Differences between the rates of 241 Pu decay in the free ions Pu I + -Pu 5 + are predicted. Two mechanisms modifying these differences when the ions are present in an ionic solid are elucidated. 241 Pu is predicted to decay 0.3% faster in solid PuO than in solid Pu0 2 . Chemical variations of this magnitude may be a contributing factor to the observed discrepancies between different experimental determinations of the half-life of 241 Pu but are unlikely on their own to account completely for the anomaly.PACS numbers: 23.40. Bw, 31.90.+s Chemical effects on nuclear p~ decay have recently become the subject of renewed interest for a variety of reasons. Not only are accurate predictions of f3~-decay spectra needed to interpret experiments searching for antineutrino mass 1 but also /3-decay half-lives are being measured with increasing precision. Accurate knowledge of the half-life of 241 Pu, for instance, including any possible chemical dependence, is important in calculations of nuclear-fuel assay and in the standardization of measurements of heavy-element isotopes. Values of the half-life measured by a number of different methods show anomalously large variations. 2,3 The very low 241 Pu decay energy prompted the suggestion 4 that these discrepancies might originate from chemical effects via the process known as bound-state fi decay in which the /3 electron is created in a bound orbital. The object of this Letter is to investigate this suggestion quantitatively and in so doing to elucidate how chemical effects in free-ion decays are modified in an ionic crystal. 241 Pu undergoes a first forbidden (nonunique) decay to 241 Am with a maximum /3-particle energy (endpoint energy, E 0 ) of 20.8 keV. In general the decay rate for this class of transitions can be expressed as a sum of terms due to six nuclear matrix elements (to first order) having various functions of electron and antineutrino wave functions as coefficients. For 24l Pu, however, the high nuclear charge, Z, and low endpoint energy cause the factor £; = Z/E 0 R to be very much greater than unity (R is the nuclear radius and all quantities are in atomic units). Therefore, in the spirit of the £ approximation, 5 we neglect combinations of lepton wave functions which are smaller than the leading terms by a factor of £. Three of the surviving nuclear-lepton combinations contain terms of order unity in the nucleon velocities and have as coefficient the same leptonic factor involving either the small component of the s-electron wave function or the large component of the p one. The two remaining combinations, which are smaller, being of order v/c in the nucleon velocities (c being the velocity of light), involve either the large component of an electronic s orbital or the small component of a /? orbital. Since the ratio of the large to small components at the nuclear radius is essentially independent of orbital, all of the five nuclear-lepton combinations can be expressed in a form having the same lepton factor as coefficient. This causes the energy depe...
The transfer of negative pions in mixtures of bromodecane and carbon tetrachloride has been investigated by combined measurements of /r 0, s produced from nuclear capture on hydrogen and pionic x rays following Coulomb capture on bromine and chlorine. A significant fraction of pion transfer occurs via an external-transfer process. The results represent the first direct observation of transfer in condensed matter. PACS numbers: 36. lO.GvIn recent years a considerable quantity of data has been acquired on the capture of negative pions in molecular systems. Despite this, a number of important questions regarding the molecular interactions of pions remain unanswered. Chief among these are the distribution of pions within the molecule immediately following molecular capture, and the nature and significance of subsequent processes involving transfer of pions from hydrogen to atoms of higher nuclear charge. The study of transfer processes involving pionic hydrogen is useful in understanding similar processes for muonic atoms. This is especially significant because pion transfer occurs only from excited states of the exotic atom, the analogous process of muon transfer from excited states being critical in muon-catalyzed fusion.Experiments which measure charge-exchange (K~,TT°) probabilities in simple systems such as mixtures of hydrogen with noble or other gases suggest that a significant proportion of pions which are initially captured on hydrogen are subsequently transferred to higher-Z atoms before nuclear capture occurs. 1,2 Such a transfer is believed to take place by a process in which the small neutral pn ~ atom first breaks away from the hydrogen molecule (in which "atomic" capture initially occurs) and then transfers the pion to a higher-Z atom in a subsequent collision, 3,4 i.e., pn~+ Z-• Zn~+p. The large increase in binding energy once the n~ is transferred to a higher-Z atom ensures that the transfer is essentially one-way. This process is called an "external-transfer" process if the pion is transferred outside of the molecule onto which it was originally captured.In more complex molecular systems (e.g., organic compounds) more involved mechanisms come into play. Whether the pion undergoes atomic capture on hydrogen instead of on other atoms in the molecule depends on the local electronic wave functions, but only the broad principles are understood. The details of the subsequent pion transfer process are even less well understood and there is dispute about the nature of the transfer mechanism itself. It has been suggested 5,6 that an alternative pion transfer mechanism is important in these systems. In
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