THE COSMIC-RAY PATH-LENGTH (AGE) DISTRIBUTION IN THE GALAXY - EXPERIMENTAL LIMITS ON THE DEVIATION FROM AN EXPONENTIAL

New experimental evidence relating to the deviation of the cosmic-ray path length distribution in the Galaxy from a simple exponential is examined-in particular the possibility that there is a (truncation) of short path lengths as has been suggested previously. A comparison of the secondary/primary ratios B/C and Z = (21-23)/Fe at low energies calculated using new cross sections with recent measurements suggests that the value of the truncation parameter which describes this deficiency is 0. 1 3 +/- 0.07. However, comparison of calculations and measurements of the energy dependence of these same two ratios at high energies along with the shape of the high-energy electron spectrum both set limits on the truncation parameter of less-than-or-equal-to 0.05. Thus the path length distribution is remarkably like an exponential with a possible deficiency of short pathlengths at low energies where ionization energy loss is important. From this it is argued that the cosmic-ray source distribution must be quite uniform, probably on a scale approximately 50 pc or less near the Sun. Extending this uniformity to the Galaxy as a whole would imply greater-than-or-equal-to 10(5) active sources. The possible departure from the exponential path length distribution at low energies could be due to nonuniform propagation conditions in which some of the sources are embedded in, for example, giant molecular clouds or other high-density regions. In any case, because the departure from an exponential path length distribution appears to be small, the cosmic-ray propagation can be described quite well by a Leaky Box Model which assumes a uniform source distribution with uniform propagation in a region with a leaky boundary in which the probability of escape is uniform in time during the propagation. This model in which all details of the propagation, escape, and source distribution are obscured, is equivalent to observing these particles through a dense fog which may, in fact, be the reality of the situation.