Radiation measured for MATROSHKA-1 experiment with passive dosimeters

Acta Astronautica 66 (2010) 301 -- 308

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Radiation measured for MATROSHKA-1 experiment with passive dosimeters D. Zhoua, b, ∗ , E. Semonesa , D. O'Sullivanc , N. Zappa , M. Weylanda , G. Reitzd , T. Bergerd , E.R. Bentone a

Johnson Space Center - NASA, 2101 Nasa Parkway, Houston, TX 77058, USA Universities Space Research Association, 3600 Bay Area Boulevard, Houston, TX 77058, USA Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland d German Aerospace Center, DLR, DE-51147 Cologne, Germany e Eril Research Inc., 1110 Innovation Way, Suite 100, Stillwater, OK 74074, USA b c

A R T I C L E

I N F O

Article history: Received 27 November 2008 Received in revised form 16 April 2009 Accepted 2 June 2009 Available online 22 July 2009 Keywords: Space radiation MATROSHKA-1 CR-39 detectors LET spectrum

A B S T R A C T

The radiation field in low Earth orbit (LEO) and deep space is complicated. The radiation impact on astronauts depends strongly on the particles' linear energy transfer (LET) and is dominated by high LET radiation. Radiation risk is a key concern for human space flight and can be estimated with radiation LET spectra measured for the different organs of an astronaut phantom. At present the best passive personal dosimeters used for astronauts are thermoluminescence dosimeters (TLDs) and optically stimulated luminescence dosimeters (OSLDs) for low LET and CR-39 plastic nuclear track detectors (PNTDs) for high LET. CR-39 PNTDs, TLDs and OSLDs were used for the MATROSHKA-1 experiment to measure radiation experienced by astronauts outside the international space station (ISS). LET spectra and radiation field quantities (differential and integral fluence, absorbed dose and dose equivalent) were measured for the different organs and skin locations of the MAROSHKA phantom using CR-39 PNTDs and TLDs. The spectra and results can be used to determine the radiation quantities for astronaut's extra vehicular activity (EVA) and for the further in-depth study of the radiation risk for astronauts. Sensitivity fading of CR-39 detectors was observed for the MATROSHKA experiment and a practical method was developed to correct it. This paper presents the radiation LET spectra measured with CR-39 PNTDs and the total radiation quantities combined from results measured with CR-39 PNTDs and TLDs. Published by Elsevier Ltd.

1. Introduction Previous studies have indicated that the radiation environment in LEO is mainly contributed by galactic cosmic rays (GCR), solar energetic particles, electrons and protons in the Earth's radiation belts, and albedo neutrons and protons from the Earth's atmosphere. During EVA, astronauts are exposed to this complicated radiation field and

∗ Corresponding author at: Johnson Space Center - NASA, 2101 Nasa Parkway, Houston, TX 77058, USA. E-mail address: [email protected] (D. Zhou). 0094-5765/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.actaastro.2009.06.014

can experience significant radiation doses. Despite many years of human activity in space, these doses and their impact on sensitive human organs are not very well known. In order to determine the radiation hazard from cosmic and solar particles it is essential to investigate the whole range of LET involved and measure the radiation dose at different depths in a human phantom in a simulated EVA. The MATROSHKA—a European Space Agency (ESA) research facility under the coordination and project lead of DLR—is an experiment unit for investigating the depth distribution of the absorbed dose in different organs of astronaut during an EVA, and currently the biggest international— more than 19 groups from all over the world—radiation

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D. Zhou et al. / Acta Astronautica 66 (2010) 301 -- 308

Fig. 2. MATROSHKA facility outside the ISS.

