Fission fragments from the reaction 237Np(p ,y f) have been measured in coincidence with muonic X-rays. The efficiency of the fission fragment detector is determined from (p , 7f)-data of the same experiment. The total fission probability per p-stop Pt has been measured as well as the fission probabilities Pf of the non-radiative muonic (3d~ ls)-and (2p~ls)-transitions; the latter has been divided into two parts leading to different mean excitation energies E: Pt=(54_+17)%, Pf(3d--*ls)=(41__21)%, Pf(2p--~ls, E= 6.218 MeV) = (61 _+ 19)%, andPf(2p~ls, E = 6.525 MeV) = (57 _+ 18)%. The influence of the muon on the fission barrier is discussed. The fission probability after muon capture is compared with a calculated value using a distribution of nuclear excitation energies following muon capture and the fission probability as measured in a a38U (3He, ~f)-reaction.

The probability for delayed muon induced fission of 2~ has been determined from a ~ -,flf2) measurement. The measured fission probability Pr = (4.2 _+0.7)x 10 -5 is compared with theoretical predictions. The high fission threshold reaction seems well suited for studying the influence of two-body meson-exchange currents in nuclear muon capture.

Abstract:The probability for non-radiative (n.r.) excitations in muonic 2~ was determined from a (#-,72C)-measurement by comparing the intensities of muonic X-ray transitions in single and coincidence spectra. The values of P,. ~.(3p--~ls) =(17.9• and P,.~.(3d-~ls) were studied in continuation of the preceding experiments on zCSpb, 232Th, and 23su [9].The experiment was performed using the/zE4 channel of the Paul-Scherrer-Institut (PSI). The experimental setup is described in more detail in ref. 10, which contains the result of the simultaneously measured fission probability of muonic 2~The beam intensity, measured with a scintillator telescope, was 2• muons per second. Two large volume Ge-detectors each with a BGO-Compton suppression system were used to detect the muonic X-rays. A large CsF-crystal (5"x5") was employed in addition to define coincidences between muonic X-rays.Following the conventional nomenclature, the probabilities for a certain transition refer to the population of the initial level. The formalism to determine the fraction of n.r. transition strengths between the levels of the muonic cascade has been outlined by RSsel et al. [9]. The probability for a non-radiative (2p--~ls)-transition P~.~.(2p--ls) has been determined from the analysis of the X-ray spectra recorded by the CsF-crystal. In order to evaluate the probability P,.~.(3p-~ls) it is necessary to analyze muonic transitions which populate the 3p-level. tn muonic 2~ the (4d~3p)-transitions clearly identified in the Ge-detectors have been used for this purpose. * Present address:By considering all radiative and n.r. decays of the 3d-level Pn.T.(3d~ls) has been obtained. The total transition probability P(3d~2p) has been evaluated by comparing the muonic X-ray spectra measured as single events with those measured in coincidence with the (2p-*ls)-transition. For muonic 2~ the results for P(3d--*2p) show significant deviations for the different fine structure transitions. This effect has previously been observed for 2~[9]. For 2~ it was possible to analyze the n.r. decay width F .... (3d-~2p). For this purpose the formalism described in ref. 9 has been extended accordingly: P,~.r.(3d--+ls) = 1 -P(3d~2p) -P,.,~a(3d---~ls) = 1 -P(3d--~2p) -P(3d~2p)r,aa(3d~ls) X rTo.(3d-2p) + r ... . (3d-,2p)The ratio of the radiative decay widths Fr~d(3d---~ls) / Fr, has been calculated by Lohs et al. [11]. Taking into account the non-radiative E3-transitions (3ds/2~2p3/~) and (3d~/z-~2pl/2) due to the E3-resonance in 2~ [12,13] the non-radiative decay width F .... (3d---~2p) has been estimated. This has been done here by the determination of the intensities of the nuclear 2c-rays belonging to the decay of the resonance. We have neglected other n.r, (3d---+2p)-transition modes and a possible population of these isomer shifted nuclear levels, following energetically higher nuclear excitations by other n.r. transitions. Therefore this gives only an approximation for the probability of a n,r. (3d-+2p)-transition.In table 1 the results for the n.r. transition pr...

The ratios of prompt to delayed fission yields for the isotopes 233U, 234U, 235U, 236U, 238U, 237Np, 242pu, and 244pu and the fission probabilities relative to each other have been investigated experimentally. Using the value of the total fission probability for 237Np the absolute probabilities for prompt and delayed fission have been determined. The fission probabilities per muon capture P1~ have been derived for all the isotopes and compared with an evaluation based on excitation functions from theory.

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...