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Astron. Astrophys. 336, L61-L64 (1998)

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1. Introduction

It has been recognized now for over 20 years that electron capture isotopes - whose decay depends on the attachment of a K-shell electron - produced as secondaries during the propagation of cosmic rays in the galaxy will decay during the lifetime of the cosmic rays arriving at the solar system. This attachment is strongly energy dependent and so provides information on the conditions of propagation and the interstellar energy at the time of attachment (Raisbeck & Yiou, 1971). A comparison of the measured abundance of these K-capture isotopes with detailed predictions of their expected abundance based on cross section measurements and propagation models may thus be used to determine how this attachment energy is related to the present energy for these nuclei - and so to provide information on whether the cosmic ray acceleration process actually occurs only over a short initial time or whether significant acceleration occurs throughout the lifetime of cosmic rays during which time the secondaries are produced. In addition, because the local interstellar energy can be estimated from the amount of decay of these isotopes, the amount of interplanetary energy loss can be estimated. This is important because most models of cosmic ray modulation in the heliosphere predict a "real" energy loss of a few hundred MeV/nuc during the course of this modulation (Gleeson & Axford, 1967; Goldstein, Fisk & Ramaty, 1970).

Several K capture isotopes can be studied to determine if this attachment and subsequent decay has occurred. Examples are: 57Co [FORMULA] 57Fe, 55Fe [FORMULA] 55Mn, 54Mn [FORMULA] 54Cr, 51Cr [FORMULA] 51V and 49V [FORMULA] 49Ti. The lifetimes of these isotopes against decay by K-capture is shorter than that for stripping of the attached K-electron even in the dense cold phase of the interstellar medium with densities [FORMULA] 102-103 atoms/cm3. Stripping is uneffective for preventing decay once attachment has occurred.

Previous attempts to observe this decay have been inconclusive for a number of reasons. Partly because the appropriate cross sections were not known accurately enough and partly because of the limitations of the cosmic ray data on the abundance of these isotopes. Now because of the addition of new Voyager data (Lukasiak et al., 1997a) to the already existing ISEE data base (Leske, 1993) on the isotopic composition of elements in the range Z=20-28, as well as improved cross section measurements of 56Fe fragmentation (Webber, Kish and Schrier, 1990a) and improved predictive formulae for the unmeasured cross sections (Webber, Kish and Schrier, 1990b), it is now possible to examine this problem with a new level of precision. In this paper we concentrate on the Vanadium isotopes with comments as to why the other K-capture decay isotopes are more difficult to observe. For Vanadium, if electron capture decay has indeed occurred, there will be a maximum observable effect with respect to the isotopic composition predicted if no decay has occurred. This charge has 3 isotopes in cosmic rays; if decay occurs 51V will be enhanced because of the decay of 51Cr, 50V will be unaffected and 49V will be depleted as it decays into 49Ti. The vanadium isotopes are mostly of secondary origin and the value of the predicted interstellar 51V/49V ratio at energies below 1 Gev/nuc is weakly affected by the uncertainties of the abundances at the cosmic ray source.

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© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998