- Email: [email protected]
Independent yields of 82Br, 86Rb, 136Cs and 150Pm in spontaneous fission of 252Cf
J. inorg,nucI.Chem.,1969,Vol.31,pp. 3357to 3361. PergamonPress. Printedin Great Bdtain
INDEPENDENT Y I E L D S O F S2Br, 86Rb, 136Cs and 15°pm I N S P O N T A N E O U S F I S S I O N O F 252Cf* H. R. VON G U N T E N , t K. F. F L Y N N and L. E. G L E N D E N I N Argonne National Laboratory, Argonne, 111.60439
(Received 26 February 1969) Ahstraet-Independent fission yields in the spontaneous fission of 25~Cf were determined by radiochemical techniques for the shielded nuclides S~Br, SeRb, 136Cs and ~5°Pm. They were found to be < 3.7 × 10-6 per cent, 5.4 × 10-3 per cent, 2"2 × 10-2 per cent and 2"0 × 10-2 per cent, respectively. The corresponding fractional chain yields lead to empirical Zp (most probable charge) values which are in good agreement with Z p values determined by physical methods (K X-rays).
SEVERAL postulates have been proposed for the most probable division of nuclear charge in low energy fission of heavy nuclei. Recent measurements[l, 2] by physical methods (K X-rays) showed that in the spontaneous fission of 252Cfonly the equal charge displacement (ECD) function [3] is in reasonable agreement with the experimental data. Radiochemical data for the division of nuclear charge in the fission of zszCf based on fractional chain yield measurements [4, 5] for la6Cs, 139Xe, x4°Xe, and 141Xe are in good agreement with physical data (K X-rays) but cover only a small range of fragment masses (5 mass units) in the region of the heavy mass peak. In order to extend the radiochemical method over a wider range of mass and charge division, independent yields for S2Br, S6Rb, 1~6Cs and xs°Pm were determined. EXPERIMENTAL Fission source The recoil technique described by Nervik [5] was used to collect the fission products from an essentially weightless source of purified 2s2Cf on an aluminum plate. The activity of the source (2.6 x l0 s fissions per min) was determined by comparing its gamma-my intensity above 3 MeV with that of a standard 252Cf source of known fission rate. Fission yields determined for SaBr, 9gMo, x311, xsaI and lSgBawere in good agreement with Nervik's [5] values indicating that the determination of the fission rate was accurate to about 10 per cent and that the fission products were collected essentially quantitatively (2~r) in the aluminum catcher foil. In each experiment the foil was exposed to the zszCf source for a length of time corresponding to about one half-life of the nuclide of interest. *Work performed under the auspices of the U.S. Atomic Energy Commission. tResident Research Associate from Eidg. Institut fOr Reaktorforschung, Wiirenlingen, Switzerland. 1. L. E. Glendenin and J. P. Unik, Phys. Rev. 140, B 1301 (1965). 2. S. S. Kapoor, H. R. Bowman and S. G. Thompson, Phys. Rev. 140, BI310 (1965). 3. L. E. Glendenin, C. D. Coryell and R. R. Edwards, Radiochemical Studies: The Fission Products (Edited by C. D. Coryell and N. Sugarman). Nat. nucl. Energy Ser. Voi. 9, Div. IV, p. 489. McGraw-Hill, New York (1951). 4. A. C. Wahl, R. L. Ferguson, D. R. Nethaway, D. E. Troutner and K. Woifsberg, Phys. Rev. 126, 1112(1962). 5. W. E. Nervik, Phys. Rev. 119, 1685 (1960). 3357
3358
H.R.
VON G U N T E N , K. F. F L Y N N and L. E. G L E N D E N I N
Chemical separations After each exposure the catcher foil was dissolved in 6N HCI containing one drop of H F with the appropriate carriers present (no carrier for 15°pm). The nuclides of interest were then separated by the following procedures.
Bromine Bromine and iodine carriers were added. The bromine was separated from iodine by a KMnOANaHSO3 oxidation-reduction cycle[6]. Remaining traces of iodine were extracted into CC14 after addition of carrier and oxidation with NaNO2. The iodine removal was repeated nine times. Further purification of the bromine was accomplished by two more oxidation-reduction cycles with iodine carrier present. The bromine was mounted as AgBr.
