Abstract

Several passive detectors were used to estimate dosimetry and microdosimetry characteristics of radiation field onboard spacecraft, namely: thermoluminescent detectors (TLDs), mainly to appreciate the contribution of radiation with low-linear energy transfer (LET); Si diode, to try to establish the contribution of fast neutrons; an LET spectrometer based on the chemically etched polyallyldiglycolcarbonate etched track detectors (PADC-TEDs). Detectors have been exposed onboard MIR and International Space Station (ISS) since 1997, they were also used during the MESSAGE 2 biological experiment, October 2003. The results are presented, analysed and discussed. Particular attention is devoted to the possibility of estimating neutron contribution based on data obtained with PADC-TED spectrometer of LET.

INTRODUCTION

Radiation risk onboard spacecraft represents one of the largest problems related to further space missions in space. It is therefore necessary to accumulate new data on dosimetric and microdosimetric characteristics of onboard spacecraft radiation fields.

Passive dosemeter systems could help to fulfil this task. They are small and not too heavy, and can accumulate data for long periods of time without external power. In this contribution, we present the results of studies with several passive dosimetry systems, onboard several types of spacecraft. They were exposed onboard MIR and ISS and during an individual space mission. The results are presented and analysed. An attempt to distinguish between the contributions of different particles to onboard dose and dose equivalent is described.

MATERIALS AND METHODS

Passive detector systems used

A spectrometer of the linear energy transfer (LET) based on a chemically etched polyallyldiglycolcarbonate etched track detector (PADC-TED) can measure dose and dose equivalent distributions in LET between ∼10 and 700 keV per μm in tissue(1). Page, 0.5 mm thick, and Tastrak, 0.5 and 1 mm thick, were used as PADC-TEDs. The etching time in 5N NaOH at 70°C is, for all PADC used, 18 h; the removed layer is ∼17 μm thick on each side of the detector. The spectrometer can measure dose equivalents from ∼1 to 100 mSv. To determine the LET value of a particle, the etch rate ratio V (=VT/VB; where VB is the bulk etching rate and VT is the track etching rate) is established, based upon the track parameters measurement, with an automatic optical image analyser LUCIA G. The V-spectra obtained are corrected for the critical registration angle and transformed to LET spectra of registered particle tracks using a calibration curve. Recently, an upgrading of spectrometer calibration curves was achieved for heavier charged particles with LET between ∼8 and 200 keV μm−1(2). Dose characteristics (D,H) due to particles registered by spectrometer can be calculated from the LET spectra as:

$D\ =\ {\int}\ \left(\frac{\mathrm{d}N}{\mathrm{d}L}\right)L\ \mathrm{d}L,$
respectively,
$H\ =\ {\int}\ \left(\frac{\mathrm{d}N}{\mathrm{d}L}\right)LQ\left(L\right)\mathrm{d}L,$
where dN/dL is the number of tracks per unit area in a LET interval; L is the value of LET; Q(L) is the ICRP 60 quality factor(3). It was found that the integral values obtained for neutrons agree with data measured with tissue-equivalent proportional counter at CERN, an Am–Be source, and onboard Concorde(4).

Thermoluminescent detectors (TLDs) Al2O3:C(5) were used to estimate the contribution of onboard radiation component with low-LET. They permit one to measure the dose equivalent due to low-LET radiation from ∼1 μGy. Their thermoluminescence yield (light conversion factor) decreases with the increase in LET above few keV μm−1 more rapidly than for other ‘classical’ TLDs(6).

Si-diode, passive fast-neutron dosemeters have been used for several years for fast-neutron dosimetry in the case of accidental exposure. Si diodes developed in the Czech Republic(7) were used to determine the dosimetric characteristics of the fast-neutron component of radiation field. The response of more sensitive, Si-2 type, diode to 252Cf neutrons is ∼1 V per 1 Gy of fast-neutron kerma in tissue. Their response to high-energy charged particles is only few per cent of the response to mentioned neutrons(8).

Space missions covered

Passive detectors were exposed during four long-term missions onboard MIR, between 1998 and 2000, in total for 594 d. Since the year 2001, they have been exposed twice onboard ISS stations, for 484 d. They were also exposed during ESA MESSAGE biological experiment on the ISS; October 2003; total flight time 10 d, on the ISS–8 d.

