Experimental evidence for muonicX-rays from fission fragments

Abstract

Abstract:In the spectrum of muonic X-rays of 238U measured in coincidence with prompt fission events a structure has been found which is attributed to 2p--*ls transitions of the muon attached to heavy fragments. The intensity Is of this structure relative to the strength Ipi observed for prompt fission has been determined to be I,/Iv. f = (6.0-{-2.1) %. Although the experimental significance for the appeareance of this phenomenon is weak, this is the first experimental indication for its occurence. 36.10.Dr Non-radiatlve (n.r.) transitions in heavy muonlc atoms excite the nucleus which then may decay via prompt fission. Contrary to delayed fission occuring as a result of p-capture by the nucleus, the muon survives the prompt fission process and is attached predominantly to the heavy fission fragment [1] forming an atom with the muon in the ls state. In several theoretical investigations it has been stated that the muon canpopulate also excited states [2][3][4][5][6][7]. The p-populates an excited level of a fission fragment either due to internal conversion of the excitation energy of the fragment to the p- [2,3] or as a consequence of the muon transitions during the fission fragment formation. In the first case the probability depends on the energy and multipole composition of the electromagnetic transition of the fragment. In the latter the population strength turns out to be influenced by the dynamics of the fission process. Theoretical predictions for the population probability of the 2p level of the heavy fragment muonic atom range between 1% [6] up to 10% per prompt fission [3,4]. No X-rays from such levels have been reported so far. PACS:Recently, the results of a 23SU(p-, 7f) experiment devoted to the study of prompt fission have been published [8]. The experimental set-up consisted of two Germanium diodes with BGO-Compton suppression for the X-ray detection and a multi parallel plate avalanche counter filled with metallic ~3SU foils, which served as an active target. The muonlc X-rays from the 23sU target have been recorded in coincidence with fragments of prompt as well as delayed fission. The muonic X-ray spectrum coincident with prompt fission should also Present address: * Heuft-Systemtechnik GmbH, W-5475 Burgbrohl, Germany ~'* Klinik und Poliklinik ffir Nuklearmedizin der Universiffit Wfirzburg, W-8700 Wfirzburg, Germany *** Bayer AG, W-5090 Leverkusen, Germany + Gesetlschaft ffr ProzeBsteuerungs-und Informationssysteme mbH, W-8750 Aschaffenburg, Germany contain the muonic X-rays from fission fragments. The energies of both lines of the 2p--,ls transition complex of the most probable isotope 140v s4 Me are lying at 3.7 and 3.8 MeV, respectively. Due to the charge distribution the 2p--,ls transitions of the muonie heavy fragments should occur within an energy interval of about 700 keV [3]. An additional complexity results from the mass distribution. Furthermore, Doppler broadening smears out the separate lines [4]. Fig. 1 shows the spectrum recorded in coincidence with prompt fission by one of th...