Rubidium and cesium Rubidium and cesium carriers were added and precipitated with HC10417]. Three Fe(OH)a scavengings and two precipitations of BaCOa and SrCOa were performed. Rubidium was separated from cesium by eight precipitations of CsaBi2Ig. Two RbCIO4 precipitations were made after boiling off 12 and precipitating Bi2Sa. The last RbCIO4 precipitate was mounted for counting. Four rubidium blanks were prepared simultaneously by the same procedure. Samples and blanks were measured for 130 days ( - 7 half-fives of S6Rb). The first CsaBi219 precipitate was dissolved in HNO3, the I2 boiled off, and the bismuth removed by precipitation as Bi2S3. The cesium was purified by two CsCIO4 precipitations and mounted for counting. The activity of the samples was followed for 100 days.
Promethium Promethium was separated by a carrier-free ion exchange procedure with a Dowex-1 column (elutriant 10 per cent 6NHNOa-90 per cent Methanol) followed by reversed-phase chromatography with di-(2-ethylhexyl) orthophosphoric acid (HDEHP) on a diatomaceous silica column[8]. The activity of the effluent from the columns was monitored with a plastic scintillator to aid in locating the promethium fraction. Lanthanum cartier was added to this fraction, and the promethium was co-precipitated with La~(C204)a.
Counting procedures The S2Br and SrRb samples were beta counted in a low-background, end-window, proportional counter. This counter had a background of 0-6 counts per min (cpm). The efficiency of the counter was determined by counting aliquots of standardized S2Br and SeRb solutions under the same conditions as used for counting the fission product samples. All SrRb samples were counted through 60 mg/cm2 of aluminum to absorb the 0.27 MeV beta radiation from the S7Rb present in the carrier. •The 136Cs samples were beta counted in an end-window proportional counter (background 9 pm). The counter efficiency was determined in a manner similar to that used for the S2Br and ~Rb samples. The radiochemical purity of the l~Cs samples was checked by y-ray spectrometry with a 3 x 3 in. NaI detector and multichannel analyzer. The 15°Pm sample was counted in a gamma-gamma coincidence counter (two 3 x 3 in. NaI detectors) with windows set on the coincident 1.17 MeV and 0.88 MeV photopeaks of 15°Pm.The efficiency of this detector was determined by counting a sample of 4nSc (ya = 1.12 MeV, ~/2 = 0.89 MeV, in coincidence) which had been standardized by absolute beta counting. The 15°Pm y branching ratio used for the efficiency calculation was taken from Lederer, Hollander and Perlman [9]. 6. 7. 8. 9.
J. Kieinberg and G. A. Cowan, Nat. A cad. Sci. monograph NAS-N S 3005, 21 (1960). G.W. Leddicotte, Nat.Acad. Sci. monograph NAS-NS 3053, 20 (1962). J. W. Winchester, J. Chromat. 10, 502 (1963). C. M. Lederer, J. M. Hollander and I. Perlman, Table of Isotopes 6th Edn. Wiley, New York (1967).
