J. S. George

MEASUREMENT OF THE SECONDARY RADIONUCLIDES 10Be, 26Al, 36Cl, 54Mn, AND 14C AND IMPLICATIONS FOR THE GALACTIC COSMIC-RAY AGE

We report on abundance measurements of ^(10)Be, ^(26)Al, ^(36)Cl, and ^(54)Mn in the Galactic cosmic rays (GCRs) using the Cosmic-Ray Isotope Spectrometer (CRIS) instrument aboard the Advanced Composition Explorer spacecraft at energies from ~70 to ~400 MeV nucleon^(-1). We also report an upper limit on the abundance of GCR ^(14)C. The high statistical significance of these measurements allows the energy dependence of their relative abundances to be studied. A steady-state, leaky-box propagation model, incorporating observations of the local interstellar medium (ISM) composition and density and recent partial fragmentation cross section measurements, is used to interpret these abundances. Using this model, the individual galactic confinement times derived using data for each species are consistent with a unique confinement time value of τ_(esc) = 15.0 ± 1.6 Myr. The CRIS abundance measurements are consistent with propagation through an average ISM hydrogen number density n_H = 0.34 ± 0.04 H atoms cm^(-3). The surviving fractions, f, for each radioactive species have been calculated. From predictions of the diffusion models of Ptuskin & Soutoul, the values of f indicate an interstellar cosmic-ray diffusion coefficient of D = (3.5 ± 2.0) × 10^(28) cm^2 s^(-1).

On the Low Energy Decrease in Galactic Cosmic Ray Secondary/Primary Ratios

Galactic cosmic ray (GCR) secondary/primary ratios such as B/C and (Sc+Ti+V)/Fe are commonly used to determine the mean amount of interstellar material through which cosmic rays travel before escaping from the Galaxy (Λ_(esc)). These ratios are observed to be energy-dependent, with a relative maximum at ~1 GeV/nucleon, implying a corresponding peak in Λ_(esc). The decrease in Λ_(esc) at energies above 1 GeV/nucleon is commonly taken to indicate that higher energy cosmic rays escape more easily from the Galaxy. The decrease in Λ_(esc) at energies <1 GeV/nuc is more controversial; suggested possibilities include the effects of a galactic wind or the effects of distributed acceleration of cosmic rays as they pass through the interstellar medium. We consider two possible explanations for the low energy decrease in Λ_(esc) and attempt to fit the combined, high-resolution measurements of secondary/primary ratios from ~0.1 to 35 GeV/nuc made with the CRIS instrument on ACE and the C2 experiment on HEAO-3. The first possibility, which hypothesizes an additional, local component of low-energy cosmic rays that has passed through very little material, is found to have difficulty simultaneously accounting for the abundance of both B and the Fe-secondaries. The second possibility, suggested by Soutoul and Ptuskin, involves a new form for Λ_(esc) motivated by their diffusion-convection model of cosmic rays in the Galaxy. Their suggested form for Λ_(esc)(E) is found to provide an excellent fit to the combined ACE and HEAO data sets.

Observations of the Li, Be, and B isotopes and constraints on cosmic-ray propagation

The abundance of Li, Be, and B isotopes in galactic cosmic rays (GCRs) between E = 50 and 200 MeV/nucleon has been observed by the Cosmic Ray Isotope Spectrometer (CRIS) on NASAs ACE mission since 1997 with high statistical accuracy. Precise observations of Li, Be, and B can be used to constrain GCR propagation models. We find that a diffusive reacceleration model with parameters that best match CRIS results (e.g., B/C, Li/C, etc.) are also consistent with other GCR observations. A ˜15-20% overproduction of Li and Be in the model predictions is attributed to uncertainties in the production cross-section data. The latter becomes a significant limitation to the study of rare GCR species that are generated predominantly via spallation.

THE ORIGIN OF PRIMARY COSMIC RAYS: CONSTRAINTS FROM ACE ELEMENTAL AND ISOTOPIC COMPOSITION OBSERVATIONS

Cosmic-ray isotope observations from NASA’s Advanced Composition Explorer (ACE) mission have been used to investigate the composition of cosmic-ray source material. Source abundances relative to 56Fe are reported for eleven isotopes of Ca, Fe, Co, and Ni, including the very rare isotopes 48Ca and 64Ni. Although the source abundances range over a factor ∼104, most of the ratios to 56Fe are consistent with solar-system values to within ∼20%. However, there are some notable differences, the most significant being an excess of ∼(70±30)% relative to the solar system for the cosmic-ray source ratio 58Fe/56Fe. The possible association of such an excess with a contribution to the cosmic-ray source from Wolf–Rayet star ejecta is discussed.

