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Radiation exposure in the moon environment
Planetary and Space Science 74 (2012) 78–83
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Planetary and Space Science journal homepage: www.elsevier.com/locate/pss
Radiation exposure in the moon environment Guenther Reitz n, Thomas Berger, Daniel Matthiae German Aerospace Center, Linder Hoehe, 51147 Koeln, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 March 2012 Received in revised form 7 July 2012 Accepted 13 July 2012 Available online 31 July 2012
During a stay on the moon humans are exposed to elevated radiation levels due to the lack of substantial atmospheric and magnetic shielding compared to the Earth’s surface. The absence of magnetic and atmospheric shielding allows cosmic rays of all energies to impinge on the lunar surface. Beside the continuous exposure to galactic cosmic rays (GCR), which increases the risk of cancer mortality, exposure through particles emitted in sudden nonpredictable solar particle events (SPE) may occur. SPEs show an enormous variability in particle flux and energy spectra and have the potential to expose space crew to life threatening doses. On Earth, the contribution to the annual terrestrial dose of natural ionizing radiation of 2.4 mSv by cosmic radiation is about 1/6, whereas the annual exposure caused by GCR on the lunar surface is roughly 380 mSv (solar minimum) and 110 mSv (solar maximum). The analysis of worst case scenarios has indicated that SPE may lead to an exposure of about 1 Sv. The only efficient measure to reduce radiation exposure is the provision of radiation shelters. Measurements on the lunar surface performed during the Apollo missions cover only a small energy band for thermal neutrons and are not sufficient to estimate the exposure. Very recently some data were added by the Radiation Dose Monitoring (RADOM) instrument operated during the Indian Chandrayaan Mission and the Cosmic Ray Telescope (CRaTER) instrument of the NASA LRO (Lunar Reconnaisance Orbiter) mission. These measurements need to be complemented by surface measurements. Models and simulations that exist describe the approximate radiation exposure in space and on the lunar surface. The knowledge on the radiation exposure at the lunar surface is exclusively based on calculations applying radiation transport codes in combination with environmental models. Own calculations are presented using Monte-Carlo simulations to calculate the radiation environment on the moon and organ doses on the surface of the moon for an astronaut in an EVA suit and are compared with measurements. Since it is necessary to verify/validate such calculations with measurement on the lunar surface, a description is given of a radiation detector for future detailed surface measurements. This device is proposed for the ESA Lunar Lander Mission and is capable to characterize the radiation field concerning particle fluencies, dose rates and energy transfer spectra for ionizing particles and to measure the dose contribution of secondary neutrons. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Moon Radiation field Radiation exposure
1. Introduction Each celestial planet is embedded in the space radiation environment composed of energetic particles of galactic and solar origin. These particles are modulated by interstellar magnetic fields, the solar magnetic field and the magnetic field of the planet (if the latter exists and is strong enough to deflect charged particles) before they interact with the molecules and atoms of the planetary atmosphere and soil. The galactic cosmic rays are composed of energetic particles which cover a broad spectrum of
n
Corresponding author. Tel.: þ49 22 03 6013137. E-mail address: [email protected] (G. Reitz).
0032-0633/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2012.07.014
energy and mass values. About 98% are atomic nuclei and 2% electrons and positrons. The nuclear component consists of about 87% protons, about 12% helium ions and about 1% nuclei of Z42, the so-called HZE particles (Simpson, 1983). These nuclei are stripped off all their orbital electrons and have travelled for several million years through the galaxy before entering the solar system. When these charged particles enter the solar system, they interact with the outbound stream of the solar wind. Cosmic ray fluxes in the heliosphere are not constant; they vary between two extremes which correspond in time to the maximum and minimum solar activity. Solar activity and cosmic ray fluxes are anticorrelated, the maximum of the particle intensity occurs during solar minimum conditions and the minimum exposure is reached at times of large solar activity.
G. Reitz et al. / Planetary and Space Science 74 (2012) 78–83
Besides electromagnetic radiation, the sun emits continuously particle radiation, consisting mainly of protons and electrons, the so-called solar wind. The intensities of these low energy particles vary during the 11 year solar cycle by two orders of magnitude between around some 1010 and 1012 particles cm 2 s 1 sr 1 (Wilson, 1978). The related energies are so low (for a proton between a few hundred electron volt and a few kilo electron volt), that the particles are stopped within the first few hundred Angstrom of tissue or shielding material. Occasionally, the surface of the sun releases large amounts of energy in sudden local outbursts of gamma rays, hard and soft X-rays, radio waves in a wide frequency band and highly energetic particles, mainly protons. In these solar particle events (SPEs) large currents and moving magnetic fields in the solar corona accelerate solar matter (Bothmer and Zhukov, 2007). Coronal particles with energies up to several giga electron volt escape into the interplanetary space spiraling around the interplanetary magnetic field lines. Within the ecliptic plane field lines expand from the sun into the interplanetary medium like the beam of a rotating garden hose. They connect the Earth/moon system with a certain point on the western part of the sun. The number and energy spectrum of particles observed in SPEs at Earth among others depend on the size of the SPE and its location on the sun relative to this connection. SPEs show an enormous variability in particle flux and energy spectra and have the potential to expose space crew to life threatening