Cosmic Ray Telescope For The Effects Of Radiation Research Articles

Signatures of volatiles in the lunar proton albedo

We find evidence for hydrated material in the lunar regolith using “albedo protons” measured with the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO). Fluxes of these albedo protons, which are emitted from the regolith due to steady bombardment by high energy radiation (Galactic Cosmic Rays), are observed to peak near the poles, and are inconsistent with the latitude trends of heavy element enrichment (e.g., enhanced Fe abundance). The latitudinal distribution of albedo protons anti-correlates with that of epithermal or high energy neutrons. The high latitude enhancement may be due to the conversion of upward directed secondary neutrons from the lunar regolith into tertiary protons due to neutron–proton collisions in hydrated regolith that is more prevalent near the poles. The CRaTER instrument may thus provide important measurements of volatile distributions within regolith at the Moon and potentially, with similar sensors and observations, at other bodies within the Solar System.

Deep dielectric charging and breakdown of lunar polar regolith

Galactic cosmic rays (GCRs) and solar energetic particles (SEPs) penetrate the regolith (layer of soil and dust) covering the Moon's surface and cause deep dielectric charging. To gain insight into this process, we have developed a data-driven, deep dielectric charging model using data from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER), which is onboard the Lunar Reconnaissance Orbiter (LRO), and the Electron, Proton, and Alpha Monitor (EPAM) on the Advanced Composition Explorer (ACE). The model results indicate that GCRs produce a persistent electric field (∼700 V m-1) within the top tens of centimeters of regolith, while large SEP events could potentially generate episodic subsurface electric fields (≥ 106 V m-1) capable of causing dielectric breakdown within the top millimeter of regolith. We also propose that this “breakdown weathering” may have significantly affected the regolith in the Moon's permanently shadowed regions (PSRs).

Update on Radiation Dose From Galactic and Solar Protons at the Moon Using the LRO/CRaTER Microdosimeter

The NASA Lunar Reconnaissance Orbiter (LRO) has been exploring the lunar surface and radiation environment since June 2009. In Mazur et al. [2011] we discussed the first 6months of mission data from a microdosimeter that is housed within the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument onboard LRO. The CRaTER microdosimeter is an early version of what is now a commercially available hybrid that accurately measures total ionizing radiation dose in a silicon target (http://www.teledynemicro.com/product/radiationdosimeter). This brief report updates the transition from a deep solar minimum radiation environment to the current weak solar maximum as witnessed with the microdosimeter. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) microdosimeter is behind about 4.4 g/cm equivalent aluminum, corresponding to the range of ~55MeV protons. Energetic protons are the dominant source of ionizing radiation at the Moon. The measured dose includes contributions from the low-energy part of the galactic cosmic ray proton spectrum and the high-energy portion of typical solar energetic particle events. Figure 1 is the history to date of the total ionizing dose averaged over 12 h intervals. The figure includes the entire Lunar Reconnaissance Orbiter (LRO) orbital history that ranged from ~50 km circular tomore elliptical orbits with aposelene above ~150 km. We did not correct the observed dose rate for the changing amount of solid angle blocked by the Moon in Figure 1 as one would need to do to derive an interplanetary rate. When averaged over 12 h, the variations among the various orbit modes have less than a 5% and 20% effect on the galactic cosmic ray dose rate and peak solar proton dose rate, respectively.

Does the worsening galactic cosmic radiation environment observed by CRaTER preclude future manned deep space exploration?

AbstractThe Sun and its solar wind are currently exhibiting extremely low densities and magnetic field strengths, representing states that have never been observed during the space age. The highly abnormal solar activity between cycles 23 and 24 has caused the longest solar minimum in over 80 years and continues into the unusually small solar maximum of cycle 24. As a result of the remarkably weak solar activity, we have also observed the highest fluxes of galactic cosmic rays in the space age and relatively small solar energetic particle events. We use observations from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter to examine the implications of these highly unusual solar conditions for human space exploration. We show that while these conditions are not a show stopper for long‐duration missions (e.g., to the Moon, an asteroid, or Mars), galactic cosmic ray radiation remains a significant and worsening factor that limits mission durations. While solar energetic particle events in cycle 24 present some hazard, the accumulated doses for astronauts behind 10 g/cm2 shielding are well below current dose limits. Galactic cosmic radiation presents a more significant challenge: the time to 3% risk of exposure‐induced death (REID) in interplanetary space was less than 400 days for a 30 year old male and less than 300 days for a 30 year old female in the last cycle 23–24 minimum. The time to 3% REID is estimated to be ∼20% lower in the coming cycle 24–25 minimum. If the heliospheric magnetic field continues to weaken over time, as is likely, then allowable mission durations will decrease correspondingly. Thus, we estimate exposures in extreme solar minimum conditions and the corresponding effects on allowable durations.

