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Nov 19, 2013 - Thomas Berger,a,1 Paweł Bilski,b Michael Hajek,c,1,2,3 Monika Puchalskad,4 ... Box 100, 1400 Vienna, Austria; e-mail: M[email protected].
RADIATION RESEARCH

180, 622–637 (2013)

0033-7587/13 $15.00 Ó2013 by Radiation Research Society. All rights of reproduction in any form reserved. DOI: 10.1667/RR13148.1

The MATROSHKA Experiment: Results and Comparison from Extravehicular Activity (MTR-1) and Intravehicular Activity (MTR-2A/2B) Exposure Thomas Berger,a,1 Paweł Bilski,b Michael Hajek,c,1,2,3 Monika Puchalskad,4 and Gu¨nther Reitza a

Institute of Aerospace Medicine, German Aerospace Center, 51147 Cologne, Germany; b Institute of Nuclear Physics, Polish Academy of Sciences, 31-342 Krakow, Poland; c Institute of Atomic and Subatomic Physics, Vienna University of Technology, 1020 Vienna, Austria; and d Nuclear Engineering, Applied Physics, Chalmers University of Technology, 41296 Gothenburg, Sweden

ed stays of astronauts on the surface of other celestial bodies all have the challenge of protecting the astronauts from radiation risks (1). These risks arise from a number of sources, including solar particle events (SPEs), galactic cosmic rays (GCRs), secondary radiations from surface impacts such as on spacecraft shielding material and body tissue, or from nuclear power sources transported on board. Radiation exposure, especially the heavy ions of the GCR, is known to produce distinct biological damage compared with radiation exposure on Earth (2). In addition, dose levels may easily exceed those permissible for terrestrial occupational exposure (3–5), and therefore, the potential health risks (6) such as acute effects or late effects that may follow a successful mission must be better understood. One of the foremost objectives of cosmic-ray dosimetry is to characterize the complex space radiation environments to provide reliable data for risk projection. The use of human phantoms, simulating a space traveler’s body, provides detailed information of the depth-dose distribution inside the human body, which is essential for evaluating the doses absorbed in the different organs and tissues. Phantom experiments performed so far (Table 1) included a waterfilled spherical phantom aboard the Mir orbital station (7– 9), as well as an anthropomorphic head (10) and upper torso aboard the Space Shuttle (11, 12). The latter was also exposed in the U.S. laboratory of the ISS during Expedition 2 (13, 14). Another spherical phantom of a polyurethane material (MATROSHKA-R) started recording data in the Russian Segment of the ISS in 2004 (15–21), around the same time the MATROSHKA (MTR) anthropomorphic phantom torso was installed on the exterior of the ISS. The multilateral scientific experiments using the MATROSHKA phantom were coordinated by the German Aerospace Center (DLR) and represent the most comprehensive effort to date in radiation protection dosimetry in space. Dose measurements were performed using thermoluminescence (TL) phosphors arranged in a regular grid inside the phantom torso. The results were combined with detailed numerical models of the human body to provide the data

Berger, T., Bilski, P., Hajek, M., Puchalska, M. and Reitz, G. The MATROSHKA Experiment: Results and Comparison from Extravehicular Activity (MTR-1) and Intravehicular Activity (MTR-2A/2B) Exposure. Radiat. Res. 180, 622–637 (2013).

Astronauts working and living in space are exposed to considerably higher doses and different qualities of ionizing radiation than people on Earth. The multilateral MATROSHKA (MTR) experiment, coordinated by the German Aerospace Center, represents the most comprehensive effort to date in radiation protection dosimetry in space using an anthropomorphic upper-torso phantom used for radiotherapy treatment planning. The anthropomorphic upper-torso phantom maps the radiation distribution as a simulated human body installed outside (MTR-1) and inside different compartments (MTR-2A: Pirs; MTR-2B: Zvezda) of the Russian Segment of the International Space Station. Thermoluminescence dosimeters arranged in a 2.54 cm orthogonal grid, at the site of vital organs and on the surface of the phantom allow for visualization of the absorbed dose distribution with superior spatial resolution. These results should help improve the estimation of radiation risks for long-term human space exploration and support benchmarking of radiation transport codes. Ó 2013 by Radiation Research Society

INTRODUCTION

Current missions to the International Space Station (ISS) and potential future exploration missions involving extend1 Address for correspondence: Institute of Aerospace Medicine, German Aerospace Center, Linder Hoehe, 51147 Cologne, Germany; e-mail: [email protected]; and International Atomic Energy Agency, Division of Radiation, Transport and Waste Safety, PO Box 100, 1400 Vienna, Austria; e-mail: [email protected]. 2 Joint senior authorship. 3 Current affiliation: International Atomic Energy Agency, Division of Radiation, Transport and Waste Safety, 1400 Vienna, Austria. 4 Formerly at: Institute of Nuclear Physics, Polish Academy of Sciences, 31-342 Krakow, Poland.

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DOSE DISTRIBUTION WITHIN AN ANTHROPOMORPHIC PHANTOM ON BOARD THE ISS

FIG. 1. The urethane-based phantom body of the MATROSHKA facility (panel A) is dressed with a Nomext skin substitute, also know as the hood and poncho (panel B), a carbon fiber container (panel C), thermally protected by a multilayer thermal insulation (MLI) (panel D), resembles the shielding properties of an EVA spacesuit. (Used with the permission of DLR)

needed for cancer risk projections for long-term human space exploration (22) and support benchmarking of radiation transport codes, such as PHITS (23–25), GEANT4 or FLUKA. The current article focuses on the studies completed within the MATROSHKA-1 (26–28), MATROSHKA-2A and B experiments phases inside and outside the ISS. MATERIALS AND METHODS The MATROSHKA Facility MATROSHKA is a European Space Agency (ESA) facility that was developed within the European Programme for Life and Physical Sciences (ELIPS) and applications for use on the ISS. It housed an Alderson anthropomorphic upper-torso phantom similar to those used in radiation therapy treatment planning (RANDOt, The Phantom Laboratory, Salem, NY). It is composed of a natural human skeleton cast inside a proprietary urethane formulation that is radiological equivalent to soft tissue (29–31). The phantom ‘‘lungs’’ are designed of lower density material to simulate human lungs in a median respiratory state and molded to match the contours of the natural human rib cage. Unlike previous phantom experiments on different space vehicles, MATROSHKA is covered by a Nomext skin substitute, a hood and a poncho, to provide for skin doses, as well as a carbon fiber container of ;0.5 g/cm2 shielding mass and a multilayer thermal insulation (MLI) to resemble the shielding properties of an astronaut’s extravehicular activity (EVA) spacesuit (Fig. 1). The phantom is sliced into 33 sections, 2.54 cm thick and contains a complete set of active and passive instrumentation to account for the cosmic-ray charge and energy spectra. A base structure houses the electronics for data acquisition and transmission of the internal temperature and pressure as well as dosimetric data from the active radiation sensors.

