Search

Measurements of the abundances of cosmic-ray ^(59)Ni and ^(59)Co are reported from the Cosmic-Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer. These nuclides form a parent-daughter pair in a radioactive decay which can occur only by electron capture. This decay cannot occur once the nuclei are accelerated to high energies and stripped of their electrons. The CRIS data indicate that the decay of ^(59)Ni to ^(59)Co has occurred, leading to the conclusion that a time longer than the 7.6 × 10^4 yr half-life of ^(59)Ni elapsed before the particles were accelerated. Such long delays indicate the acceleration of old, stellar or interstellar material rather than fresh supernova ejecta. For cosmic-ray source material to have the composition of supernova ejecta would require that these ejecta not undergo significant mixing with normal interstellar gas before ~10^5 yr has elapsed.

Galactic cosmic rays (GCR) interact to produce secondary fragments as they pass through the interstellar medium (ISM). Abundances of the long-lived radioactive secondaries ^(10)Be, ^(26)Al, ^(36)Cl, and ^(54)Mn can be used to a derive the confinement time of cosmic rays in the galaxy. Abundances for these species have been measured recently using the Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE) spacecraft. To interpret this data we have modeled the production and propagation of the radioactive secondaries, taking into account recently published isotopic production cross-sections. Abundances for all species are consistent with a confinement time of π_(esc) ~22 x 10^6 years.

We present measurements of the nickel and cobalt isotopes from the Cosmic Ray Isotope Spectrometer (CRJS) which was launched in August, 1997 aboard the NASA Advanced Composition Explorer (ACE). The objectives of CRJS are to measure the isotopic abundances and energy spectra of galactic cosmic rays ~GCRs) with 3 ≤ Z ≤ 30. We find that for these nuclei the measured ^5Ni/^(60)Ni and ^(59)Co/^(60)Ni ratios imply a time delay between nucleosynthesis and acceleration to cosmic ray energies of > 10^5 y.

The Cosmic Ray Isotope Spectrometer (CRIS) instrument on board the Advanced Composition Explorer (ACE) spacecraft is making new measurements of nuclides just beyond the iron-nickel peak in the galactic cosmic rays. Isotopes of copper and zinc have been resolved for the first time. Elemental abundances for nuclei with 29≤Z≤34 are reported with good separation between species. Several of these elements are useful for studying fractionation processes which may depend on the first ionization potential or volatility. Source abundances are estimated using a prior propagation calculation and their potential for distinguishing between source models are discussed.

The elemental abundances of galactic cosmic rays (GCR) observed near Earth provide information about the composition of the cosmic ray sources as well as their propagation history. The Cosmic Ray Isotope Spectrometer (CRIS) onboard the Advanced Composition Explorer (ACE) spacecraft measures the elemental and isotopic composition of GCRs with energies '"" 50 - 500 Me V /nucleon with high statistical accuracy ('"" 5000 stopping nuclei heavier than helium per day) due to its large geometrical factor. The CRIS data are used to derive cosmic ray abundances at the lowest level of solar activity during the last solar minimum. We present elemental abundances measured by CRIS, compare them with previous measurements, and discuss the plausible origins of the disagreement.

Cosmic rays that are produced by nuclear interactions during propagation and decay only by electron capture can be used to determine the level of cosmic ray reacceleration that has occurred. The high resolution, large statistical sample of cosmic rays collected by the Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE) spacecraft provides a data set that allows precision measurements of these isotopic abundances. We discuss electron capture decay, the signatures of reacceleration, and the sensitivity with which reacceleration models can be tested.

The time dependences of the intensities of low-energy galactic cosmic ray (GCR) nuclei have been investigated using instrumentation carried aboard the Advanced Composition Explorer (ACE). The intensities of these low energy particles near Earth began to decline from solar minimum levels in late-1997 /early-1998, shortly after the launch of ACE, and have been continuously monitored by the Cosmic Ray Isotope Spectrometer (CRIS) instrument since that time. We find that the variations are well correlated with variations of higher energy cosmic rays, as indicated by neutron monitor observations. The relative magnitude of the GCR variations is significantly less than previously reported for anomalous cosmic rays. We also compare the fluctuations observed for different energies and different nuclides.