Radiation environment at the Moon: Comparisons of transport code modeling and measurements from the CRaTER instrument

AbstractThe Cosmic Ray Telescope for the Effects of Radiation (CRaTER), an instrument carried on the Lunar Reconnaissance Orbiter spacecraft, directly measures the energy depositions by solar and galactic cosmic radiations in its silicon wafer detectors. These energy depositions are converted to linear energy transfer (LET) spectra. High LET particles, which are mainly high‐energy heavy ions found in the incident cosmic ray spectrum, or target fragments and recoils produced by protons and heavier ions, are of particular importance because of their potential to cause significant damage to human tissue and electronic components. Aside from providing LET data useful for space radiation risk analyses for lunar missions, the observed LET spectra can also be used to help validate space radiation transport codes, used for shielding design and risk assessment applications, which is a major thrust of this work. In this work the Monte Carlo transport code HETC‐HEDS (High‐Energy Transport Code‐Human Exploration and Development in Space) is used to estimate LET contributions from the incident primary ions and their charged secondaries produced by nuclear collisions as they pass through the three pairs of silicon detectors. Also in this work, the contributions to the LET of the primary ions and their charged secondaries are analyzed and compared with estimates obtained using the deterministic space radiation code HZETRN 2010, developed at NASA Langley Research Center. LET estimates obtained from the two transport codes are compared with measurements of LET from the CRaTER instrument during the mission. Overall, a comparison of the LET predictions of the HETC‐HEDS code to the predictions of the HZETRN code displays good agreement. The code predictions are also in good agreement with the CRaTER LET measurements above 15 keV/µm but differ from the measurements for smaller values of LET. A possible reason for this disagreement between measured and calculated spectra below 15 keV/µm is an inadequate representation of the light ion spectra in HETC‐HEDS and HZETRN code calculations. It is also clear from the results of this work that Vavilov distributions need to be incorporated into the HETC‐HJEDS code before it will be able to recreate the observed LET spectra measured by the CRaTER instrument.

Lunar cosmic ray radiation environments during Luna and Lunar Reconnaissance Orbiter missions

The RV-2N-series instruments onboard Luna missions and the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument onboard Lunar Reconnaissance Orbiter (LRO) were designed to characterize the global lunar radiation environment and its biological impacts by measuring cosmic ray (CR) intensity. In this study, we have shown that the RV-2N-series instruments onboard of Russian Luna missions and the CRaTER reliably detect both background CRs and solar proton events (SPEs) in the lunar radiation environment using the proton intensity measured by the RV-2N-series onboard Luna missions out of the Russian Luna program for the exploration of the Moon (November 1970–August 1975) and the CR intensity on the Moon observed by the CRaTER (June 2009–March 2011). Those were compared with the CR intensities observed by neutron monitors (McMurdo, Thule, Oulu) on the Earth. The sunspot number is used as the index of solar activity (NOAA National Geophysical Data Center). As a result, the background CR intensities on the Moon turned out to have a good anti-correlation with the solar activity. We have also identified the proton intensity increasing events on the Moon which have the similar profiles to those observed by neutron monitors on the Earth. Most of these events show the significant increase of proton intensities in the lunar radiation environment when the SPEs associated with solar eruptions are verified. Therefore, most of the proton intensity increasing events are associated with the energetic solar particles in the lunar environment.

Radiation modeling in the Earth and Mars atmospheres using LRO/CRaTER with the EMMREM Module

We expand upon the efforts of Joyce et al. (2013), who computed the modulation potential at the Moon using measurements from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument on the Lunar Reconnaissance Orbiter (LRO) spacecraft along with data products from the Earth-Moon-Mars Radiation Environment Module (EMMREM). Using the computed modulation potential, we calculate galactic cosmic ray (GCR) dose and dose equivalent rates in the Earth and Mars atmospheres for various altitudes over the course of the LRO mission. While we cannot validate these predictions by directly comparable measurement, we find that our results conform to expectations and are in good agreement with the nearest available measurements and therefore may be used as reasonable estimates for use in efforts in risk assessment in the planning of future space missions as well as in the study of GCRs. PREDICCS (Predictions of radiation from REleASE, EMMREM, and Data Incorporating the CRaTER, COSTEP, and other solar energetic particles measurements) is an online system designed to provide the scientific community with a comprehensive resource on the radiation environments of the inner heliosphere. The data products shown here will be incorporated into PREDICCS in order to further this effort and daily updates will be made available on the PREDICCS website (http://prediccs.sr.unh.edu).