Independent yields in spontaneous fission RESULTS
3359
AND DISCUSSION
The bromine samples decayed with a half-life of 2-4 hr (S3Br) into a long-lived component of about 0.1 counts per min (cpm) above the background of the counter. Although the 35.3 hr SZBr was not detectable, an upper limit for its independent fission yield was established. The iodine samples from the last extraction cycle contained an activity of about 0-4 cpm. Since the distribution coefficient for iodine in the bromine extraction cycle is expected to be 0.02, the contribution of iodine in the bromine sample can be estimated to be < 0.01 cpm. The activity of the rubidium samples (0-5 cpm) decayed with the half-life of 18.7-day 86Rb into an unidentified long-lived component of about the same activity as the four blanks (0.6 cpm). The activity of the blanks remained constant during the time of the investigation. The counting rate at zero time for 136Cs was found by graphical resolution of the composite decay curve which contained both 136Cs and 137Cs. The results of the two independent determinations were in very good agreement. The counting rate of the 15°Pm at zero time was found from the gamma-gamma coincidence decay curve. After subtraction of a long-lived tail (0.7 cpm), 2.7 hr ~5°Pm was detectable. The beta decay curve of the sample could be resolved into a long-lived component 11.1-day 147Nd and a 44-hr component due to the mixture of 149pm and ~5~pm. The activity of 15°pm was determined relative to the activities of ~49pm and ~ P m in the sample. The fission yield of 15°pm was then calculated using the known fission yields for 149pm and ~5~pm[5]. The results of the independent yield determinations are given in Table 1. The saturation activities of the samples given are calculated for 2rr collection of fission fragments. The chain yields used for the calculation of the fractional chain yields were taken from the smooth mass-yield curve [5]. The empirical Zp values were calculated by the method of Wahl [4] with a width parameter[10] c of 0-86___0-15 in the Gaussian charge dispersion function. The average primary mass A' was obtained by increasing the product mass-number A by the average number of neutrons emitted by the fragment [ 11 ]. In Fig. 1 the deviation of the most probable nuclear charge from unchanged charge distribution (UCD), Zp-A '(ZflAF), is plotted against the primary fragment mass A'. The figure shows our radiochemical points (circles), Wahl's [4] values for some xenon isotopes (squares), and Nervik's [5] value for 136Cs (triangle). The errors include the uncertainty in the value of c and estimated experimental errors. The figure also compares charge division from radiochemical data with that from the K-X-ray data[l]. The measurements (physical and radiochemical) are in good agreement with the rule of equal charge displacement (ECD) [3], but they disagree considerably with the calculated functions for maximum energy release [11, 12] in fission. Wolfsberg[13] has shown that the deviation from U C D , at 10. A. E. Norris and A. C. Wahl, Phys. Rev. 146, 926 (1966). 11. H. R. Bowman, J. C. D. Milton, S. G. Thompson and W. J. Swiatecki, Phys. Rev. 129, 2133 (1963). 12. P. Armbruster, Proc. Syrup. Phys. Chem. Fission, Vol. I, 103. International Atomic Energy Agency, Vienna (1965). 13. K. Wolfsberg, Phys. R ev. 137, B929 (1965).
3360
H.R.
V O N G U N T E N , K. F. F L Y N N and L. E. G L E N D E N I N
+l = ,~ R ,", .'i"
+1'~" . ~ .
~.
I,,.~I. T x ÷1
~ + I Y ~
~
+1 Y ~ ~ +1 •
~
V ~
V
V ~
V
o~o6
v
v~
~ o =~.~e,," ~' = o : E ~
6
+
Independent yields in spontaneous fission
3361
A~ 126
130 i
134 i
138 i
142 .
i
146 i
150 i
154 I
158 i
162 i
166 i
170 I
0"8
-0.8
0"6
-0'6
L
0"4
-0'4
.,.J ,¢I !
0"2
-0"2
<~
L ,¢
N
.Z <[
k
N
0
0
-0"2 126
0"2 I
I
I
I
I
I
I
I
I
I
I
122
118
114
II0
106
102
98
94
90
86
82
Fig. 1. Difference of the most probable nuclear charge from that given by the charge density of the fissioning nucleus, Zp-A'(ZF/Ar), as a function of initial fragment mass; O radiochemical data from this work, [] radiochemical data from Wahl et al.[4], A radiochemical data from Nervik[5]. The line is based on K-X-ray measurements[l]; the shaded area represents the error band (90 per cent confidence level) for the line.
least for mass numbers with high yield, is ~ 0-6 charge unit in all cases of lowenergy neutron-induced fission (E, < 14 MeV). The radiochemical and physical data for 2s2cf show an average deviation from U C D of ~- 0-4 charge units. Acknowledgements-The authors wish to thank the Argonne heavy element group for preparing the ~zCf source, M. A. Wahlgren, F. R. Lawless, and M. A. Essling for assistance during the promethium separations and Dr. H. C. Griffin for his suggestions regarding the '5°Pro determination. One of us (H.R.v.G.) expresses his thanks to Argonne National Laboratory for making his stay at the Laboratory possible.
