PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = "2007-08-14 Peter Ford Initial version; 2010-03-03, M.J. Golightly, --updated INSTRUMENT_REFERENCE_INFO." OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "LRO" INSTRUMENT_ID = "CRAT" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "COSMIC RAY TELESCOPE FOR THE EFFECTS OF RADIATION" INSTRUMENT_TYPE = "ENERGETIC PARTICLE DETECTOR" INSTRUMENT_DESC = " Abstract: ========= The investigation hardware consists of a single, integrated sensor and electronics box with simple electronic and mechanical interfaces to the LRO spacecraft. The CRaTER sensor front-end design is based on standard stacked-detector, cosmic ray telescope systems that have been flown for decades, using detectors developed for other NASA flight programs. The analog electronics design is virtually identical to the robust and flight-proven design of the NASA/POLAR Imaging Proton Spectrometer that has been operating flawlessly on orbit since 1996. The digital processing unit is a simple and straightforward design also based on similar instruments with excellent spaceflight heritage. No new technology developments or supporting research are required for the final design, fabrication, and operation of this instrument.. The text of this instrument description has been abstracted from the instrument paper [CHINETAL2007] and from a preliminary draft of the CRaTER Instrument Calibration Plan [KASPER2007]. Scientific Objectives: ====================== The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) is designed to answer key questions to enable future human exploration of the Solar System, and to address one of the prime objectives of LRO. Specifically, CRaTER addresses an objective required by NASA's Exploration Initiative to safely return humans to the Moon; CRaTER is designed to achieve 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 will fill knowledge gaps regarding radiation effects, provide fundamental progress in knowledge of the Moon's radiation environment, and provide specific path-finding benefits for future planned human exploration. CRaTER's primary measurement goal is to measure directly the linear energy transfer (LET) spectra caused by space radiation penetrating shielding material. Such LET spectra are a missing link, currently derived by models which require experimental measurements to provide ground truth. CRaTER will provide this essential information about the lunar radiation environment. LET is defined as the mean energy absorbed locally, per unit path length, when an energetic particle traverses material. A LET spectrometer measures the amount of energy deposited in a detector of known thickness and material property when an energetic particle passes through it, usually without stopping. LET measurements behind various thicknesses and types of material are of great importance to spacecraft engineers and radiation health specialists. Such measurements are especially important to modelers who study the impacts of the penetrating radiation; LET is one of the most important inputs for predictive models of human health risks and radiation effects in electronic devices. While LET spectrometers do not necessarily resolve particle mass, LET measurements do include all the species, with the possible exception of neutrons, that are relevant to the energy deposited behind a known amount of spacecraft shielding. A LET spectrometer essentially provides the key direct measurement needed to bridge the gap between well measured cosmic ray intensities (that will be available from other spacecraft) and specific energy deposition behind shielding materials, exploration-enabling knowledge vital to the safety of humans working in the harsh space radiation environment. Accordingly, CRaTER is designed to measure this important quantity and thereby provide critical closure between measurements, theory, and modeling. CRaTER will measure LET spectra produced by incident galactic cosmic rays (GCRs) and solar energetic protons (SEPs). GCRs and SEPs with energies >10 MeV have sufficient energy to penetrate even moderate shielding. When they interact with matter, they leave behind energy, damaging the human tissue and electronic parts they pass through. GCRs and SEPs possess both short and long timescale variations (see Fig. 10), some of which are predictable and others that are not presently predictable. GCRs are a slowly-varying and uniform source of cosmic radiation that bathes the solar system. SEPs are episodic and rare, but come in extreme bursts associated with intense solar magnetic activity. Both GCRs and SEPs pose serious risks to humans venturing above the relative safety of low-Earth orbit and the Earths powerful magnetic shield; areas including the Moon and the interplanetary space between Earth and Mars may be dangerous to humans. In order to achieve the LRO radiation mission requirement, CRaTER is designed to return the following required data products: * Measure and characterize that aspect of the deep space radiation environment, LET spectra of galactic and solar cosmic rays (particularly above 10 MeV), most critically important to the engineering and modeling communities to assure safe, long-term, human presence in space. * Investigate the effects of shielding by measuring LET spectra behind different amounts and types of areal density, including tissue-equivalent plastic. The CRaTER measurement concept is shown in the see-thru telescope drawing below (Fig. 11). The investigation hardware consists of a single, integrated telescope and electronics box with straightforward electronic and mechanical interfaces to the spacecraft. The zenith-nadir viewing telescope employs a stack of three pairs of detectors embedded within aluminum structure and tissue-equivalent plastic (TEP) to establish the LET spectra of cosmic radiation relevant for human health and electronics part concerns. Primary GCRs and SEPs enter the telescope through the zenith, deep-space entrance, depositing energy in the telescope stack through ionizing radiation and producing secondary particles through nuclear interactions. The primary and secondary particles interact with one or more of the six detectors through the stack: the thin (thick) detectors are optimized for high (low) LET interactions. Events with sufficient energy deposition in a detector