Proton flux and radiation dose from galactic cosmic rays in the lunar regolith and implications for organic synthesis at the poles of the Moon and Mercury

Galactic cosmic rays are a potential energy source to stimulate organic synthesis from simple ices. The recent detection of organic molecules at the polar regions of the Moon by LCROSS (Colaprete, A. et al. [2010]. Science 330, 463–468, http://dx.doi.org/10.1126/science.1186986), and possibly at the poles of Mercury (Paige, D.A. et al. [2013]. Science 339, 300–303, http://dx.doi.org/10.1126/science.1231106), introduces the question of whether the organics were delivered by impact or formed in situ. Laboratory experiments show that high energy particles can cause organic production from simple ices. We use a Monte Carlo particle scattering code (MCNPX) to model and report the flux of GCR protons at the surface of the Moon and report radiation dose rates and absorbed doses at the Moon’s surface and with depth as a result of GCR protons and secondary particles, and apply scaling factors to account for contributions to dose from heavier ions. We compare our results with dose rate measurements by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) experiment on Lunar Reconnaissance Orbiter (Schwadron, N.A. et al. [2012]. J. Geophys. Res. 117, E00H13, http://dx.doi.org/10.1029/2011JE003978) and find them in good agreement, indicating that MCNPX can be confidently applied to studies of radiation dose at and within the surface of the Moon. We use our dose rate calculations to conclude that organic synthesis is plausible well within the age of the lunar polar cold traps, and that organics detected at the poles of the Moon may have been produced in situ. Our dose rate calculations also indicate that galactic cosmic rays can induce organic synthesis within the estimated age of the dark deposits at the pole of Mercury that may contain organics.

Validation of PREDICCS using LRO/CRaTER observations during three major solar events in 2012

AbstractPREDICCS (Predictions of Radiation from Release, EMMREM, and Data Incorporating the CRaTER, COSTEP and other SEP measurements, prediccs.sr.unh.edu) is an online system designed to provide a near real‐time characterization of the radiation environment of the inner heliosphere. PREDICCS utilizes data from various satellites in conjunction with numerical models such as the Earth‐Moon‐Mars Radiation Environment Module (EMMREM) to produce dose rate and particle flux data at the Earth, Moon and Mars. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument launched aboard the Lunar Reconnaissance Orbiter (LRO) spacecraft in 2009 and designed to measure energetic particle radiation, offers an opportunity to test the capability of PREDICCS to accurately describe the lunar radiation environment. We provide comparisons between dose rates produced by PREDICCS with those measured by CRaTER during three major solar energetic particle (SEP) events that occurred in 2012. In addition, using EMMREM data products together with our archive of measured CRaTER dose rates, we compute the modulation potential at the Moon throughout the LRO mission and, using this, compute the background GCR dose rate during each event. We demonstrate reasonable agreement between PREDICCS and CRaTER dose rates and come to the conclusion that PREDICCS provides credible characterization of the lunar radiation environment. This study represents the first multi‐event validation, via in situ measurement, of radiation models such as EMMREM, which should prove to be valuable in future efforts in risk assessment and in the study of radiation in the inner heliosphere.

The deep space galactic cosmic ray lineal energy spectrum at solar minimum

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument is an energetic particle telescope on board the Lunar Reconnaissance Orbiter spacecraft. CRaTER measures energetic charged particles that have sufficient energy to penetrate the outer shielding of the instrument (about 12 MeV/nucleon). Galactic cosmic rays (GCR) with these energies are the primary radiation concern for spacecraft and astronauts outside of the Earth's magnetosphere during times of minimal solar activity. These particles can easily penetrate typical shielding and damage electronics, causing increased electronics failure rates and single event upsets. When this radiation impacts biological cells, it causes an increased risk of cancer. The CRaTER instrument was built to characterize the radiation dose and lineal energy with unprecedented time and energy resolution and was fortuitously flown during a period of time that coincided with the highest GCR fluxes in the modern space age. We report here this worst‐case GCR lineal energy spectrum. Observations are made behind a thin aluminum window and different thicknesses of tissue‐equivalent plastic. These measurements provide important observational data points to compare with current model predictions of the dose deposited by energetic particles within a tissue‐like material.

