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Independent Yields of Kr and Хе Fragments in the Photofission Of Physics of Atomic Nuclei, Vol. 68, No. 9, 2005, pp. 1417–1420. Translated from Yadernaya Fizika, Vol. 68, No. 9, 2005, pp. 1475–1478. Original Russian Text Copyright c 2005 by Gangrsky, Zhemenik, Mishinsky, Penionzhkevich. NUCLEI Experiment Independent Yields of Kr and Хе Fragments in the Photofission of 237Np and 243Am Odd Nuclei Yu.P.Gangrsky*, V. I. Zhemenik, G. V. Mishinsky, and Yu. E. Penionzhkevich Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia Received August 5, 2004 Abstract—Results are presented that were obtained by measuring the independent yields of Kr (A = 89−93)andХе(A = 135−142) appearing as fragments in the photofission of 237Np and 243Am odd nuclei. The respective experiments were performed in a beam of bremsstrahlung photons from electrons accelerated to an energy of 25 MeV at the microtron of the Laboratory of Nuclear Reactions at the Joint Institute for Nuclear Research (JINR, Dubna). Use was made of the procedure involving the transportation of fragments emitted from the target by a gas flow along a capillary and the condensation of inert gases in a cryostat at liquid-nitrogen temperature. The identification of Kr and Хе appearing as fragments was performed by the gamma spectra of their daughter products. The mass-number distributions of the independent yields of Kr and Хе isotopes were obtained, along with those for the complementary fragments (Y and La in the fission of 237Np and Nb and Pr in the fission of 243Am). c 2005 Pleiades Publishing, Inc. INTRODUCTION Хе inert gases—originating from the photofission of 237 243 Measurement of fragment yields and of their de- Np and Am odd nuclei and is a continuation 232 238 pendences on various features of fissile nuclei and of similar experiments performed with Th, U, fragments formed (nucleonic composition, excitation and 244Pu even–even nuclei and reported in [3, 4]. energy, angular momentum) is one of the lines of in- An analysis of the measured fragment-mass-number vestigation into the mechanism of the nuclear-fission dependences of the yields with allowance for known process. Measurement of the yields of primary frag- similar data on even nuclei would make it possible to ments (independent yields)—that is, those fragments explore the effect of an odd particle in a fissile nucleus that were formed upon the scission of a fissile nucleus on the formation of fragments. There are virtually and neutron emission, but which have not yet under- no data on the independent yields of fragments in gone beta decay—is of particular interest. Such mea- the photofission of nuclei featuring an odd number of surements furnish important information about the protons or neutrons (we can only indicate the study formation of the nucleonic composition of fragments reported in [5]). Moreover, the use of known data on in the process of their transition from the saddle to the the number of prompt fission neutrons would make scission point. it possible to deduce additional information about the However, data on this process are obviously in- yields of the complementary fragments as well—Nb sufficient and refer predominantly to the neutron- and Pr in the photofission of 243Am and Y and La in induced fission of U and Pu nuclei and to the spon- the fission of 237Np. taneous fission of 252Cf [1, 2]. It is of interest to extend the range of such investigations by including in them fission reactions caused by other bombarding EXPERIMENTAL PROCEDURE particles—for example, photons. Special features of photofission reactions (a fixed angular momentum In the present experiments, we employed the same that is introduced in a fissile nucleus by a photon and procedure as in the studies of our group that were the absence of Coulomb energy and binding energy) devoted to the photofission of even–even nuclei and make it possible to obtain new information about the which were reported in [3, 4]. Within this procedure, effect of external conditions on the formation of the one implements an efficient separation of Kr and Хе nucleonic composition of fragments. isotopes from other fission fragments by using the fact The present study is devoted to measuring the that the chemical properties of the former are strongly independent yields of fragments—isotopes of Kr and different from the chemical properties of the latter. Krypton and xenon nuclei originating as fission frag- *e-mail: [email protected] ments from a target irradiated with bremsstrahlung 1063-7788/05/6809-1417$26.00 c 2005 Pleiades Publishing, Inc. 1418 GANGRSKY et al. Table 1. Independent yields of Kr and Хе fragments after the scission of a fissile nucleus and neutron emission, but which did not have time to undergo beta 237Np (γ, f) 243Am (γ, f) decay. The measurements were performed in a room Frag- that was protected from microtron radiation—that is, ment −1 −1 Yrel, % Yabs, fiss. Yrel, % Yabs, fiss. under conditions of a low gamma-ray and neutron 89Kr 65(8) 0.0019(2) 81(3) 0.019(2) background. 