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A comprehensive study of the heavy-particle cosmic-ray exposure received by the individual astronauts during the nine lunar Apollo missions reveals a significant variation in the exposure as a function of shielding and the phase of the solar cycle. The data are useful in planning for future long-range missions and in estimating the expected biological damage.
The first InterComparison for Cosmic-ray with Heavy Ion Beams At NIRS (ICCHIBAN) project is an ongoing, international collaboration organized at the National Institute of Radiological Sciences (NIRS), Japan, for the purpose of characterizing and comparing at a controlled, ground-based heavy ion facility the radiation response of instruments used aboard piloted spacecraft for crew and area dosimetry. We present preliminary results from the first set of ICCHIBAN exposures made at HIMAC heavy ion accelerator in February 2002. The initial series of exposures (1st ICCHIBAN run) was designed to establish the response of active detectors to two well-characterized heavy ion beams; 400 MeV/nucleon 12C and 400 MeV/nucleon 56Fe. These beams are representative in charge and energy of two of the most significant heavy ion components present in the galactic cosmic radiation spectrum. The properties of the incident beam, including intensity, profile, charge and total energy, were characterized using several different detector systems, including silicon detectors, CR-39 plastic nuclear track detectors and plastic scintillation counters. Once the response of each detector to heavy ion beams of known composition has been measured, results from on-orbit measurements made by the different instruments can be more meaningfully compared. We conclude by discussing plans for future ICCHIBAN runs, including next 2nd ICCHIBAN run for passive detectors in early summer 2002.
Using the Langley Research Center galactic cosmic ray (GCR) transport computer code (HZETRN) and the computerized anatomical man (CAM) model, crew radiation levels inside manned spacecraft on interplanetary missions are estimated. These radiation-level estimates include particle fluxes, LET (linear energy transfer) spectra, absorbed dose, and dose equivalent within various organs of interest in GCR protection studies. Changes in these radiation levels resulting from the use of various different types of shield materials are presented.
The chemistry that occurs in the interstellar medium in response to cosmic ray ionization is summarized, and a review of the ionization rates that have been derived from measurements of molecular abundances is presented. The successful detection of large abundances of H(3)(+) in diffuse clouds and the recognition that dissociative recombination of H(3)(+) is fast has led to an upward revision of the derived ionization rates. In dense clouds the molecular abundances are sensitive to the depletion of carbon monoxide, atomic oxygen, nitrogen, water, and metals and the presence of large molecules and grains. Measurements of the relative abundances of deuterated species provide information about the ion removal mechanisms, but uncertainties remain. The models, both of dense and diffuse clouds, that are used to interpret the observations may be seriously inadequate. Nevertheless, it appears that the ionization rates differ in dense and diffuse clouds and in the intercloud medium.
Numerical calculations were done of the indoor exposure rate due to the cosmic rays. Only muons were considered, and the calculations were done within a two-dimensional framework under the non-scattering approximation. The effects of various structural parameters on the level and the distribution of the exposure rate were studied by a case control method. It was shown that a building's dimensions and the floor thickness were important in all cases. However, the effect of partition walls would become significant only when the partition number is large. The effect of neighboring buildings would be especially important in the lower floors of tall buildings. In such a case, attention is necessary not only to the buildings' internal structures but also to the relationship between that particular building and its neighborhood. The maximum amplitude of the possible variation of the exposure rate due to the change of any single structural parameter would be 30% in ordinary Japanese housing conditions.
The mean free path for nuclear interactions of galactic cosmic-rays is comparable to shielding and tissue thicknesses present in human interplanetary exploration, resulting in a significant fraction of nuclear reaction products at depth. In order to characterize the radiation field, the energy spectrum, the angular distribution, and the multiplicity of each type of secondary particles must also be known as a function of depth. Reactions can take place anywhere in a thick absorber; therefore, it is necessary to know these quantities as a function of particle energy for all particles produced. HZE transport methods are used to predict the radiation field; they are dependent on models of the interaction of man-made systems with the space environment to an even greater extent than methods used for other types of radiation. Hence, there is a major need to validate these transport codes by comparison with experimental data. The most cost-effective method of validation is a comparison with ground-based experimental measurements. A research program to provide such validation measurements using neon, iron and other accelerated heavy ion beams will be discussed and illustrated using results from ongoing experiments and their comparison with current transport codes. The extent to which physical measurements yield radiobiological predictions will be discussed.
If, as recently proposed by Farrar and Piran, Cen A is the source of cosmic rays detected above the Greisen-Zatsepin-Kuz'min cutoff, neutrons are approximately 140 more probable than protons to be observed along its line of sight. This is because the proton flux is rendered nearly isotropic by O(microG) intergalactic magnetic fields. With the anticipated aperture of the Southern Auger Observatory, one may expect on the order of 2 neutron events /year above 10(20) eV in the line of sight of Cen A.
We describe theoretical considerations for developing models of heavy ion fragmentation appropriate for galactic cosmic ray studies. Previous models have been based on parametric fits to limited experimental data or models that ignored some aspects of the reaction dynamics, including nuclear cluster effects. The abrasion-ablation description of the fragmentation process is re-formulated to describe the excitation spectrum of pre-fragment nuclei. The resulting spectrum is shown to be related to the many-body response of the nuclear ground-state and excited states, and an approach to simplify this function is discussed. An analytic solution to the nuclear de-excitation process is described which includes a realistic level spectrum of the GCR nuclei (A < 60). Comparisons are made to experiments for fragmentation of 24Mg, 32S, and 56Fe beams on several targets and results are discussed.