To facilitate detailed dose mapping throughout the phantom torso, channels were drilled through the soft-tissue material to accommodate a total number of up to 5,800 luminescence dosimeters located at equidistant points in 354 polyethylene tubes, which enabled the measurement of the depth distribution of absorbed dose to be determined in a 2.54 cm orthogonal grid. Combinations of further luminescence and plastic nuclear track etch detectors (PNTDs), which are capable of measuring the linear energy transfer (LET) spectrum 10 keV/lm, were implemented into polyethylene boxes of 60 3 40 3 25 mm3 at the sites of selected organs (eye, lung, stomach, kidney and intestine; Fig. 2 and Table 2A) as well as in the poncho (at mid thorax, upper abdomen, lateral right, lateral left, mid dorsal and lumbar positions; Fig. 3 and Table 2B). Skin dose could be evaluated from luminescence dosimeter stripes that were sewn in the poncho to measure the dose at a depth of 0.6 mm (Table 2C). Seven active radiation detectors provided real-time dose monitoring. Five silicon scintillation detectors (SSDs) were installed at the sites of the organs

TABLE 1 Overview of Phantom Experiments in Space Experiment Water-filled spherical phantom Anthropomorphic phantom head Anthropomorphic phantom torso ‘‘Fred’’ Anthropomorphic phantom torso ‘‘Fred’’ Spherical phantom MATROSHKA-R Anthropomorphic phantom torso MATROSHKA

Date

Location

Ref.(s)

1997–1999

Mir

(7–9)

1989–1990

Space Shuttle

(10)

1998

Space Shuttle

(11, 12)

2001

ISS

(13–14)

2004–ongoing

ISS

(15–21)

2004–2011

ISS

(21–31)

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FIG. 2. Polyethylene tubes containing luminescence detectors are accommodated in a regular grid throughout the phantom body (see example the rectangle in panel C). Organ detector boxes (see example the rectangle in panel A) and silicon scintillation detectors (see example the rectangle in panel B) monitor the doses at the sites of selected organs, as shown for the eye (panel A), the lung (panel B) and the stomach (panel C). (Used with the permission of DLR)

mentioned in Table 2A to measure the interior heavy-ion and neutron component. A silicon-based dosimetry telescope (DOSTEL) on top of the phantom head and a tissue-equivalent proportional counter (TEPC) in front of the phantom torso assessed the ambient exposure. Additional reference detector boxes facilitated comparison of the results measured in the MATROSHKA phantom with dose rates inside the ISS. During MTR-1, these reference detector boxes were stored in the Russian Service Module Zvezda. During MTR-2A and MTR-2B, they were attached to the surface of the MATROSHKA container. The MATROSHKA Experiment Phases The timeline of the MATROSHKA experiment phases is summarized in Table 3. MATROSHKA was launched to the International Space Station on 29 January 2004 with an unmanned Russian resupply spacecraft (Progress M1-11, also known as Progress 13 or 13P) from Baikonur, Kazakhstan and mounted outside the Russian Service Module (Zvezda) on 26 February 2004. After 539 days of exposure (MTR-1, Fig. 4A) the facility was retrieved and transferred to the inside of the ISS on 18 August 2005. Since then, MATROSHKA assessed the radiation exposure under intravehicular activity (IVA) conditions. To assess the influence of variable shielding configurations, the facility was moved to different segments of the ISS: the Pirs docking compartment (MTR-2A, Fig. 4B), the Zvezda Service Module (MTR-2B, Fig. 4C) and most recently, the Japanese Experiment Module (JEM) Kibo¯ (MTR-2 KIBO). After each experiment phase, all passive detectors were removed and returned to Earth by Soyuz (TMA) or Space Shuttle (STS) for processing and evaluation in the participating laboratories. Data from active instrumentation were stored on memory cards or (partly) transmitted directly to Earth by U.S. voice link. The passive detectors had to be stored inside the ISS before loading into the facility and after disassembly before the return to Earth. The accumulated background exposure was assessed by reference detector boxes and later subtracted from the overall detector reading. Storage times, however, were kept to a minimum: 81 days for MTR-1 and 27 days for MTR-2A. In the MTR-2B experiment phase, dismounting of passive detectors was accomplished in two steps. While the detector-loaded polyethylene tubes contained in slices no. 3, 15, 20, 22 and 27, as well as all of the organ and poncho detector boxes were removed on 25 November 2008 and downloaded by STS-126 three days later, the remaining tubes along with the dosimeter stripes for skin dose assessment and the reference detector boxes were disassembled on 18 March 2009 and returned to Earth by STS-119 in the following week. The storage times for the MTR-2B experiment phase therefore account for 11 and 15 days, respectively.