91Kr 100 0.025(3)∗ 100 0.023(2)∗ For targets subjected to irradiation, we used 50-µg/cm2-thick layers made from Np and Am 92 Kr 44(7) 0.011(1) 42(2) 0.0095(9) oxides and deposited onto an aluminum substrate 93Kr 18(4) 0.0045(5) 20 µm thick. At these layer and substrate thick- 135 nesses, half of the fragments formed in a target were Xe 61(3) 0.023(3) 42(5) 0.015(2) emitted from it and were moderated in the gas. 137 ∗ ∗ Xe 100 0.038(4) 100 0.035(4) In order to measure the gamma spectra in ques- 138Xe 80(10) 0.030(3) 76(8) 0.027(3) tion,weemployedanHpGedetectorofvolume 100 cm3 and resolution 2.1 keV for the 1332-keV 139 31(4) 0.012(1) 35(3) 0.012(1) Xe gamma line of 60Со. The resulting spectra were saved 140Xe 8.0(9) 0.0030(3) 9.8(6) 0.0034(3) in the memory of a PC for a subsequent analysis with 141Xe 3.0(3) 0.0011(1) 6.0(6) 0.0021(2) theaidoftheAKTIVcode[7].Theyieldsofidentified Kr and Хе fragments were determined from the areas 142Xe 2.0(3) 0.00076(8) 3.4(7) 0.0012(1) of their gamma lines (or the gamma lines of their ∗ Estimated yield values. daughter products) in the spectra with allowance for the detection efficiency, the intensity per decay event, the irradiation time, the time of transportation along photons were moderated in a gas and were trans- the capillary, and the time of the measurements. ported by its flow along a Teflon capillary to a cryostat, The experiments were performed at an accelerated- where they were condensed on the walls of a 1-m- electron energy of 25 MeV,the corresponding average long copper pipe coiled into a spiral and placed in excitation energy of a fissile nucleus being 13 MeV (it a Dewar flask that was filled with liquid nitrogen. was determined from the shape of the bremsstrahlung All other fragments stopped in the gas were blocked spectrum [8] and the excitation function for the by a filter at the inlet of the capillary. Thus, only Kr photofission of 237Np and 243Am nuclei under the and Хе isotopes, whose half-lives were longer than assumption that this function is similar to the well- 0.2 s (the time of fragment transportation along the known dependence that was determined in [9] for capillary was about 0.5 s), and the products of their 238U). beta decay reached the copper pipe. For the carrier gas, we employed pure helium at a pressure of 2.5 atm in the chamber. This gas was characterized by the RESULTS OF THE MEASUREMENTS highest velocity of fragment transportation and was not activated by neutrons or bremsstrahlung from the By analyzing the measured gamma spectra, we microtron used. were able to identify four krypton isotopes and seven xenon isotopes. The ratios of the independent yields In identifying Kr and Хе isotopes and in determin- 91 137 ing their yields, we relied on the spectra of gamma of these isotopes to the yields of Kr and Хе, radiation that they emitted (for data on these spectra, respectively, are given in Table 1. In order to deter- see [6]). The identification of short-lived Kr and Хе mine these yields per fission event, we measured the isotopes (in the case of half-lives shorter than 1 min) cumulative yields of fragments characterized by the was based on the gamma spectra of their daughter mass numbers of A =91and A = 137 and estimated the fractions of Kr and Хе (fractional yields) in these products (Rb and Sr for Kr and Ва and La for Хе). isobars. The cumulative yields were obtained from These daughter products, which were formed directly 91 in the scission of a fissile nucleus, were blocked by the intensities of the gamma lines of the Sr and 137 the filter at the inlet of the capillary. Therefore, they Cs isotopes, which are at the end of the beta-decay did not contribute to the measured yields of Kr and chains for these A values. The results proved to be Хе fragments. Also, the filter blocked Se, Br, Te, and 4.2(2)% and 6.3(3)% of the number of fission events. I fragments, whose beta decay could produce Kr and The fractional yields of 91Kr and 137Хеfragments Хе isotopes being studied. Thus, one can see that, in were estimated by using the systematics of yields in order to measure gamma spectra, we separated only the neutron-induced fission of U and Pu nuclei [1] those Kr and Хе fragments that emerged immediately and were found to be 0.60(6) for 237Np and 0.55(6) PHYSICS OF ATOMIC NUCLEI Vol.
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