Fig. 1. MATROSHKA torso with poncho and hood.

experiment onboard the ISS. MATROSHKA consists of a contained human upper torso, allowing the accommodation of dedicated radiation measurement devices [1–3]. Passive detectors of MATROSHKA comprised TL and OSL detectors in polyethylene tubes for the depth dose distribution measurement as well as a combination of TL, OSL and CR-39 dosimeters positioned in 5 so called “organ dose boxes”—at the sites of radiation sensitive organs (eye, lung, stomach, kidney and intestine). In addition the phantom was dressed by a “hood” and a “poncho”, made of nomex with sewed in TLDs to measure the skin dose. Skin doses were also measured on the phantom surface with so called “poncho boxes” (mid thorax, upper abdomen, lateral left, lateral right, mid dorsal and lumbar and marked as ponchos 1–6 respectively). Fig. 1 shows the MAROSHKA torso with poncho and hood equipped with passive detector systems. Active devices of MATROSHKA included five silicon scintillation detector (SSD), a silicon telescope DOSTEL (DOSimetry TELescope) [2,4] and a tissue equivalent proportional counter (TEPC). The DOSTEL was located on the top of the phantom head. In total, the MATROSHKA-1 experiment spent 616 days in orbit with 77 days inside the ISS and 539 days outside the Russian Zvezda module between 29 January 2004 and 10 October 2005. Measurements were undertaken by the JSCSRAG (Space Radiation Analysis Group) and DIAS (Dublin Institute for Advanced Studies) using TL, OSL and CR-39 dosimeters. CR-39 provided by DIAS was scanned by DIAS and SRAG and CR-39 provided by Eril Research Inc. was

scanned by SRAG. The data analysis work was conducted by SRAG. A set of two reference detectors was also placed inside the ISS during the whole experimental phase (616 days) to account for the dose received inside the space station. Fig. 2 shows the location of MATROSHKA experiment, the facility (encircled) was mounted outside the Zvezda module on the ISS. Following the post-flight processing of the CR-39 detectors and subsequent analysis of the nuclear particle tracks (to be described in detail later) it was discovered that the doses observed at all seven locations were much less than those expected for the orbital inclination, altitude and stage of solar cycle involved. The variations in pressure and temperature during the mission could not explain the low values and the main suspect was a sensitivity fading over the very long period of exposure. Consequently a formula for correcting the sensitivity fading of CR-39 was developed using the measured MATROSHKA (616 days) data and the results obtained from the ISS Expedition-12 (30 September 05–8 April 2006, 190 days), using a method of internal LET calibration based on the GCR iron peak at ∼1 GeV/n. The radiation dose for the low LET component was measured with TLDs by SRAG. After correction of the CR-39 sensitivity, the LET spectra of radiation (fluence, dose and dose equivalent) for high LET ( > 5 KeV/ m water) were obtained with the CR-39 PNTDs. Then the total dose and dose equivalent were obtained by combining both sets of results. These values were then used for comparison with the results obtained by the active silicon telescope DOSTEL A detailed description of the physical principles and methods for the passive TLD dosimeters and CR-39 can be found in [5–10]. This paper describes the role of high LET radiation in radiobiology, the LET spectrum method and LET calibration using CR-39 detectors, the correction method for the fading of CR-39 sensitivity and presents the measured and combined results from SRAG and DIAS for the MATROSHKA-1 space mission. 2. The role of high LET radiation in radiobiology Research in radiobiology indicates that the RBE (relative biological effectiveness) for chromosomal, cellular and tissue

D. Zhou et al. / Acta Astronautica 66 (2010) 301 -- 308

Radiation Risk Cross Section (Curtis et al., 1995)

LET Calibration of CR-39 (Version of November 05)

100

100

Total Stomach Colon and Lung Bone Marrow Breast Bladder and Esophagus

Comparison of etch rate ratio for CR-39 detectors with different time of oxygen exposure Data (plastic film removed when stack prepared) Data Fit Data (keep plastic film on CR-39 plates until etch) Data Fit

10 Reduced Etch Rate Ratio (S-1)

Risk Cross Section (µm2)

10

303

1

0.1

1

0.1

0.01

0.001 1

10 100 LET (keV/µm tissue)

1000

0.01 1

10 100 LET200 (keV/µm CR-39)