ISS Altitude and Galactic Cosmic Ray Environment Comparing the data from the MATROSHKA experiments over a timespan of more than five years requires understanding of various parameters influencing the recorded doses. The temporal variation of the ISS altitude (Fig. 5A) represents one of these parameters. While the average altitude was 358 km for MTR-1, it dropped to 342 km for MTR-2A and slightly increased again to 346 km for the MTR-2B experiment phase. Assuming similar shielding conditions, the contribution of galactic cosmic-ray particles is almost independent of the station altitude, while the fluence of trapped protons increases with altitude (32), as does the duration of South Atlantic Anomaly (SAA) passages. The solar activity decreased during the time of the MATROSHKA experiments, reaching almost a solar minimum for the MTR-2B experimental phase. Figure 5B shows the relative deviation of the Oulu neutron monitor count rate (http://cosmicrays.oulu.fi) from the long-term average for the period of 2003–2010, with an average of

TABLE 2 Part A: Locations of organ detector boxes Organ detector box no. 1 2 3 4 5 6

Slice no. 3 15 20 22 27 –

Organ Eye Lungs Stomach Kidney Intestine Top of head

Part B: Locations of poncho detector boxes Poncho detector box no. 1 2 3 4 5 6

Location Front (top) Front (bottom) Right Left Back (top) Back (bottom)

Mid thorax Upper abdomen Lateral right Lateral left Mid dorsal Lumbar

Part C: Locations of detector stripes for skin dosimetry Skin detector stripes no. Top Middle Bottom

Slice no. 18 23 29

Location Upper torso Mid torso Lower torso

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FIG. 3. Skin doses are measured on the phantom’s surface by means of sewn-in dosimeter stripes and dedicated poncho detector boxes [mid thorax and upper abdomen (panel A), lateral right (panel B), lateral left (panel C), mid dorsal and lumbar (panel D)]. (Used with the permission of DLR)

2.9% (MTR-1), þ2.7% (MTR-2A) and þ5.9% (MTR-2B), respectively. Due to reduced deflection by the planetary magnetic field, decreasing solar activity causes increasing dose contribution from galactic cosmic rays. Detector Systems Thermoluminescence detectors (TLDs) are passive, integrating radiation sensors. Their operating principle is based on the storage of energy from ionizing radiation through trapping of charge carriers in metastable energy states within the band gap of insulating crystals

(33). Upon heating, the trapped charge carriers are released and recombine resulting in the emission of light, the intensity of which is proportional to the absorbed dose. TLDs have been applied for dosimetric purposes since the early 1950s (34). They have been used also for dosimetry of cosmic radiation since the beginning of the space era, from Gemini through Apollo, to the Space Shuttle missions and in programs using orbital space stations such as Skylab, Salyut and Mir (35, 36). In the last decade, many experiments aimed at radiation dosimetry using TLDs were conducted aboard the ISS (37, 38). TLDs are also employed for operational dosimetry of astronauts (39).

TABLE 3 MATROSHKA Experiment Phase Timeline Experiment phase

Event

Date

MTR-1 (2004–2005)

Launch of MTR facility (Progress M1-11) Activation of active instruments Extravehicular exposure: 539 days Dismounting of passive detectors Passive detector download (Soyuz TMA-6)

29 January 2004 April 2004 26 February 2004–18 August 2005 14 September 2005 10 October 2005 (undocking)

MTR-2A (2006)

Passive detector upload (Progress M-55) Integration of passive detectors Intravehicular exposure: 336 days Dismounting of passive detectors Passive detector download (STS-116)

21 December 2005 5 January 2006 5 January 2006–7 December 2006 7 December 2006 19 December 2006 (undocking)

MTR-2B (2007–2009)

Passive detector upload (Soyuz TMA-11) Integration of passive detectors Intravehicular exposure: 517 days Dismounting of passive detectors I Passive detector download I (STS-126) Dismounting of passive detectors II Passive detector download II (STS-119)

10 18 18 25 28 18 25

October 2007 October 2007 October 2007–18 March 2009 November 2008 November 2008 (undocking) March 2009 March 2009 (undocking)

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FIG. 4. Panel A: MTR-1 extravehicular exposure at the outer surface of the ISS Russian Service Module (Zvezda). Panel B: MTR-2A intravehicular exposure in the ISS Russian Docking Compartment (Pirs). Panel C: MTR-2B intravehicular exposure in the ISS Russian Service Module (Zvezda). (Used with the permission of NASA and Roskomos)

Within the MATROSHKA experiment, all participating laboratories used TLDs based on lithium fluoride, activated with magnesium and titanium (LiF:Mg,Ti). TLDs were prepared using lithium of two different isotopic compositions: highly enriched in 6 Li or 7Li isotopes. The Institute of Nuclear Physics (IFJ), Krakow, Poland, applied self-produced pellets of 6LiF:Mg,Ti (MTS-6) and 7 LiF:Mg,Ti (MTS-7) with a diameter of 4.5 mm and 0.6 mm thickness (0.4 mm for exposures in the MLI). The German Aerospace Center, Cologne, Germany, and the Institute of Atomic and Subatomic Physics (ATI), Vienna, Austria, used extruded ribbons of 6LiF:Mg,Ti (TLD-600) and 7LiF:Mg,Ti (TLD-700) available from Thermo Fisher Scientific Inc., Waltham, MA (former Harshaw Chemical Co., Solon, OH), in the dimension of 3.2 3 3.2 3 0.9 mm3. The measured thermoluminescent signals were converted to units of absorbed dose in water through calibrations performed with secondary-standard gamma-ray radiation sources (60Co for ATI, and 137Cs for DLR and IFJ). In total, the three MTR experimental phases described in the current paper

required well over 20,000 TLD readouts (including calibrations). The experimental protocols used by the individual laboratories for detector preparation and readout are summarized in Table 4. As described above, the MATROSHKA TLD data were obtained by three investigators using six TLD batches, which had been produced by two different manufactures. Compilation of these data into a single dataset thus required that the characteristics of all used TLD systems were compatible. To achieve this goal, an extensive program of investigation and inter-comparison was conducted that mainly focused on the TLD response to high-energy ions as encountered in cosmic radiation. During these studies, several irradiation campaigns at accelerator facilities around the world including the: Heavy Ion Medical Accelerator, National Institute of Radiological Sciences, Chiba, Japan; Heavy Ion Synchrotron, GSI Helmholtz Center for Heavy Ion Research, Darmstadt, Germany; and NASA Space Radiation Laboratory, Brookhaven National Laboratory, Upton, NY were performed (41–44). Further topics under study were the long-term stability of the TLD signal (fading) and a gamma-ray

FIG. 5. Panel A: Altitude of the International Space Station (ISS) for the time between 2003 and 2010. Panel B: Relative count rate of the Oulo Neutron Monitor for the time between 2003 and 2010. Note. The relevant MTR experiment phases are given in black in both plots.

