradiation. Aliquots were taken from the stock solution at various time intervals, and the fission yields of bromine, iodine, molybdenum, strontium, silver and barium isotopes were measured. Radiocbemical procedures used were essentially the same as those employed by BROOM~5~ at the University of Arkansas in the studies of fast neutron-induced fission of uranium and thorium. Prior to the irradiation of the 200-g sample of uranyl acetate, a 20-g sample was also irradiated for about 1 hr with 700-MeV bremsstrahlung and radiochemical measurements of a number of fission products were carried out. However, the activities obtained were too low in the latter case and the data presented in this report were obtained after the sample size was increased from 20 g to 200 g. Although the irradiation of the 200-g sample was done only once, radiochemical measurements of the fission yields were carried out in duplicate, except in the case of silver and molybdenum isotopes. A control sample (50 g of uranyl acetate) was placed near the bath of bremsstrahlung during the irradiation period. Iodine isotopes were isolated from the control sample and counted. An activity of less than one count per minute was observed in the iodine fraction isolated from the control sample. The contribution of the fast-neutron induced fission resulting from (y, n) reactions under similar experimental conditions was reported to be about 1 per cent by SCHMrrT and SUGARMAN.I1) It is possible that the contribution from secondary fission is substantially larger at 650-MeV because of the increase in neutron multiplicity at this energy. However, our data on the yields on iodine isotope are quite different from the yields of iodine isotopes from the 14"5 MeV neutron-induced fission of U ~38 reported by BROOM.~5~ A Geiger-Miiller counter (PC-50 type, Kobe K6gby Corp., with SA-250 scaler) was used at the Institute for Nuclear Study for the radioactivity measurements. This counter was calibrated against the CE-14SL Low Background Beta Counter of the University of Arkansas, by counting a number of reference standards in both counters. The M e 99 yield was assumed to be 6'6 per cent and the yields for other mass chains were calculated relative to the Me 99 yield. From a knowledge of the absolute activity at the end of the irradiation after correcting for parent-daughter growths and decays, branching decays and counting efficiency, the total number of atoms of a particular species produced during the irradiation was determined by means of the following equation: AtotaI = [A2a/(1 -- e-~at)] t, where Atotal is the total number of atoms of species A produced during the irradiation, A2a is the activity of species A at the end of the irradiation, and t is the length of the bombardment. Then, 6'6 Yield of A (per cent) - (Atotal) MOgatota1 ") R. A. SCHMITTand N. SUGARMAN)Phys. Rev. 95, 1260 (1954). c9) H. G. RICHTERand C. D. CORYELL,Phys. Rev. 95, 1550 (1954). (a) R. B. DUFFIELD, R. A. SCHMITI"and R. A, SHARP, Paper P/678, P. 202 Prec. 2nd U.N. Conf. Peaceful Uses of Atomic Energy 15, Geneva, 1958. ") Institute for Nuclear Study, University of Tokyo, INSJ Annual Report, April 1960-March 1962, p. 22 (March 31, 1962). (5) K. M. BROOM,Phys. Rev. 126, 627 (1962).
680
Notes TABLE 1.---650-MeV PHOTOFISSION YIELDS FOR URANIUM
Mass
Isotope
83 89 99 111 131 132 133 134 135 140
Br Sr Mo Ag I I I I I Ba
Fission yield (~o) 1"8 ± 0"6 3"7 -4- 1-1 6"6 ( = standard) 1"8 ± 0"4 1"5 -4- 0"5 2"6 4- 0-5 2"0 4- 0-4 3"9 4- 0"8 2'5 -4- 0"5 5.7 :E 1.7
Since the number of neutrons emitted per fission was unknown, the iodine yields were calculated from the counting data assuming the ~H = 3. In calculating the mirror points, a value of P = 5 was assumed. The values of ~ arc reported to be 3 at 10- and 16-MeV and 4 at 48-MeV. ~8~ Table 1 and Fig. 1 show the experimental results. The area under the mass-yield curve shown in Fig. 1 is approximately 200 per cent. It has to be noted here that the molybdenum-99 yield was determined only once and it was taken to be 6.6 per cent. The error limit indicated is a combination
T
T Z t,l.I C, IE i.i 0,.
