Energy of 242Am and 242mAm fission fragments

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Nuclear Physics A108 (1968) 689--696; ~ ) North-HollandPublishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

ENERGY OF 242Am A N D 242mAm FISSION F R A G M E N T S B. H. E R K K I L A and R. B. L E A C H M A N t Los Alamos Scientific Laboratory, University of CaliJbrnia, Los Alamos, New Mexico tt Received 3 November 1967

Abstract: The energy spectra of single fragments from spontaneous fission of the 14 msec 242mAm isomer and from prompt fission of ~4ZAm were measured. The isomer was produced by the 2a2Pu(d, 2n)242mAm reaction, and the prompt fissions were produced by the 24°Pu(d, f) reaction. The fragments from delayed fission were observed by semiconductor detectors after the compound nucleus had recoiled to a rotating catcher wheel. The usual doubly peaked spectra of fragment energies from heavy element fission were observed in both cases. The valley of the spectrum from spontaneous fission of the isomer is equally deep as the valley of the spectrum from prompt fission induced by 12 MeV deuterons. Energies of the peaks were the same for prompt fission and for spontaneous fission of the isomer. The former energies were constant over the 7.6 to 14 MeV span of incident deuteron energy. Comparisons of these fragment energies with those from ~2Cf spontaneous fission and from deuteron-induced fission of other heavy elements agree with ZZ/A~ systematics.

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N U C L E A R REACTIONS, FISSION ~z~°Pu(d,f), E -- 7.6-14 MeV; measured a(E; Etragment) "~3°Th,2aau, 24~'Pu(d, f), E = 14 MeV; measured a(Erragment). RADIOACTIVITY, FISSION 242mAm(sf-isomer) [from ~42pu(d, 2n)]; measured T~, Efragm~nt. 252Cf measured Etragment.

1. Introduction I s o m e r s o f 3 M e V e x c i t a t i o n e n e r g y 1) h a v e b e e n s h o w n to h a v e a significant p r o b a bility o f d e c a y by fission r a t h e r t h a n by g a m m a rays 2) o r by a l p h a p a r t i c l e s 3). F u r t h e r m o r e , studies m a d e w i t h different b o m b a r d i n g particles h a v e s h o w n t h a t the s p i n o f t h e i s o m e r is l o w 4), a n d so t h e l i f e t i m e o f the i s o m e r is n o t likely to r e s u l t f r o m a spin f o r b i d d e n n e s s a g a i n s t decay. T h i s has g i v e n rise to s o m e s p e c u l a t i o n s 5) t h a t t h e i s o m e r is o f a h i g h l y d e f o r m e d n u c l e u s w h i c h t h u s has a r e d u c e d fission barrier to p e n e t r a t e a n d c a n n o t r e a d i l y d e c a y to the stable s h a p e by g a m m a - r a y e m i s s i o n . O n e o f the m o s t s t u d i e d o f t h e s e i s o m e r s is the a p p r o x i m a t e l y 3 M e V m e t a s t a b l e state o f 242mAm. T h i s h i g h - e n e r g y i s o m e r i s r e a d i l y p r o d u c e d by t h e 242pu(d, 2n)a42mAm r e a c t i o n w i t h a 14 m s e c half=life a n d a n 8 p b cross s e c t i o n for fission by 12 M e V d e u * Present address: Physics Department, Kansas State University, Manhattan, Kansas 66502. ** Work performed under the auspices of the U.S. Atomic Energy Commission. 689

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terons 1). In the present investigation we have used this reaction to observe the kinetic energies of single fission fragments from 242"Am fission. The spectra of these energies are compared with the fragment energies from zsaCf spontaneous fission and with fragment energies from 24ZAm fission from the reaction 24°pu(d, f). Any significant difference in the nuclear shape or the velocities of the nascent fragments at scission for p r o m p t fission compared to spontaneous fission of the isomer should be detected by this comparison. Furthermore, the peak-to-valley ratios of these spectra give information, although only qualitative, on the relative probabilities for asymmetric and symmetric fission for prompt fission compared to spontaneous fission of the isomer.

