neutron dose study with bubble detectors aboard the ... - Oxford Journals

Radiation Protection Dosimetry (2009), Vol. 133, No. 4, pp. 200–207 Advance Access publication 23 March 2009

doi:10.1093/rpd/ncp039

NEUTRON DOSE STUDY WITH BUBBLE DETECTORS ABOARD THE INTERNATIONAL SPACE STATION AS PART OF THE MATROSHKA-R EXPERIMENT R. Machrafi1, *, K. Garrow1, H. Ing1, M. B. Smith1, H. R. Andrews1, Yu. Akatov2, V. Arkhangelsky2, I. Chernykh2, V. Mitrikas2, V. Petrov2, V. Shurshakov2, L. Tomi3, I. Kartsev4 and V. Lyagushin5 1 Bubble Technology Industries (BTI), 31278 Highway, 17 Chalk River, Ont., Canada K0J 1J0 2 Institute of Biomedical Problems, State Scientific Center of Russian Federation, Russian Academy of Sciences (IMBP-RAN), Moscow, Russia 3 Canadian Space Agency, Montreal, Que., Canada 4 Scientific Center “SNIIP”, Moscow, Russia 5 Rocket Space Corporation “Energia”, Moscow, Russia

Received March 31 2008, revised February 19 2009, accepted February 24 2009 As part of the Matroshka-R experiments, a spherical phantom and space bubble detectors (SBDs) were used on board the International Space Station to characterise the neutron radiation field. Seven experimental sessions with SBDs were carried out during expeditions ISS-13, ISS-14 and ISS-15. The detectors were positioned at various places throughout the Space Station, in order to determine dose variations with location and on/in the phantom in order to establish the relationship between the neutron dose measured externally to the body and the dose received internally. Experimental data on/in the phantom and at different locations are presented.

INTRODUCTION The radiation environment inside the International Space Station (ISS) is an extremely complex mixture of photons, charged and neutral particles. In lowEarth orbit, the radiation levels of these photons and particles are dependent on the phase of the solar activity cycle, as well as geophysical and orbital parameters. The Matroshka-R project is a multi-stage space programme, which arose from the complexity of the space radiation field and its impact on the safety of astronauts during long-duration manned space flights. Experimental study of the radiation field in different modules of the ISS is one of the purposes of the Matroshka-R experiments. To characterise the neutron component of the radiation field, a spherical phantom(1), special space bubble detectors (SBDs) and a mini reader for automatic bubble counting were transported to the ISS. The spherical phantom was designed by the Institute of Biomedical Problems (IBMP) of the Russian Academy of Sciences, and the SBDs and mini reader were developed by Bubble Technology Industries (BTI). The equipment was used in three separate expeditions of the ISS. The SBDs were positioned on and in the spherical phantom in order to investigate the relationship between the neutron dose measured externally to the body and the dose received internally. Also, the phantom was located at *Corresponding author: [email protected]

various places throughout the ISS to evaluate the influence of its shielding. Starting in January 2006, seven experimental sessions were carried out during expeditions ISS-13, ISS-14 and ISS-15. This period coincided with a decreased phase of the current solar activity cycle 23 (solar flare activity was low). In this paper, the data collected from the SBDs, their analysis and subsequent conclusions are presented. EXPERIMENTAL DESCRIPTION Experimental conditions The Matroshka-R experimental sessions were carried out during a period of undisturbed space radiation conditions. The doses received in space flight on the ISS depend on solar activity, the geophysical parameters describing conditions in the Earth’s magnetosphere and orbital parameters of the ISS, such as inclination and altitude of the orbit. Parameters for the measurements conducted, such as radio emission flux, Wolf ’s number, planetary geomagnetic activity index, amplitude of the ring current, and proton and electron fluxes, are given in reference (2) for each expedition. Experimental apparatus For the Matroshka-R experiments, special space-type bubble detectors were produced with a firmer

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MATROSHKA-R NEUTRON DOSE STUDY

polymer so that the growth of the formed bubbles is slower, allowing the detector to be used for a longer period between recompressions. The detectors have an active volume of 10 ml with 104 microscopic droplets giving a low sensitivity from 140 to 200 bubbles per mSv (for neutrons from an AmBe source). The bubble counting was done automatically using a lightweight mini reader. A total of 16 bubble detectors were launched aboard the Progress M-356 in April 2006. Each detector’s exposure was for 5 days with counting immediately after exposure. A photograph of the SBD is shown in Figure 1. The spherical phantom to simulate the human body is 350 mm in diameter, 32 kg in mass and has 13 tissue-equivalent slices (see Figure 2). A working jacket with 32 pockets and radial holes was designed to facilitate the mounting of detectors on the surface and insertion inside the phantom at a depth of 105– 165 mm, to approximate the measured dose in critical organs. Preliminary data obtained with this phantom were reported in reference (3).

