Physics of Atomic Nuclei, Vol. 66, No. 7, 2003, pp. 1211–1218. Translatedfrom Yadernaya Fizika, Vol. 66, No. 7, 2003, pp. 1251–1258. Original Russian Text Copyright c 2003 by Gangrsky, Zhemenik, Maslova, Mishinsky, Penionzhkevich, Sz¨oll¨os. NUCLEI Experiment Independent Yields of Kr and Xe Isotopes in the Photofission of Heavy Nuclei Yu.P.Gangrsky*, V.I.Zhemenik,N.Yu.Maslova, G. V. Mishinsky, Yu. E. Penionzhkevich, and O. Szoll¨ os¨ Joint Institute for Nuclear Research, Dubna, Moscow oblast, 141980 Russia Received July 15, 2002 Abstract—The yields of Kr (A =87–93)andXe(A = 138–143) primary fission fragments produced in 232Th, 238U,and244Pu photofission upon the scission of a target nucleus and neutron emission were measured in an experiment with bremsstrahlung from electrons accelerated to 25 MeV by a microtron, and the results of these measurements are presented. The experimental procedure used involved the transportation of fragments that escaped from the target by a gas flow through a capillary and the condensation of Kr and Xe inert gases in a cryostat at liquid-nitrogen temperature. The fragments of all other elements were retained with a filter at the capillary inlet. The isotopes of Kr and Xe were identified by the γ spectra of their daughter products. The mass-number distributions of the independent yields of Kr and Xe isotopes are obtained and compared with similar data on fission induced by thermal and fast neutrons; the shifts of the fragment charges with respect to the undistorted charge distribution are determined. Prospects for using photofission fragments in studying the structure of highly neutron-rich nuclei are discussed. c 2003 MAIK “Nauka/Interperiodica”. INTRODUCTION At the same time, a wider range of experimental data and new theoretical approaches to describing The distributions of product fragments with re- them are required for obtaining deeper insight into spect to their mass numbers A and charge numbers Z the dynamics of the fission process. Measurement of are among the main properties of the nuclear-fission the isotopic and isobaric distributions of fragments process. The shape of these distributions is controlled originating from reactions induced by γ rays is one by the dynamics of the fission process from the saddle of the promising lines of investigation in this region to the scission point. This intricate process depends of nuclear fission since such reactions possess some on a number of factors, including the relief of the special features: energy surface, the configurations of nuclear shapes γ at the instant of scission, and the nuclear viscosity (1) The interaction of radiation with nuclei is of collective motion. Therefore, measurements of the purely electromagnetic, and its properties are well isotopic and isobaric distributions of fragments (that known; therefore, an adequate calculation of this in- is, those in A at given Z and those in Z at given A, teraction can be performed without resort to addi- respectively) provide important data on the dynamics tional model concepts. of the fission process. The distributions of fragments (2) In the energy range 10–16 MeV, the interac- produced after fissile-nucleus scission and the emis- tion of photons with nuclei is of resonance origin (this sion of neutrons (primary fragments) are the most is the region of a giant dipole resonance); the energy of informative of them. this resonance corresponds to the frequency of proton oscillations with respect to neutrons in a nucleus. However, measurement of such distributions in- volves some difficulties. Each of the techniques used (3) The absence of the binding energy and of the has its limitations and can be applied efficiently only Coulomb barrier allows one to obtain fissile nuclei of to a certain range of fission fragments. Therefore, iso- any (even extremely low) excitation energy immedi- topic and isobaric distributions of primary fragments ately after photon absorption. have received adequate study only for the fission of the (4) Over a wide range of γ-ray energies, the Th, U, and Pu isotopes that is induced by low-energy angular-momentum transfer to the irradiated nucleus neutrons and for the spontaneous fission of 252Cf. undergoes virtually no change—it is as low as 1 in Data on these distributions are compiled in [1, 2]. the dipole absorption of photons. These features peculiar to reactions induced by γ *e-mail: [email protected] rays allow one to obtain new information about the 1063-7788/03/6607-1211$24.00 c 2003 MAIK “Nauka/Interperiodica” 1212 GANGRSKY et al. Ar 238U,and244Pu fission induced by γ rays of energy corresponding to a giant dipole resonance and to compare these yields with similar data