Estimation of the time scale of last chance alpha emission using an ‘‘atomic clock’’

Abstract

The probability of Slling a K-vacancy, created on the incoming part of the coQision, before nparticle emission is used to time the decay of Yb compound nuclei. These nuclei were produced in the fusion reaction 250 MeV Ni + Mo. In general the nuclear decays are too fast to be timed by this clock, however, n-particle emitting compound nucleus states have lifetimes sufBciently long for this technique to work when both the a-particle has low energy and the compound nucleus spin is large. This supports the existence of last, or near last, chance o;-particle emission in the deexcitation of high spin compound nuclei.PACS number(s): 24.60. Dr, 25.70.Gh Thirty years ago Gugelot [1] pointed out that one can calibrate the decay width (I', ) of a compound nucleus in terms of the K x-ray width (I' ) for atomic vacancies. All one has to do is prepare an atoxn with an excited nucleus and a K vacancy and then measure the branching ratio I' /I', . In practice, there are several problems in applying this idea. The K-vacancy production probabilities P~are low (of the order of 1%) for typical low energy fusion reactions and, in contrast to atomic collisions below the Coulomb barrier, the x rays of interest must be detected in a large background resulting &om internal conversion x rays and p rays emitted by excited nuclear reaction products. Another important issue is photon production from the filling of vacancies in a charged particle exit channel during the time after exnission but before the emitted particle is well outside the K-shell Bohr radius (molecular orbital emission). Due to these problems, this old idea has met with success only recently [2][3][4].This technique is best suited to the study of the lifetimes of last or near last chance decays of compound systems in the bottom portion (high charge) of the periodic table. Only under these conditions does the K-vacancy lifetime approach the compound nuclear lifetime. Two interesting topics for which this technique is quite promising are: (1) the study of slow fission components and (2) slow "yrast" a-particle emission. The former involves actinide nuclei for which the K-vacancy lifetime is less than 10~s, and the latter, heavy rare earth nuclei with Kvacancy lifetixnes between 10 and 10 s. The recent paper by Molitoris et al. [4] addresses the first of these topics and this work deals with the second.Statistical model codes predict that the emission of a particles competes with p-ray emission at very low excitation energies when the nuclei are trapped near the yrast line at high to moderate spins [5,6]. Grover and Gilat first discussed this process in work published 25 years ago [5]. These early calculations, as well as more recent ones [7], predict that these "stretched" and "last chance" o. particles would be emitted &om states with lifetimes similar to the K-vacancy lifetime and therefore present an ideal application of the atoxnic clock xnethod. The predicted a-particle energies are low for these "last chance" particle emissions which, despite the angular moxnentum re...