空间辐射剂量测量技术发展CSCD

简要介绍了用于空间辐射剂量测量的被动式和主动式仪器,并进行了对比。被动式仪器有荧光探测器、径迹蚀刻探测器、核乳胶探测器、金属箔探测器、气泡探测器和MOSFET剂量计;主动式仪器有硅半导体探测器(Liulin-4)、硅望远镜(DOSTEL、RRMD、Liulin-5、CPDS)、气体探测器(TEPC、R-16电离室)和中子谱仪(Bonner球中子谱仪、层叠闪烁谱仪)。利用多种测量方法互相补充和验证,为空间辐射剂量提供了更可靠的数据。     

Radiation measured during ISS-Expedition 13 with different dosimeters

Radiation in low Earth orbit (LEO) is mainly composed of galactic cosmic rays (GCR), solar energetic particles and particles in SAA (South Atlantic Anomaly). The biological impact of space radiation to astronauts depends strongly on the particles’ linear energy transfer (LET) and is dominated by high LET radiation. It is important to measure the LET spectrum for the space radiation field and to investigate the influence of radiation on astronauts. At present, the preferred active dosimeters sensitive to all LET are the tissue equivalent proportional counter (TEPC) and the silicon detectors in various configurations; the preferred passive dosimeters are CR-39 plastic nuclear track detectors (PNTDs) sensitive to high LET and thermoluminescence dosimeters (TLDs) as well as optically stimulated luminescence dosimeters (OSLDs) sensitive to low LET. The TEPC, CR-39 PNTDs, TLDs and OSLDs were used to investigate the radiation field for the ISS mission Expedition 13 (ISS-12S) in LEO. LET spectra and radiation quantities (fluence, absorbed dose, dose equivalent and quality factor) were measured for the space mission with different dosimeters. This paper introduces the role of high LET radiation in radiobiology, the operational principles for the different dosimeters, the LET spectrum method using CR-39 detectors, the method to combine the results measured with TLDs/OSLDs and CR-39 PNTDs, and presents the LET spectra and the radiation quantities measured and combined.     

Comparison of measured cosmic ray LET spectra with models and predictions.

Radiation effects of cosmic ray nuclei are generally described as a function of the particle LET. For a large number of space missions LET spectra have been measured and models have been developed to calculate these spectra that include the effects of geomagnetic shielding and shielding provided by material. In this paper we compare measured and calculated LET spectra. For low earth orbits events with high local energy deposition, i.e., short range secondaries, contribute significantly to the measured spectra. These events are produced by nuclear interactions, mainly induced by protons from the south atlantic anomaly. The technique to include these contributions in the models depends on the size of radiation sensitive volumes. For sizes comparable to or larger than the range of target secondaries it is essential to separate contributions by target interactions from those of cosmic rays. This separation is possible in experiments which use stacks of plastic nuclear track detectors. The yield of short range events generated by protons and measured in the detector can be calibrated from accelerator experimental data. We present first results for CR-39 detectors.     

The measured radiation environment within Spacelabs 1 and 2 and comparison with predictions

To measure the radiation environment in the Spacelab (SL) module and on the pallet, a set of passive and active radiation detectors was flown as part of the Verification Flight Instrumentation (VFI). SL 1 carried 4 passive and 2 active detector packages which, with the data from the 26 passive detectors of Experiment INS006, provided a comprehensive survey of the radiation environment within the spacecraft. SL 2 carried 2 passive VFI units on the pallet. Thermoluminescent dosimeters (TLDs) measured the low linear energy transfer (LET) dose component; the HZE fluence and LET spectra were mapped with CR-39 track detectors; thermal and epithermal neutrons were measured with the use of fission foils; metal samples analyzed by gamma ray spectroscopy measured low levels of several activation lines. The TLDs registered from 97 to 143 mrad in the SL 1 module. Dose equivalents of 330±70 mrem in the SL 1 module and 537±37 mrem on the SL 2 pallet were measured. The active units in the SL 1 module each contained an integrating tissue-equivalent ion chamber and two differently-shielded xenon-filled proportional counters. The ion chambers accumulated 125 and 128 mrads for the mission with 17 and 12 mrads accumulated during passages through the South Atlantic Anomaly (SAA). The proportional counter rates (1 cps at sea level) were 100 cps in the middle of the SAA (mostly protons), 35 cps at large geomagnetic latitudes (cosmic rays) and 100 cps in the South Horn of the electron belts (mostly bremsstrahlung). Detailed results of the measurements and comparison with calculated values are described.     