cross a trigger threshold. Digital logic then compares multi-detection coincidences with predefined event masks to identify desirable events. Pulse height analysis is performed on every detector to measure LET at each point in the stack. The measurement team will use observations taken during the mission to construct LET spectra behind the different amounts of material, including TEP, as a function of particle environment (GCR vs. SEP; foreshock vs. magnetotail vs. solar wind; etc.). The team will also test models of radiation effects and shielding by verifying/validating deterministic models. Model predictions of energy transport of incident GCR and SEP spectra (available contemporaneously on other missions) through the CRaTER instrument will be compared to the measured LET spectra. Thus, CRaTER will provide not only direct measurements of LET in the lunar environment, but will also better constrain radiation effects models that are being used to assess the effects of other radiation environments, including in interplanetary space and at Mars. Calibration: ============ The calibration of the CRaTER Flight Models is based on lessons learned from experiences with the Engineering Model: the relationship between the value returned by the detectors and the original energy deposited is essentially linear. Gains and offsets for each detector are determined with high precision by calibrating the instrument with a beam of high energy protons produced by the Northeast Proton Therapy Center (NPTC) of Massachusetts General Hospital (MGH) in Boston. A 300 MeV proton beam at MGH is degraded in energy using sheets of plastic until a beam is produced with large energy dispersion and a peak energy tuned to the response of a pair of thin and thick CRaTER detectors. The dispersed beam produces a characteristic track in energy deposition in the pair of detectors. The gains and offsets for each of the detectors is then determined by iteratively varying the free parameters of the instrument response until the measurements match the predictions of GEANT numerical simulations of the energy loss. Operational Modes: =========================== The CRaTER instrument has only two modes: powered up and powered down. There are no operational constraints on these modes. In particular, CRaTER can be powered up during the cruise phase of the mission, and will return scientifically useful data. Sensors: ========== The investigation hardware consists of a single, integrated sensor and electronics box with simple electronic and mechanical interfaces to the LRO spacecraft. The CRaTER sensor front-end design is based on standard stacked-detector, cosmic ray telescope systems that have been flown for decades, using detectors developed for other NASA flight programs. The analog electronics design is virtually identical to the robust and flight- proven design of the NASA/POLAR Imaging Proton Spectrometer that has been operating flawlessly on orbit since 1996. The digital processing unit is a simple and straightforward design also based on similar instruments with excellent spaceflight heritage. No new technology developments or supporting research are required for the final design, fabrication, and operation of this instrument. The CRaTER telescope consists of six ion-implanted silicon detectors, mounted on detector boards, and separated by pieces of tissue- equivalent plastic, hereinafter referred to as TEP. All six of the silicon detectors are 2 cm in diameter. Detectors 1, 3, and 5 are 140m thick; the others are 1000m thick. TEP (such as A-150 manufactured by Standard Imaging) simulates soft body tissue (muscle) and has been used for both ground-based as well as space-based (i.e., Space Station) experiments. Solid-state detectors use semi-conducting crystals (in CRaTERs case, silicon) with n-type (electron-rich, electron conducting) and p-type (electron-deficient, hole conducting) regions. When a reversed bias voltage is applied across the junction, the un-bonded electrons in the semiconductor are pushed away from the voltage source, while the holes are pulled towards it. This leaves a neutral area void of charge and current at the junction of the sectors, called the depletion region. As incoming radiation (e.g., a solar proton or cosmic ray particle) collides with the depletion region, electron-hole pairs are formed in the material (where a once bonded electron is freed from its atom, leaving a hole). The electron and the hole respond to the applied voltage, and a small current is created. This current can be detected and later analyzed. A cold environment greatly reduces the transmission of thermal signals. In addition, the solid state of the semiconducting material makes it easier to detect those signals attributable to freed electrons. Tissue equivalent plastic (or TEP) is a plastic recipe designed to simulate human tissue. It includes hydrogen and nitrogen percentages-by-composition that are similar to that found in human skin and muscle. Scientists can use the atomic-level effects that radiation has on the TEP to deduce what sort of similar effects may occur in humans. Electronics: ============ The front-end analog electronics utilize charge amplifiers to collect signals from the six silicon detectors, amplify, and pulse-shape them for high-level processing. The backend digital electronics receives the six signals converts them in parallel to 12-bit digital quantities and, using a programmable coincidence mask, filters out the events of interest. Those events are then packed into standard CCSDS data packets and forwarded to the spacecraft data system for storage and eventual telemetry to the ground. A field programmable logic array contains all of the digital circuitry that receives commands from the spacecraft and handles telemetry packet formatting as well as collecting useful secondary science and analog housekeeping information. There is no processor or software within the instrument. The maximum event rate that the instrument can telemeter is limited to 1200 events/second, enforced separately for every 1 second interval; excess events are discarded. The instrument has an internal deadtime of 12 microseconds. Events are time-tagged to an accuracy of 1 second." END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "SPENCEETAL2010" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END