1000

Fig. 3. Radiation risk as a function of LET [12]. Fig. 4. LET calibration for CR-39 detectors.

increases with LET [11–16]. Fig. 3 shows the relationship between the risk cross section for a fatal tumor and the LET of the particles. The figure shows that the tumor risk strongly depends on the particle's LET, and high LET radiation ( ⱖ 10 keV/ m) is dominant. That means, no matter how accurate the radiation dose for low LET was measured with active and/or passive dosimeters, its contribution to the biological risk estimation was not significant. Therefore radiation measurement for high LET particles should be emphasized and systematic measurements of the LET spectrum of space radiation should be conducted, using appropriate detectors. The probability of a tumor being induced by high LET radiation can be estimated using radiation risk cross section and the LET spectrum measured with personal dosimeters. The method is essentially experimental. CR-39 PNTDs are so far the only personal dosimeters which are capable of measuring LET spectrum for radiation field with LET higher than 5 keV/ m water. JSC-SRAG has successfully measured radiation LET spectra for all the astronauts since space mission STS-114 and ISS-Expedition 12, including the LET spectra for the different organs of the MATROSHKA phantoms [5–10]. These experimental results are of great significance for the in-depth investigation of radiation risk for astronauts. Although the simulation method for risk estimation has its unique advantages—flexibility to obtain radiation estimates for organs and for locations beyond LEO, radiation risk calculated by experimental method is more reliable than that obtained from a simulation method. The reasons are: (a) except for the sleeping time astronauts are always in a moving state, the shielding and the environment around the astronaut is changed accordingly, and the simulation method is

unable to track the variation properly; (b) the energy spectra of GCR used for simulation are inferred from limited experimental data and may differ from the real spectra, especially for GCR with higher charges; (c) the interaction cross section between the target tissue and the incident particles is not fully known yet; (d) there is no standard approach to energy loss formulae. Therefore, the experimental method to determine radiation risk is preferable. 3. LET spectrum method and LET calibration using CR-39 detectors After exposure and recovery, the CR-39 detectors were chemically etched, data were scanned and analyzed and the LET value was calculated using LET calibration of CR-39 detectors. The LET spectra (differential and integral particle fluence, absorbed dose and dose equivalent) were then generated with LET spectrum method using CR-39 detectors [5–10,17–23]. The CR-39 surface is covered by a plastic film when it is manufactured, in order to protect the CR-39 material from damage and to decrease the background radiation. Fig. 4 shows the LET calibration obtained by JSC-SRAG [24] for CR39 detectors with and without plastic film on the CR-39 surface during the radiation exposure. The CR-39 material used for the calibration was manufactured by American Technical Plastics. Calibration experiments conducted by the JSC-SRAG group confirmed that the sensitivity of the CR-39 was influenced by environmental oxygen. Fig. 4 shows that the sensitivity of CR-39 detectors in good oxygen environment (plastic film was removed when CR-39 stack was prepared)

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Table 1 Comparison of LET before and after correction of CR-39 sensitivity. CR-39 detector at position

Major/minor axes (m)

LET before correction (keV/ m water)

LET after correction (keV/ m water)

33.3/23.3 39/7

115.45 116.04

135.83 136.47

Poncho5

33.8/21

116.88

137.38

Eye

36.5/17 38/12

118.05 117.20

138.64 137.72

37.5/20.7 37.7/12

128.50 129.76

136.82 138.12

MATROSHKA-1 Poncho1

ISS-Expedition 12

is higher than those in poor oxygen environment (plastic film kept on CR-39 surface until etch). 2.25