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DOSE DISTRIBUTION WITHIN AN ANTHROPOMORPHIC PHANTOM ON BOARD THE ISS

TABLE 4 Experimental Protocols for TLD Readout and Preparation Parameter

ATI

DLR

IFJ

TL reader Heating method Photomultiplier Neutral gas flow Heating rate Pre-heat Annealing cycle Cooling rate Calibration method Calibration source Glow-curve evaluation method

TL-DAT.II (40) Contact (Nikrothal 80 planchet) Thorn EMI 9635 QB Nitrogen 58C/s 1208C (30 min) 4008C (1 h) Slow Single-chip 60 Co Peak height

Harshaw 5500 Hot nitrogen gas Hamamatsu RC095 HA Nitrogen 58C/s No preheat 4008C (1 h), 1008C (2 h) Slow Single-chip 137 Cs Peak height

RA’94 (Mikrolab) Contact (platinum planchet) Thorn EMI 9789 QB Argon 108C/s 1208C (30 min) 4008C (1 h), 1008C (2 h) Fast Separate group of TLDs 137 Cs Region of interest

cross-calibration. The results of these investigations indicated an agreement within 65% between the diverse applied TLD systems (45). MLI Measurements Besides the depth-dose measurements inside the phantom, an additional experiment was performed within the MTR-1 experiment phase at the outermost layers of the MATROSHKA facility. In the outside pockets of the multilayer thermal insulation (MLI), stacks of TLDs were placed (see example rectangle in Fig. 1D) to measure absorbed dose behind very low shielding thicknesses. These stacks of different TLD phosphors were placed in cylindrical aluminum holders of 2 cm height to accomplish depth-dose curves up to a maximum shielding mass of ;4 g/cm2. While the detectors provided by DLR (TLD-700) and IFJ (MTS-7) were located at the front side of the phantom, dosimeters supplied by ATI (TLD-600 and TLD-700) were attached to the backside of the phantom. Depth-Dose Distribution The depth-dose distribution inside the phantom was derived from measurements with TLDs located in polyethylene tubes and forming a 2.54 cm orthogonal grid over the phantom volume. For each location, the minimum distance between detector position and the phantom surface was calculated. The experimental point dose rates were further interpolated to determine a quasi-continuous 3D distribution. In these calculations, results of TLDs located at the surface of the phantom (‘‘skin’’ doses) were taken into account as well (46). The interpolations were done according to a method initially proposed by Shepard (47) and modified by Liszka (48) where the values between the grid points were calculated with a weighted average of the values available at the known grid points. The steep dose gradient between the phantom’s surface (the skin dose) and the nearest TLDs required a different interpolation method namely, an exponential attenuation function was adopted. Neutron Dose Estimation Neutron dose estimation was based on the comparison of signals from 6LiF/7LiF TLD pairs (49). 6Li possesses a very high crosssection for (n,a) reactions at neutron energies below 200 keV in contrast to 7Li. It may therefore be assumed that the difference between the 6LiF and the 7LiF TLD signals indicates the presence of slow neutrons. This neutron signal was expressed as gammaequivalent neutron absorbed dose, i.e., a gamma-ray dose that would be needed to produce a thermoluminescent signal of equal intensity.

RESULTS AND DISCUSSION

Reference Detectors (MTR-1/-2A/-2B)

The evaluated absorbed doses included contributions accumulated during storage of the detectors on board the ISS, when the dosimeters had not yet been installed in the MATROSHKA facility or had already been removed from the phantom. To subtract these contributions, the ambient background dose was assessed by means of separate reference detector boxes (reference boxes) stored on board and shipped along with the MATROSHKA TLD sets. Since in every mission detector installation took place only a couple of days after launch, and download to Earth was accomplished shortly after disintegration, the doses to be subtracted from the MATROSHKA TLD reading were minor: 13.6 6 0.2 mGy for MTR-1; 6.3 6 0.2 mGy for MTR-2A; as well as 2.0 6 0.1 mGy and 2.7 6 0.1 mGy for the fractionated detector removal of MTR-2B, respectively. These values are based on a mission daily average. Further, the reference detectors provided long-term dose rate measurements inside the particular modules of the ISS Russian Segment (Table 5). The average dose rate recorded in the less shielded Pirs Docking Compartment (MTR-2A) was about 40% higher than the dose rates measured in the Zvezda Service Module (MTR-1, MTR-2B). These findings are in line with experimental data from Hungarian PilleMKS CaSO4:Dy bulb dosimeters (50) attached to the MATROSHKA container and with results from area monitoring in the ISS Russian Segment carried out within the MATROSHKA-R experiment (51, 52). The Pille-MKS system has also been employed for routine dosimetry in the ISS Russian Segment since October 2003. During Expeditions 8–10, nine dosimeters were distributed throughout the Zvezda Service Module and read out once a month. The mission-averaged absorbed dose rates of 168 6 6 lGy/d (MTR-1) and 180 6 3 lGy/d (MTR-2B) evaluated from 7 LiF:Mg,Ti TL phosphors contained in the MATROSHKA reference detector boxes fit well to the average absorbed dose rate of 194 6 23 lGy/d recorded by the Pille-MKS

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TABLE 5 Absorbed Dose Rate in lGy/Day Measured with TLDs in the MATROSHKA Facility during MTR-1, MTR-2A and MTR-2B Experiment Phases. Uncertainties Represent One Standard Deviation of the TLD Results 6