RESULTS AND DISCUSSION

Fast neutron measurements with Si diodes

Si diodes, as passive fast-neutron dosemeters, were exposed on the ISS during the period 2001–2002. Their readings were, however, statistically indistinguishable from the background. The longest exposure lasted ∼1 y, the threshold of fast-neutron detection was estimated to be ∼16 mGy(9). It follows that the fast-neutron contribution should be lower than ∼50 μGy d−1. Another, much longer, exposure onboard ISS is now in progress to get a more precise estimate.

Thermoluminescent detectors for low-LET radiation

Thermoluminescent detectors were exposed during several space missions. The results of evaluation are given in Table 1, the average value is equal to 163 ± 30 μSv d−1.

Table 1.

Results of TLD's evaluation during some space missions.

Mission

Period

Shield (g cm−2)

H*(10) (μSv d−1)

MIR 28 06/04/00–16/06/00 157 ± 11(a)
15 140 ± 10
ISS 30/11/01–03/11/02 20 212 ± 15
MESSAGE 17/10/03–28/10/03 — 166 ± 8
Mission

Period

Shield (g cm−2)

H*(10) (μSv d−1)

MIR 28 06/04/00–16/06/00 157 ± 11(a)
15 140 ± 10
ISS 30/11/01–03/11/02 20 212 ± 15
MESSAGE 17/10/03–28/10/03 — 166 ± 8
(a)

Here and in all cases uncertainty expressed as 1 S.D. (standard deviation)

LET spectrometer based on chemically etched PADC-TED

PADC-TEDs were exposed onboard spacecraft during all missions mentioned above. Typical microdosimetry distributions of particle track numbers L*N(L), absorbed doses L*D(L) and ambient dose equivalents with ICRP 60 quality factors L*H60(L) are presented in Figure 1. One can see there that:

1. L*N(L) is a steeply decreasing function of the LET.

2. Both L*D(L) and L*H60(L) exhibit a maximum, situated at LET of ∼100 keV μm−1.

Figure 1.

Microdosimetric distributions of particle track numbers and corresponding doses and ambient dose equivalents (ICRP 60); ISS exposure.

Figure 1.

Microdosimetric distributions of particle track numbers and corresponding doses and ambient dose equivalents (ICRP 60); ISS exposure.

Integral dosimetry characteristics were calculated from measured LET distributions using Equations 1 and 2, some of the results are presented in Table 2. It should be remembered that these characteristics correspond to particles with LET > 10 keV μm−1 whose tracks are registered in PADC-TED. There are three possible origins of such tracks:

1. Primary heavier cosmic particles.

2. Secondary particles created in the TED and its surroundings by nuclear reactions of mostly high-energy protons of cosmic origin.

3. Secondary particles created in the TED and its surroundings by neutron-induced nuclear reactions.

Table 2.

Integral dosimetric characteristics of particles with LET >10 keV μm−1 as measured during some space mission with PADC-TED LET spectrometer.

Mission

Shield

D (μGy d−1)

H60 (μSv d−1)

MIR 28 16.5 ± 1.2(a) 118 ± 7
15 13.1 ± 0.7 85 ± 5
ISS 20 22 ± 2 202 ± 12
MESSAGE — 16 ± 2 223 ± 22
Mission

Shield

D (μGy d−1)

H60 (μSv d−1)

MIR 28 16.5 ± 1.2(a) 118 ± 7
15 13.1 ± 0.7 85 ± 5
ISS 20 22 ± 2 202 ± 12
MESSAGE — 16 ± 2 223 ± 22
(a)

Here and in all cases uncertainty expressed as 1 S.D. (standard deviation)

Mostly short-range secondary particles are registered by means of our evaluation method, the contribution of primary heavier cosmic particles in the presented integral values is rather limited. It could be estimated from the analysis presented in a recent review article(10) that this contribution does not exceed ∼10% in N(L), it is lower in D(L), and/or H60(L).