Charge Distribution (Matroshka-1 and ISS-Expedition 12: GCR Iron Events) Ni

4. Fading of CR-39 sensitivity and correction 4.1. Fading of CR-39 sensitivity

Ti Ca Ar

2.00

S Si Mg Seff

When charged particles pass through CR-39 detector, they lose energy by ionization and break the molecular bonds of the CR-39 material to form damage trails along their paths in the detector. Studies on the sensitivity of CR-39 detectors indicate (1) that environmental oxygen around the CR-39 detectors tends to combine with ions and radicals, thus preventing their recombination and preventing a change in the sensitivity of the CR-39 detectors; (2) higher temperature makes the recombination of the ionization easier and faster; (3) the longer the time between the particles' passing through the detector and the chemical etch of the detector, the more fading [25–26]. The CR-39 used by JSC-SRAG and DIAS as well as Eril Research Inc. was manufactured by the same company. The fading problem of the CR-39 sensitivity was found for MATROSHKA-1 and Expedition 12 [27–31]. Because of sensitivity fading, the etch rate ratio and the LET value is smaller and subsequently the absorbed dose and dose equivalent are under-estimated. As a result, the LET for the GCR iron peak (∼1 GeV/n) dropped below its normal value 137 keV/ m water.

Fe Cr

Data with CR-39 sensitivity correction Data no CR-39 sensitivity correction Data with CR-39 sensitivity correction Data no CR-39 sensitivity correction

1.75 Ne

1.50

1.25 10-3

10-2

10-1 -1

2

Gradient (g cm ) Fig. 5. Charge identification with G–Seff method for test events.

The formula can be expressed as 4.2. Correction for the fading of CR-39 sensitivity In 2006 the JSC-SRAG team developed a practical method to correct the sensitivity fading of CR-39 detectors which were exposed to radiation for a period longer than several months. The method is essentially based on internal LET calibration using the GCR iron peak at ∼1 GeV/n. The sensitivity of CR-39 detectors can be represented by the etch rate ratio. Thus, the fading problem for CR-39 detectors can be corrected through this parameter. A correction formula for the etch rate ratio was found using data from both the Expedition 12 and the MATROSHKA data sets. The effect of the correction is to fix the GCR iron peak at ∼ 137 keV/ m water.

Sc = So/[1 − (2.4424 × T + 9.5394) × 10−3 ]

(1)

where So is the etch rate ratio without sensitivity correction, Sc is the etch rate ratio after sensitivity correction and T is the exposure time of CR-39 detector in months. The formula indicates that the average decrease of CR-39 sensitivity is ∼0.4% per month. Details of the method used to determine the correction formula for the sensitivity fading of CR-39 can be found in Zhou et al. [8]. For several possible GCR iron events, the LET values before and after sensitivity correction are listed in Table 1. Table 1 shows that the LET values after correction are in the range of 135.83–138.64 keV/ m water and are consistent

D. Zhou et al. / Acta Astronautica 66 (2010) 301 -- 308

305

Table 2 Results measured with CR-39 PNTDs (MATROSHKA-1, ICRP 60, ⱖ 10 keV/ m water). Detector location

Dose (mGy)

Dose rate (Gy/d)

Poncho 1 (total) Poncho 1 (HZE) Poncho 2 Poncho 5 (total) Poncho 5 (HZE) Eye (total) Eye (HZE) Eyea (total) Stomach (total) Stomacha (total) Reference1a (total) Reference2 (total) Reference2 (HZE)

34.8 ± 1.5 6.4 ± 0.6 30.1 ± 1.6 31.5 ± 1.6 6.1 ± 0.7 19.4 ± 1.1 3.4 ± 0.5 19.3 ± 1.4 16.7 ± 1.1 16.8 ± 1.0 17.9 ± 1.0 17.6 ± 0.9 3.0 ± 0.4

64 12 56 58 11 36 6 36 31 31 29 28 5

a

± ± ± ± ± ± ± ± ± ± ± ± ±

3 1 3 3 1 2 1 3 2 2 2 1 1

Dose equivalent (mSv)

Dose equivalent rate (Sv/d)

411.0 ± 125.3 ± 356.4 ± 376.8 ± 115.6 ± 248.5 ± 73.3 ± 247.4 ± 219.5 ± 220.5 ± 217.3 ± 215.0 ± 60.3 ±

763 233 661 699 214 461 136 459 407 409 353 349 98

17.4 12.7 18.4 19.6 13.9 14.6 11.2 18.4 14.2 13.3 11.6 11.0 7.4

± ± ± ± ± ± ± ± ± ± ± ± ±

Q factor

32 24 34 36 26 27 21 34 26 25 19 18 12

11.83 19.46 11.83 11.95 19.07 12.80 21.63 12.84 13.14 13.16 12.12 12.25 19.92

CR-39 PNTDs from Eril Research Inc.

with the GCR iron peak as measured in the preflight calibration.

