Io encounters past and present: A heavy ion comparison

The heavy ion counter (HIC) on the Galileo spacecraft consists of two low-energy telescopes that were proof-test models of the telescopes used for the cosmic ray subsystem (CRS) on Voyager. The telescopes on HIC were improved to better withstand the intense radiation environment of the Jovian system and to measure higher-energy particles with greater sensitivity. The similarity of these two instruments makes it natural to compare the data in an effort to glean insight on the behavior of heavy ion fluxes at higher energies and how the inner Jovian magnetosphere has changed in the last 18 years. Inside L = 6, the oxygen spectrum suggests a spectral break at ∼ 7500 MeV/Gauss, corresponding to ions with gyroradii of ∼ 1.5 R_(Io) at L = 6. A comparison of the phase space densities show remarkable agreement between the two time periods outside of the Io orbit, with differential spectra consistent with γ ∼ −8 above ∼ 6 MeV/nucleon. Near Io Galileo observed a large density gradient not apparent in the Voyager data that may be due to local time asymmetries or a decrease in the radial diffusion rate

Fragmentation cross sections of relativistic ^(84)_(36)Kr and ^(109)_(47)Ag nuclei in targets from hydrogen to lead

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

Anisotropy of Galactic Iron of Energy 30 to 500 GeV/amu Studied by HEAO-3

The anisotropy of cosmic ray iron observed by the Heavy Nuclei Experiment [1] on the HEA0-3 spacecraft has been studied. A high rigidity data set was chosen by requiring the Stoermer cutoff be greater than 7 GV, and the energy of individual events was determined by relativistic rise in the ion chamber signal [2]. Events which have estimated rigidity well above their Stoermer cutoff rigidity were chosen in order to reduce the effect of the geomagnetic field on the cosmic ray trajectories. Selecting events with estimated rigidity greater than ~58 GV from eight months of data yields 2459 events. This data set allows an anisotropy measurement with a statistical uncertainty of 3%. We will continue to try increasing the size. of the selected data set while limiting systematic errors due to the geomagnetic and interplanetary fields

Nuclear Interaction Cross Sections for UltraHeavy Nuclei

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

Implications of Source Abundances of Ultraheavy Cosmic Rays

The ratio of cosmic ray source abundance to solar-system abundance was examined for individual elements. Correlations of these ratios with first-ionization potential (FIP) and the expected mass-to-charge ratio (A/Q) of the elements in a million-degree plasma are analyzed. The FIP correlation was examined and it is shown that the correlation is affected by the choice of C2 or C1 chondritic meteorites as the solar-system standard for comparison. An A/Q correlation is suggested as a consequence of the shock acceleration model in the hot interstellar medium. The correlations are presented

Response of Scintillators to UH Nuclei

In order to evaluate the performance of plastic scintillators for the detection of Ultra-Heavy cosmic ray nuclei, as envisaged in paper OG 10.1.14P, we have conducted experiments at the LBL Bevalac in which we exposed NE-114 and acrylic scintillators to beams of 47Ag ions and its interaction fragments. As a result we have calibrated these scintillators over the charge range 31 ≤ Z ≤ 47. Our results show that a combination of Cherenkov and scintillator detectors can resolve individual charges over this charge range. The resolution obtained in scintillator was 0.24 and 0.28 cu for NE-114 and acrylic scintillator respectively. In addition the light emission is shown to be linear to a good approximation with dE/dx over this charge range

Fragmentation of UH Nuclei

We have measured the total charge changing cross sections as a function of energy for projectile _(36)Kr nuclei in a wide range of targets ranging from polyethylene to lead. These cross sections are energy dependent and the dependence increases as the target mass increases

Response of Ionization Chambers and Cherenkov Counters to Relativistic Ultraheavy Nuclei

We shall report results from a calibration of a set of high-resolution ionization chambers and Cherenkov counters at the Lawrence Berkeley Laboratory Bevalac, in November 1986. We obtained exposures to beams of 26Fe, 57La, 67Ho, and 79Au, at several diffyfent energies, ranging from -300 MeV/amu to a maximum between 1 and 1.6 Ge V lamu. The detectors were basically similar to those used in the HEA0-3 HNE (Binns et al. 1981) and these exposures were intended to calibrate the response of the HNE detectors to UH nuclei in the cosmic radiation. For nuclei of low .atomic number (Z), the response of these detectors scales like Z2; however, at high Z this scaling is expected to break down (Ahlen 1980,1982; Derrickson et til. 1981)

Energy Dependence of the Fragmentation of UH-Nuclei

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

Abundances of 'secondary' elements among the ultraheavy cosmic rays: Results from HEAO-3

The HEAO-3 Heavy Nuclei Experiment has measured elemental abundances of ultraheavy cosmic rays near earth. The elements with atomic number (Z) in the intervals 44≤Z≤48 and 62≤Z≤74 arriving at earth are expected to have significant secondary components. However, their source abundances are unlikely to be low enough to warrant treating them as pure secondaries. Our results are consistent with solar system abundances modified for first ionization potential with possibly some enhancement of tho r to s ratio