doses. Their occurrence is linked to the solar cycle with a higher probability at the end of a solar maximum (Stassinopoulus, 1988; Kim et al., 2011). In the vicinity of a planet, the flux of primary nuclei may be modulated by the magnetic field of the planet, if existing. This causes, on the one hand, deflection of particles having low energies, the so-called cut-off effect; on the other hand, the capture of light solar wind particles and protons and electrons from the decay of neutrons produced by the primary particles with the atoms of the atmosphere, may lead to the formation of radiation belts of trapped particles. The Earth is surrounded by partly overlapping radiation belts consisting mainly of electrons and protons and to a much smaller extent of heavier ions. The trapped proton population expands from about 500 km to about 13,000 km above Earth’s surface and contains protons with energies up to a few hundred mega electron volt. The electron population is separated in an inner belt and an outer belt containing particles with energies up to a few mega electron volt (inner belt) and a few hundred mega electron volt (outer belt). In the so-called South Atlantic Anomaly (SAA) the trapped particles reach particularly low altitudes and lead to significantly increased radiation exposure at the orbit of the International Space Station (ISS). For fast transits, however, the radiation exposure from trapped particles is of minor importance. The moon has no global magnetic field, and therefore no radiation belt, although some surprisingly high local magnetic-field intensities are detected. Due to the missing magnetosphere and atmosphere particles of galactic and solar origin reach the surface of moon unattenuated. Dwellings or radiation shelters, e.g. built from regolith, are required to reduce the exposure to safe levels for prolonged human presence on the surface of the moon. Even much more of concern are solar particle events occurring during the cruise to moon where doses of more than 1 Sv can occur. The hazards due to SPEs have been identified as one of the major risks during human interplanetary missions. In colliding with the atoms and nuclei in lunar soil, the primary particles loose energy in ionization interactions and in nuclear interactions secondary radiation is generated, like hadrons (e.g. helium and heavier ions, protons, neutrons, p and K mesons) and leptons (e.g. muons and electrons) (Wilson et al., 1991). This causes a radiation environment comprising a complex
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mixture of primary and secondary particles of all types in a wide energy range. All particles may undergo further interactions or decay to other particles in the human body. An additional radiation exposure comes from emissions from the planetary surfaces due to the primordial radio nuclides 40K, 235U, the 238U series and the 232Th series (Surkov, 1981). The radio nuclides contribute to a total of 0.3 mSv/a to the dose equivalent on the Moon surface which is less that 1% of that resulting from cosmic rays. Despite of the numerous measurements and simulations performed in low Earth orbit and in interplanetary space our knowledge on the radiation exposure on the lunar surface is rather limited. The lunar lander mission can give important insights on the dose rates on the moon and will delivery valuable measurements for the validation of the simulated radiation exposures. In this work we summarize the current knowledge on the radiation exposure in lunar orbit from recent missions and from simulations performed for the lunar surface. Additionally, we present estimates of the radiation exposure by calculating organ dose rates based on Monte-Carlo simulations and using the ICRP human phantom in a low shielding environment on the lunar surface. In addition we present the design of the Moon Ionizing Radiation Sensor (MIRS) intended to be integrated in the lunar lander mission.
2. Materials and methods 2.1. Radiation exposures in space missions Numerous manned space missions have already been performed in which the astronaut’s exposure was determined by measurements. Based on these measurements effective doses as a quantity of radiation exposure were calculated by Cucinotta et al. (2001) and are given in Fig. 1 which summarizes the effective dose rates to be expected on a large number of different space flight missions. The highest doses were measured in the high altitude rocket and Shuttle flights, where a high contribution to the exposure is due to protons of the earth radiation belt. The dependence of the exposure on the solar activity is obvious for the Shuttle flights. The deep space mission of the Apollo flights range from 0.7 to about 3 mSv/d. No major exposures due to a SPE were experienced by astronauts so far; fortunately the large SPE (Ground Level Event/ GLE 24) occurred fortunately between Apollo 16 and 17 and not during the missions. GLE 24 would have led to severe radiation exposure of the Apollo astronauts. The exposure by primary particles on the lunar surface is expected to be about in the same scale as during the Apollo missions. Of course, there is a further modification of the exposure through differences in the solar activity conditions, the production of secondary particles in the shielding material and the lunar soil and the higher shadowing effect by the moon for galactic cosmic particles. Measurements in the moon subsurface has been performed for low energy neutrons only during Apollo (Woolum et al., 1975). Measurements in the moon orbit were provided recently by the Radiation Dose Monitor (RADOM) onboard the Chandrayaan-1 mission (Dachev et al., 2011). The spacecraft reached on November 12, 2008 its operational 100 km circular orbit. Measurements showed a dose rate of 0.227 mGy d 1 averaged over 3545 h of measurement time (20/11/2008–18/5/2009). During the last three months of the mission (20/05/2009–28/08/2009) the spacecraft reached a 200 km orbit. The dose rate increased to 0.257 mGy d 1 owing to the reduction of the lunar shadow effect for cosmic rays and to the increase of the cosmic ray flux related
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G. Reitz et al. / Planetary and Space Science 74 (2012) 78–83
Fig. 1. Effective dose rates calculated for a number of NASA missions during the last 40 years (Cucinotta et al., 2001).