Solar minimum spectra of galactic cosmic rays and their implications for models of the near-earth radiation environment

The radiation dose from galactic cosmic rays during a manned mission to Mars is expected to be comparable to the allowable limit for space shuttle astronauts. Most of this dose would be due to galactic cosmic rays with energies < 1 GeV nucleon^(−1), with important contributions from heavy nuclei in spite of their low abundance relative to H and He. Using instruments on NASAs Advanced Composition Explorer (ACE) spacecraft, we have made the most statistically precise measurements to date of the solar minimum energy spectra of cosmic ray nuclei with charge Z = 4–28 in the energy range ∼ 40–500 MeV nucleon^(−1). We compare these measurements obtained during the 1997–1998 solar minimum period with measurements from previous solar minima and with models of the near-Earth radiation environment currently used to perform shielding and dose calculations. We find that the cosmic ray heavy-element spectra measured by ACE are as much as 20% higher than previously published solar minimum measurements. We also find significant differences between the ACE measurements and the predictions of available models of the near-Earth radiation environment, suggesting that these models need revision. We describe a cosmic ray interstellar propagation and solar modulation model that provides an improved fit to the ACE measurements compared to radiation environment models currently in use.

Cosmic ray energy loss in the heliosphere: Direct evidence from electron‐capture‐decay secondary isotopes

Measurements by the Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE) spacecraft provide direct evidence that galactic cosmic rays lose energy as a result of their interactions with magnetic fields expanding with the solar wind. The secondary isotopes ^(49)V and ^(51)Cr can decay to ^(49)Ti and ^(51)V, respectively, only by electron capture. The observed abundances of these isotopes are directly related to the probability of attaching an electron from the interstellar medium; this probability decreases strongly with increasing energy around a few hundred MeV/nucleon. At the highest energies observed by CRIS, electron attachment on these nuclides is very unlikely, and thus ^(49)V and ^(51)Cr are essentially stable. At lower energies, attachment and decay do occur. Comparison of the energy dependence of the daughter/parent ratios ^(49)Ti/^(49)V and ^(51)V/^(51)Cr during solar minimum and solar maximum conditions confirms that increased energy loss occurs during solar maximum. This analysis indicates an increase in the modulation parameter ϕ of about 400 to 700 MV corresponding to an increase in average energy loss for these elements of about 200 to 300 MeV/nucleon.

Secondary electron-capture-decay isotopes and implications for the propagation of galactic cosmic rays

We report the first observation of an energy dependence in the titanium, vanadium, and chromium isotopic abundances in cosmic rays. The observations were made in the 100–500 MeV/nucleon energy interval using data from the Cosmic Ray Isotope Spectrometer on the ACE spacecraft. The ^(51)Cr and ^(49)V isotopes in cosmic rays are produced by fragmentation of heavier cosmic ray nuclides and decay only by electron capture. The observations indicate that electron-capture decay occurred primarily at the lower energies measured, and that there is a resulting energy dependence in the abundances of these isotopes and their decay products.

The isotopic source composition of cosmic-ray iron, cobalt, and nickel

The isotopic composition of cosmic-ray Fe, Co, and Ni at energies ∼150–500 MeV/nucleon has been measured with the Cosmic-Ray Isotope Spectrometer (CRIS) instrument on ACE. Source abundances have been derived from a leaky-box propagation model using secondary species in the range 41 ≤ A ≤ 55 to constrain the calculated secondary contributions to the Fe, Co, and Ni isotopes. The derived relative source abundances bear a strong resemblance to solar-system values. The most significant difference, an excess of ∼ (70 ± 30)%, is found for the source abundance ratio ^(58)Fe/^(56)Fe. Some implications of the derived source composition for the origin of cosmic rays are discussed.

Cosmic-ray time scales using radioactive clocks

Radionuclides in the galactic cosmic rays serve as chronometers for measuring the characteristic time of physical processes affecting cosmic ray energy spectra and composition. The radionuclide ^(59)Ni, present in the ejecta of supernovae, will decay to ^(59)Co via electron-capture with a halflife of T_(1/2) = 7.6 × 10^4 yr. However, if the cosmic ray acceleration time scale is shorter than the decay halflife, ^(59)Ni will become fully-stripped of electrons and will be present in the cosmic rays. Abundances of cosmic ray ^(59)Ni and ^(59)Co measured with the Cosmic Ray Isotope Spectrometer (CRIS) are consistent with the decay of all source ^(59)Ni, implying an acceleration time delay > 10^5 yr. Abundances of the β-decay radioactive secondaries, produced by fragmentation of the cosmic rays during transport in the interstellar medium (ISM), depend on the time scales for spallation and escape from the Galaxy. Consequently, measurement of these abundances can be used to derive the galactic confinement time, τ_(esc), for cosmic rays. Using the abundances of the β-decay species ^(10)Be, ^(26)Al, ^(36)Cl, and ^(54)Mn measured by CRIS, we find a confinement time τ_(esc) ∼ 15 Myr.

Cosmic ray source abundances and the acceleration of cosmic rays

The galactic cosmic ray elemental source abundances display a fractionation that is possibly based on first ionization potential (FIP) or volatility. A few elements break the general correlation of FIP and volatility and the abundances of these may help to distinguish between models for the origin of the cosmic ray source material. Data from the Cosmic Ray Isotope Spectrometer instrument on NASA’s Advanced Composition Explorer spacecraft were used to derive source abundances for several of these elements (Na, Cu, Zn, Ga, Ge). Three (Na, Cu, Ge) show depletions which could be consistent with a volatility-based source fractionation model.