Radiation measurements on the flight of IML-2.

The second flight of the International Microgravity Laboratory (IML-2) on Space Shuttle flight STS-65 provided a unique opportunity for the intercomparison of a wide variety of radiation measurement techniques. Although this was not a coordinated or planned campaign, by sheer chance, a number of space radiation experiments from several countries were flown on this mission. There were active radiation measuring instruments from Japan and US, and passive detectors from US, Russia, Japan, and Germany. These detectors were distributed throughout the Space Shuttle volume: payload bay, middeck, flight deck, and Spacelab. STS-65 was launched on July 8, 1994, in a 28.45 degrees x 306 km orbit for a duration of 14 d 17 hr and 55 min. The crew doses varied from 0.935 mGy to 1.235 mGy. A factor of two variation was observed between various passive detectors mounted inside the habitable Shuttle volume. There is reasonable agreement between the galactic cosmic ray dose, dose equivalent and LET spectra measured by the tissue equivalent proportional counter flown in the payload bay with model calculations. There are significant differences in the measurements of LET spectra measured by different groups. The neutron spectrum in the 1-20 MeV region was measured. Using fluence-dose conversion factors, the neutron dose and dose equivalent rates were 11 +/- 2.7 microGy/day and 95 +/- 23.5 microSv/day respectively. The average east-west asymmetry of trapped proton (>3OMeV) and (>60 MeV) dose rate was 3.3 and 1.9 respectively.     

Dosimetry during the first IBIS facility flight.

The dosimetry of cosmic rays was performed during the first experimental flight of the IBIS facility. Different thermoluminescent detectors (TLD) have been used to measure the contribution of the low linear energy transfer component (LET < 10 keV/micrometer) and plastic nuclear track detectors (PNTD) for the high linear energy tranfer (LET) component. Several parameters of tracks have been measured to determine the LET spectra of primary and secondary charged particles. The total absorbed dose rate (TLD+PNTD) during the flight was 0.23 mGy/day and the dose equivalent rate using the ICRP 60 was 0.52 mSv/day. The corresponding mean quality factor was 2.4. These results are in agreement with those obtained aboard the MIR station with a tissue equivalent proportional counter.     

Summary of current radiation dosimetry results on manned spacecraft.

Measurements of radiation exposures aboard manned space flights of various altitudes, orbital inclinations and durations were performed by means of passive radiation detectors, thermoluminescent detectors (TLD's), and in some cases by active electronic counters. The TLD's and electronic counters covered the lower portion of the LET (linear energy transfer) spectra, while the nuclear track detectors measured high-LET produced by HZE particles. In Spacelab (SL-1), TLD's recorded a range of 102 to 190-millirad, yielding an average low-LET dose rate of 11.2 mrad per day inside the module, about twice the dose rate measured on previous space shuttle flights. Because of a higher inclination of the SL-1 orbit (57 degrees versus 28.5 degrees for previous shuttle flights), substantial fluxes of highly ionizing HZE particles were also observed, yielding an overall average mission dose-equivalent of about 135 millirem, about three times higher than measured an previous shuttle missions. A dose rate more than an order of magnitude higher than for any other space shuttle light was obtained for mission STS-41C, reflecting the highest orbital altitude to date of 519 km.     

Microscopic track structure of equal-LET heavy ions.

The spatial distributions of ionization and energy deposition produced by high-velocity heavy ions are crucial to an understanding of their radiation quality as exhibited eg., in track segment experiments of cell survival and chromosome aberrations of mammalian cells. The stopping power (or LET) of a high velocity ion is proportional to the ratio z2/v2, apart from a slowly varying logarithmic factor. The maximum delta-ray energy that an ion can produce is proportional to v2 (non-relativistically). Therefore, two HZE ions having the same LET, but in general differing z and v will have different maximum delta-ray energies and consequently will produce different spatial patterns of energy deposition along their paths. To begin to explore the implications of this fact for the microscopic dosimetry of heavy ions, we have calculated radial distributions in energy imparted and ionization for iron and neon ions of approximately equal LET in order to make a direct comparison of their delta-ray track structure. Monte Carlo techniques are used for the charged particle radiation transport simulation.     