Differential LET Spectrum (Flux) (Matroshka-1) 101 Poncho1 Poncho1-hze Poncho5 Poncho5-hze Eye Eye-hze Stomach Reference2 Reference2-hze

Charged particles can be identified by measuring effective etch rate ratio Seff and fractional etch rate gradient G (g−1 cm2 ), as described in O'Sullivan et al. [19], Zhou et al. [5,8,17,20,22] and Fowler et al. [32]. The charge of the test events listed in Table 1 were found to be around 26 with this method and are shown in Fig. 5. The CR-39 PNTDs from Eril Research Inc. were not covered with plastic film while all others were. Experimental data indicate that fading for CR-39 with film covered is stronger than for uncovered CR-39. Therefore an additional correction of sensitivity fading had to be applied. The additional correction factor can be determined by comparing the results of dose equivalent, obtained from the two types after the common fading correction. The additional correction factor is found to be about 13%. All results presented in the following sections were obtained using JSC-SRAG LET calibration and sensitivity correction. The final results measured by CR-39 detectors, whether covered with plastic film or not—are in combination with the TLD data—comparable to the data from the active DOSTEL instrument.

Particles/(cm2.d.sr.keV/µm water)

4.3. Charge determination for the test GCR events

100

10-1

10-2

10-3 101

102 LETinf.(keV/µm water)

103

Fig. 6. Differential spectra of flux for MATROSHKA-1.

5. Radiation results measured with CR-39 detectors The net radiation quantities can be obtained by subtracting the data from the reference radiation package (77 days inside the ISS) from the total radiation (616 days: 77 days inside ISS+539 days outside ISS) measured at different locations. The net radiation results measured with CR-39 PNTDs for MATROSHKA-1 are collected in Table 2. In the table all CR-39 detectors are from DIAS and scanned manually at either JSC-SRAG or DIAS, except for those from Eril Research Inc., which were scanned manually at JSC-SRAG and are marked with a star. Manual scanning can collect all events (primary and secondary products) and HZE particles (mainly GCR) in the same scan procedure. Table 2 indicates: (1) the highest radiation dose is at poncho 1 and the lowest dose is at the stomach location and

from skin to stomach nearly half of the dose was absorbed by body tissue itself; (2) the dose measured at different poncho locations are consistent; (3) dose contributed by HZE particles is an important part of total dose, namely, 30.5%, 30.7%, 29.5% and 28.0% for ponchos 1, 5, eye and reference2, respectively. Fig. 6 shows the differential LET spectra of the flux measured with DIAS CR-39 PNTDs for MATROSHKA-1 experiment. In the figure and Fig. 7 the solid lines represent LET spectra for all radiation including HZE particles and the dotted lines represent LET spectra for HZE particles. Fig. 7 shows the integral LET spectra of the dose equivalent (ICRP 60) measured with DIAS CR-39 detectors for MATROSHKA-1 experiment. The highest dose equivalent for

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The Model 5500 reader—a Harshaw (now Thermo Fisher Scientific) instrument is used for the data readout. The reader uses a hot N2 -gas heating system and is connected to a supply of oxygen-free N2 gas with a pressure of 30–40 psi. The source of N2 gas in the SRD Lab is the in-house supply which is derived from liquid N2 . Setup of a TLD reader requires setup of the time temperature profile (TTP), for TLD-300 used on STS and ISS space missions consists of a linear ramp of 5 ◦ C/s from an initial temperature of 100 ◦ C to a maximum temperature of 400 ◦ C. A total acquired time of 50 s is used, which leaves the maximum temperature on for 10 s. During readout, the net value of the TL is determined by taking the initial TL reading and subtracting a second reading of the same TLD (the intrinsic noise of the TLD).