The formation of molecular hydrogen from water ice in the lunar regolith by energetic charged particles

On 9 October 2009, the Lunar Crater Observation and Sensing Satellite (LCROSS) mission impacted a spent Centaur rocket into the permanently shadowed region (PSR) within Cabeus crater and detected water vapor and ice, as well as other volatiles, in the ejecta plume. The Lyman Alpha Mapping Project (LAMP), a far ultraviolet (FUV) imaging spectrograph on board the Lunar Reconnaissance Orbiter (LRO), observed this plume as FUV emissions from the fluorescence of sunlight by molecular hydrogen (H2) and other constituents. Energetic charged particles, such as galactic cosmic rays (GCRs) and solar energetic particles (SEPs), can dissociate the molecules in water ice to form H2. We examine how much H2can be formed by these types of particle radiation interacting with water ice sequestered in the regolith within PSRs, and we assess whether it can account for the H2 observed by LAMP. To estimate H2formation, we use the GCR and SEP radiation dose rates measured by the LRO Cosmic Ray Telescope for the Effects of Radiation (CRaTER). The exposure time of the ice is calculated by considering meteoritic gardening and the penetration depth of the energetic particles. We find that GCRs and SEPs could convert at least 1–7% of the original water molecules into H2. Therefore, given the amount of water detected by LCROSS, such particle radiation‒induced dissociation of water ice could likely account for a significant percentage (10–100%) of the H2measured by LAMP.

Characterizing the Moon's radiation environment

The radiation environment near the Moon could be damaging to humans and electronics on future missions. To characterize this potentially hazardous environment, the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on board the Lunar Reconnaissance Orbiter mission, which orbits at 50 kilometers above the Moon's surface, measures the radiation that would be absorbed by either electronic parts or human tissue behind the shielding of a spacecraft.

Measurements of galactic cosmic ray shielding with the CRaTER instrument

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument aboard the Lunar Reconnaissance Orbiter has been measuring energetic charged particles from the galactic cosmic rays (GCRs) and solar particle events in lunar orbit since 2009. CRaTER includes three pairs of silicon detectors, separated by pieces of tissue‐equivalent plastic that shield two of the three pairs from particles incident at the zenith‐facing end of the telescope. Heavy‐ion beams studied in previous ground‐based work have been shown to be reasonable proxies for the GCRs when their energies are sufficiently high. That work, which included GCR simulations, led to predictions for the amount of dose reduction that would be observed by CRaTER. Those predictions are compared to flight data obtained by CRaTER in 2010–2011.