EXPERIMENTAL SETUP

Figure 1. Space bubble detector.

Measurements were carried out over three expeditions (ISS-13, ISS-14 and ISS-15) with two to three sessions each. During these sessions, the bubble detectors were placed at various locations on board the ISS and on/in the phantom located in the service module (SM) and in the docking module (DM).

Figure 2. Spherical phantom used to simulate the human body.

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Bubble detector calibration and dose calculation The space-type bubble detectors were calibrated in the same fashion as the commercial bubble detectors used for terrestrial dosimetry. The procedure used an AmBe source with an emission rate of 1.13 107 s21 and a conversion coefficient of 4.11 1024 mSv cm2 (according to ICRU Report 66). The calibration was performed at different temperatures at a distance from the source and a time period that produced 100 –150 bubbles. The bubbles that result from exposure to the calibration neutron field give the sensitivity of the detectors in bubbles per mSv(4). As the energy response of the bubble detector is isotropic, the error analysis only includes systematic uncertainty in detector sensitivity and the statistical fluctuation in the number of bubbles formed. In terrestrial nuclear activities, bubble detectors have been used for decades for routine monitoring of personal neutron doses, and operational procedures are well established. An overview of bubble detectors, including energy dependence, is given in references (5,6). For terrestrial use, bubble detectors are calibrated in terms of the quantity personal dose equivalent, Hp(10), as defined in ICRU Report 51. For space applications, it is not clear whether the quantity Hp(10) is relevant, or whether the ambient dose equivalent, H*(10), is more appropriate. In this paper, the convention introduced in reference (7) is followed, and the bubble-detector sensitivity measured with the AmBe source is scaled by a factor of 1.62 for use in the space environment. This scaling factor is based on theoretical calculations of

the bubble-detector response, which have been verified experimentally using the CERN-EU highenergy reference field (CERF). The CERF(8) is a mixed radiation field and its neutron component resembles the neutron component of the radiation field encountered on the ISS. The CERF has become a standard reference for testing and calibrating radiation monitors intended for use in the space environment. In the approach of Green et al. (7), the quantity reported is H*(10), although the difference between Hp(10) and H*(10) is very small for an AmBe source. It should be noted that the results reported in this paper are relative measurements, and so the distinction between Hp(10) and H*(10) does not affect the conclusions presented.

Measurement on the surface of the phantom Measurements with bubble detectors on the surface of the phantom were performed in two sessions in ISS-13 with the phantom located in the right crew cabin in the SM. In the first session, two bubble detectors were fixed on the surface, at the equatorial level, of the phantom shown in Figure 3a, and facing the living area of the cabin towards the centre of the SM. In the second session, the same set was used, but the detectors were relocated to the back of the phantom facing the outer wall of the crew cabin (Figure 3b). The experimental data from these sessions are listed in Table 1. As the astronauts spend most of their time in the crew cabin, Sessions 3 and 4 of ISS-14 employed a set

Figure 3. Location of the bubble detectors on the phantom: (a) facing the living area and (b) facing the outer wall.

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MATROSHKA-R NEUTRON DOSE STUDY Table 1. Experimental data from Sessions 1 and 2. Detector label

A01

Sensitivity (bubbles per mSv) Location Exposure time (min) Number of bubbles Dose rate (mSv per day)

A02

86 + 9 86 + 9 Facing the living area 7040 7040 41 52 97 + 18 123 + 26

A01

A02

86 + 9 86 + 9 Facing the outer wall 7598 7597 33 37 72 + 15 81 + 18

Figure 4. Diagram of the right astronaut cabin and location of the bubble detectors.