INDEPENDENT Y I E L D S O F S2Br, 86Rb, 136Cs and 15°pm I N S P O N T A N E O U S F I S S I O N O F 252Cf* H. R. VON G U N T E N , t K. F. F L Y N N and L. E. G L E N D E N I N Argonne National Laboratory, Argonne, 111.60439
(Received 26 February 1969) Ahstraet-Independent fission yields in the spontaneous fission of 25~Cf were determined by radiochemical techniques for the shielded nuclides S~Br, SeRb, 136Cs and ~5°Pm. They were found to be < 3.7 × 10-6 per cent, 5.4 × 10-3 per cent, 2"2 × 10-2 per cent and 2"0 × 10-2 per cent, respectively. The corresponding fractional chain yields lead to empirical Zp (most probable charge) values which are in good agreement with Z p values determined by physical methods (K X-rays).
SEVERAL postulates have been proposed for the most probable division of nuclear charge in low energy fission of heavy nuclei. Recent measurements[l, 2] by physical methods (K X-rays) showed that in the spontaneous fission of 252Cfonly the equal charge displacement (ECD) function [3] is in reasonable agreement with the experimental data. Radiochemical data for the division of nuclear charge in the fission of zszCf based on fractional chain yield measurements [4, 5] for la6Cs, 139Xe, x4°Xe, and 141Xe are in good agreement with physical data (K X-rays) but cover only a small range of fragment masses (5 mass units) in the region of the heavy mass peak. In order to extend the radiochemical method over a wider range of mass and charge division, independent yields for S2Br, S6Rb, 1~6Cs and xs°Pm were determined. EXPERIMENTAL Fission source The recoil technique described by Nervik [5] was used to collect the fission products from an essentially weightless source of purified 2s2Cf on an aluminum plate. The activity of the source (2.6 x l0 s fissions per min) was determined by comparing its gamma-my intensity above 3 MeV with that of a standard 252Cf source of known fission rate. Fission yields determined for SaBr, 9gMo, x311, xsaI and lSgBawere in good agreement with Nervik's [5] values indicating that the determination of the fission rate was accurate to about 10 per cent and that the fission products were collected essentially quantitatively (2~r) in the aluminum catcher foil. In each experiment the foil was exposed to the zszCf source for a length of time corresponding to about one half-life of the nuclide of interest. *Work performed under the auspices of the U.S. Atomic Energy Commission. tResident Research Associate from Eidg. Institut fOr Reaktorforschung, Wiirenlingen, Switzerland. 1. L. E. Glendenin and J. P. Unik, Phys. Rev. 140, B 1301 (1965). 2. S. S. Kapoor, H. R. Bowman and S. G. Thompson, Phys. Rev. 140, BI310 (1965). 3. L. E. Glendenin, C. D. Coryell and R. R. Edwards, Radiochemical Studies: The Fission Products (Edited by C. D. Coryell and N. Sugarman). Nat. nucl. Energy Ser. Voi. 9, Div. IV, p. 489. McGraw-Hill, New York (1951). 4. A. C. Wahl, R. L. Ferguson, D. R. Nethaway, D. E. Troutner and K. Woifsberg, Phys. Rev. 126, 1112(1962). 5. W. E. Nervik, Phys. Rev. 119, 1685 (1960). 3357
3358
H.R.
VON G U N T E N , K. F. F L Y N N and L. E. G L E N D E N I N
Chemical separations After each exposure the catcher foil was dissolved in 6N HCI containing one drop of H F with the appropriate carriers present (no carrier for 15°pm). The nuclides of interest were then separated by the following procedures.
Bromine Bromine and iodine carriers were added. The bromine was separated from iodine by a KMnOANaHSO3 oxidation-reduction cycle[6]. Remaining traces of iodine were extracted into CC14 after addition of carrier and oxidation with NaNO2. The iodine removal was repeated nine times. Further purification of the bromine was accomplished by two more oxidation-reduction cycles with iodine carrier present. The bromine was mounted as AgBr.