2. Experimental procedure The experimental technique for observing spontaneous fission of the isomer was based on the recoil method shown in fig. 1. A 12.5 MeV deuteron beam from the Los Alamos variable energy cyclotron traversed a 50 #g/cm / 242pu deposit on a 60/~g/cm 2

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COLLECTOR WHEEL

F i g . 1. S c h e m a t i c d i a g r a m o f r o t a t i n g c o l l e c t o r w h e e l u s e d f o r m e a s u r i n g f r a g m e n t e n e r g i e s a n d i s o m e r l i f e t i m e s f r o m s p o n t a n e o u s fissions o f 242mAre n u c l e i . R e c o i l i n g 242'hAm a t o m s f r o m t h e s t a t i o n a r y 242pu t a r g e t w e r e c o l l e c t e d o n t h e r o t a t i n g w h e e l . O n l y o n e d e t e c t o r a t t h e t = 7.5 m s e c position was used for energy measurements.

carbon backing. The recoiling 242Am atoms were stopped on a thin aluminium foil, which rotated at 1700 r.p.m, and was traversed by the deuteron beam. The rotating foil presented the 242Am atoms in front of the surface barrier detectors mounted in an arc opposite this foil. To compensate for the small effective thickness of the talget

Am FISSIONFRAGMENTS

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and for the low cross section, large-area counters were used without collimators, even though this resulted in a sacrifice in energy resolution. For measurements of the halflife, several detectors were used. For energy spectrum measurements, only one detector was used. Neutrons from the deuteron beam resulted in a large rate of small pulses, and so the effect of neutroninduced fatigue on the response of this single counter was carefully monitored. This was done by inserting a z s z c f spontaneous fission source in front of this detector at intervals during the isomer runs. [Fragment energies were normalized to the 2 5 2 C f energies reported by Whetstone 6)]. Even during deuteron bombardment, the peakto-valley ratio of the fragment energy spectrum did not change, and the energy positions of the peaks decreased by only 3 MeV during the entire experiment. In most measurements of 2 4 2 m A m fission a "slow" electronics system (2 llsec pulses) was used. To verify that pile-up of neutron-induced pulses was negligible, a much faster system (0.02/~sec pulses) was used *. A half-life measurement of 14.0_ 0.2 msec confirmed the reported half-life of 242mAm formed by neutrons 7) and by deuterons 8,9). The energy of the deuteron beam was chosen to be 12.5 MeV for this experiment since excitation functions reported by others 9) peaked at this energy. Fissions from the 24°pu(d, f) reaction were used for observing the energy spectra of fragments from prompt fission of 242Am. For these prompt fission measurements, the detector was placed at 150 ° in a conventional scattering chamber, and various incident energies of deuterons were used to produce the fission events. The 2 4 ° p u target was a 100/~g/cm 2 deposit on 70/~g/cm 2 carbon backing. Similar thicknesses of 2 3 ° T h , 2 3 3 U and 242pu targets were used with the same apparatus and reaction for measurements of fragment energies of other nuclei. Instrumental effects resulted in the same dispersion for energy spectrum measurements for both prompt and delayed fissions. This was confirmed by repeated observations of the spectrum from 2S2Cf fission under these different conditions. This dispersion decreased observed peak-to-valley ratios, and thus the absolute measured ratios are changed from true ratios. Nevertheless, the relative values of this ratio for different conditions of fission are significant.

3. Results and discussion

Spectra from the energy measurements are illustrated in figs. 2 and 3. Average energies of the peaks and peak-to-valley ratios are presented in table 1. Comparisons * The following system, which was designed by R. D. Hiebert, was used. Fission pulses of 0.02 ktsec duration were obtained f r o m the semiconductor detector by using adequately small time constants and a fast circuit to convert the counter impedence to the input impedence of a fast distributed amplifier. Those pulses corresponding to about 10 MeV or greater passed a threshold circuit and generated gates o f 0.04/~sec duration. A fast linear gate similar to that of Barna and Marshall 10 passed those pulses after having been appropriately delayed for gating. Those pulses which were passed were then stretched in time to match usual/~sec amplifiers and analysers which followed.

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B. H . E R K K I L A A N D R. B. L E A C H M A N

are made between spectra from spontaneous fissions of the 242roAm i s o m e r , 2 4 2 A m fissions and 252Cf spontaneous fissions *. All the spectra measured were doubly peaked. Similarly, corrections for the changes of counter sensitivity during the isomer runs are also included. The S K E W E D computer code 1o) was used to fit two skewed

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Gaussian peaks to the 242roAm, 2 4 2 A m and 252Cf spectra. These were of the form N Y(() - ~/~o" [ 1 - ½ ~ + ~ ° ~ a ] exp(-½~2)' t The peak-to-valley ratios given by Schmitt and Pleasanton ~9) for 252Cf fragments were not achieved in the present experiment. Contributing factors explaining this are possibly the large area of the counter, counter quality, bias voltage, pulse shaping and the lack of a collimator.