to the reduced solar activity. The minimum of solar activity was reached shortly after the end of the mission in December 2009 and it is not expected that significantly larger radiation exposures due to galactic cosmic rays than the 0.257 mGy d 1 are to be expected in the specific orbits. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) (Spence et al., 2010a) is currently flying in lunar orbit aboard the Lunar Reconnaissance Orbiter (LRO) spacecraft; the nominal mapping orbit is 50 km up. CRaTER consists of a stack of six circular silicon detectors mounted in three pairs. The design includes slices of tissue-equivalent plastic and allows measurements corresponding to the depth of the blood-forming organs in the second pair, while the first pair provides the measurement of the incident cosmic radiation and the third pair also detects albedo particles from the moon surface. The science objectives in the Lunar Exploration Program (LEP) are described accordingly: ‘‘Characterization of the global lunar radiation environment and its biological impacts and potential mitigation, as well as investigation of shielding capabilities and validation of other deep space radiation mitigation strategies involving materials.’’ CRaTER measured a radiation exposure of about 0.22–0.27 mGy d 1 (Spence et al., 2010b). In order to determine the radiation exposure on the lunar surface numerically, the radiation environment in space described by models of the galactic cosmic rays, solar energetic particles and other sources adding to the exposure are used as an input for different radiation transport codes which calculate the transport through the shielding geometry and the target and the resulting dose quantities. Typically, either deterministic transport codes, e.g. HZETRN, based on a numerical solution of the Boltzmann transport equation or codes based on the Monte-Carlo technique (e.g. GEANT4, PHITS, and FLUKA) are used. The advantage of the application of numerical models is their capability to perform shielding studies or predictions for situations which are, at present, experimentally not accessible, such as the lunar surface. However, in order to rely on numerical modeling for the radiation exposure thorough benchmarking is inevitable. Wilson et al. (1997) and Clowdsley et al. (2003) have used the HZETRN code to estimate the radiation environment caused by GCR and SPE on the lunar surface and the resulting exposure of blood forming organs (BFO). They calculated the dose equivalent
rates behind a spherical shielding of different materials and thicknesses corresponding to areal densities between 0 g cm 2 and 100 g cm 2 from GCR during solar minimum and maximum and for a worst case of SPE scenario. The dose equivalent rate of the BFO for GCR particles is estimated to range from 0.3 mSv d 1 during solar maximum to 1 mSv d 1 during solar minimum with no or little shielding. The reduction of the exposure behind certain shielding is very much dependent on the shielding material with liquid hydrogen providing the best shielding properties and aluminum offering the least reduction of the materials under investigation for the same area density. For the worst case of SPE scenario the exposure of the BFO amounts to around 900 mSv with little or no shielding. This exposure is drastically reduced by applying different shielding materials and reaches values between 25 mSv (liquid hydrogen) and 150 mSv (Al) at 25 g/cm2 shielding and is further reduced to almost 0 mSv (liquid hydrogen) to 40 mSv (Al) for 100 g/cm2. 2.2. Numerical estimation of organ and effective doses In this work, in order to estimate the radiation exposure on the surface of the Moon numerical simulations were performed using the Monte-Carlo framework GEANT4. To obtain results for a worst-case scenario for the exposure caused by galactic cosmic rays, solar minimum conditions were chosen in combination with a minimal cylindrical shielding of 0.5 g/cm2 polycarbonate. The latter was taken as a simple description of a space suit based on information given by Anderson et al. (2003). Backscattered albedo particles were considered by including a lunar soil with a depth of 10 m. The atomic composition of the soil was implemented as described by Lindsay (1976) for Mare soil: 60.3% O, 0.4% Na, 5.1% Mg, 6.5% Al, 16.9% Si, 4.7% Ca, 1.1% Ti, and 4.4% Fe. The usage of the anthropomorphic ICRP male phantom (International Commission on Radiological Protection, 2004) allows calculating the exposure of individual organs as well as the effective dose as the weighted sum over all relevant organs. The simulation geometry is illustrated in the top panel of Fig. 2. The cylindrical shielding containing the phantom is irradiated from above with a source described by a half sphere with a radius of 2 m and indicated by the red half circle in the figure. The source
G. Reitz et al. / Planetary and Space Science 74 (2012) 78–83
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Fig. 3. Organ absorbed dose rates dD/dt (lower line) and dose equivalent rates dH/dt (upper line) from galactic cosmic rays for solar minimum conditions on the lunar surface.
Fig. 2. Top—Simulation scenario for the estimation of the radiation exposure on the surface of Moon. Bottom—Galactic cosmic ray energy spectra (Matthiae et al., under review) for selected nuclei during solar minimum. The oxygen and iron spectra are compared to ACE/CRIS data during the very deep solar minimum in the end of 2009. (For interpretation of the reference to color in this figure, the reader is referred to the web version of this article.)
particles are GCR nuclei from hydrogen to iron (atomic numbers Z ¼1–26) with energies between 10 MeV/n and 100 GeV/n. The energy distribution of H, He, O, and Fe is illustrated in the lower panel of Fig. 2 and compared to experimental data from the Cosmic ray isotope spectrometer (CRIS) instrument onboard the ACE spacecraft (Stone et al., 1998). CRIS measures the isotopic composition of cosmic rays and the spectra of nuclei between Li and Ni at an energy range between 100 MeV/n and 500 MeV/n. The energy spectra of the GCR nuclei are based on Matthiae et al. (under review) and were taken for the end of the year 2009 which was the time of the maximum of the particle intensity during the past very deep solar minimum. Particle intensities at that time exceeded the maxima reached during previous solar minimum periods and can be considered as the maximum GCR fluxes reached during at least the last 50 years. The model describing the CGR input spectra is derived from the ISO-model of GCR (ISO, 2004). The solar cycle dependent modulation in this model is derived from Oulu neutron monitor count rates and ACE measurements. The ICRP male phantom used in this work consists of almost 2 million voxel with sizes of 2.137 2.137 8.0 cm3 filled with 53 different materials and densities and grouped into 141 different organs (see International Commission on Radiological Protection 2004 for details). The energy deposition, dose and dose equivalent in each of these voxel is calculated for all primary particle types and the organs defined in the ICRP phantom are mapped and grouped together to the 15 organs contributing with different weighting factors to the effective dose (International Commission on Radiological Protection, 2007). The effective dose is calculated from the organ doses by applying the weighting factors to the values of the dose equivalent and by summing over all relevant organs.