Rubidium and cesium Rubidium and cesium carriers were added and precipitated with HC10417]. Three Fe(OH)a scavengings and two precipitations of BaCOa and SrCOa were performed. Rubidium was separated from cesium by eight precipitations of CsaBi2Ig. Two RbCIO4 precipitations were made after boiling off 12 and precipitating Bi2Sa. The last RbCIO4 precipitate was mounted for counting. Four rubidium blanks were prepared simultaneously by the same procedure. Samples and blanks were measured for 130 days ( - 7 half-fives of S6Rb). The first CsaBi219 precipitate was dissolved in HNO3, the I2 boiled off, and the bismuth removed by precipitation as Bi2S3. The cesium was purified by two CsCIO4 precipitations and mounted for counting. The activity of the samples was followed for 100 days.
Promethium Promethium was separated by a carrier-free ion exchange procedure with a Dowex-1 column (elutriant 10 per cent 6NHNOa-90 per cent Methanol) followed by reversed-phase chromatography with di-(2-ethylhexyl) orthophosphoric acid (HDEHP) on a diatomaceous silica column[8]. The activity of the effluent from the columns was monitored with a plastic scintillator to aid in locating the promethium fraction. Lanthanum cartier was added to this fraction, and the promethium was co-precipitated with La~(C204)a.
Counting procedures The S2Br and SrRb samples were beta counted in a low-background, end-window, proportional counter. This counter had a background of 0-6 counts per min (cpm). The efficiency of the counter was determined by counting aliquots of standardized S2Br and SeRb solutions under the same conditions as used for counting the fission product samples. All SrRb samples were counted through 60 mg/cm2 of aluminum to absorb the 0.27 MeV beta radiation from the S7Rb present in the carrier. •The 136Cs samples were beta counted in an end-window proportional counter (background 9 pm). The counter efficiency was determined in a manner similar to that used for the S2Br and ~Rb samples. The radiochemical purity of the l~Cs samples was checked by y-ray spectrometry with a 3 x 3 in. NaI detector and multichannel analyzer. The 15°Pm sample was counted in a gamma-gamma coincidence counter (two 3 x 3 in. NaI detectors) with windows set on the coincident 1.17 MeV and 0.88 MeV photopeaks of 15°Pm.The efficiency of this detector was determined by counting a sample of 4nSc (ya = 1.12 MeV, ~/2 = 0.89 MeV, in coincidence) which had been standardized by absolute beta counting. The 15°Pm y branching ratio used for the efficiency calculation was taken from Lederer, Hollander and Perlman [9]. 6. 7. 8. 9.
J. Kieinberg and G. A. Cowan, Nat. A cad. Sci. monograph NAS-N S 3005, 21 (1960). G.W. Leddicotte, Nat.Acad. Sci. monograph NAS-NS 3053, 20 (1962). J. W. Winchester, J. Chromat. 10, 502 (1963). C. M. Lederer, J. M. Hollander and I. Perlman, Table of Isotopes 6th Edn. Wiley, New York (1967).