of six detectors to measure the dose in the cabin. In Session 3, three detectors (A01–A03), shown in Figure 4, were fixed at different locations in the cabin. One of the other detectors, A04, was fixed symmetrically to the A01 detector, on the left crew cabin, in order to compare the symmetry of the neutron field. Two other detectors (A05 and A06) were on the crew desk and in close proximity to an illuminator (illuminator is a window on the floor of the ISS) on the floor, respectively (see Figure 5). To check the consistency of the data, the measurements were repeated in Session 4 with the same experimental setup. Summaries of these data are given in Tables 2 and 3. Measurements inside the phantom The measurements inside the phantom were conducted during the ISS-15 expedition, where Sessions

5 –7 were performed with another set of six bubble detectors referred to as B01 –B06. For these measurements, the phantom was located in the DM. In Session 5, in order to measure the dose received by critical organs (internal dose), three bubble detectors (B01 –B03) were inserted into the three radial channels at the centre of the phantom at a depth of 105–165 mm. The measurement was repeated in Session 6 to assess data consistency. The experimental setup of this measurement is shown in Figure 6a. Two other detectors (B05 and B06) were fixed on the internal wall of the cabin, as shown in Figure 6b, and the last detector, B04, was fixed behind the phantom on the outer wall of the cabin. Data obtained in these measurements are summarised in Tables 4 and 5, respectively. Session 7, also in the DM, compared the dose measured on the surface of the phantom with the

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Figure 5. Location of the bubble detector A06 close to the illuminator on the floor. Table 2. Experimental data from Session 3. Detector name Sensitivity (bubbles per mSv) Exposure time (min) Number of bubbles Dose rate (mSv per day)

A01

A02

A03

A04

A05

A06

86+9 7518 36 80 + 16

86+9 7523 47 104 + 22

111+4 7496 58 100 + 14

86+6 7520 50 111 + 18

99 + 15 7515 65 126 + 25

93 + 6 7516 48 99 + 16

Table 3. Experimental data from Session 4. Detector label Sensitivity (bubbles per mSv) Exposure time (min) Number of bubbles Dose rate (mSv per day)

A01

A02

A03

A04

A05

A06

86+9 9134 47 86 + 15

86+9 9133 54 99 + 17

111+4 9137 59 84 + 14

86+6 9135 43 78 + 14

99 + 15 9136 47 75 + 13

93 + 6 9136 47 80 + 14

internal dose measurements. During this experiment, detectors B01–B03 were relocated to the surface of the phantom, whereas the other detectors remained in the same positions as in Sessions 5 and 6. Data from this session are given in Table 6. DATA ANALYSIS AND DISCUSSION From ISS-13 expedition data, the dose rates read by detector A01 (Table 1) corresponding to two

opposite sides of the phantom are slightly larger on the inboard side of the phantom with limited statistical certainty. The detector read a value of 97 + 18 mSv per day when it was facing the living area of the astronaut cabin and 72 + 15 mSv per day when it was facing the outer wall. This behaviour is consistent with the reading given by A02, which was in close proximity to A01 (Figure 7a). To investigate the asymmetry of the field relative to the central axis of the SM, two detectors (A01 and

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Figure 6. Position of the bubble detectors in the phantom (detector B04 is not shown): (a) inside the phantom and (b) on the internal wall. Table 4. Experimental data from Session 5. Detector label Sensitivity (bubbles per mSv) Exposure time (min) Number of bubbles Dose rate (mSv per day)

B01

B02

B03

B04

B05

B06

123 + 12 8679 70 94 + 14

117 + 19 8679 59 83 + 17

111 + 12 8679 65 97 + 16

111 + 21 8677 79 118 + 26

105 + 4 8673 59 93 + 13

99 + 9 8670 63 106 + 16


B01

B02

B03

B04

B05

B06

123 + 12 8369 67 93 + 14

117 + 19 8376 60 88 + 18

111 + 12 8373 60 93 + 15

111 + 21 8371 63 98 + 22

105 + 4 8368 84 138 + 16

99 + 9 8366 77 134 + 19


B01

B02

B03

B04

B05

B06

123 + 12 10144 99 114 + 16

117 + 19 10145 98 119 + 23

111 + 12 10139 94 120 + 18

111 + 21 10141 106 135 + 29

105 + 4 10145 104 141 + 15

99 + 9 10138 69 99 + 15

A04) were fixed symmetrically on the outer wall of each cabin (Figure 4) and gave comparable doses (Tables 2 and 3) within statistical uncertainties. The average value of the dose, in the living area from the measured data in Sessions 3 and 4 (A01 –A03), shows

that the dose rate is similar within about 20%. Each experiment was conducted twice to verify the reproducibility of the data, and the data are consistent. In the ISS-15 expedition, the dose equivalent at a depth of 105–165 mm (critical-organ dose) and

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Figure 8. Bubble-detector readings in different locations (with different shielding).