Cosmic rays that are produced by nuclear interactions during propagation and decay only by electroncapture can be used to determine the level of cosmic ray reacceleration that has occurred. The high resolution, large statistical sample of cosmic rays collected by the Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE) spacecraft provides a data set that allows precision measurements of these isotopic abundances. We discuss electron capture decay, the signatures of reacceleration, and the sensitivity with which reacceleration models can be tested.

Supernovae should produce the radioactive nuclide ^(59)Ni, and in the ejecta of the explosions these particles will decay by electron capture with a halflife of 7.6 x 10^5 yr to produce ^(59)Co. However, if the ^(59)Ni nuclei are accelerated to cosmic-ray energies on a time scale short compared to this halflife, they are stripped of their electrons and decay is prevented. Thus the abundances of ^(59)Ni and ^(59)Co can be used to determine whether the time between nucleosynthesis and cosmic-ray acceleration is short or long compared to the 59Ni halflife (Soutoul, Casse, & Juliusson 1978). We have used the Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE) to measure the abundances of 59Ni and 59Co in galactic cosmic rays, and find that the data are consistent with complete decay of 59Ni indicating a time delay 2 10^5 yr. We present the observations and discuss their significance for models of cosmic ray origin and acceleration.

Radioactive cosmic ray nuclei produced by nuclear interactions during cosmic ray propagation through the galaxy can be used to study the mean interstellar gas density in the propagation volume and the time scales associated with the propagation process. The Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE) has made high-resolution abundance measurements of the beta-decay secondary isotopes ^(10)Be, ^(26)Al, ^(36)Cl, and ^(54)Mn over the energy range 70-400 MeV/nuc. The large geometrical factor of CRIS (~250 cm^2sr) and the 20 months of data collection at near solar minimum conditions have made it possible since launch in August, 1997 to accumulate data samples considerably larger than previous missions. The isotopic abundances derived from these data are presented and compared with previous measurements.

With the addition of krypton and silver projectiles we have extended our previous studies of the fragmentation of heavy relativistic nuclei in targets ranging in mass from hydrogen to lead. These projectiles were studied at a number of discrete energies between 450 and 1500A MeV. The total and partial charge-changing cross sections were determined for each energy, target, and projectile, and the values compared with previous predictions. A new parametrization of the dependence of the total charge-changing cross sections on the target and projectile is introduced, based on nuclear charge radii derived from electron scattering. We have also parametrized the energy dependence of the total cross sections over the range of energies studied. New parameters were found for a previous representation of the partial charge-changing cross sections in hydrogen and a new parametrization has been introduced for the nonhydrogen targets. The evidence that limiting fragmentation has been attained for these relatively light projectile nuclei at Bevalac energies is shown to be inconclusive, and further measurements at higher energies will be needed to address this question.

We have measured the charge-changing cross sections of 10.6 GeV/nucleon ^(197)Au nuclei interacting in targets of CH_2 (polyethylene), C, Al, Cu, Sn, and Pb. Cross sections for H are calculated from those measured in C and CH_2. The total charge-changing cross sections are higher than those measured at energies of ≤1 GeV/nucleon. The measured cross sections for the heavier targets are somewhat larger than those predicted by a model based on data taken at lower energies with lighter targets. Partial charge-changing cross sections for the production of fragments from the incident Au projectiles were measured for charge changes (ΔZ) from ΔZ=+1_(,80)Hg, down to approximately ΔZ=-29_(,50)Sn. In comparison to lower energy measurements, these partial cross sections are found to be smaller for small ΔZ and larger or the same for large ΔZ. The H partial cross sections are found to follow a power law in ΔZ similar to that for heavier targets, instead of the exponential form observed at lower energies. Factorization is found to hold for all partial cross sections with ΔZ greater than two. In the heavier targets, the cross sections for one and two proton removal have significant contributions from electromagnetic dissociation. The electromagnetic dissociation contribution to the total cross section is derived and found to be relatively small, but with a strong dependence on the charge of the target nuclei of the form Z_T^(1.75±0.01).