3. Results and discussion The resulting absorbed dose rates and dose equivalent rates for the relevant organs are illustrated in Fig. 3. The daily absorbed
organ doses at the lunar surface during solar minimum behind a shielding of 0.5 g/cm2 are estimated to be between 0.16 mGy and 0.22 mGy, and the daily organ dose equivalent to be between 0.44 mSv and 0.82 mSv. The mean quality factor in the different organs lies between QE2.4 (bladder) and QE4.3 (skin). The effective dose rates are calculated from the dose equivalent rates in Fig. 3 by applying the tissue weighting factors recommended by International Commission on Radiological Protection (2007). The contributions of the different galactic cosmic ray nuclei to the total estimated effective dose rate, dE/ dt¼0.6 mSv d 1, are illustrated in absolute numbers in the top panel of Fig. 4 and in percentage contribution in the lower panel of Fig. 4. The main contributors are hydrogen (Z¼1, 0.19 mSv d 1, 31%), iron (Z ¼26, 0.10 mSv d 1, 16%) and helium (Z¼2, 0.056 mSv d 1, 9.4%) followed by oxygen (Z ¼8, 0.039 mSv d 1, 6.4%), silicon (Z ¼14, 0.029, 4.8%), magnesium (Z ¼12, 0.026 mSv d 1, 4.4%) and chromium (Z ¼24, 0.025 mSv d 1, 4.1%). The backscattered albedo particles including neutrons from the lunar soil are contained in the contribution of the primary particle producing the backscattering. Dachev et al. (2011) measured a dose rate in silicon of E0.26 mGy d 1 in a 200 km lunar orbit for the time period between June 2009 and August 2009. In order to compare the results of this work with the measurements, the differences in the shadowing effect have to be accounted for. The ratio of the solid angles on ground and in 200 km altitude not shadowed by the moon is 0.69. Additionally, the measured dose rates in silicon can be approximately converted to dose rates in water or tissue by applying a factor of 1.23 (Beaujean et al., 2002). If these scaling factors are applied to the measurements by Dachev et al., the result is a dose rate of about 0.22 mGy d 1 which is close to the estimated values of the organ doses (between 0.16 mGy d 1 and 0.22 mGy d 1). Final validation of the calculated doses can only be done with data from direct measurements on the lunar surface. A detector system having in hands was developed for the ExoMars Mission and will serve as prototype for a Moon Ionizing Radiation Sensor (MIRS). In the current design the sensor head consists of four segmented planar silicon PIN-detectors (300 mm thickness). Each detector is divided in an inner and outer sector. The distance between the upper two detectors is 25 mm and defines a 601 half angle zenith-pointing field of view for charged particle. Detectors B–D are mounted very tight together. Requiring coincidence between measurements in detectors A–C, only
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The detector head including charge sensitive amplifiers is located in the upper part of the container in Fig. 5. The instrument is composed of a prism shaped box with a size of 133 123 96 mm3 housing the detector head (upper part) and the electronics (lower part). The mass of the current system is 600 g; the power consumption about 4 W. The energy range covered is from 75 keV to 270 MeV. The corresponding linear energy transfer (LET) is 0.11 to 424 keV/mm. The instrument delivers time resolved data of charged and neutral components of galactic and solar particle radiation, with their dose rates and Linear Energy Transfer (LET) spectra, in order to provide the information required for establishing radiation protection guidelines for future human interplanetary missions.
4. Summary In summary, only results from two missions in Lunar orbit are available with dose values depending on altitude of around 200– 300 mGy d 1. The current knowledge about the radiation environment on the surface of moon is exclusively based on calculations using radiation transport models with input parameters from models for the galactic cosmic ray spectra and for solar particle events. We presented new calculations for the radiation exposure and described an instrument which can provide the data needed to validate calculations. The ESA Lunar lander mission provides the ideal platform for detailed radiation measurements on the surface of Moon. The instrument will provide, for the first time, a detailed, continuous and long-term characterization of the lunar radiation environment and is essential for placing in context measurements performed by the rest of the scientific payload, assessing lunar surface habitability, identifying and quantifying hazards to humans on Moon and providing ground truth for lunar environment models. References Fig. 4. Effective dose rates dE/dt from galactic cosmic rays for solar minimum conditions on the lunar surface from different GCR nuclei with atomic number Z (top) and their percentage contribution (bottom).
Fig. 5. Conceptual design of the MIRS instrument (left) and the detection principle (right).
those particles entering from the top or from below can be selected. For these, the path length in the detectors is known sufficiently well so that the linear energy transfer (LET) can be measured accurately enough for dosimetry purposes. The inner part of the third detector records neutron interactions by using its outer part plus the second and fourth detector as anticoincidence. Two additional PIN diodes are added in the electronic part beneath the detector head to provide measurements in the x and y directions. Using the inner segment of the C detector in anti-coincidence with all other detectors allow to measure the flux and dose from neutral particles (neutrons and gamma rays).
