Space radiation quality factor investigations with “NAUSICAA”-device on board the “MIR” space station

The estimation of radiation quality factor in space flights is a serious problem of space dosimetry. The solution of this problem is based on LET-spectra measurements. The “NAUSICAA”-device installed on the “MIR” station has a detector with a large geometric factor, that gives the possibility of measuring LET-spectra with sufficient statistic in relatively short time interval and hence the corresponding Q. LET-spectra are accumulated through 24 hour periods and equivalent dose (H), absorbed dose (D), quality factor and flux (F) are measured every 10 minutes. The obtained data permit the estimate of the diurnal Q and Q in South Atlantic Anomaly. These values vary in the range of 1.4 – 2.2. The analysis of these values including their comparison with the calculated results is given in this paper.     

Galactic cosmic ray abundances and spectra behind defined shielding.

LET spectra have been measured for lunar missions and for several near Earth orbits ranging from 28 degrees to 83 degrees inclination. In some of the experiments the flux of GCR was determined separately from contributions caused by interactions in the detector material. Results of these experiments are compared to model calculations. The general agreement justifies the use of the model to calculate GCR fluxes. The magnitude of variations caused by solar modulation, geomagnetic shielding, and shielding by matter determined from calculated LET spectra is generally in agreement with experimental data. However, more detailed investigations show that there are some weak points in modeling solar modulation and shielding by material. These points are discussed in more detail.     

Comparison of measured cosmic ray LET spectra with models and predictions.

Radiation effects of cosmic ray nuclei are generally described as a function of the particle LET. For a large number of space missions LET spectra have been measured and models have been developed to calculate these spectra that include the effects of geomagnetic shielding and shielding provided by material. In this paper we compare measured and calculated LET spectra. For low earth orbits events with high local energy deposition, i.e., short range secondaries, contribute significantly to the measured spectra. These events are produced by nuclear interactions, mainly induced by protons from the south atlantic anomaly. The technique to include these contributions in the models depends on the size of radiation sensitive volumes. For sizes comparable to or larger than the range of target secondaries it is essential to separate contributions by target interactions from those of cosmic rays. This separation is possible in experiments which use stacks of plastic nuclear track detectors. The yield of short range events generated by protons and measured in the detector can be calibrated from accelerator experimental data. We present first results for CR-39 detectors.     

Analysis of radiation risk from alpha particle component of solar particle events.

The solar particle events (SPE) will contain a primary alpha particle component, representing a possible increase in the potential risk to astronauts during an SPE over the often studied proton component. We discuss the physical interactions of alpha particles important in describing the transport of these particles through spacecraft and body shielding. Models of light ion reactions are presented and their effects on energy and linear energy transfer (LET) spectra in shielding discussed. We present predictions of particle spectra, dose, and dose equivalent in organs of interest for SPE spectra typical of those occurring in recent solar cycles. The large events of solar cycle 19 are found to have substantial increase in biological risk from alpha particles, including a large increase in secondary neutron production from alpha particle breakup.     

Secondary protons and neutrons generated by galactic and solar cosmic ray particles behind 1–100 g/cm aluminium shielding

Proton and neutron energy spectra behind AI shielding affected by galactic and solar cosmic ray particles are Monte-Carlo calculated using the multipurpose hadron transport code (SHIELD).     

