Photofission of 238 U in the giant-resonance region
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CLEI Experiment
Photofission of
238
U in the Giant-Resonance Region
L. Z. Dzhilavyan* and V. G. Nedorezov Institute for Nuclear Research, Russian Academy of Sciences, pr. Shestidesyatiletiya Oktyabrya 7a, Moscow, 117312 Russia Received January 14, 2013
Abstract—Data on cross sections for the reaction 238 U(γ, F ) in the giant-resonance region were analyzed in connection with the preparation of new experiments aimed at studying 238 U photofission—in particular, in beams of photons from in-flight positron annihilation on internal targets of positron storage rings. These data were taken from measurements also performed with annihilation photons but from positron beams external to the accelerators used. The procedures applied in such measurements and based both on processing the multiplicity of detected neutrons and on detecting fission fragments were also analyzed. DOI: 10.1134/S1063778813120041
1. INTRODUCTION The fission of nuclei in (γ, F ) photonuclear reactions at incident photon energies corresponding to the excitation of giant resonances [overwhelmingly dominated—at least integrally—by the isovector electric giant dipole resonance, which, unless otherwise stated, is referred to below as a giant resonance (GR), for the sake of brevity] is an important deexcitation channel for actinide nuclei. It follows that, for such nuclei, (γ, F ) cross sections are data of importance for studying the physics of GR excitation and decay. Moreover, the possibility of studying giant resonances of other multipolarities (first of all, electric quadrupole resonances) in electronuclear reactions of actinide fission induced by electron and/or positron beams depends greatly on the accuracy of these data. On the other hand, data on the cross section for precisely the reaction 238 U(γ, F ) in the GR region are of importance for some fundamental and applied investigations that are not related (at least directly) to studying the physics of giant multipole resonances. Among such investigations, we would like to mention first of all experiments that we plan in order to study 238 U photofission induced by incident photons of energy about a few hundred MeV units. These experiments will also explore photon-induced processes involving low energy and momentum transfers and proceeding simultaneously with the reaction 238 U(γ, F ) (see [1, 2]), as well as processes initiated by powerful pulsed femtosecond lasers and accompanied by the production of relatively hard gamma radiation [3], which admits separation against a heavy background *
E-mail: [email protected]
of softer electromagnetic radiation by means of detecting fission events. Related topics also include investigation into physics problems associated with the operation of nuclear reactors and fast-neutron sources based on pulsed electron accelerators (see, for example, [4]). The cross section for the photofission reaction 238 U(γ, F ) in the GR energy region has been measured repeatedly (see [5, 6] and references therein) with various photon sources including the braking of electrons in radia
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