.I.
l/ P1
a l hi
Z
1.0
o
(/)
_.m l,
0
E X P '.'RIME ~ITAL
X
MIRF OR (~'-5
°'~o
80
~o
~oo , o MASS
12o 13o
POIN
JOINT
ASSUME( )
I,o
zo
16o 17o
NUMBER
FIG. 1.--Mass-yield curve for 650-MeV photofission of U m .
Notes
681
of the counting error and the errors introduced in the calculations of the fission yields. The peak-tovalley yield ratio for the 650-MeV photofission of uranium, taken to be equal to the Mo'e/Agm ratio, is 3"7 + 0"8. This value is in line with the general trend of the decrease of the peak-to-valley ratio reported by previous investigators: >300 at 7-MeV, 200 at 10-MeV, 38 at 16-MeV, 23 at 21-MeV, 11 at 48-MeV, 8 at 100-MeV, and 4 at 300-MeV. clj The peak-to-valley ratio for the fission spectrum neutron-induced fission of U ~38 is 200, and the ratio for the 14-MeV neutron-induced fission of U ~38 is about 10. ~5~ The resemblance between the shapes of mass-yield curves for the photofission and fast neutron-induced fission of uranium is remarkable in the energy range mentioned above. HYD~~6~ has pointed out that, because of the dominance of the giant resonance in the excitation function for photofission, the majority of the fissioning nuclei are probably excited to about 14-MeV even when the maximum energy of the bremsstrahlung beam is considerably greater. Due to the low activity of iodine isotopes produced, and also due to the uncertainty on the average number of neutrons emitted per fission, it was impossible to show unequivocally whether or not fine structure exists in the mass 131 to 135 region. Our data indicate, however, that the mass 133 yield is not unusually high, as compared with the yields for the neighboring masses at 132 and 134.
Acknowledgement--We are grateful to Professor ITARU NONAKA, Director of the Institute for Nuclear Study, University of Tokyo, for allowing one of us (P. K. K.) to spend the summer of 1964 at the Institute to carry out this work. Our special thanks are due to Professor SEIXAROYAMA~UCI-U, Dr. t-hROSaI SASAKI,and the members of the High Energy Physics Division for their kind assistance in the irradiation experiments; to Professor SHIOEO TANAKA,Dr. MICHIKO TSUCHIMOTO,Mr. T. INOUE of the Chemistry Division for valuable suggestions and assistances during the course of this work; to Mr. B. D. PALMERof the University of Arkansas for his assistance in radiochemical work. This work was supported in part by the U.S. Atomic Energy Commission Contract No. AT-(40-1)3235.
Institute for Nuclear Study University of Tokyo
K. SAKAMTOTO
Tokyo, Japan Department of Chemistry P . K . KURODA University of Arkansas Fayetteville, Arkansas c6~E. K. HYDE, The Nuclear Properties of the Heavy Elements. III. Fission Phenomena, p. 487. Prentice-Hall, Englewood Cliffs, New Jersey (1964).
J. Inorg. Nucl. Chem.. 1966. Vol. 28. pp. 681 to 682. PergamonPress Ltd. Printedin Northern Ireland
Two Y rays in the decay of 3.0-min 86Br
(Received 10 August 1965) THE DECAYof 3"0-min SSBr has been reported ~1~to proceed solely by a/~ transition (of 2"5 MeV endpoint energy) to the ground state of SSKr. In a study ~ of the short-lived bromine fission products 54-see SeBr and 55-sec STBr, ~, rays of 0.81 and 0.92 MeV were observed to decay with a half-life of approximately 3 min, on taking successive ~, spectra of the chemically pure bromine samples. From consideration of half-life alone, we could not rule out the possibility that the 7 rays arise in the decay ~3~ of 6-rain s4'~Br although the 55-sec and 30-min bromine activities were definitely excluded. A chemical separation procedure c2~ was used in which all bromine formed independently and by the decay of short-lived precursors was removed, and then bromine was allowed to grow in from the selenium ~I~ Nuclear Data Sheets, compiled by K. WA~ et aL (Printing and Publishing Office, National Academy of Sciences--National Research Council, Washington 25, D.C.), NRC 60-3-39. ~2~E. T. WILLIAMSand C. D. COR'~LL, To be published. ~3~j. E. SATnZAt-rN, J. D. KNxorrr and M. KAHN,J. lnorg. NucL Chem. 12, 206 (1960).