Anderson, B.M., Nealy, J.E., Kim, M.H., Qualls, G.D., Wilson, J.W., 2003. Analysis of a radiation model of the shuttle space suit. NASA/TP. Beaujean, R., Kopp, J., Burmeister, S., Petersen, F., Reitz, G., 2002. Dosimetry inside MIR station using a silicon detector telescope (DOSTEL). Radiation Measurements 35, 433–438. Bothmer, V., Zhukov, A., 2007. The sun as the prime source of space weather. In: Bothmer, V., Daglis, I.A. (Eds.), Space Weather Physics and Effects. SpringerPraxis Books in Environmental Sciences, Heidelberg, pp. 31–92. Clowdsley, M.S., De Angelis, G., Badavi, F.F., Wilson, J.W., Singleterry, R., Thibeault, S.A., 2003. Surface environments for exploration. In: AIP Conference Proceedings, Space Technology and Applications Forum—Staif, vol. 654, pp. 1034– 1045. Cucinotta, F.A., Schimmerling, W., Wilson, J.W., Peterson, L.E., Badhwar, G.D., Saganti, P.B., Dicello, J.F., 2001. Space Radiation Cancer Risk Projections for Explorative Missions: Uncertainty Reduction and Mitigation, NASA JSC Document (JSC-29295). NASA, Washington, DC. Dachev, T.P., Tomov, B.T., Matviichuk, Yu,N., Dimitrov, P.S., Vadawale, S.V., Goswami, J.N., De Angelis, G., Girish, V., 2011. An overview of RADOM results for earth and moon radiation environment on Chandrayaan-1 satellite. Advances in Space Research 48, 779–791. International Commission on Radiological Protection, 2004. Adult Reference Computational Phantom, ICRP Publication 110, Annals of the ICRP. Elsevier. International Commission on Radiological Protection, 2007. The 2007 Recommendations of the International Commission on Radiological Protection, ICRP Publication 103. Elsevier. ISO 15390, 2004. Space Environment (natural and artificial)—galactic cosmic ray model. Kim, Y., De Angelis, G., Cucinotta, F.A., 2011. Probalistic assessment of radiation risk of astronauts in space missions. Acta Astronautica 68, 747–759. Lindsay, J.F., 1976. Lunar Stratigraphy and Sedimentology,. Elsevier, ISBN0-444414443-6. Matthiae, D., Mrigakshi, A., Berger, T., Reitz, G. A ready-to-use galactic cosmic ray model. Advances in Space Research, under review. Surkov, Yu.A., 1981. Radioactivity of the moon and planets. Advances in Space Research 1 (1), 21–38. Simpson, J.A., 1983. Elemental and isotopic composition of the galactic cosmic rays. Annual Review of Nuclear and Particle Science 33, 323–381.
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Contents lists available at SciVerse ScienceDirect
Planetary and Space Science journal homepage: www.elsevier.com/locate/pss
Radiation exposure in the moon environment Guenther Reitz n, Thomas Berger, Daniel Matthiae German Aerospace Center, Linder Hoehe, 51147 Koeln, Germany
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 March 2012 Received in revised form 7 July 2012 Accepted 13 July 2012 Available online 31 July 2012
During a stay on the moon humans are exposed to elevated radiation levels due to the lack of substantial atmospheric and magnetic shielding compared to the Earth’s surface. The absence of magnetic and atmospheric shielding allows cosmic rays of all energies to impinge on the lunar surface. Beside the continuous exposure to galactic cosmic rays (GCR), which increases the risk of cancer mortality, exposure through particles emitted in sudden nonpredictable solar particle events (SPE) may occur. SPEs show an enormous variability in particle flux and energy spectra and have the potential to expose space crew to life threatening doses. On Earth, the contribution to the annual terrestrial dose of natural ionizing radiation of 2.4 mSv by cosmic radiation is about 1/6, whereas the annual exposure caused by GCR on the lunar surface is roughly 380 mSv (solar minimum) and 110 mSv (solar maximum). The analysis of worst case scenarios has indicated that SPE may lead to an exposure of about 1 Sv. The only efficient measure to reduce radiation exposure is the provision of radiation shelters. Measurements on the lunar surface performed during the Apollo missions cover only a small energy band for thermal neutrons and are not sufficient to estimate the exposure. Very recently some data were added by the Radiation Dose Monitoring (RADOM) instrument operated during the Indian Chandrayaan Mission and the Cosmic Ray Telescope (CRaTER) instrument of the NASA LRO (Lunar Reconnaisance Orbiter) mission. These measurements need to be complemented by surface measurements. Models and simulations that exist describe the approximate radiation exposure in space and on the lunar surface. The knowledge on the radiation exposure at the lunar surface is exclusively based on calculations applying radiation transport codes in combination with environmental models. Own calculations are presented using Monte-Carlo simulations to calculate the radiation environment on the moon and organ doses on the surface of the moon for an astronaut in an EVA suit and are compared with measurements. Since it is necessary to verify/validate such calculations with measurement on the lunar surface, a description is given of a radiation detector for future detailed surface measurements. This device is proposed for the ESA Lunar Lander Mission and is capable to characterize the radiation field concerning particle fluencies, dose rates and energy transfer spectra for ionizing particles and to measure the dose contribution of secondary neutrons. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Moon Radiation field Radiation exposure
1. Introduction Each celestial planet is embedded in the space radiation environment composed of energetic particles of galactic and solar origin. These particles are modulated by interstellar magnetic fields, the solar magnetic field and the magnetic field of the planet (if the latter exists and is strong enough to deflect charged particles) before they interact with the molecules and atoms of the planetary atmosphere and soil. The galactic cosmic rays are composed of energetic particles which cover a broad spectrum of
n
Corresponding author. Tel.: þ49 22 03 6013137. E-mail address: [email protected] (G. Reitz).
0032-0633/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pss.2012.07.014
energy and mass values. About 98% are atomic nuclei and 2% electrons and positrons. The nuclear component consists of about 87% protons, about 12% helium ions and about 1% nuclei of Z42, the so-called HZE particles (Simpson, 1983). These nuclei are stripped off all their orbital electrons and have travelled for several million years through the galaxy before entering the solar system. When these charged particles enter the solar system, they interact with the outbound stream of the solar wind. Cosmic ray fluxes in the heliosphere are not constant; they vary between two extremes which correspond in time to the maximum and minimum solar activity. Solar activity and cosmic ray fluxes are anticorrelated, the maximum of the particle intensity occurs during solar minimum conditions and the minimum exposure is reached at times of large solar activity.