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where Y-values are the yields, and ~ = (E-EP)/a is proportional to the difference between the energy E and the median energies E P of the Gaussians before being

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Fig. 3. P r o m p t fission f r a g m e n t energy spectra p r o d u c e d in the b o m b a r d m e n t o f 24°pu with deuterons at four different deuteron energies E D. Curves are the c o m b i n a t i o n o f two skewed G a u s s i a n s fitted to the data. TABLE 1 Results f r o m the fission f r a g m e n t spectra o f 242Am, z42mAm a n d 252Cf fission Compound nucleus 252Cf 242roAm Z4ZAm 242Arn 24ZAm 242Am

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80.0 76.4~2 73.5±3 75.2-~2 75.9 ± 1 74.1 ± 1

(EL) (MeV) 105.7 101.7±2 102.7~3 102.8 ± 2 102.6 -- 1 101.7~ 1

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~0.2 ±0.6 ~1 ±0.5 ± 0.4 ±-0.4

T h e average energies f r o m the skewed G a u s s i a n fits to the spectra are designated by (EH~ a n d ( E L ) , Nrl/N L is the ratio o f the n u m b e r o f events in these peaks, a n d P / V is the peak-to-valley ratio. T h e incident deuteron energy is E D. ECN* is the c o m p o u n d - n u c l e u s energy for the resulting 2~2Am. Fission f r a g m e n t energies are n o r m a l i s e d to k n o w n energies for z52Cf fission.

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13. H . E R K K I L A A N D R. B. L E A C H M A N

skewed. The code searches for the standard deviations a, the numbers N and the common value of skewness ~ to give the best fit to the two peaks simultaneously. The deviations from the true NH/NL = 1 condition for fission are a measure of the inadequacy of the two skewed Gaussians in representing the fission spectra. (Subscripts H and L refer to the heavy and light fragments.) Uncertainties in table 1 result from counting statistics. Uncertainties resulting from counter fatigue were negligibly small. Before comparisons are made between the spectra from delayed and prompt fission of 24ZAm, consideration should first be given to the possible energy spectrum changes caused by contributions from 241Am fission and, for the two higher energies of deuterons,/4°Am fission following neutron emission from the Z42Am compound nucleus. Since the probability of 24°pu(d, nf) and, for the two higher energies of deuterons, a4°pu(d, 2nf) fissions from direct reactions is small compared to compound-nucleus reactions, the systematics 11) of F,/Ff data, where Fn is the neutron emission width and Ff the fission width, for compound-nucleus reactions can be used to estimate the probabilities of these other fissions. The approximate probabilities are then 0.61 for 242Am fission, 0.27 for 241Am fission and 0.09 for 24°Am fission. As is discussed below, the kinetic energies from 241Am and 24°Am fission are expected from systematics to be progressively higher than from 242Am fission, but the change will be shown to be insignificant compared to measurement uncertainties. Therefore, a direct comparison between the kinetic energies from prompt fission from 24°pu(d, f ) a n d from spontaneous fission of the 242mAm isomer can be made. Effects on the mass distribution of the fragments are expected to be more severe. Not only does the valley (probability of symmetric fission) for mass yield distributions decrease rapidly with the decreasing compound-nucleus energy associated with these sequential reactions (fissions following neutron emission), but the valley depth for a given excitation energy varies from one fissioning nucleus to another. For the approximately 8 MeV decrease in compound-nucleus excitation energy associated with each neutron emission (6 MeV binding energy plus 2 MeV kinetic energy), the valley change with energy probably exceeds the change with nuclear identity. Thus, the valley of the mass distribution observed for all the fissions from 24°pu(d, f) is probably deeper than for prompt fission of only 242Am at the original compound-nucleus excitation energy. We now compare fission fragment energies. Previous studies 12) have shown that fragment kinetic energies are essentially independent of the compound-nucleus energy inducing fission. The present results for deuteron-induced fission are confirmatory. On this basis, the agreement between energies of spontaneous fission of the isomer and prompt fission indicates that the co.nbined effect of nuclear elongation and nascent fragment motion at scission is the same in the two cases. Systematics for heavy elements t3) indicate that the most probable kinetic energy released in fission increases in propoltion to Z2/A ~. Since only single fragments were measured, the present comparison of the fragment energy EK with Za/A ~ is for the average total energy (EK) = ( E H ) + (EL). In fig. 4, least-squares fits to the present data give E K = (O.123+_O.O04)ZZ/A+ MeV. This agrees with E K = (0.124+0.004)

695

A m FISSION FRAGMENTS

ZZ/A ~;MeV from our fit to the data of Bennett and Stein i4) which are average energies of fragment pairs from compound nuclei in the mass region of interest. These results and the probabilities of 24IAm and Z4°Am fission estimated above for 24°Pu(d, f) fission indicate that the fragment energy from prompt fission of 242Am is actually only 0.1 ~ higher than observed for 24°Pu(d, f), but this correction is not included in the results in table 1 and fig. 4. The ratios of the average yield of fragments with energies in the two peaks to the yield of fragments with energies in the valley of the spectra are also given in table 1. These peak-to-valley ratios qualitatively show the usual ~5) decrease with increasing