Integral LET Spectrum (Dose Equivalent, ICRP 60) (Matroshka-1)

Dose Equi. (>LETinf.)(Sv/d)

10-3

10-4

Poncho1 Ponch1-hze Poncho5 Poncho5-hze Eye Eye-hze Stomach Reference2 Reference2-hze

10-5

6.2. Combination of TLD and CR-39 data

10-6 101

102 LETinf.(keV/µm water)

103

Fig. 7. Integral LET spectra of dose equivalent for MATROSHKA-1.

the outside exposure is at the poncho 1, and the lowest is at stomach location. 6. Radiation results combined from those measured with CR-39 PNTDs and TLDs 6.1. TLD read out and annealing procedure JSC-SRAG has been using different TLDs for measuring absorbed dose for STS and ISS space missions. TLD-300 (CaF2 :Tm) was used by JSC-SRAG for MATROSHKA-1 experiment. The best data fit for the LET dependent TL-detection efficiency of TLD-300 is:

 = 0.94 + 0.20 log(LET) − 0.14 log2 (LET)

(2)

The annealing procedure used in the SRAG-SRD (space radiation dosimetry) Lab for the CaF2 dosimeters consists of a 400 ◦ C anneal for 1 h inside the Thermolyne Type 4800 Furnace, followed by a 2 h anneal at 100 ◦ C inside a preheated microprocessor-controlled oven (Fisher Scientific Isotemp Premium Ovens, 700 Series). After TLD exposure to a radiation field in space or during calibration, and just prior to readout, a post-irradiation anneal is performed with the TLD inside the readout disc at 100 ◦ C for 30 min (this anneal is 20 min if the TLD are in beakers). This short anneal removes the low temperature TL peaks, which ae more sensitive to fading effects. This anneal is performed by placing the TLD in the 50 ml beaker (using the vacuum pickup or plastic tweezers) and placing the beaker in the preheated microprocessor-controlled oven (same as above).

The radiation quantities for total LET can be obtained by combining the results measured with TLDs (low LET) and CR-39 detectors (high LET). The combined method for results measured with CR-39 detectors and TLDs is described briefly in NCRP [16], the detail of the method can be found in Doke et al. [33] and Zhou et al. [7,8,10]. The experimental results obtained by TLDs and CR-39 PNTDs indicate that the combination point of LET can be chosen as 10 keV/ m water. Thus, the total dose and total dose equivalent can be obtained by combining the contributions from TLDs (low LET: < 10 keV/m water) and CR-39 PNTDs (high LET: ⱖ 10 keV/ m water). Table 3 is a collection of results measured with CR-39 PNTDs and TLD300 for the MATROSHKA-1 exposure. Table 3 indicates: (1) the contribution of the dose measured with TLD (low LET) and CR-39 (high LET) is 84% and 16% of total dose for stomach and 89% and 11% of total dose for poncho 1 respectively; and (2) the contribution of dose equivalent measured with TLD and CR-39 is 29% and 71% of total dose equivalent for stomach and 41% and 59% of total dose equivalent for poncho 1 respectively. The net dose equivalent accumulated within 539 days at the poncho 1 location is 1299 Sv/day, of which 763 Sv/day is measured by CR-39 detectors and represents the high LET ( ⱖ 10 keV/ m water) component. It comprises ∼59% of the total dose equivalent. This number is lower than the ratio of ∼71% for location reference2 and indicates that at poncho 1, TLDs measured a higher dose at low LET region which may have been contributed by energetic GCR protons, solar energetic particles and particles in the Earth's radiation belts. The quality factor obtained from the combined dose and dose equivalent for ponchos 1, 2 and 5 is lower than the value of 2.48 measured by DOSTEL. A possible explanation is that TLDs tend to measure more low LET radiation than DOSTEL [1], therefore the quality factor is lower. The dose equivalent measured by JSC-DIAS passive dosimeters at location poncho 1 is ∼ 1299 Sv/day, well consistent with the value of 1265 Sv/day measured by DOSTEL [1] located on the top of the phantom head. The higher value measured by passive dosimeters is reasonable and may be partly from TLDs which are more sensitive for very low LET radiation than silicon dosimeters or from CR-39 PNTDs which are sensitive to neutrons.