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Besides electromagnetic radiation, the sun emits continuously particle radiation, consisting mainly of protons and electrons, the so-called solar wind. The intensities of these low energy particles vary during the 11 year solar cycle by two orders of magnitude between around some 1010 and 1012 particles cm 2 s 1 sr 1 (Wilson, 1978). The related energies are so low (for a proton between a few hundred electron volt and a few kilo electron volt), that the particles are stopped within the first few hundred Angstrom of tissue or shielding material. Occasionally, the surface of the sun releases large amounts of energy in sudden local outbursts of gamma rays, hard and soft X-rays, radio waves in a wide frequency band and highly energetic particles, mainly protons. In these solar particle events (SPEs) large currents and moving magnetic fields in the solar corona accelerate solar matter (Bothmer and Zhukov, 2007). Coronal particles with energies up to several giga electron volt escape into the interplanetary space spiraling around the interplanetary magnetic field lines. Within the ecliptic plane field lines expand from the sun into the interplanetary medium like the beam of a rotating garden hose. They connect the Earth/moon system with a certain point on the western part of the sun. The number and energy spectrum of particles observed in SPEs at Earth among others depend on the size of the SPE and its location on the sun relative to this connection. SPEs show an enormous variability in particle flux and energy spectra and have the potential to expose space crew to life threatening doses. Their occurrence is linked to the solar cycle with a higher probability at the end of a solar maximum (Stassinopoulus, 1988; Kim et al., 2011). In the vicinity of a planet, the flux of primary nuclei may be modulated by the magnetic field of the planet, if existing. This causes, on the one hand, deflection of particles having low energies, the so-called cut-off effect; on the other hand, the capture of light solar wind particles and protons and electrons from the decay of neutrons produced by the primary particles with the atoms of the atmosphere, may lead to the formation of radiation belts of trapped particles. The Earth is surrounded by partly overlapping radiation belts consisting mainly of electrons and protons and to a much smaller extent of heavier ions. The trapped proton population expands from about 500 km to about 13,000 km above Earth’s surface and contains protons with energies up to a few hundred mega electron volt. The electron population is separated in an inner belt and an outer belt containing particles with energies up to a few mega electron volt (inner belt) and a few hundred mega electron volt (outer belt). In the so-called South Atlantic Anomaly (SAA) the trapped particles reach particularly low altitudes and lead to significantly increased radiation exposure at the orbit of the International Space Station (ISS). For fast transits, however, the radiation exposure from trapped particles is of minor importance. The moon has no global magnetic field, and therefore no radiation belt, although some surprisingly high local magnetic-field intensities are detected. Due to the missing magnetosphere and atmosphere particles of galactic and solar origin reach the surface of moon unattenuated. Dwellings or radiation shelters, e.g. built from regolith, are required to reduce the exposure to safe levels for prolonged human presence on the surface of the moon. Even much more of concern are solar particle events occurring during the cruise to moon where doses of more than 1 Sv can occur. The hazards due to SPEs have been identified as one of the major risks during human interplanetary missions. In colliding with the atoms and nuclei in lunar soil, the primary particles loose energy in ionization interactions and in nuclear interactions secondary radiation is generated, like hadrons (e.g. helium and heavier ions, protons, neutrons, p and K mesons) and leptons (e.g. muons and electrons) (Wilson et al., 1991). This causes a radiation environment comprising a complex
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mixture of primary and secondary particles of all types in a wide energy range. All particles may undergo further interactions or decay to other particles in the human body. An additional radiation exposure comes from emissions from the planetary surfaces due to the primordial radio nuclides 40K, 235U, the 238U series and the 232Th series (Surkov, 1981). The radio nuclides contribute to a total of 0.3 mSv/a to the dose equivalent on the Moon surface which is less that 1% of that resulting from cosmic rays. Despite of the numerous measurements and simulations performed in low Earth orbit and in interplanetary space our knowledge on the radiation exposure on the lunar surface is rather limited. The lunar lander mission can give important insights on the dose rates on the moon and will delivery valuable measurements for the validation of the simulated radiation exposures. In this work we summarize the current knowledge on the radiation exposure in lunar orbit from recent missions and from simulations performed for the lunar surface. Additionally, we present estimates of the radiation exposure by calculating organ dose rates based on Monte-Carlo simulations and using the ICRP human phantom in a low shielding environment on the lunar surface. In addition we present the design of the Moon Ionizing Radiation Sensor (MIRS) intended to be integrated in the lunar lander mission.
2. Materials and methods 2.1. Radiation exposures in space missions Numerous manned space missions have already been performed in which the astronaut’s exposure was determined by measurements. Based on these measurements effective doses as a quantity of radiation exposure were calculated by Cucinotta et al. (2001) and are given in Fig. 1 which summarizes the effective dose rates to be expected on a large number of different space flight missions. The highest doses were measured in the high altitude rocket and Shuttle flights, where a high contribution to the exposure is due to protons of the earth radiation belt. The dependence of the exposure on the solar activity is obvious for the Shuttle flights. The deep space mission of the Apollo flights range from 0.7 to about 3 mSv/d. No major exposures due to a SPE were experienced by astronauts so far; fortunately the large SPE (Ground Level Event/ GLE 24) occurred fortunately between Apollo 16 and 17 and not during the missions. GLE 24 would have led to severe radiation exposure of the Apollo astronauts. The exposure by primary particles on the lunar surface is expected to be about in the same scale as during the Apollo missions. Of course, there is a further modification of the exposure through differences in the solar activity conditions, the production of secondary particles in the shielding material and the lunar soil and the higher shadowing effect by the moon for galactic cosmic particles. Measurements in the moon subsurface has been performed for low energy neutrons only during Apollo (Woolum et al., 1975). Measurements in the moon orbit were provided recently by the Radiation Dose Monitor (RADOM) onboard the Chandrayaan-1 mission (Dachev et al., 2011). The spacecraft reached on November 12, 2008 its operational 100 km circular orbit. Measurements showed a dose rate of 0.227 mGy d 1 averaged over 3545 h of measurement time (20/11/2008–18/5/2009). During the last three months of the mission (20/05/2009–28/08/2009) the spacecraft reached a 200 km orbit. The dose rate increased to 0.257 mGy d 1 owing to the reduction of the lunar shadow effect for cosmic rays and to the increase of the cosmic ray flux related
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Fig. 1. Effective dose rates calculated for a number of NASA missions during the last 40 years (Cucinotta et al., 2001).