A predictive correlation between hard cosmic ray flux (HCRF) near the Earth's surface and sudden increases in the number of cases of cardiovascular disease (CVD) was studied in Vilnius city. Gamma spectrometers were used to measure HCRF. Changes in the total number of impulses registered by these devices were taken as the physical parameter for prediction of an increase in the number of CVD cases. For quantitative assessment of CVD dynamics two empirical criteria for HCRF were proposed: (1) a consistent fall in the HCRF by 200 or more impulses for 4 or more hours per day; (2) the difference (no less than 15 impulses per hour) between the average number of impulses registered in 2 (subsequent and previous) days. The periods analyzed were those when the number of CVD cases exceeded the average monthly level by 10% and 15%. HCRF fluctuations were used as an indirect indicator of variations in the geomagnetic field, and a leap in CVD cases was predicted 1-3 days following a change in the HCRF according to the two criteria mentioned above. The predictive reliability of an increase in CVD cases within 2-3 days exceeded 80%, and within 1-2 days 70%. No essential difference in predictive reliability was observed when the number of CVD cases exceeded the average monthly level by 10% or 15%. Geomagnetic field variations contribute only insignificantly to leaps in the number of CVD cases, which are evoked by the sum effects of other factors. The results of the present study should be regarded as preliminary.
We have measured the cosmic ray spectrum above 10(17.2) eV using the two air-fluorescence detectors of the High Resolution Fly's Eye observatory operating in monocular mode. We describe the detector, phototube, and atmospheric calibrations, as well as the analysis techniques for the two detectors. We fit the spectrum to a model consisting of galactic and extragalactic sources.
We present a new measurement of the cosmic-ray positron fraction at energies between 5 and 15 GeV with the balloon-borne HEAT-pbar instrument in the spring of 2000. The data presented here are compatible with our previous measurements, obtained with a different instrument. The combined data from the three HEAT flights indicate a small positron flux of nonstandard origin above 5 GeV. We compare the new measurement with earlier data obtained with the HEAT-e(+/-) instrument, during the opposite epoch of the solar cycle, and conclude that our measurements do not support predictions of charge sign dependent solar modulation of the positron abundance at 5 GeV.
The spectra of secondary particles resulting from interactions of primary galactic cosmic rays with the nuclei in the atmosphere have been calculated using the Monte Carlo transport code FLUKA. The simulations have been carried out at solar minimum and solar maximum activity, for several values of the vertical geomagnetic cut-off. The effective dose rate and the ambient dose equivalent rate as a function of geomagnetic cut-off and altitude have been obtained using appropriate sets of conversion coefficients. The calculated results are discussed and compared with experimental data and other calculations. A simple method is proposed to calculate the radiation exposure at aircraft altitudes.
Crews of manned interplanetary missions may accumulate significant radiation exposures from the galactic cosmic ray (GCR) environment in space. Estimates of how these dose levels are affected by the assumed temporal and spatial variations in the composition of the GCR environment, and by the effects of the spacecraft and body self-shielding on the transported radiation fields are presented. In this work, the physical processes through which shielding alters the transported radiation fields are described. We then present estimates of the effects on model calculations of (1) nuclear fragmentation model uncertainties, (2) solar modulation, (3) variations between solar cycles, and (4) proposed changes to the quality factors which relate dose equivalent to absorbed dose.
Computer simulations on the exposure indoors from cosmic ray muons were practiced in the framework of non-scattering and non-cascade assumptions. The model buildings were two-dimensional, rectangular, and were made of a normal concrete. A stratified structure was assumed in each building, where no mezzanine was considered. Walls were not taken into account yet. The distributions of the exposure rates in 26-story buildings were illustrated in contour maps for various sets of parameters. All of them gave basically archlike patterns. Analyses of the results showed that the exposure rate is affected most largely by the floor board thickness. The ceiling height would be an insignificant factor for short buildings. The min/max ratio of the muon exposure rate in a moderate size building was estimated to be more than 0.7.
The angular distribution of the secondary radiation produced by the galactic component of cosmic rays has been determined by simulating the penetration of the primary spectra in the Earth's atmosphere. The simulations have been carried out with the latest version of the FLUKA code. Particles have been scored at various altitudes according to their angle of incidence for some significant values of vertical cut-off rigidity and solar modulation parameter. The calculated results at typical cruise altitudes for a civil aircraft are presented. The data at 10.7 km have been fitted with simple mathematical equations. It has been demonstrated that the major contribution to the doses at aviation altitudes arises from downward-directed particles. The isotropic irradiation usually assumed for the evaluation of aircrew exposure could be a very poor approximation.
The average mass composition of cosmic rays with primary energies between 10(17) and 10(18) eV has been studied using a hybrid detector consisting of the High Resolution Fly's Eye (HiRes) prototype and the MIA muon array. Measurements have been made of the change in the depth of shower maximum and the muon density as a function of energy. The results show that the composition is changing from a heavy to lighter mix as the energy increases.
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