An update about recent developments of the PHITS code

PHITS (Particle and Heavy-Ion Transport code System) is a general-purpose three-dimensional Monte Carlo code, developed and maintained by RIST, JAEA and KEK in Japan together with Sihver et al. at Chalmers in Sweden. PHITS can deal with the transports of all varieties of hadrons and heavy ions with energies up to around 100 GeV/nucleon, and in this paper the current status of PHITS is presented. We introduce a relativistically covariant version of JQMD, called R-JQMD, that features an improved ground state initialization algorithm, and we will present the introduction of electron and photon transport in PHITS using EGS5, which have increased the energy region for the photon and energy transport from up to around 3 GeV to up to several hundred GeV depending on the atomic number of the target. We show how the accuracy in dose and fluence calculations can be improved by using tabulated cross sections. Benchmarking of shielding and irradiation effects of high energy protons in different materials relevant for shielding of accelerator facilities is also presented. In particular, we show that PHITS can be used for estimating the dose received by aircrews and personnel in space. In recent years, many countries have issued regulations or recommendations to set annual dose limitations for aircrews. Since estimation of cosmic-ray spectra in the atmosphere is an essential issue for the evaluation of aviation doses, we have calculated these spectra using PHITS. The accuracy of the atmospheric propagation simulation of cosmic-ray performed by PHITS has been well verified by experimental cosmic-ray spectra taken under various conditions. Based on a comprehensive analysis of the simulation results, an analytical model called “PARMA” has been proposed for instantaneously estimating the atmospheric cosmic-ray spectra below the altitude of 20 km. We have also performed preliminary simulations of long-term dose distribution measurements at the ISS performed with the joint ESA-FSA experiment MATROSHKA-R (MTR-R) led by the Russian Federation Institute of Biomedical Problems (IMBP) and the ESA supported experiment MATROSHKA (MTR), led by the German Aerospace Center (DLR). For the purpose of examining the applicability of PHITS to the shielding design in space, the absorbed doses in a tissue equivalent water phantom inside an imaginary space vessel has been estimated for different shielding materials of different thicknesses. The results confirm previous results which indicate that PHITS is a suitable tool when performing shielding design studies of spacecrafts.     

HZE-dosimetry on MIR92, IML-1 and D-2

Nuclear track detectors were used to measure the integral Linear Energy Transfer (LET) spectra above 1 GeV per cm water behind the complex material shielding inside a spacecraft. The measurements are compared with predictions of the contribution of high charge, high energy HZE particles of the galactic cosmic radiation taking into account the influence of solar and geomagnetic modulation and shielding by matter.     

The low earth orbit environment observed using CREAM and CREDO.

The Cosmic Radiation Environment and Dosimetry experiment (CREDO) has been operational on board the Advanced Photovoltaics & Electronics Experiment Spacecraft since August 1994. Extensive measurements of cosmic ray linear energy transfer spectra (using data to January 1996) and total dose (using data to November 1994) have been made, and compared with predictions of standard models. Detailed consideration of spacecraft shielding effects have been made. Predictions are shown to overestimate the measured linear energy transfer spectra. The CREAM experiment was flown on STS-63 in the SpaceHab module. Results show penetration of high energy electrons into the SpaceHab module.     

Role of shielding in modulating the effects of solar particle events: Monte Carlo calculation of absorbed dose and DNA complex lesions in different organs.

Distributions of absorbed dose and DNA clustered damage yields in various organs and tissues following the October 1989 solar particle event (SPE) were calculated by coupling the FLUKA Monte Carlo transport code with two anthropomorphic phantoms (a mathematical model and a voxel model), with the main aim of quantifying the role of the shielding features in modulating organ doses. The phantoms, which were assumed to be in deep space, were inserted into a shielding box of variable thickness and material and were irradiated with the proton spectra of the October 1989 event. Average numbers of DNA lesions per cell in different organs were calculated by adopting a technique already tested in previous works, consisting of integrating into "condensed-history" Monte Carlo transport codes--such as FLUKA--yields of radiobiological damage, either calculated with "event-by-event" track structure simulations, or taken from experimental works available in the literature. More specifically, the yields of "Complex Lesions" (or "CL", defined and calculated as a clustered DNA damage in a previous work) per unit dose and DNA mass (CL Gy-1 Da-1) due to the various beam components, including those derived from nuclear interactions with the shielding and the human body, were integrated in FLUKA. This provided spatial distributions of CL/cell yields in different organs, as well as distributions of absorbed doses. The contributions of primary protons and secondary hadrons were calculated separately, and the simulations were repeated for values of Al shielding thickness ranging between 1 and 20 g/cm2. Slight differences were found between the two phantom types. Skin and eye lenses were found to receive larger doses with respect to internal organs; however, shielding was more effective for skin and lenses. Secondary particles arising from nuclear interactions were found to have a minor role, although their relative contribution was found to be larger for the Complex Lesions than for the absorbed dose, due to their higher LET and thus higher biological effectiveness.     

Hze-particles measured during IML-1 mission

Three stacks of CR-39 plastic nuclear track detectors were exposed to cosmic radiation during IML-1 mission. They were mounted with their axes parallel to the shuttle's axes to allow measurements of LET-spectra for different impinging directions. First results of this experiment are reported.     

An assessment of galactic cosmic radiation quality considering heavy ion track structures within the cellular environment.