DLR

7

LiF:Mg,Ti ATI

IFJ

DLR

LiF:Mg,Ti ATI

7

LiF:Mg,Ti Average

IFJ

MTR-1 Organ box no. 1 2 3 4 5 6

320 249 279 219 258 500

6 6 6 6 6 6

13 7 9 2 13 21

306 276 278 265 254 482

6 6 6 6 6 6

4 15 6 10 11 27

315 285 272 284 257 499

6 6 6 6 6 6

8 4 6 8 3 5

337 234 253 206 222 535

6 6 6 6 6 6

9 3 3 3 3 38

311 244 229 234 208 490

6 6 6 6 6 6

9 10 11 6 2 12

309 231 224 241 204 585

6 6 6 6 6 6

12 2 6 9 4 61

319 236 235 227 211 537

6 6 6 6 6 6

16 7 16 19 9 48

Poncho box no. 1 2 3 4 5 6

599 470 414 441 447 424

6 6 6 6 6 6

22 21 20 16 28 25

711 610 521 551 581 527

6 6 6 6 6 6

32 62 13 12 38 36

699 600 501 532 551 524

6 6 6 6 6 6

46 39 27 29 44 40

587 472 402 408 406 397

6 6 6 6 6 6

28 16 13 11 9 10

622 554 459 483 481 466

6 6 6 6 6 6

29 28 4 14 14 18

665 606 493 502 488 470

6 6 6 6 6 6

23 57 27 27 30 24

625 544 451 464 458 444

6 6 6 6 6 6

39 68 46 50 45 41

Reference box no. 1 2

182 6 4 181 6 3

203 6 8 213 6 9

196 6 1 188 6 4

163 6 5 165 6 6

172 6 7 181 6 5

162 6 1 162 6 3

166 6 6 169 6 10

MTR-2A Organ box no. 1 2 3 4 5 6

227 199 210 201 204 218

6 6 6 6 6 6

3 5 3 0 7 4

214 197 211 203 217 230

6 6 6 6 6 6

9 11 21 19 12 11

247 247 241 240 225 239

6 6 6 6 6 6

6 3 9 4 2 5

202 165 178 173 161 207

6 6 6 6 6 6

5 3 4 3 3 3

195 180 170 175 163 222

6 6 6 6 6 6

21 22 15 5 19 25

199 175 179 161 164 216

6 6 6 6 6 6

3 3 5 4 3 3

199 173 176 170 163 215

6 6 6 6 6 6

4 8 5 8 2 8

Poncho box no. 1 2 3 4 5 6

245 232 245 243 232 226

6 6 6 6 6 6

6 4 5 4 3 1

240 239 262 239 243 253

6 6 6 6 6 6

9 18 19 6 13 11

260 259 260 257 251 251

6 6 6 6 6 6

2 4 4 4 3 5

223 218 220 210 214 212

6 6 6 6 6 6

3 3 1 3 3 7

232 254 247 228 216 221

6 6 6 6 6 6

22 10 8 14 12 14

224 225 231 223 228 229

6 6 6 6 6 6

3 3 6 1 3 4

226 232 233 220 219 221

6 6 6 6 6 6

5 19 14 9 8 9

Reference box no. 1 2

247 6 8 253 6 7

279 6 15 291 6 15

271 6 6 268 6 6

236 6 4 240 6 9

243 6 31 227 6 22

232 6 4 231 6 2

237 6 6 233 6 7

MTR-2B Organ box no. 1 2 3 4 5 6

249 236 230 241 233 221

6 6 6 6 6 6

6 3 1 14 3 6

225 237 237 236 236 222

6 6 6 6 6 6

9 15 2 6 14 5

231 247 224 249 232 206

6 6 6 6 6 6

16 10 16 9 15 8

160 164 163 165 161 194

6 6 6 6 6 6

4 3 4 7 1 2

166 161 174 170 164 190

6 6 6 6 6 6

1 3 10 4 5 3

161 164 155 174 156 182

6 6 6 6 6 6

5 5 5 6 6 7

162 163 164 170 160 189

6 6 6 6 6 6

3 2 10 5 4 6

Poncho box no. 1 2 3 4 5 6

216 208 221 223 240 239

6 6 6 6 6 6

2 3 1 6 3 8

211 201 226 227 244 241

6 6 6 6 6 6

4 5 4 11 5 5

190 194 211 203 216 213

6 6 6 6 6 6

14 8 8 9 7 7

173 166 182 173 189 197

6 6 6 6 6 6

1 2 2 3 4 5

168 162 185 171 196 198

6 6 6 6 6 6

5 2 7 8 6 3

171 170 185 175 194 189

6 6 6 6 6 6

6 7 7 12 7 7

171 166 184 173 193 195

6 6 6 6 6 6

3 4 2 2 4 5

Continued on next page

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TABLE 5 Absorbed Dose Rate in lGy/Day Measured with TLDs in the MATROSHKA Facility during MTR-1, MTR-2A and MTR-2B Experiment Phases. Uncertainties Represent One Standard Deviation of the TLD Results 6

Reference box no. 1 2

7

LiF:Mg,Ti

LiF:Mg,Ti

7

DLR

ATI

IFJ

DLR

ATI

IFJ

LiF:Mg,Ti Average

215 6 1 221 6 2

222 6 6 220 6 8

193 6 8 213 6 9

186 6 2 181 6 2

175 6 3 179 6 6

177 6 6 179 6 5

179 6 6 180 6 1

Note. The last column represents the averaged absorbed dose rate in lGy/day measured with 7LiF:Mg,Ti (TLD-700 by ATI and DLR and MTS7 by IFJ). The data represent mean dose rates calculated from three independent measurements conducted by ATI, DLR and IFJ.

dosimeters for the period from October 2003 to April 2005 (53). The results obtained from 6LiF:Mg,Ti (MTS-6, TLD-600) and 7LiF:Mg,Ti (MTS-7, TLD-700) dosimeters contained in the organ and poncho detector boxes as well as the reference detector boxes are summarized in Table 5 for the MTR-1, MTR-2A and MTR-2B experiment phases. The overall agreement of data measured by the different investigators can be traced back to similar detector properties regarding heavy-ion TL response and long-term fading characteristics, as it was demonstrated in ground-based experiments (41– 45). As a consequence, the further discussion will be based on mean dose rates calculated from three independent measurements (ATI, DLR and IFJ), shown in the last column of Table 5 for the results mentioned above. MLI Depth-Dose Measurements (MTR-1)

The measured distribution of absorbed dose rate with increasing depth for the MTR-1 experiment phase MLI TLD stack measurements are shown in Fig. 6A and B.