Independent yields in spontaneous fission RESULTS
3359
AND DISCUSSION
The bromine samples decayed with a half-life of 2-4 hr (S3Br) into a long-lived component of about 0.1 counts per min (cpm) above the background of the counter. Although the 35.3 hr SZBr was not detectable, an upper limit for its independent fission yield was established. The iodine samples from the last extraction cycle contained an activity of about 0-4 cpm. Since the distribution coefficient for iodine in the bromine extraction cycle is expected to be 0.02, the contribution of iodine in the bromine sample can be estimated to be < 0.01 cpm. The activity of the rubidium samples (0-5 cpm) decayed with the half-life of 18.7-day 86Rb into an unidentified long-lived component of about the same activity as the four blanks (0.6 cpm). The activity of the blanks remained constant during the time of the investigation. The counting rate at zero time for 136Cs was found by graphical resolution of the composite decay curve which contained both 136Cs and 137Cs. The results of the two independent determinations were in very good agreement. The counting rate of the 15°Pm at zero time was found from the gamma-gamma coincidence decay curve. After subtraction of a long-lived tail (0.7 cpm), 2.7 hr ~5°Pm was detectable. The beta decay curve of the sample could be resolved into a long-lived component 11.1-day 147Nd and a 44-hr component due to the mixture of 149pm and ~5~pm. The activity of 15°pm was determined relative to the activities of ~49pm and ~ P m in the sample. The fission yield of 15°pm was then calculated using the known fission yields for 149pm and ~5~pm[5]. The results of the independent yield determinations are given in Table 1. The saturation activities of the samples given are calculated for 2rr collection of fission fragments. The chain yields used for the calculation of the fractional chain yields were taken from the smooth mass-yield curve [5]. The empirical Zp values were calculated by the method of Wahl [4] with a width parameter[10] c of 0-86___0-15 in the Gaussian charge dispersion function. The average primary mass A' was obtained by increasing the product mass-number A by the average number of neutrons emitted by the fragment [ 11 ]. In Fig. 1 the deviation of the most probable nuclear charge from unchanged charge distribution (UCD), Zp-A '(ZflAF), is plotted against the primary fragment mass A'. The figure shows our radiochemical points (circles), Wahl's [4] values for some xenon isotopes (squares), and Nervik's [5] value for 136Cs (triangle). The errors include the uncertainty in the value of c and estimated experimental errors. The figure also compares charge division from radiochemical data with that from the K-X-ray data[l]. The measurements (physical and radiochemical) are in good agreement with the rule of equal charge displacement (ECD) [3], but they disagree considerably with the calculated functions for maximum energy release [11, 12] in fission. Wolfsberg[13] has shown that the deviation from U C D , at 10. A. E. Norris and A. C. Wahl, Phys. Rev. 146, 926 (1966). 11. H. R. Bowman, J. C. D. Milton, S. G. Thompson and W. J. Swiatecki, Phys. Rev. 129, 2133 (1963). 12. P. Armbruster, Proc. Syrup. Phys. Chem. Fission, Vol. I, 103. International Atomic Energy Agency, Vienna (1965). 13. K. Wolfsberg, Phys. R ev. 137, B929 (1965).
3360
H.R.
V O N G U N T E N , K. F. F L Y N N and L. E. G L E N D E N I N
+l = ,~ R ,", .'i"
+1'~" . ~ .
~.
I,,.~I. T x ÷1
~ + I Y ~
~
+1 Y ~ ~ +1 •
~
V ~
V
V ~
V
o~o6
v
v~
~ o =~.~e,," ~' = o : E ~
6
+
Independent yields in spontaneous fission
3361
A~ 126
130 i
134 i
138 i
142 .
i
146 i
150 i
154 I
158 i
162 i
166 i
170 I
0"8
-0.8
0"6
-0'6
L
0"4
-0'4
.,.J ,¢I !
0"2
-0"2
<~
L ,¢
N
.Z <[
k
N
0
0
-0"2 126
0"2 I
I
I
I
I
I
I
I
I
I
I
122
118
114
II0
106
102
98
94
90
86
82
Fig. 1. Difference of the most probable nuclear charge from that given by the charge density of the fissioning nucleus, Zp-A'(ZF/Ar), as a function of initial fragment mass; O radiochemical data from this work, [] radiochemical data from Wahl et al.[4], A radiochemical data from Nervik[5]. The line is based on K-X-ray measurements[l]; the shaded area represents the error band (90 per cent confidence level) for the line.
least for mass numbers with high yield, is ~ 0-6 charge unit in all cases of lowenergy neutron-induced fission (E, < 14 MeV). The radiochemical and physical data for 2s2cf show an average deviation from U C D of ~- 0-4 charge units. Acknowledgements-The authors wish to thank the Argonne heavy element group for preparing the ~zCf source, M. A. Wahlgren, F. R. Lawless, and M. A. Essling for assistance during the promethium separations and Dr. H. C. Griffin for his suggestions regarding the '5°Pro determination. One of us (H.R.v.G.) expresses his thanks to Argonne National Laboratory for making his stay at the Laboratory possible.