Near‐Real‐Time Situational Awareness of Space Radiation Hazards

Galactic cosmic rays and solar energetic particles (SEPs) present formidable radiation hazards [e.g., Cucinotta et al., 2010] for human and robotic operations beyond low Earth orbit (LEO). As new plans are conceived for human exploration beyond LEO, these space radiation hazards create critical needs for accurate situational awareness. A new near-real-time tool called PREDICCS (Predictions of Radiation From REleASE, EMMREM, and Data Incorporating the CRaTER, COSTEP, and Other SEP Measurements, http://prediccs.sr.unh.edu) provides the first online system for predicting and forecasting the radiation environment in near-Earth, lunar, and Martian space environments. PREDICCS integrates a host of near-real-time measurements being made by satellites currently in space with numerical models (e.g., EMMREM [Schwadron et al., 2010]) to determine radiation doses and dose equivalents that characterize biological impacts and energetic particle propagation codes that can accurately project radiation levels through the inner solar system and out to Mars. PREDICCS provides updates of the radiation environment on an hourly basis and archives the data weekly, monthly, and yearly to provide a clear historical record of the space radiation environment. The PREDICCS tool provides in-depth data to characterize how hazardous SEP events are throughout the solar cycle, provides detailed information on how often large events that have significant risk associated with them are likely to be observed, and facilitates comparison of the near-Earth and lunar radiation environments to that near Mars. As an example, Table 1 shows the lens doses (in centigrays; 1 centigray = 1 rad) and dose equivalents (in centisieverts; 1 centisievert = 1 rem) for full-sphere radiation exposure from PREDICCS over the last 12 months. The PREDICCS lens dose quantifies the energy per mass deposited in 1 gram per square centimeter of water, which is a proxy for the biological impact to the human eye. Large SEP events on 23 January 2012, on 27 March 2012, and in the summer of 2012 contributed significantly to the hazards of the space environment over this interval. We observe that the dose rate limit of 200 centigrays for the lens dose [Cucinotta et al., 2010] is exceeded for nominal spacecraft shielding. Among the satellite measurements used by PREDICCS are data from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument on NASA's Lunar Reconnaissance Orbiter [Spence et al., 2010]. For the SEP events of 23 January (Figure 1) and 27 March 2012, PREDICCS results are typically within 20%-30% of what CRaTER observed. These highly accurate comparisons have never before been available over such long periods of time. CRaTER not only provides validation that PREDICCS models are accurate but also directly measures the biological impacts of radiation by observing linear energy transfer spectra behind different thicknesses of tissue-equivalent plastic [Spence et al., 2010]. Accurate assessment of the biological impacts of space radiation is vital to fully characterizing the radiation hazards of the space environment. The near-real-time situational awareness provided by PREDICCs will help to enable future human space exploration endeavors, including a return to the Moon and possible missions to Mars. Nathan Schwadron is an associate professor at the University of New Hampshire and the Institute for the Study of Earth, Oceans and Space. His email is n.schwadron@unh.edu.

Lunar radiation environment and space weathering from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER)

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) measures linear energy transfer by Galactic Cosmic Rays (GCRs) and Solar Energetic Particles (SEPs) on the Lunar Reconnaissance Orbiter (LRO) Mission in a circular, polar lunar orbit. GCR fluxes remain at the highest levels ever observed during the space age. One of the largest SEP events observed by CRaTER during the LRO mission occurred on June 7, 2011. We compare model predictions by the Earth‐Moon‐Mars Radiation Environment Module (EMMREM) for both dose rates from GCRs and SEPs during this event with results from CRaTER. We find agreement between these models and the CRaTER dose rates, which together demonstrate the accuracy of EMMREM, and its suitability for a real‐time space weather system. We utilize CRaTER to test forecasts made by the Relativistic Electron Alert System for Exploration (REleASE), which successfully predicts the June 7th event. At the maximum CRaTER‐observed GCR dose rate (∼11.7 cGy/yr where Gy is a unit indicating energy deposition per unit mass, 1 Gy = 1 J/kg), GCRs deposit ∼88 eV/molecule in water over 4 billion years, causing significant change in molecular composition and physical structure (e.g., density, color, crystallinity) of water ice, loss of molecular hydrogen, and production of more complex molecules linking carbon and other elements in the irradiated ice. This shows that space weathering by GCRs may be extremely important for chemical evolution of ice on the Moon. Thus, we show comprehensive observations from the CRaTER instrument on the Lunar Reconnaissance Orbiter that characterizes the radiation environment and space weathering on the Moon.

New measurements of total ionizing dose in the lunar environment

We report new measurements of solar minimum ionizing radiation dose at the Moon onboard the Lunar Reconnaissance Orbiter (LRO) from June 2009 through May 2010. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument on LRO houses a compact and highly precise microdosimeter whose design allows measurements of dose rates below 1 micro‐Rad per second in silicon achieved with minimal resources (20 g, ∼250 milliwatts, and ∼3 bits/second). We envision the use of such a small yet accurate dosimeter in many future spaceflight applications where volume, mass, and power are highly constrained. As this was the first operation of the microdosimeter in a space environment, the goal of this study is to verify its response by using simultaneous measurements of the galactic cosmic ray ionizing environment at LRO, at L1, and with other concurrent dosimeter measurements and model predictions. The microdosimeter measured the same short timescale modulations in the galactic cosmic rays as the other independent measurements, thus verifying its response to a known source of minimum‐ionizing particles. The total dose for the LRO mission over the first 333 days was only 12.2 Rads behind ∼130 mils of aluminum because of the delayed rise of solar activity in solar cycle 24 and the corresponding lack of intense solar energetic particle events. The dose rate in a 50 km lunar orbit was about 30 percent lower than the interplanetary rate, as one would expect from lunar obstruction of the visible sky.