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compound-nucleus energy. Uncertainties were determined from the counting statistics when the skewed Gaussians were fitted to the data. It is interesting to note that the peak-to-valley ratio for spontaneous fission of the 3 MeV isomer is about the same as for prompt fissions from 12 MeV deuterons, which for 242Am fission is at 19.8 MeV compound-nucleus energy. If the distribution of the nuclear shapes for prompt fission apply, systematics 15) of the increasing probability of symmetric fission with increasing compound-nucleus energy would indicate spontaneous fission of the isomer to have the deepest valley and thus the greatest peak-to-valley ratio. However, the results in table 1 do not show a significantly greater peak-to-valley ratio for spontaneous fission of the isomer than for deuteron-induced fission. However, a small part of this could be explained by

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fissions and (for higher energies) 2 4 ° A m fissions that probably increase the observed peak-to-valley ratio compared to the ratio for prompt fission of 242Am alone. The combination of the uncertainties arising from these 241Am and 24°Am fissions, counting statistics and the fact that the energy spectrum of single fragments cannot readily be interpreted in terms of the mass distribution is, however, reason to stress the more qualitative conclusion about fission asymmetry deduced from the present measurements: namely, spontaneous fission of the isomer is, like low-energy p r o m p t fission of heavy elements, predominantly asymmetric. Nevertheless, the indication from the data that spontaneous fission of the isomer probably has an unexpectedly high probability for symmetric fission is reason to speculate whether the distribution in the nuclear shape and in the motion after traversing this fission barrier is basically different than for the nucleus with the same potential energy after a prompt-fission passage over the fission barrier. This result is not surprising since the barrier against fission of the isomer is likely to be quite different from the barrier for a nucleus of equilibrium shape. The fact that the nuclei are the same in the prompt and delayed fission cases argues that scission effects, such as densities of fragment states, are not likely to cause the difference. It is interesting to note that the present change of the probability of symmetric fission appears to be large compared to the change attributed to spin, which has been observed from yields for different resonances of neutron-induced fission 16) and for different orbital angular momenta imparted 15,17). 2~lAm

The authors thank S. Bj0rnholm, H. C. Britt and I. Halpern for helpful discussions and W. S. Hall for assistance in computer fits to data. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16)

17) 18) 19)

S. Bjornholm, J. Borggreen, L. Westgaard and V. A. Karnaukov, Nuclear Physics A95 (1967) 513 A. Ghiorso, private communication (1965) R. B. Leachman and B. H. Erkkila, Bull. Am. Phys. Soc. 10 (1965) 1204 G. N. Flerov et al., I U C N D u b n a Report P7-3065 (1966) V. M. Strutinsky, Nuclear Physics A95 (1967) 420 S. L. Whetstone, Jr., Phys. Rev. 131 (1963) 1232 A. F. Linev, B. N. Markov, A. A. Pleve and S. M. Polikanov, Nuclear Physics 63 (1965) 173 G. N. Flerov et al., JETP (Sov. Phys.) 18 (1964) 964 G. N. Flerov et al., Rev. Roum. Phys. 10 (1965) 217; D. S. Brenner, L. Westgaard and S. Bjornholm, Nuclear Physics 89 (1966) 267 P. McWilliams, W. S. Hall and H. E. Wegner, Rev. Sci. Instr. 33 (1962) 70 J. R. Huizenga and R. Vandenbosch, in Nuclear reactions, Vol. II, ed. by P. M. Endt and P. B. Smith (North-Holland Publ. Co., Amsterdam, 1962) p. 42 S. L. Whetstone, Jr., Phys. Rev. 133 (1964) B613 V. E. Viola, Jr. and G. T. Seaborg, J. Inorg. Nucl. Chem. 28 (1966) 697 M. J. Bennett and W. E. Stein, Phys. Rev. 156 (1967) 1277 G. P. Ford and R. B. Leachman, Phys. Rev. 137 (1965) B826 G. A. Cowan, B. P. Bayhurst, R. J. Prestwood, J. S. Gilmore and G. W. Knobeloch, Phys. Rev. 144 (1966) 979; G. A. Cowan, B. P. Bayhurst, and R. J. Prestwood, Phys. Rev. 130 (1963) 2380 J. C. Cuninghame, K. Fritze, J. E. Lynn and C. B. Webster, Nuclear Physics 84 (1966) 49; J. C. Cuninghame, G. P. Kitt and E. R. Rae, Nuclear Physics 27 (1961) 154 A. Barna and J. H. Marshall, Rev. Sci. Instr. 35 (1964) 881 H. W. Schmitt and F. Pleasanton, Nucl. Instr. 40 (1966) 204