to the reduced solar activity. The minimum of solar activity was reached shortly after the end of the mission in December 2009 and it is not expected that significantly larger radiation exposures due to galactic cosmic rays than the 0.257 mGy d 1 are to be expected in the specific orbits. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) (Spence et al., 2010a) is currently flying in lunar orbit aboard the Lunar Reconnaissance Orbiter (LRO) spacecraft; the nominal mapping orbit is 50 km up. CRaTER consists of a stack of six circular silicon detectors mounted in three pairs. The design includes slices of tissue-equivalent plastic and allows measurements corresponding to the depth of the blood-forming organs in the second pair, while the first pair provides the measurement of the incident cosmic radiation and the third pair also detects albedo particles from the moon surface. The science objectives in the Lunar Exploration Program (LEP) are described accordingly: ‘‘Characterization of the global lunar radiation environment and its biological impacts and potential mitigation, as well as investigation of shielding capabilities and validation of other deep space radiation mitigation strategies involving materials.’’ CRaTER measured a radiation exposure of about 0.22–0.27 mGy d 1 (Spence et al., 2010b). In order to determine the radiation exposure on the lunar surface numerically, the radiation environment in space described by models of the galactic cosmic rays, solar energetic particles and other sources adding to the exposure are used as an input for different radiation transport codes which calculate the transport through the shielding geometry and the target and the resulting dose quantities. Typically, either deterministic transport codes, e.g. HZETRN, based on a numerical solution of the Boltzmann transport equation or codes based on the Monte-Carlo technique (e.g. GEANT4, PHITS, and FLUKA) are used. The advantage of the application of numerical models is their capability to perform shielding studies or predictions for situations which are, at present, experimentally not accessible, such as the lunar surface. However, in order to rely on numerical modeling for the radiation exposure thorough benchmarking is inevitable. Wilson et al. (1997) and Clowdsley et al. (2003) have used the HZETRN code to estimate the radiation environment caused by GCR and SPE on the lunar surface and the resulting exposure of blood forming organs (BFO). They calculated the dose equivalent
rates behind a spherical shielding of different materials and thicknesses corresponding to areal densities between 0 g cm 2 and 100 g cm 2 from GCR during solar minimum and maximum and for a worst case of SPE scenario. The dose equivalent rate of the BFO for GCR particles is estimated to range from 0.3 mSv d 1 during solar maximum to 1 mSv d 1 during solar minimum with no or little shielding. The reduction of the exposure behind certain shielding is very much dependent on the shielding material with liquid hydrogen providing the best shielding properties and aluminum offering the least reduction of the materials under investigation for the same area density. For the worst case of SPE scenario the exposure of the BFO amounts to around 900 mSv with little or no shielding. This exposure is drastically reduced by applying different shielding materials and reaches values between 25 mSv (liquid hydrogen) and 150 mSv (Al) at 25 g/cm2 shielding and is further reduced to almost 0 mSv (liquid hydrogen) to 40 mSv (Al) for 100 g/cm2. 2.2. Numerical estimation of organ and effective doses In this work, in order to estimate the radiation exposure on the surface of the Moon numerical simulations were performed using the Monte-Carlo framework GEANT4. To obtain results for a worst-case scenario for the exposure caused by galactic cosmic rays, solar minimum conditions were chosen in combination with a minimal cylindrical shielding of 0.5 g/cm2 polycarbonate. The latter was taken as a simple description of a space suit based on information given by Anderson et al. (2003). Backscattered albedo particles were considered by including a lunar soil with a depth of 10 m. The atomic composition of the soil was implemented as described by Lindsay (1976) for Mare soil: 60.3% O, 0.4% Na, 5.1% Mg, 6.5% Al, 16.9% Si, 4.7% Ca, 1.1% Ti, and 4.4% Fe. The usage of the anthropomorphic ICRP male phantom (International Commission on Radiological Protection, 2004) allows calculating the exposure of individual organs as well as the effective dose as the weighted sum over all relevant organs. The simulation geometry is illustrated in the top panel of Fig. 2. The cylindrical shielding containing the phantom is irradiated from above with a source described by a half sphere with a radius of 2 m and indicated by the red half circle in the figure. The source
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Fig. 3. Organ absorbed dose rates dD/dt (lower line) and dose equivalent rates dH/dt (upper line) from galactic cosmic rays for solar minimum conditions on the lunar surface.
Fig. 2. Top—Simulation scenario for the estimation of the radiation exposure on the surface of Moon. Bottom—Galactic cosmic ray energy spectra (Matthiae et al., under review) for selected nuclei during solar minimum. The oxygen and iron spectra are compared to ACE/CRIS data during the very deep solar minimum in the end of 2009. (For interpretation of the reference to color in this figure, the reader is referred to the web version of this article.)
particles are GCR nuclei from hydrogen to iron (atomic numbers Z ¼1–26) with energies between 10 MeV/n and 100 GeV/n. The energy distribution of H, He, O, and Fe is illustrated in the lower panel of Fig. 2 and compared to experimental data from the Cosmic ray isotope spectrometer (CRIS) instrument onboard the ACE spacecraft (Stone et al., 1998). CRIS measures the isotopic composition of cosmic rays and the spectra of nuclei between Li and Ni at an energy range between 100 MeV/n and 500 MeV/n. The energy spectra of the GCR nuclei are based on Matthiae et al. (under review) and were taken for the end of the year 2009 which was the time of the maximum of the particle intensity during the past very deep solar minimum. Particle intensities at that time exceeded the maxima reached during previous solar minimum periods and can be considered as the maximum GCR fluxes reached during at least the last 50 years. The model describing the CGR input spectra is derived from the ISO-model of GCR (ISO, 2004). The solar cycle dependent modulation in this model is derived from Oulu neutron monitor count rates and ACE measurements. The ICRP male phantom used in this work consists of almost 2 million voxel with sizes of 2.137 2.137 8.0 cm3 filled with 53 different materials and densities and grouped into 141 different organs (see International Commission on Radiological Protection 2004 for details). The energy deposition, dose and dose equivalent in each of these voxel is calculated for all primary particle types and the organs defined in the ICRP phantom are mapped and grouped together to the 15 organs contributing with different weighting factors to the effective dose (International Commission on Radiological Protection, 2007). The effective dose is calculated from the organ doses by applying the weighting factors to the values of the dose equivalent and by summing over all relevant organs.