Verification of shielding effect by the water-filled materials for space radiation in the International Space Station using passive dosimeters

The dose reduction effects for space radiation by installation of water shielding material (“protective curtain”) of a stack board consisting of the hygienic wipes and towels have been experimentally evaluated in the International Space Station by using passive dosimeters. The averaged water thickness of the protective curtain was 6.3 g/cm². The passive dosimeters consisted of a combination of thermoluminescent detectors (TLDs) and plastic nuclear track detectors (PNTDs). Totally 12 passive dosimeter packages were installed in the Russian Service Module during late 2010. Half of the packages were located at the protective curtain surface and the other half were at the crew cabin wall behind or aside the protective curtain. The mean absorbed dose and dose equivalent rates are measured to be 327 μGy/day and 821 μSv/day for the unprotected packages and 224 μGy/day and 575 μSv/day for the protected packages, respectively. The observed dose reduction rate with protective curtain was found to be 37 ± 7% in dose equivalent, which was consistent with the calculation in the spherical water phantom by PHITS. The contributions due to low and high LET particles were found to be comparable in observed dose reduction rate. The protective curtain would be effective shielding material for not only trapped particles (several 10 MeV) but also for low energy galactic cosmic rays (several 100 MeV/n). The properly utilized protective curtain will effectively reduce the radiation dose for crew living in space station and prolong long-term mission in the future.     

Space radiation dosimetry using bubble detectors.

Bubble detectors--a new development in radiation detection--has only recently been used for radiation measurements in space. One important characteristic of the bubble detector is that it operates on a phenomenon which bears considerable resemblance to biological response. Recent experimental results from irradiating bubble detectors with high-energy heavy ions point to the need to re-examine the methodology used for assessing space radiation and the relevance of conventional quantities such as dose equivalent for space dosimetry. It may be that biological hazard associated with the intensely ionizing events--associated with nuclear fragmentation but delivering relatively small dose equivalent--may be much more important than that associated with lightly ionizing events which comprise the bulk of the conventional radiation dose equivalent.     

Dosimetric mapping inside BIORACK

The experiment was flown in different locations inside BIORACK on the D1 mission. It contained different plastic detectors (cellulose nitrate, Lexan, and CR 39) and emulsions to measure the high LET components of the radiation environment. For low LET measurements thermoluminescence dosimeters (L iF) were used. The paper gives data about total dose, charge, energy, and LET spectra so far obtained. These data are compared with data of previous spaceflights.     

Fragmentation of 1 GeV/nucleon iron ions in thick targets relevant for space exploration.

We have measured charged nuclear fragments produced by 1 GeV/nucleon 56Fe ions interacting with aluminium, polyethylene and lead. These materials are relevant for assessment of radiation risk for manned space flight. The data will be presented in a form suitable for comparison with models of nuclear fragmentation and transport, including linear energy transfer (LET) spectrum, fluence for iron and fragments, event-tack- and event-dose-averaged LET, total dose and iron contribution to dose.     

Analytical-HZETRN model for rapid assessment of active magnetic radiation shielding

The use of active radiation shielding designs has the potential to reduce the radiation exposure received by astronauts on deep-space missions at a significantly lower mass penalty than designs utilizing only passive shielding. Unfortunately, the determination of the radiation exposure inside these shielded environments often involves lengthy and computationally intensive Monte Carlo analysis. In order to evaluate the large trade space of design parameters associated with a magnetic radiation shield design, an analytical model was developed for the determination of flux inside a solenoid magnetic field due to the Galactic Cosmic Radiation (GCR) radiation environment. This analytical model was then coupled with NASA’s radiation transport code, HZETRN, to account for the effects of passive/structural shielding mass. The resulting model can rapidly obtain results for a given configuration and can therefore be used to analyze an entire trade space of potential variables in less time than is required for even a single Monte Carlo run. Analyzing this trade space for a solenoid magnetic shield design indicates that active shield bending powers greater than ∼15 Tm and passive/structural shielding thicknesses greater than 40 g/cm² have a limited impact on reducing dose equivalent values. Also, it is shown that higher magnetic field strengths are more effective than thicker magnetic fields at reducing dose equivalent.     