The depth-dose curves determined by DLR and IFJ are consistent (Fig. 6A), yielding dose rates of ;0.1 mGy/d behind a shielding mass of 2.5 g/cm2. The dose rate measured by ATI behind equal shielding (Fig. 6B), however, amounted to significantly higher values of ;0.7 mGy/day. Moreover, the dose rates recorded by DLR and IFJ on the phantom’s front side begin to increase again from ;3 g/cm2 onwards. This behavior is explained by the considerable exposure of the front-side detector badges to solar infrared radiation causing temperature enhancement (also evident from partial melting of polystyrene used in neighboring detector holders). The dosimeter stacks attached at the backside of the phantom had not been subject to such high temperature levels. The increased temperatures of the front-side detectors induced notable fading of the TL signal (particularly glow peaks 4 and 5 in LiF:Mg,Ti) and, consequently, lead to underestimation of the dose rate for the first few g/cm2 of shielding mass. The increase in dose rate observed from the DLR and IFJ data beyond ;3 g/cm2 reflects better temperature shielding, thus that the fading of the TL signal become less significant. Nevertheless, the data

FIG. 6. Depth dose rate measured at the front side (panel A) (data: DLR, IFJ) and the backside side (panel B) (data: ATI) of the MATROSHKA phantom during the MTR-1 experiment phase.

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FIG. 7. Schematic illustration of TLD positions within the detector stripes sewed into the MATROSHKA poncho used for skin dose measurements.

from DLR and IFJ have to be interpreted as being strongly influenced by the high temperature, which is why only the ATI data should be taken for comparison with other spaceborne measurements. A qualitatively and quantitatively similar depth-dose distribution that validates the MATROSHKA findings was measured during experiments on Russian Foton satellites (54) and in the sample trays of the Expose-E facility that was mounted on the Columbus external European Technology Exposure Facility (EuTEF) platform from February 2008 to September 2009 (55). Skin Dose (MTR-1/-2A/-2B)

Skin dose measurements were performed by TLDs sewn into polyethylene stripes on the inside of the poncho. These dosimeters are closely attached to the skin, while the poncho detector boxes built up of alternating layers of TLDs and PNTDs acted as surrogates for personal monitoring badges. The locations of the skin dose stripes are shown in Table 2C. TLD positions within a particular stripe are shown schematically in Fig. 7. The skin dose rates measured with 7LiF:Mg,Ti at different locations around the MATROSHKA phantom are shown in Fig. 8 for the MTR-1, MTR-2A and MTR-2B experiment phases. From repeated calibrations, the statistical uncertainty of these single-chip measurements was estimated to be less than 5%. Figure 8A shows the skin dose distribution around the phantom obtained for MTR-1, while Fig. 8B compares the skin dose rates measured in the MTR-2A (closed symbols) and MTR-2B (open symbols) experiment phases. As it could be expected, the skin dose rate determined for the extravehicular exposure (MTR-1) is about one order of magnitude higher than for intravehicular conditions (MTR-2A/2B). The resulting average skin dose rates are 1529 6 404 lGy/d (MTR-1), 242 6 16 lGy/d

(MTR-2A) and 178 6 14 lGy/d (MTR-2B). These numbers not only reflect the different shielding conditions, but for intravehicular exposure they are close to the dose rates acquired from the reference detector boxes and the poncho detector boxes (see Table 5). From the results of the measurements obtained for MTR-1 (Fig. 8A), a wide range of dose rates from 0.5–2.5 mGy/day become apparent. This result is due to the highly inhomogeneous shielding of the individual TLD positions within a stripe. For example, positions 12 and 13 at the front part of the phantom were shielded by the NASA TEPC, thus yielding the dose rate minimum of 0.5 mGy/day. The range of skin dose rates determined for the intravehicular MTR-2A (200 to 280 lGy/day) and MTR2B (150 to 210 lGy/day) experiment phases reveals relative variation of ;40% that is evident in all stripes. The variation is due to the fact that one side of the phantom sees only the shielding of the spacecraft hull, whereas the remaining parts experience additional shielding from the spacecraft interior. The distributions recorded in the MTR2A and MTR-2B experiment phases, indicate that during MTR-2A (Pirs) the phantom’s right shoulder (positions 9 and 10) was located closest to the spacecraft hull (highest dose rate), while for MTR-2B (Zvezda) the backside of MATROSHKA was facing the wall. Nevertheless, during the MTR-2A experiment phase a decrease in dose due to shielding by the NASA TEPC that was placed in front of the abdomen was still visible for positions 12 and 13 in the bottom stripe and thus confirms the lower shielding provided by the Pirs docking compartment in comparison with the Zvezda Service Module. Computing the average of TLD measurements at a particular position in the top, middle and bottom stripes, show that the resulting distribution of the mean skin dose rate around the body may be approximated by a sinusoidal fit (Fig. 8C). Dose measurements on the surface of the Russian polyurethanebased spherical phantom MATROSHKA-R that had been exposed in both the Pirs and Zvezda module revealed more pronounced angular variations [see figure 2 from ref. (56)] than those seen for MATROSHKA, which may be attributed to the different storage locations of the phantoms. The minimum skin dose rates measured in the MTR-2A and MTR-2B experiments in the Zvezda and Pirs modules of the ISS, however, were in excellent agreement (56). Figure 9 further shows the skin dose distribution recorded for MTR2A and MTR-2B in a three-dimensional plot, showing the clear maximum at the point in closest proximity to the hull of the space station and a relative minimum at the opposite site. Poncho and Organ Detector Boxes (MTR-1/-2A/-2B)

The poncho detector boxes sewed to the poncho (Table 2B) simulate a personal dosimeter badge worn by an astronaut at the surface of his body. The results for the various MTR phases obtained from TLDs contained in the