Since many years we have studied the response of our PADC-TED LET spectrometer to high-energy protons(11). Recent analysis of all results obtained during the past decade and earlier showed that the ratio of the dose due to high-LET secondary particles to the ionisation collision dose of primary protons with energies between 70 and 1000 MeV is 0.027 ± 0.003, similar ratio for H60 is 0.35 ± 0.05 Sv Gy. If these ratios are considered and supposing that the readings of TLDs are mostly given by high-energy protons, the dosimetry characteristics measured by the PADC-TED LET spectrometer could be divided into the contributions due to neutrons and/or high-energy protons. The results of such an attempt are presented in Table 3. It could be stated that:

1. The neutron contribution to total values of D and H60 measured with LET spectrometer is higher than that of high-energy protons. Considering good correlation of H*(10) for neutrons measured with LET spectrometer and tissue-equivalent proportional counter already stated(4), the values obtained characterise real neutron contribution. It should be reminded that the Si diode evaluation gave that this contribution is <50 μGy (∼500 μSv) per day, correlating with the data in Table 3.

2. All values obtained during the twenty eighth basic expedition of MIR are lower than those obtained onboard ISS. It corresponds to lower flight altitude of MIR at this expedition (∼330 km) as compared to ISS (∼400 km).

Table 3.

Estimation of proton and neutron contribution to the dosimetry characteristics measured with PADC-TED LET spectrometer during some space missions.

Mission TLD dose D-PADC (LET ≥ 10 keV μm−1)

H60-PADC (LET ≥ 10 keV μm−1)

Total

protons (μGy d−1)

neutrons

total

protons (μGy d−1)

neutrons

MIR 28, transit 157 ± 11(a) 16.5 ± 1.2 4.2 ± 0.5 12.3 ± 1.3 118 ± 7 41 ± 6 77 ± 9
MIR 28, living 140 ± 10 13.1 ± 0.9 3.8 ± 0.5 9.3 ± 1.1 85 ± 5 30 ± 4 55 ± 6
ISS 212 ± 15 22 ± 2 5.7 ± 0.6 16 ± 2 202 ± 12 74 ± 11 128 ± 16
MESSAGE 166 ± 8 16 ± 2 4.4 ± 0.5 12 ± 2 223 ± 22 58 ± 8 165 ± 23
Mission TLD dose D-PADC (LET ≥ 10 keV μm−1)

H60-PADC (LET ≥ 10 keV μm−1)

Total

protons (μGy d−1)

neutrons

total

protons (μGy d−1)

neutrons

MIR 28, transit 157 ± 11(a) 16.5 ± 1.2 4.2 ± 0.5 12.3 ± 1.3 118 ± 7 41 ± 6 77 ± 9
MIR 28, living 140 ± 10 13.1 ± 0.9 3.8 ± 0.5 9.3 ± 1.1 85 ± 5 30 ± 4 55 ± 6
ISS 212 ± 15 22 ± 2 5.7 ± 0.6 16 ± 2 202 ± 12 74 ± 11 128 ± 16
MESSAGE 166 ± 8 16 ± 2 4.4 ± 0.5 12 ± 2 223 ± 22 58 ± 8 165 ± 23
(a)

Here and in all cases uncertainty expressed as 1 S.D. (standard deviation)

CONCLUSIONS

The results of our studies and their analysis could be summarised as:

1. The TLDs and PADC-TED LET spectrometer both proved that they can be used for onboard spacecraft measurement for missions lasting 10 d and more. The highest measurable H*(10) is ∼100 mSv. It corresponds for 400 km flight altitude to the exposure of ∼500 d. Such limit for TLDs would be at least about two orders of magnitude higher. As far as Si diodes are concerned, it could be deduced from Table 3 that ∼3–4 y of exposure would be needed to obtain statistically reliable reading.

2. When the values of H60 are compared, our value for LET ∼10 keV μm−1 represents ∼60% of the average value established in the case of the twenty third expedition of MIR and LET >5 keV μm−1(10). We do not know for the moment the exact reasons for this difference. It could be related to the actual positioning of detector sets. Other possibility could be related to the fact that the value obtained in review article(10) considers also primary heavier charged particles of cosmic origin.

3. An attempt was made to deduce the neutron contribution to the total D and H60 measured with PADC-TED LET spectrometer. It was shown that it constitutes ∼73% of D and ∼67% of H60.

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