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307

Table 3 Results combined from those measured with TLD300 and CR-39 PNTDs (MATROSHKA-1, ICRP 60, combined at 10 keV/ m water). Dosimeter location

Dose TLD low LET Q = 1 (mGy)

Poncho 1 Poncho 2 Poncho 5 Eye Stomach Reference 2

289.2 248.4 225.0 118.1 90.4 86.7

± ± ± ± ± ±

5.0 5.0 5.0 1.0 1.0 1.0

Dose CR-39 high LET Q > 11 (mGy)

34.8 30.1 31.5 19.4 16.7 17.6

± ± ± ± ± ±

1.5 1.6 1.6 1.1 1.1 0.9

Dose all LET (mGy)

324.0 278.5 256.5 137.5 107.1 104.3

± ± ± ± ± ±

5.2 5.2 5.3 1.5 1.5 1.3

7. Conclusions Several conclusions can be drawn from this work: (1) The experimental approach for JSC-SRAG using passive dosimeter CR-39 PNTDs and TLDs is successful: the LET spectra with high LET can be measured with CR-39 PNTDs and the absorbed dose can be measured with TLDs. (2) The LET spectrum method using CR-39 PNTDs and the JSC LET calibration for CR-39 detectors are successful and reliable and will be used in NASA's future space missions. The combination method for results measured by CR-39 detectors and by TLDs is successful too and will be also used in the future work. (3) The fading effect of sensitivity for CR-39 detectors used for long time exposures was observed and the method of “internal LET calibration using GCR iron peak” is used to deduce the correction formula for the sensitivity fading. After the correction of sensitivity fading for CR-39 detectors and the combination for results measured with CR-39 PNTDs and TLDs, the final dose equivalent is consistent with that measured by DOSTEL. (4) The radiation LET spectra measured for the different organs of the phantom can be used to estimate the radiation risk based on the experimental approach for the risk determination. (5) Further work in the framework of the MATROSHKA project will include the intercomparison of CR-39 LET spectra measured by other MATROSHKA investigators as well as joint measurements inside the ISS. Acknowledgments The authors wish to thank all colleagues for the results compared with and all those who assisted them in their work at NSRL, BNL, TAMU, HIMAC and for the MATROSHKA1 experiment as well as for Expedition 12. References [1] G. Reitz, T. Berger, The MATROSHKA facility—dose determination during an EVA, Radiation Protection Dosimetry 120 (2006) 442–445. [2] J. Dettmann, G. Reitz, G. Gianfiglio, MATROSHKA—the first ESA external payload on the international space station, Acta Astronautica 60 (2007) 17–23. [3] G. Reitz, T. Berger, P. Bilski, et al., Astronaut's organ doses inferred from measurements in a human phantom outside the international space station, Radiation Research 171 (2009) 225–235.

Dose rate all LET (Gy/d)

601 517 476 255 199 169

± ± ± ± ± ±

10 10 10 3 3 2

Total dose equivalent all LET (ICRP 60) (mSv) 700.2 604.8 601.8 366.6 310.0 301.8

± ± ± ± ± ±

18.1 19.0 20.2 14.6 14.2 11.1

Dose equivalent rate all LET (ICRP60) (Sv/d) 1299 1122 1116 680 575 490

± ± ± ± ± ±

34 35 38 27 26 18

Q all LET

2.16 2.17 2.35 2.68 2.89 2.89

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