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poncho detector boxes are provided numerically in Table 5 and shown in Fig. 10A. While for the extravehicular exposure (MTR-1) the measured doses ranged from 444– 625 lGy/day, the doses recorded inside the station were substantially lower: 219–233 lGy/day for MTR-2A (Pirs) and 166–195 lGy/day for MTR-2B (Zvezda), reflecting the different shielding conditions and the reduced contribution of trapped protons. Taking into account the pronounced dose gradient observed for extravehicular exposure, the considerable statistical uncertainties in the MTR-1 data of up to 10% reflect the local shielding of the individual detectors at the phantom surface. The doses measured for intravehicular exposure (MTR-2A/2B) match the results obtained from the reference detector boxes stored in proximity of the phantom. Figure 10B shows the average dose rate measured in the five organ detector boxes (Table 2A). Due to self-shielding within the phantom body, the organ dose rates obtained for MTR-1 are significantly lower than the dose rates measured in the poncho, with a maximum of 319 6 16 lGy/day at the location of the eye (no. 1) and a minimum of 211 6 9 lGy/ day at the intestine (no. 5). The dose rates determined for the intravehicular exposures are very similar due to selfshielding of the body and the additional shielding provided by the hull of the space station. Depth-Dose Distribution (MTR-1/-2A/-2B)

Figure 11A–C show the discrete distribution of absorbed dose rate at the 1,600 measurement points of the 2.54-cm orthogonal grid of the MATROSHKA facility, obtained from combining the readings of 7LiF:Mg,Ti detectors from ATI, IFJ and DLR. The three-dimensional depth-dose distribution is plotted for the MTR-1 (Fig. 11A), MTR-2A (Fig. 11B) and MTR-2B (Fig. 11C) experiment phases. While the dose rate ranges from 0.1–0.5 mGy/day for MTR1, the gradient’s observed for the heavier shielded MTR-2A (Pirs) and MTR-2B (Zvezda) experiment phases are much smaller. For the purpose of clarity, the two latter plots are using the same scale. To better compare the results of the three experiments, Fig. 12 visualizes the dose rate dependence on the minimum distance from the phantom surface, thus presenting a depth dose distribution for the three experiment phases. While Fig. 12A, C and E shows the data acquired from 7LiF:Mg,Ti detectors, Fig. 12 B, D and F show the dose rates measured by neutron sensitive 6LiF:Mg,Ti dosimeters. For MTR-1

FIG. 8. Panel A: Distribution of skin dose rate measured during the MTR-1 experiment phase in three detector stripes sewed into the poncho (top, middle, bottom). Panel B: Distribution of skin dose rate measured during the MTR-2A (closed symbols) and MTR-2B (open symbols) experiment phase in the three detector stripes (circles: top; triangles: middle; squares: bottom). Panel C: Mean skin dose rate around the body and sinusoidal fit for MTR-2A (closed circles) and MTR-2B (open circles).

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FIG. 9. Three-dimensional skin dose distribution measured during the (panel A) MTR-2A (Pirs) and (panel B) MTR-2B (Zvezda) experiment phases. Note. Panel A: Different scale used for MTR-2A as a consequence of the higher dose rate.

(Fig. 12A and B), both detector systems initially show decreasing dose rate due to the contribution of low-energy electrons and protons. The depth dose decrease is still evident from the 7LiF:Mg,Ti measurements during MTR-2A (Fig. 12C) but much less during MTR-2B (Fig. 12E), thus reflecting the increase in overall shielding thickness from Pirs to Zvezda. In contrast to extravehicular exposure, the dose rate measured for the MTR-2 experiments with 6LiF:Mg,Ti (Fig. 12D and F) is increasing with depth due to the heavier shielding provided by the spacecraft structures and the

additional self-shielding of the body. Both factors stimulate enhanced neutron production, visible in the 6LiF:Mg,Ti readings. This is clearly seen looking at the neutron dose only, which is given in Fig. 13. Figure 13A presents the gamma-equivalent neutron dose rate over the absorbed dose rate measured with 7LiF:Mg,Ti detectors, and thus illustrates the relative contribution of low-energy neutrons to the total absorbed dose. Clearly, this contribution increases with shielding thickness: it is highest for the intravehicular exposure in Zvezda (MTR-2B) and lowest for the extravehicular exposure (MTR-1). Comparison of results obtained from 6LiF:Mg,Ti detectors, however, is not straightforward due to the fact that shape and thickness of TLDs used by IFJ (circular, 0.6 mm thick) were not identical with those applied by DLR and ATI (square, 0.9 mm thick). While these differences do not influence the response to deeply penetrating radiation, such as gamma rays or high-energy charged particles, the situation is different for low-energy neutrons. The very high crosssection of 6Li for neutron absorption causes significant selfabsorption within not even a millimeter of 6LiF. This effect brings about a higher relative response of thin detectors to low-energy neutrons. Burgkhardt et al. (57) compared the response of various TLDs to thermal neutrons, including the IFJ-produced 0.6-mm thick MTS-6 pellets and TLD-600 chips obtained from Thermo Fisher Scientific, by exposing them to a reference neutron field directed perpendicularly to the TLD surface. They found that the response of thin MTS6 is 1.72-times higher than that of standard TLD-600. Applying this value as a correction factor to the TLD-600 results, improved the agreement between data measured by ATI, DLR and IFJ, as shown in Fig. 13B for the MTR-2A experiment phase. Nevertheless to focus on one, primary and uncorrected set of 6LiF:Mg,Ti detectors data, results given in Fig. 12 B, D and F and in Fig. 13A show IFJ data only. Comparing the determined depth dose distributions with the findings from the MATROSHKA-R spherical phantom reveals a similar dose gradient for exposure in the Zvezda Service Module (a decrease in dose rate by 20–5%), but a steeper dose gradient for MATROSHKA-R when exposed in the Pirs docking compartment [see figure 1 from ref. (56)]. The latter is a consequence of the ISS altitude, which was higher by 10–20 km in 2007–2008 (MATROSHKA-R) than in 2006 (MTR-2A) and thus additional dose is added by trapped protons of the South Atlantic Anomaly (SAA). Due to considerably less shielding, the dose gradient of ;60% determined for the extravehicular exposure of MTR1 is much more pronounced. Continuous Depth-Dose Distribution (MTR-1/-2A/-2B)