We have measured the charge-changing cross sections of 10.6 GeV/nucleon ^(197)Au nuclei interacting in targets of CH2 (polyethylene), C, Al, Cu, Sn, and Pb. Cross sections for H are calculated from those measured in C and CH_2. The total charge-changing cross sections are higher than those measured at energies of ≤1 GeV/nucleon. The measured cross sections for the heavier targets are somewhat larger than those predicted by a model based on data taken at lower energies with lighter targets. Partial charge-changing cross sections for the production of fragments from the incident Au projectiles were measured for charge changes (ΔZ) from ΔZ=+1, _(80)Hg, down to approximately ΔZ=-29, _(50)Sn. In comparison to lower energy measurements, these partial cross sections are found to be smaller for small ΔZ and larger or the same for large ΔZ. The H partial cross sections are found to follow a power law in ΔZ similar to that for heavier targets, instead of the exponential form observed at lower energies. Factorization is found to hold for all partial cross sections with ΔZ greater than two. In the heavier targets, the cross sections for one and two proton removal have significant contributions from electromagnetic dissociation. The electromagnetic dissociation contribution to the total cross section is derived and found to be relatively small, but with a strong dependence on the charge of the target nuclei of the form Z__T^(1.75±0.01).

We summarize additions to our data base of charge-changing cross sections for relativistic ultraheavy nuclei interacting in targets ranging from H to Pb. We have improved parametric fits to those cross sections as functions of energy and of projectile, target, and fragment charge. At high energies, we have determined cross sections for Au projectiles at 10.6 GeV /nucleon in targets of H, CH_2, C, Al, Cu, Sn, and Pb. Compared with cross sections at 1 GeV /n, fragment production is substantially changed, especially for the H target. These changes have important implications for calculations of interstellar propagation of ultraheavy nuclei. At lower energies, we have added Kr and Ag to our list of projectiles. Analysis of these data has led to a better understanding of the systematics of these cross sections, hence more physically meaningful parameterizations for fragmentation at high energies and for charge pickup.

In 1990 we exposed a wide range of targets to beams of relativistic krypton and silver nuclei at the LBL Bevalac at energies from 0.5 to 1.5 GeV/nucleon. Incoming and fragmented nuclei were detected in an array of ion chambers, Cherenkov detectors, and multiwire proportional chambers. The total and partial cross sections for charge-changing interactions have been measured, with emphasis on those of cosmic ray interest. These cross sections are compared to those from parametric fits.

Measurements of the detection efficiency of a Scintillating Optical Fiber Trajectory (SOFT) detector for 660 MeV/n Carbon, Boron, Beryllium and Lithium nuclei and for 300 MeV/n Fe nuclei are described. Detection efficiencies of 99.8%, 98.7%, 97.5%, and 93.8% for C, B, Be, and Li respectively were obtained for a single layer of fibers at an angle of incidence of 30°. These fibers had a black coating of extramural absorber (EMA) to optically decouple the fibers. Somewhat better detection efficiencies were obtained for optically coupled fibers (no EMA). The detection efficiencies for iron nuclei were ~99.7%. We have used these results to estimate the event detection efficiencies of the fiber hodoscope which is planned for use in the Cosmic Ray Isotope Spectrometer (CRIS) (Stone, et al., 1989) experiment on the Advanced Composition Explorer (ACE). We find that for the highest energy Lithium nuclei which will stop in CRIS, the detection efficiency is about 89% for 6 plane events and about 99% for 5 plane events at 30°.

The fragmentation of 10.6 GeV/n Au in CH_2. C, Al, Cu, Sn, and Pb targets has been studied using an array of ion chambers, multi-wire proportional counters (MWPC), and Cherenkov counters. Total charge-changing cross sections were found to be monotonically increasing with target charge over cross sections measured and derived from lower energy data. Partial charge-changing cross sections yielding charge changes less than 1O were depressed from those measured at lower energy.

In December, 1990, we exposed the Scintillating Optical Fiber Isotope Experiment (SOFIE) to beams of silicon and iron nuclei at the LBL Bevalac accelerator. SOFIE is a Cerenkov-dE/dx-range balloon experiment designed to study the isotopic composition of heavy galactic cosmic rays. Newly developed detectors using scintillating optical fiber technology provide high-resolution trajectory and range measurements in a large-area detector. An Analysis of the Bevalac data is presented, including performance of the trajectory, Cerenkov and range detectors, and mass resolution at silicon and iron. We find the mass resolution to be ~0.26amu at silicon and ~ 0.4amu at iron, consistent with predictions based on the expected sources of error in the Cerenkov and range detectors.