Overview of the Martian radiation environment experiment.

Space radiation presents a hazard to astronauts, particularly those journeying outside the protective influence of the geomagnetosphere. Crews on future missions to Mars will be exposed to the harsh radiation environment of deep space during the transit between Earth and Mars. Once on Mars, they will encounter radiation that is only slightly reduced, compared to free space, by the thin Martian atmosphere. NASA is obliged to minimize, where possible, the radiation exposures received by astronauts. Thus, as a precursor to eventual human exploration, it is necessary to measure the Martian radiation environment in detail. The MARIE experiment, aboard the 2001 Mars Odyssey spacecraft, is returning the first data that bear directly on this problem. Here we provide an overview of the experiment, including introductory material on space radiation and radiation dosimetry, a description of the detector, model predictions of the radiation environment at Mars, and preliminary dose-rate data obtained at Mars.

Heavy ion observations by MARIE in cruise phase and Mars orbit.

The charged particle spectrum for nuclei from protons to neon, (charge Z=10) was observed during the cruise phase and orbit around Mars by the MARIE charged particle spectrometer on the Odyssey spacecraft. The cruise data were taken between April 23, 2001 and mid-August 2001. The Mars orbit data were taken March 5, 2002 through May 2002 and are scheduled to continue until August 2004. Charge peaks are clearly separated for charges up to Z=10. Especially prominent are the carbon and oxygen peaks, with boron and nitrogen also clearly visible. Although heavy ions are much less abundant than protons in the cosmic ray environment, it is important to determine their abundances because their ionization energy losses (proportional to Z2) are far more dangerous to humans and to instruments. Thus the higher charged nuclei make a significant contribution to dose and dose equivalent received in space. Results of the charged particle spectrum measurements will be reported.

Space radiation absorbed dose distribution in a human phantom.

The radiation risk to astronauts has always been based on measurements using passive thermoluminescent dosimeters (TLDs). The skin dose is converted to dose equivalent using an average radiation quality factor based on model calculations. The radiological risk estimates, however, are based on organ and tissue doses. This paper describes results from the first space flight (STS-91, 51.65 degrees inclination and approximately 380 km altitude) of a fully instrumented Alderson Rando phantom torso (with head) to relate the skin dose to organ doses. Spatial distributions of absorbed dose in 34 1-inch-thick sections measured using TLDs are described. There is about a 30% change in dose as one moves from the front to the back of the phantom body. Small active dosimeters were developed specifically to provide time-resolved measurements of absorbed dose rates and quality factors at five organ locations (brain, thyroid, heart/lung, stomach and colon) inside the phantom. Using these dosimeters, it was possible to separate the trapped-proton and the galactic cosmic radiation components of the doses. A tissue-equivalent proportional counter (TEPC) and a charged-particle directional spectrometer (CPDS) were flown next to the phantom torso to provide data on the incident internal radiation environment. Accurate models of the shielding distributions at the site of the TEPC, the CPDS and a scalable Computerized Anatomical Male (CAM) model of the phantom torso were developed. These measurements provided a comprehensive data set to map the dose distribution inside a human phantom, and to assess the accuracy and validity of radiation transport models throughout the human body. The results show that for the conditions in the International Space Station (ISS) orbit during periods near the solar minimum, the ratio of the blood-forming organ dose rate to the skin absorbed dose rate is about 80%, and the ratio of the dose equivalents is almost one. The results show that the GCR model dose-rate predictions are 20% lower than the observations. Assuming that the trapped-belt models lead to a correct orbit-averaged energy spectrum, the measurements of dose rates inside the phantom cannot be fully understood. Passive measurements using 6Li- and 7Li-based detectors on the astronauts and inside the brain and thyroid of the phantom show the presence of a significant contribution due to thermal neutrons, an area requiring additional study.

Neutron measurements onboard the space shuttle.