Summary of radiation dosimetry results on U.S. and Soviet manned spacecraft.

Measurements of the radiation environment aboard U.S. and Soviet manned spacecraft are reviewed and summarized. Data obtained mostly from passive and some active radiation detectors now exist for the case of low Earth-orbit missions. Major uncertainties still exist for space exposure in high altitude, high inclination, geostationary orbits, in connection with solar effects and that of shielding. Data from active detectors flown in Spacelabs 1 and 2 suggest that a variety of phenomena must be understood before the effects of long-term exposure at the space-station type of orbit and shielding can be properly assessed.     

Analysis of secondary electron emission spectra of equal-LET protons and alpha particles for purposes of radiation quality and spaceflight hazard assessment.

Amongst the great variety of heavy particles present in the galactic and solar cosmic ray spectra, hydrogen and helium nuclei are significantly more abundant than all other heavier ions and, as such, represent a major radiation hazard to humans in space. Experimental data have suggested that differences in relative biological effectiveness (RBE) exist between the two species at the same value of linear energy transfer (LET). This has consequences for heavily ionising radiation protection procedures, which currently still assume a simple dependence of radiation quality on LET. By analysing the secondary electron (delta-ray) emission spectra of protons and alpha particles, in terms of the spatial characteristics of energy deposition in cellular targets and the likelihood of complex lesion formation, a numerical quantity representing biological effectiveness is generated. When expressed relative to a reference radiation, this quantity is found to differ for protons and a particles of the same LET, demonstrating not only the ion-specific nature of RBE but also the inadequacy of specifying radiation quality as a function of LET only. Such a method for numerically assessing radiation quality may have implications for procedures for heavy ion protection in space at low doses and for understanding the initial mechanisms of radiation action.     

Issues and problems for radiobiological research in space.

The uniqueness of the space radiation field creates specific problems in the evaluation of hazards to men and materials. Comprehensive measurements of all physical parameters are necessary but not sufficient. Particular attention has to be paid to variables like solar flares by applying fast-responding active dosimetry. The assessment of biological consequences poses even more problems. There are no human data for the kinds of particles seen in space and they will presumably never be available. The only reasonable approach is therefore to use the information obtained for other radiations and check their applicability for the space situation. This involves both the study of fundamental processes in ground experiments as well as their verification in space missions. Special emphasis has to be laid on the modification of radiation effects by flight-dynamic factors and microgravity. Radiation protection guidelines for space flights cannot simply be transformed from existent regulations designed for radiation workers on earth but have to be tailored to the specific situation in space.     

Cell cycle delays induced by heavy ion irradiation of synchronous mammalian cells.

Cell cycle effects of very high LET particles on synchronous V79 Chinese Hamster cells have been studied in a track segment experiment by means of flow cytometric methods. Cells were irradiated with 10 MeV/u Pb-ions (LET = 13500 keV/micrometers) at an average fluence of 2 particles per cell nucleus, corresponding to a survival level of about 25%. Instantaneous drastic reductions of cell proliferation in all cycle phases have been observed, which affect the cell cycle for at least 50 hours after exposure to heavy ions. These findings are in clear contrast to the results from low LET radiation experiments, where significant delays can only be observed in S-phase and G2M-phase and for comparatively short time intervals of a few hours. Additionally, high LET radiation gives rise to prolonged DNA synthesis bypassing cell division, which leads to cells with DNA content greater than that of G2M-cells.     

Importance of dose-rate and cell proliferation in the evaluation of biological experimental results.

The nuclei of cells within the bodies of astronauts traveling on extended missions outside the geomagnetosphere will experience single traversals of particles with high LET (e.g., one iron ion per one hundred years, on average) superimposed on a background of tracks with low LET (approximately one proton every two to three days, and one helium ion per month). In addition, some cell populations within the body will be proliferating, thus possibly providing increasing numbers of cells with "initiated" targets for subsequent radiation hits. These temporal characteristics are not generally reproduced in laboratory experimental protocols. Implications of the differences in the temporal patterns of radiation delivery between conventionally designed radiation biology experiments and the pattern to be experienced in space are examined and the importance of dose-rate and cell proliferation are pointed out in the context of radiation risk assessment on long missions in space.