Beyond the magnetic influence of the Earth, the flux of galactic cosmic radiation (GCR) represents a radiological concern for long-term manned space missions. Current concepts of radiation quality and equivalent dose are inadequate for accurately specifying the relative biological "efficiency" of low doses of such heavily ionising radiations, based as they are on the single parameter of Linear Energy Transfer (LET). Such methods take no account of the mechanisms, nor of the highly inhomogeneous spatial structure, of energy deposition in radiation tracks. DNA damage in the cell nucleus, which ultimately leads to the death or transformation of the cell, is usually initiated by electrons liberated from surrounding molecules by the incident projectile ion. The characteristics of these emitted "delta-rays", dependent primarily upon the charge and velocity of the ion, are considered in relation to an idealised representation of the cellular environment. Theoretically calculated delta-ray energy spectra are multiplied by a series of weighting algorithms designed to represent the potential for DNA insult in this environment, both in terms of the quantity and quality of damage. By evaluating the resulting curves, and taking into account the energy spectra of heavy ions in space, a relative measure of the biological relevance of the most abundant GCR species is obtained, behind several shielding configurations. It is hoped that this method of assessing the radiation quality of galactic cosmic rays will be of value when considering the safety of long-term manned space missions.     

Dosimetric results on EURECA.

Detector packages were exposed on the European Retrievable Carrier (EURECA) as part of the Biostack experiment inside the Exobiology and Radiation Assembly (ERA) and at several locations around EURECA. The packages consist of different plastic nuclear track detectors, nuclear emulsions and thermoluminescence dosimeters (TLDs). Evaluation of these detectors yields data on absorbed dose and particle and linear energy transfer (LET) spectra. Behind a shielding thickness in front of the detectors of 0.09g cm-2 the doses range between 21.26 Gy and 0.87 Gy depending on the location of the dosimeter. Not all measurement can be explained by calculations.     

Estimates of HZE particle contributions to SPE radiation exposures on interplanetary missions.

Estimates of radiation doses resulting from possible HZE (high energy heavy ion) components of solar particle events (SPEs) are presented for crews of manned interplanetary missions. The calculations assume a model spectrum obtained by folding measured solar flare HZE particle abundances with the measured energy spectra of SPE alpha particles. These hypothetical spectra are then transported through aluminum spacecraft shielding. The results, presented as estimates of absorbed dose and dose equivalent, indicate that HZE components by themselves are not a major concern for crew protection but should be included in any overall risk assessment. The predictions are found to be sensitive to the assumed spectral hardness parameters.     

Thin structure of temporal profiles of solar flares January 15, 17 and 20, 2005 by data of AVS-F apparatus onboard CORONAS-F satellite

The count rate temporal profiles and energy spectra of the solar flares January 15, 17, 20 2005 in hard X-ray and gamma energy bands by data of AVS-F apparatus onboard CORONAS-F satellite are discussed. The energy spectra of these solar flares contain positron line and neutron capture line. Solar flares of January 17 and 20 spectra also contain some nuclear lines. Thin structure with characteristic timescales of 33–92 s is presented on flares temporal profiles in energy bands corresponding to the observed spectral features, which are confirmed by periodogram analysis (confidence level is 99%).     

A comparison of total reaction cross section models used in particle and heavy ion transport codes

Understanding the interactions and propagations of high energy protons and heavy ions are essential when trying to estimate the biological effects of Galactic Cosmic Rays (GCR) and Solar Particle Events (SPE) on personnel in space. To be able to calculate the shielding properties of different materials and radiation risks, particle and heavy ion transport codes are needed. In all particle and heavy ion transport codes, the probability function that a projectile particle will collide within a certain distance x in the matter depends on the total reaction cross sections, and the calculated partial fragmentation cross sections scale with the total reaction cross sections. It is therefore crucial that accurate total reaction cross section models are used in the transport calculations. In this paper, different models for calculating nucleon–nucleus and nucleus–nucleus total reaction cross sections are compared with each other and with measurements. The uncertainties in the calculations with the different models are discussed, as well as their overall performances with respect to the available experimental data. Finally, a new compilation of experimental data is briefly presented.     

Observations and predictions of secondary neutrons on Space Shuttle and aircraft.