The combination and interpolation of the skin dose measurements (Figs. 8 and 9) and the discrete threedimensional point dose distribution (Fig. 11) allowed generation of a continuous three-dimensional distribution

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FIG. 10. Absorbed dose rates measured with 7LiF:Mg,Ti in the (panel A) poncho and (panel B) organ detector boxes during the MTR-1, MTR2A and MTR-2B experiment phases. Note. The respective data is provided in the last column of Table 5.

of absorbed dose rate throughout the phantom body that is presented in Fig. 14A–C for the three MATROSHKA experiment phases using data from 7LiF:Mg,Ti detectors. The dose decreases from the skin towards the inner layers of the body by 80% for the head and upper torso and even

by 90% for the lower torso region at extravehicular exposure (MTR-1). The more pronounced dose gradient in the abdominal region is a consequence of the higher selfshielding of the body and additional shielding from the ISS.

FIG. 11. Three-dimensional point dose distribution measured in a 2.54 cm orthogonal grid with 7LiF:Mg,Ti detectors supplied by ATI, DLR and IFJ during the MTR-1 (panel A), MTR-2A (panel B) and MTR-2B (panel C) experimental phases. Note. Different scale used for MTR-1 as a consequence of the significantly higher dose rate.

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FIG. 12. Depth dose distribution recorded with 7LiF:Mg,Ti and 6LiF:Mg,Ti detectors during the MTR-1; panel A: 7LiF:Mg,Ti, panel B: LiF:Mg,Ti; MTR-2A; panel C: 7LiF:Mg,Ti, panel D: 6LiF:Mg,Ti; and MTR-2B; panel E: 7LiF:Mg,Ti, panel F: 6LiF:Mg,Ti experiment phases.

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FIG. 13. Panel A: Gamma-equivalent neutron absorbed dose rate plotted as a function of the dose rate measured with 7LiF:Mg,Ti detectors to illustrate the relative contribution of low-energy neutrons to the total absorbed dose for the MTR-1, MTR-2A and MTR-2B experimental phases. Panel B: Gamma-equivalent neutron absorbed dose rate plotted as a function of the minimum distance from the phantom surface for the MTR-2A experiment phase. Note. Panel B: ATI and DLR data have been scaled by a geometry factor of 1.72 according to ref. (57).

The dose decrease observed for the intravehicular exposures (MTR-2A/B) were significantly lower. For MTR-2A, the decrease in dose from the skin to the inner layers of the body was 20% in the upper part and 60% in the bottom part of the phantom. Similar dose decreases were found for the MTR-2B exposure. However, the dose rate measured at the skin (for MTR-2B) was 30% lower than those for MTR-2A, with a maximum of ;0.2 mGy/day. CONCLUSION

The ESA MATROSHKA facility is a comprehensive and overarching international dosimetry program coordinated by the German Aerospace Center, which delivers basic data for the calculation of the health risk of space crews. For the first time, depth-dose distributions were measured using an anthropomorphic phantom mounted outside and inside the International Space Station. The data generated with passive thermoluminescence detectors at over 1,600 measurement points gives an impressive view of the dose distribution through a human body in locations of different shielding distributions around the phantom. From extravehicular exposure, simulating a spacewalk, the skin dose may reach values as high as 2.5 mGy/day, while inside the ISS the dose on the surface of the body is reduced to 160 to 260 lGy/day, depending on the local shielding conditions. The dose rates encountered at deeper depths inside the phantom almost match each other for extravehicular and intravehicular exposure thereby reflecting the considerable selfshielding properties of the organs inside the human body. In

combination with results from plastic nuclear track detectors, the evaluated continuous dose distributions serve as data input to calculate organ dose equivalents and effective doses needed for radiation risk assessment. This generated data will be put into a dedicated database within the European Community’s 7th Framework Programme (EC FP7) project HAMLET (Human Model MATROSHKA for Radiation Exposure Determination of Astronauts), thereby allowing free access to the scientific community for the data usage in the frame of benchmarking radiation transport and radiation environmental models. ACKNOWLEDGMENTS The authors gratefully acknowledge the European Space Agency for providing funding for the MATROSHKA facility, Roskosmos and RSC Energia for the integration support and the launch of MATROSHKA, and the long list of astronauts and cosmonauts, who worked on the experiment: CM Foale, AY Kaleri, SK Krikalev, YV Lonchakov, WS McArthur, JL Phillips, T Reiter and VI Tokarev. Austrian participation in the project was supported by the Federal Ministry of Transport, Innovation and Technology under the Austrian Space Applications Programme (ASAP). The Polish contribution was partly supported by a research project from the Polish Ministry of Science over the years 2006 and 2007. Extensive scientific exploitation of data was made possible through the funding by the European Community’s 7th Framework Programme (FP7) under Contract No. 218817 (HAMLET, http://www.fp7-hamlet.eu). We wish to express our further gratitude for the support of the operating staffs and the following collaborators during the ground-based experiments at highenergy accelerator centers: H Kitamura, S Kodaira and Y Uchihori (National Institute of Radiological Sciences, Chiba, Japan); M Durante, C La Tessa and D Schardt (GSI Helmholtz Center for Heavy Ion Research, Darmstadt, Germany); A Rusek (NASA Space Radiation Laboratory,

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FIG. 14. Interpolated depth-dose distribution obtained from 7LiF:Mg,Ti measurements by ATI, DLR and IFJ during the (panel A) MTR-1; (panel B) MTR-2A; and (panel C) MTR-2B experimental phases. Note. Different scale used for MTR-1 as a consequence of the significantly higher dose rate. Upton, NY). The experiments at NIRS-HIMAC were performed between February 2009 and February 2011 under research grant no. 20P240. In January and February 2011, proton irradiations could be realized at the NIRS-930 Cyclotron within the NIRS International Open Laboratory programme. Investigations at GSI-SIS were conducted in August 2009 and April 2010 under Research Grant No. AO-08-IBER-12. Received: August 3, 2013; accepted: August 27, 2013; published online: November 19, 2013

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