3. Results and discussion The resulting absorbed dose rates and dose equivalent rates for the relevant organs are illustrated in Fig. 3. The daily absorbed
organ doses at the lunar surface during solar minimum behind a shielding of 0.5 g/cm2 are estimated to be between 0.16 mGy and 0.22 mGy, and the daily organ dose equivalent to be between 0.44 mSv and 0.82 mSv. The mean quality factor in the different organs lies between QE2.4 (bladder) and QE4.3 (skin). The effective dose rates are calculated from the dose equivalent rates in Fig. 3 by applying the tissue weighting factors recommended by International Commission on Radiological Protection (2007). The contributions of the different galactic cosmic ray nuclei to the total estimated effective dose rate, dE/ dt¼0.6 mSv d 1, are illustrated in absolute numbers in the top panel of Fig. 4 and in percentage contribution in the lower panel of Fig. 4. The main contributors are hydrogen (Z¼1, 0.19 mSv d 1, 31%), iron (Z ¼26, 0.10 mSv d 1, 16%) and helium (Z¼2, 0.056 mSv d 1, 9.4%) followed by oxygen (Z ¼8, 0.039 mSv d 1, 6.4%), silicon (Z ¼14, 0.029, 4.8%), magnesium (Z ¼12, 0.026 mSv d 1, 4.4%) and chromium (Z ¼24, 0.025 mSv d 1, 4.1%). The backscattered albedo particles including neutrons from the lunar soil are contained in the contribution of the primary particle producing the backscattering. Dachev et al. (2011) measured a dose rate in silicon of E0.26 mGy d 1 in a 200 km lunar orbit for the time period between June 2009 and August 2009. In order to compare the results of this work with the measurements, the differences in the shadowing effect have to be accounted for. The ratio of the solid angles on ground and in 200 km altitude not shadowed by the moon is 0.69. Additionally, the measured dose rates in silicon can be approximately converted to dose rates in water or tissue by applying a factor of 1.23 (Beaujean et al., 2002). If these scaling factors are applied to the measurements by Dachev et al., the result is a dose rate of about 0.22 mGy d 1 which is close to the estimated values of the organ doses (between 0.16 mGy d 1 and 0.22 mGy d 1). Final validation of the calculated doses can only be done with data from direct measurements on the lunar surface. A detector system having in hands was developed for the ExoMars Mission and will serve as prototype for a Moon Ionizing Radiation Sensor (MIRS). In the current design the sensor head consists of four segmented planar silicon PIN-detectors (300 mm thickness). Each detector is divided in an inner and outer sector. The distance between the upper two detectors is 25 mm and defines a 601 half angle zenith-pointing field of view for charged particle. Detectors B–D are mounted very tight together. Requiring coincidence between measurements in detectors A–C, only
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The detector head including charge sensitive amplifiers is located in the upper part of the container in Fig. 5. The instrument is composed of a prism shaped box with a size of 133 123 96 mm3 housing the detector head (upper part) and the electronics (lower part). The mass of the current system is 600 g; the power consumption about 4 W. The energy range covered is from 75 keV to 270 MeV. The corresponding linear energy transfer (LET) is 0.11 to 424 keV/mm. The instrument delivers time resolved data of charged and neutral components of galactic and solar particle radiation, with their dose rates and Linear Energy Transfer (LET) spectra, in order to provide the information required for establishing radiation protection guidelines for future human interplanetary missions.
4. Summary In summary, only results from two missions in Lunar orbit are available with dose values depending on altitude of around 200– 300 mGy d 1. The current knowledge about the radiation environment on the surface of moon is exclusively based on calculations using radiation transport models with input parameters from models for the galactic cosmic ray spectra and for solar particle events. We presented new calculations for the radiation exposure and described an instrument which can provide the data needed to validate calculations. The ESA Lunar lander mission provides the ideal platform for detailed radiation measurements on the surface of Moon. The instrument will provide, for the first time, a detailed, continuous and long-term characterization of the lunar radiation environment and is essential for placing in context measurements performed by the rest of the scientific payload, assessing lunar surface habitability, identifying and quantifying hazards to humans on Moon and providing ground truth for lunar environment models. References Fig. 4. Effective dose rates dE/dt from galactic cosmic rays for solar minimum conditions on the lunar surface from different GCR nuclei with atomic number Z (top) and their percentage contribution (bottom).
Fig. 5. Conceptual design of the MIRS instrument (left) and the detection principle (right).
those particles entering from the top or from below can be selected. For these, the path length in the detectors is known sufficiently well so that the linear energy transfer (LET) can be measured accurately enough for dosimetry purposes. The inner part of the third detector records neutron interactions by using its outer part plus the second and fourth detector as anticoincidence. Two additional PIN diodes are added in the electronic part beneath the detector head to provide measurements in the x and y directions. Using the inner segment of the C detector in anti-coincidence with all other detectors allow to measure the flux and dose from neutral particles (neutrons and gamma rays).
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