The Cosmic Radiation Effects and Activation Monitor has flown on six Shuttle flights between September 1991 and February 1995 covering the full range of inclinations as well as altitudes between 220 and 570 km, while a version has flown at supersonic altitudes on Concorde between 1988 and 1992 and at subsonic altitudes on a SAS Boeing 767 between May and August 1993. The Shuttle flights have included passive packages in addition to the active cosmic ray monitor which comprises an array of pin diodes. These are positioned at a number of locations to investigate the influence of shielding and local materials. Use of both metal activation foils and scintillator crystals enables neutron fluences to be inferred from the induced radioactivity which is observed on return to Earth. Supporting radiation transport calculations are performed to predict secondary neutron spectra and the energy deposition due to nuclear reactions in silicon pin diodes and the induced radioactivity in the various scintillator crystals. The wide variety of orbital and atmospheric locations enables investigation of the influence of shielding on cosmic ray, trapped proton and solar flare proton spectra.     

Long-term modulation of Galactic Cosmic Radiation and its model for space exploration.

As the human exploration of space has received new attention in the United States, studies find that exposure to space radiation could adversely impact the mission design. Galactic Cosmic Radiation (GCR), with its very wide range of charges and energies, is particularly important for a mission to Mars, because it imposes a stiff mass penalty for spacecraft shielding. Dose equivalent versus shielding thickness calculations, show a rapid initial drop in exposure with thickness, but an asymptotic behavior at a higher shielding thickness. Uncertainties in the radiobiology are largely unknown. For a fixed radiation risk, this leads to large uncertain ties in shielding thickness for small uncertainties in estimated dose. In this paper we investigate the application of steady-state, spherically-symmetric diffusion-convection theory of solar modulation to individual measurements of differential energy spectra from 1954 to 1989 in order to estimate the diffusion coefficient, kappa (r,t), as a function of time. We have correlated the diffusion coefficient to the Climax neutron monitor rates and show that, if the diffusion coefficient can be separated into independent functions of space and time: kappa (-r,t)=K(t)kappa 0 beta P kappa 1(r), where beta is the particle velocity and P the rigidity, then (i) The time dependent quantity 1/K(t), which is proportional to the deceleration potential, phi(r,t), is linearly related to the Climax neutron monitor counting rate. (ii) The coefficients obtained from hydrogen or helium intensity measurements are the same. (iii) There are different correlation functions for odd and even solar cycles. (iv) The correlation function for the Climax neutron monitor counting rate for given time, t, can be used to estimate mean deceleration parameter phi(t) to within +/- 15% with 90% confidence. We have shown that kappa(r,t) determined from hydrogen and/or helium data, can be used to fit the oxygen and iron differential energy spectra with a root mean square error of about +/- 10%, and essentially independent of the particle charge or energy. We have also examined the ion chamber and 14C measurements which allow the analysis to be extended from the year 1906 to 1990. Using this model we have defined reference GCR spectra at solar minimum and solar maximum. These can be used for space exploration studies and provide a quantitative estimate of the error in dose due to changes in GCR intensities.     

Neutron yields from interactions of GCR-like beams in stopping targets.

In order to accurately determine the radiation risk to astronauts from GCR, the nature of the secondary radiation field created by the fragmentation of GCR in shielding and tissue must be understood. Due to the their high penetrabilities, neutrons are an important component of the secondary radiation field, especially for astronauts protected by thick shielding on lunar or Martian bases. Neutron yields from 435A MeV and 272A MeV Nb stopping in Nb and Al targets are presented, along with some preliminary analysis of neutron yields from 155A MeV C stopping in Al. Energy spectra and angular distributions are shown for neutron energies above 20 MeV. The data provides some information about the dependence of the neutron yield on projectile energy and target mass. Comparisons of the data with BUU calculations are also shown.     

Shielding against galactic cosmic rays.

Ions of galactic origin are modified but not attenuated by the presence of shielding materials. Indeed, the number of particles and the absorbed energy behind most shield materials increases as a function of shield thickness. The modification of the galactic cosmic ray composition upon interaction with shielding is the only effective means of providing astronaut protection. This modification is intimately connected with the shield transport properties and is a strong function of shield composition. The systematic behavior of the shield properties in terms of microscopic energy absorption events will be discussed. The shield effectiveness is examined with respect to conventional protection practice and in terms of a biological endpoint: the efficiency for reduction of the probability of transformation of shielded C3H10T1/2 mouse cells. The relative advantage of developing new shielding technologies is discussed in terms of a shield performance as related to biological effect and the resulting uncertainty in estimating astronaut risk.