APPROACH:
The Matroshka-torso phantom facility was successfully mounted outside the ISS during the Extravehicular Activity (EVA) on February 26, 2004, and instrument activation was completed by the following April. The Matroshka-1 experiment spent 539 days outside the Russian Zvezda module and 77 days inside the International Space Station (ISS) for the total of 616 days in orbit between January 2004 and October 2005. The first phase of data collection provides detailed information about the depth of absorbed radiation dose for internal organs as well as skin dose in simulated EVA conditions. Temperature and pressure housekeeping data, and experimental data were frequently downloaded directly or brought back to earth stored on a memory card with Progress or Soyuz. The first memory card was delivered with the Soyuz Taxiflight at the end of April 2004.

RESULTS:
Data showed a thermally stable facility with temperatures between -20 and +20 degrees Celcius and pressure near one atmosphere inside the Matroshka container (Dettmann et al. 2007). First results from the active dosimetry telescope (DOSTEL) for April 2004 indicated a dose equivalent rate of 1.3 mSv (millisievert) per day during an EVA and revealed that the radiation exposure is about a factor of three higher than being inside the more protective environment of the ISS (people on Earth receive an average radiation dose of ~0.017 mSv per day). These results helped researchers understand the radiation risks for astronauts and cosmonauts working in space which is critical for efforts to expand space exploration (Reitz and Berger 2006).

Furthermore, the relationships between the skin and organ absorbed doses obtained from Matroshka phantom exposure showed a steep drop of about 20 times lower for the deep organs from the uppermost layer of the skin. This decrease due to the body self shielding and an associated increase of the radiation quality factor (biological damage factor) highlight the complexities of an adequate dosimetry determination for space radiation. Hence, the substantial increase in radiation exposure for an astronaut performing an EVA predominantly affects tissues and organs close to the body surface and, fortunately due to self shielding by the body, exposure of interior organs is only slightly enhanced over that inside the ISS. Organ dose rates calculated from TLD depth-dose distribution show that the surface organs (skin, eyes) receive the highest dose which decreases as it penetrates to the deeper organs (kidney, spleen) due to body shielding. Noteworthy, however, is the significant increase of the mean quality factor as the radiation quality changes upon its penetration into matter. It is also important to note that the exposure data of Matroshka during this time period was free of massive solar particle events (SPEs). As time changes with respect to the solar cycle, the orbit with respect to the radiation belts and the Earth’s magnetic shielding, or the material shielding around an astronaut, the internal dose distribution will change accordingly. A major purpose of the data gathered in the 3-dimensional dose distribution (and presently being further accumulated inside the ISS) is its application in the validation of radiation particle transport models that can calculate organ doses for the ISS orbit based on consolidated data regarding the external radiation field and on an accurate shielding model of the ISS, including the Earth’s shadow, and the self-shielding of Matroshka (Dettmann et al. 2007, Reitz et al., 2009.)

At present the best passive personal radiation dosimeters used for astronauts are thermoluminescence dosimeters (TLDs) and optically stimulated luminescence dosimeters (OSLDs) for low Linear Energy Transfer (LET), and Columbia Resin formula 39 (CR-39) plastic nuclear track detectors (PNTDs) for high LET ionizing particles detection. All three types were used in the Matroshka-torso phantom, however, following the postflight processing of the CR-39 detectors and analysis of the nuclear particle tracks, it was discovered that the doses observed were much less than expected, and the suspected cause was a sensitivity fading of CR-39 over the long period of exposure. Consequently, a method of “internal LET calibration using galactic cosmic rays (GCRs) iron peak” was used to develop the correction formula for the sensitivity fading which brings CR-39 PNTDs results, and those combined with TLDs results, in agreement with that measured by DOSTEL. The LET spectrum method and the combination CR-39 and TLDs correction method prove successful and reliable and will be used in current and future investigations (Zhou et al. 2006, 2010).

A related study compared the space radiation doses inside the Matroshka-Torso phantom with the doses in a cosmonaut body in Orlan-M spacesuit during an EVA and concluded that shielding properties of the spacesuit are essentially different from that of the Matroshka-Torso container. The calculated ratios of dose equivalents in critical organs of the Orlan-M spacesuit to those in Matroshka-Torso vary greatly dependent on the selected organ and solar cycle phase. In some practical cases considered in the analysis, Matroshka-Torso doses were well below or well above the doses in real spacesuit. The best agreement was observed for the eye lens when protected from solar light with extra screen. These results should be taken into account when transferring the data of the Matroshka-Torso experiment to the real EVA radiation conditions in Orlan-M spacesuit (Petrov et al. 2011).

Space suits

Radiation dosage

Cosmic radiation

Thermoluminescent dosimetry

Absorbed dose

Depth

Dose distribution

Dose rate

Effective dose