GCR access to the Moon as measured by the CRaTER instrument on LRO

Recent modeling efforts have yielded varying and conflicting results regarding the possibility that Earth's magnetosphere is able to shield energetic particles of >10 MeV at lunar distances. This population of particles consists of galactic cosmic rays as well as energetic particles that are accelerated by solar flares and coronal mass ejections. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) onboard the Lunar Reconnaissance Orbiter is in orbit about the Moon and is thus able to directly test these modeling results. Over the course of a month, CRaTER samples the upstream solar wind as well as various regions of Earth's magnetotail. CRaTER data from multiple lunations demonstrate that Earth's magnetosphere at lunar distances produces no measurable influence on energetic particle flux, even at the lowest energies (>14 MeV protons) where any effect should be maximized. For particles with energies of 14–30 MeV, we calculate an upper limit (determined by counting statistics) on the amount of shielding caused by the magnetosphere of 1.7%. The high energy channel (>500 MeV) provides an upper limit of 3.2%.

Galactic cosmic ray radiation hazard in the unusual extended solar minimum between solar cycles 23 and 24

[1] Galactic cosmic rays (GCRs) are extremely difficult to shield against and pose one of the most severe long-term hazards for human exploration of space. The recent solar minimum between solar cycles 23 and 24 shows a prolonged period of reduced solar activity and low interplanetary magnetic field strengths. As a result, the modulation of GCRs is very weak, and the fluxes of GCRs are near their highest levels in the last 25 years in the fall of 2009. Here we explore the dose rates of GCRs in the current prolonged solar minimum and make predictions for the Lunar Reconnaissance Orbiter (LRO) Cosmic Ray Telescope for the Effects of Radiation (CRaTER), which is now measuring GCRs in the lunar environment. Our results confirm the weak modulation of GCRs leading to the largest dose rates seen in the last 25 years over a prolonged period of little solar activity.

Parameterizations of the linear energy transfer spectrum for the CRaTER instrument during the LRO mission

[1] The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument was launched as part of the Lunar Reconnaissance Orbiter (LRO) spacecraft in June 2009. Its purpose is to measure the linear energy transfer (LET) spectrum in lunar orbit as an aid in determining risks to human crews on future lunar missions. Part of the preparations for the mission involved estimating the LET spectrum for the anticipated environment that the instrument is likely to see during the 1 year operational phase of the LRO mission. Detailed estimates of LET spectra in the six silicon detectors and two tissue equivalent plastic segments were made using the beta version of the HETC-HEDS Monte Carlo transport code. Tables of LET in each detector component, for incident particle elemental species from hydrogen through iron, were carried out at incident particle energies from 20 MeV per nucleon to 3 GeV per nucleon. The LET values in these tables have been parameterized by elemental species and energy for ease in quickly and accurately estimating the LET response for any input solar or galactic cosmic ray spectrum likely to be encountered during the lifetime of the instrument. The parameterized LET values are in excellent agreement with the HETC-HEDS calculations. Typical differences are on the order of a few percent. These parameterizations will also be useful in validation studies of the Earth-Moon-Mars Radiation Environment Module using CRaTER measurements in lunar orbit.

CRaTER: The Cosmic Ray Telescope for the Effects of Radiation Experiment on the Lunar Reconnaissance Orbiter Mission

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) characterizes the radiation environment to be experienced by humans during future lunar missions. CRaTER measures the effects of ionizing energy loss in matter due to penetrating solar energetic protons (SEP) and galactic cosmic rays (GCR), specifically in silicon solid-state detectors and after interactions with tissue-equivalent plastic (TEP), a synthetic analog of human tissue. The CRaTER investigation quantifies the linear energy transfer (LET) spectrum in these materials through direct measurements with the lunar space radiation environment, particularly the interactions of ions with energies above 10 MeV, which penetrate and are detected by CRaTER. Combined with models of radiation transport through materials, CRaTER LET measurements constrain models of the biological effects of ionizing radiation in the lunar environment as well as provide valuable information on radiation effects on electronic systems in deep space. In addition to these human exploration goals, CRaTER measurements also provide new insights on the spatial and temporal variability of the SEP and GCR populations and their interactions with the lunar surface. We present here an overview of the CRaTER science goals and investigation, including: an instrument description; observation strategies; instrument testing, characterization, and calibration; and data analysis, interpretation, and modeling plans.