The Spectra of 2H and 3He Secondary Cosmic Ray Isotopes from ~20-85 MeV/nuc as Measured Using the B-end HET Telescope on Voyager beyond the Heliopause and a Fit to These Interstellar Spectra Using a Leaky Box Propagation Model

TL;DR

This study measures cosmic ray secondary isotopes 2H and 3He beyond the heliopause using Voyager data, and fits these spectra with a Leaky Box model to infer interstellar matter traversal and cosmic ray propagation characteristics.

Contribution

It provides new measurements of secondary cosmic ray isotopes at low energies and applies a Leaky Box model to interpret the interstellar propagation parameters.

Findings

Secondary to primary ratios indicate 7-9 g/cm2 matter traversal.

Interstellar path length consistent with a Leaky Box model.

Excess intensities of 2H and Boron below 30 MeV/nuc suggest additional processes.

Abstract

We have measured the intensity and spectra of the cosmic ray secondary isotopes 2H and 3He and the primary isotopes H and 4He between 20-85 MeV/nuc during a 5 year time period after Voyager 1 (V1) crossed the heliopause. The data reported here is from the B-end high energy telescope. The ratios of the intensities of the secondary to primary spectra of these nuclei at low energies are sensitive indicators for determining the amount of matter traversed at these energies by their galactic cosmic ray progenitor nuclei, after acceleration, in this case mostly 4He nuclei. The measurements of secondary 3He abundances indicate that cosmic ray 4He of energies between 30-100 MeV/nuc have traversed between 7-9 g/cm2 of interstellar matter (90% H, 10% He) in a Leaky Box propagation model. This path length is also consistent with the production of secondary 2H nuclei between 20-50 MeV/nuc, which is also produced mainly by 4He in a LBM. The Boron abundance, studied in separate papers, is also consistent with this path length at energies >30 MeV/nuc. These secondary intensities imply that the interstellar cosmic ray path length may be described in a LBM in a manner consistent with a mean path length,gamma= 20.6 beta P-0.45 at rigidities above ~0.5 GV (30 MeV/nuc for A/Z=2 nuclei). Both 2H and Boron have an excess intensity vs. the predictions for path lengths of 9 g/cm2 below 30 MeV/nuc.