Figure 7. Phantom data comparison: (a) two opposite sides of the phantom and (b) inside and outside the phantom.

the external dose were measured by inserting the detectors deep in the phantom and fixing them on the surface. Data from Session 5, shown in Table 4, reflects the consistency of the dose-rate values of detectors B01 –B03 placed at 105– 165 mm near the centre of the phantom. The experiment was repeated in Session 6, and data shown in Table 5, for the same detectors, confirm the reproducibility of this result. An important feature is observed in the comparison of the data with the data obtained in Session 7 (Table 6), when these three detectors were moved from inside to the surface of the phantom. The dose rates measured in these experiments are essentially equal within experimental error (Figure 7b). Bubble detectors have been used in space for over a decade on various space missions(4,7,9) on the Russian BIOCOSMOS (Bion) satellites, the Space

Shuttle and the Mir Space Station. Monte Carlo calculations using the Lyagushin neutron spectrum(10) and a spherical water phantom give a ratio of about 4 for the surface to internal dose(11), in contrast to the results presented in Figure 7b. The ratio determined from Monte Carlo calculations is consistent with the anticipated reduction in dose for a common 252 Cf neutron source shielded by 15 cm of tissue (NCRP-38)(12), which is approximately the radius of the phantom used in space. However, an earlier experimental measurement from the crew cabin also indicated little difference between internal and external doses(11). The variance between the experimental results here and the theoretical prediction is likely due to interaction of cosmic-ray particles with the phantom itself to produce additional neutrons inside the phantom, thus compensating for the neutron attenuation. More experiments are needed to understand the basic mechanism behind the observed results for neutron dose inside and outside the phantom. Finally, the phantom was positioned at two different locations, namely in the SM in the ISS-13 and ISS-14 expeditions, and in the DM in the ISS-15 expedition. Detectors, for example, A01/A02 (Session 2) and B01/B02 (Session 7), on the surface of the phantom in both locations show a slight difference in the dose rate due to the difference in the shielding thicknesses between the two locations (Figure 8). The measurements presented here are in good agreement with the results of similar experiments performed on the ISS. Koshiishi et al. (13) discussed measurements, made using the Bonner Ball Neutron Detector, of the neutron field at two locations inside the US Laboratory Module of the ISS. The neutron doseequivalent rates determined were 69 and 88 mSv per

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day. These results are in good agreement with that of the experiments here, keeping in mind the variation in shielding at various locations in the ISS. The effect of charged particles on the reading of the bubble detectors is being investigated. A model that details the interaction of charged particles with the microscopic superheated droplets in the bubble detector has been developed. It is based on the use of either experimental data on bubble formation per unit track length(14) or data for particles that exceed a minimum linear energy transfer for bubble formation(15). The contribution of charged particles to the bubble count is small, and the results from these calculations will be reported in a future publication.

CONCLUSION Within the Matroshka-R project, seven experiments were carried out during three expeditions of the ISS. Two sets of special SBDs with low neutron sensitivity were used. The first set consisted of two detectors and the second set of six detectors. To simulate the human body and characterise the dose received internally and externally, a spherical phantom was used to house bubble detectors. Data taken in the DM showed that the phantom’s internal dose was about the same as its external dose. This attests that the dose read by a dosemeter worn on the human body in space is not an overestimate of the real dose received by critical organs, as it is on Earth. In the astronaut cabin, the measured doses on the outer wall and in the living area were not significantly different. The lack of data on the cosmic-ray spectra within the ISS and on the sensitivity of bubble detectors to charged particles make an estimation of the contribution of charged particles to the bubble-detector reading difficult. However from previous measurements with bubble detectors(14,15), this contribution is known to be small. The contribution of charged particles to the bubble count will be discussed in detail in a future publication. Future plans also include the measurement of the neutron spectrum with a bubble-detector spectrometer.

FUNDING The authors wish to thank the Canadian Space Agency and the Russian Space Agency for funding this work.

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