The radiation environment inside a shielded volume is highly complex, consisting of both charged and neutral particles. Since the inception of human space flights, the charged particle component has received virtually all of the attention. There is however, a significant production of secondary neutrons, particularly from the aluminum structure in low earth orbiting spacecrafts. The interactions of galactic cosmic rays (GCR), and solar energetic particles with the earth's atmosphere produce a non-isotropic distribution of albedo neutrons. Inside any reasonable habitable module, the average radiation quality factor of neutrons is about 4-5 times larger than the corresponding average quality factor of charged particles. The measurement of neutrons and their energy spectra is a difficult problem due the intense sources of charged particles. This paper reviews the results of Shuttle flight experiments (made during both solar maximum and solar minimum) to measure the contribution of neutrons to the dose equivalent, as well as theoretical calculations to estimate the appropriate range of neutron energies that contribute most to the dose equivalent.

Radiation dose from reentrant electrons.

In estimating the crew exposures during an extra vehicular activity (EVA), the contribution of reentrant electrons has always been neglected. Although the flux of these electrons is small compared to the flux of trapped electrons, their energy spectrum extends to several GeV compared to about 7 MeV for trapped electrons. This is also true of splash electrons. Using the measured reentrant electron energy spectra, it is shown that the dose contribution of these electrons to the blood forming organs (BFO) is more that 10 times greater than that from the trapped electrons. The calculations also show that the dose-depth response is a very slowly changing function of depth, and thus adding reasonable amounts of additional shielding would not significantly lower the dose to BFO.

Comparison of the SPE model with proton and heavy ion data.

Proton data from the GOES 6 and 7 satellites and heavy ion data from the IMP-8 satellite have been compared to the expected results of Nymmik's new model for solar particle event fluences. This model calculates the energy spectra of ions for protons through nickel for solar particle events, based upon the observed proton integral fluence above 30 MeV. Based upon 27 observed proton events of solar cycle 22, and three large historical events, with integral fluences above 30 MeV of greater than 10(6) particles/cm2, a reasonable agreement with model predictions is seen for more than half of the events. However, several events show a marked departure from the model predictions, leading to the conclusion that there may exist more than a single class of event, or that it may be necessary to include additional parameters within the model, such as solar disk position of the source flare, or height of disturbance in the solar corona. Data for heavy ions, (oxygen and iron), were limited to a total of six solar particle events, of which only two occurred in solar cycle 22. The agreement between data and the model predictions appeared to be quite good, however this agreement was sensitively dependent upon the value taken for the proton fluence above 30 MeV.

A sensitive single reverse passive haemagglutination test for detecting both HBsAg and anti-HBs.

A trial of a modified reverse passive haemagglutination test for HBsAg using a 0.1% cell suspension instead of the recommended 1% showed an approximately eight-fold increase in detection sensitivity. The test can be performed within 30 minutes and lends itself to mass screening techniques. Confirmation tests can be done using the 0.1% method. In addition, the same serological plates and cells used for HBsAg screening can then be used to screen for high-titre anti-HBs. This makes the overall screening for both HBsAg and high-titre anti-HBs donors cheap and convenient.

Liver enzyme concentrations as measure of possible infectivity in chronic asymptomatic carriers of hepatitis B.

Fifty-two British-born blood donors who were chronic carriers of hepatitis B surface antigen (HBsAg) were tested for the presence of hepatitis B e antigen (HBeAg) and antibody to HBeAg by an immunoradiometric assay. The presence of HBeAg was closely associated with a slight rise in serum liver enzyme concentrations, a high HBsAg titre, and male sex. We suggest that the finding of persistently raised serum liver enzyme concentrations in an asymptomatic HBsAg carrier might be useful as a likely indicator of HBeAg and high infectivity.

A comparison of different methods of screening blood donations for HBsAg.

In a retrospective comparison between countermigration immunoelectrophoresis (CIEP) and reverse passive haemagglutination (RPHA) for screening 260,500 blood donations, the latter's 10-fold increase in sensitivity resulted in 36% more HBsAg detections. In a prospective comparison between RPHA and radioimmunoassay (RIA) the latter's 40-fold increase in sensitivity over RPHA resulted in 11% more detections than RPHA in 27,094 new donors. One in 500 new donors was HBsAg-positive by RPHA, compared with 1 in 11,000 established donors who had donated and been tested previously. Acute hepatitis B infections, though uncommon, accounted for a greater proportion of the HBsAg-positive found in "established" rather than new donors. Reported post-transfusion hepatitis cases have declined following the introduction of screening tests in 1971. The feasibility of RIA testing at a transfusion centre supplied simply with the two basic RIA reagents has been demonstrated.