obtained in neutron-induced fission. The measurements of the Chamber 238 – cumulative yields of these isotopes in Uphoto fis- e γ sion are reported in [9]; preliminary results on the independent yields of Xe fragments in 232Th and 238U fission are given in [10]. Capillary Pump EXPERIMENTAL PROCEDURE The extraction of Kr and Xe isotopes from the bulk of photofission fragments is based on the use of their special chemical properties, which differ significantly Cryostat from the properties of other elements and which re- Fig. 1. Layout of the experimental setup. veal themselves in a number of phenomena, includ- ing the adsorption of their atoms on a filter and the walls of the capillary through which the fragments are nuclear-fission process. For example, experiments transported from the irradiated target to radioactive- that studied the photofission of nuclei furnished a radiation detectors. The point is that Kr and Xe inert gases are efficiently adsorbed only at liquid-nitrogen wide set of data on the structure of the fission barrier ◦ and the shape of the potential surface [3]. The above temperature (−195.8 C), while all other fragments features of γ-ray interaction with nuclei are expected are adsorbed even at room temperature. It is precisely to manifest themselves in the process of fission- this property that is employed in our experimental fragment formation as well. This can be illustrated setup, which comprises a reaction chamber, a cryo- by the results presented in [4], according to which the stat, and Teflon capillaries for a gas flow. The layout γ-ray-energy dependence of the yield of fragments of the setup is displayed in Fig. 1, and a more detailed (115Cd, 117Cd) from symmetric fission has a rather description of it is given elsewhere [11]. unusual character, with a maximum at 6 MeV and a The irradiated targets were placed in the reaction kink at 5.3 MeV. chamber, which had the shape of a cylinder 30 mm in However, data on the isotopic and especially height, its inner diameter being 40 mm. The cylinder isobaric distributions of photofission fragments (first walls had a thickness of 1.5 mm. The inlet and out- of all, on their independent yields) are considerably let holes for a buffer gas were located symmetrically scarcer than data on reactions induced by neutrons opposite each other along a diameter of the reaction and charged particles. We can mention only the chamber. One or two targets placed at the end faces of results of the experiments performed in Ghent (Bel- the cylinder were used in the experiments. The prop- gium) for a limited set of nuclei [5–8]. erties of the targets are presented in Table 1 (these are The objective of this study is to measure the in- the chemical composition, admixtures, dimensions, dependent yields of fragments (these are the isotopes layer thickness, and the substrates). of Kr and Xe inert gases) produced directly in 232Th, An inert gas was supplied from a gas-container to the reaction chamber through a polyethylene capillary 4 mm in diameter. The gas pressure in the chamber Table 1. Properties of the targets used was adjusted by a container valve and measured with a manometer at the chamber inlet. Chemically pure Target 232Th 238U 244Pu He, Ar, and N2 at a pressure of 1 to 2 atm were used as buffer gases. The choice of inert gas and pressure Material Metal U3O8 PuO2 in the chamber was determined by the conditions of Content 100 99.85 96 the experiment (the efficiency of fragment collection, (%) the rate of their transportation, the levels of the γ- radiation background caused by the activation of the Admixtures (%) – 0.15 (235U) 4(240Pu + 242Pu) inert gas owing to microtron bremsstrahlung). Thickness 25 3 0.3 2 The fission fragments were transported, together (mg/cm ) with the buffer gas, from the reaction chamber to the Dimensions (mm) ∅15 ∅20 and 30 10 × 10 cryostat through a Teflon capillary of length 10 m and inner diameter 2 mm by means of evacuation Substrate – Al Ti with a pump. A fibrous filter placed at the capillary PHYSICS OF ATOMIC NUCLEI Vol. 66 No. 7 2003 INDEPENDENT YIELDS OF Kr AND Xe 1213 A = 91 93 140 142 Se 0.37 s ↓ Br 0.64 s Br 0.1 s I 0.86 s I 0.2 s ↓ ↓ ↓ ↓ Kr 8.6 s Kr 1.3 s Xe 14 s Xe 1.2 s ↓ ↓ ↓ ↓ Rb 58 s Rb 5.8 s Cs 64 s Cs 1.8 s ↓ ↓ ↓ ↓ Sr 9.5 h Sr 7.4 min Ba 13 d Ba 11 min ↓ ↓ ↓ ↓ Y 5.8 d Y 10.5 h La 40 h La 92 min ↓ ↓ ↓ ↓ Zr stable Zr 1.5 × 106 y Ce stable Ce stable Fig. 2. Examples of the chains of β decays of the fission fragments that include Kr and Xe isotopes. The isotopes for which we measured γ spectraaregiveninboldfacetype. inlet efficiently adsorbed all fission fragments, with the cryostat. Therefore, only Kr and Xe fragments that exception of Kr and Xe.