PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2002-09-27 UA, C. Shinohara, Original; 2002-09-30 GEO:slavney Edited, reformatted; 2002-12-13 UA, GRS Team, revised" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = ODY INSTRUMENT_ID = GRS OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "GAMMA RAY SPECTROMETER" INSTRUMENT_TYPE = "SPECTROMETER" INSTRUMENT_DESC = " Instrument Overview =================== The Mars Odyssey Gamma-Ray Spectrometer is a suite of three different instruments working together to collect data that will permit the mapping of elemental concentrations of the surface of Mars. The instruments are a gamma-ray spectrometer (GS), a neutron spectrometer (NS), and a high-energy neutron detector (HEND). The instruments are complementary in that the neutron instruments have much better counting statistics and can sample to greater depths than the GS, but the GS determines the abundances of many more elements. Working together the instruments are most powerful at mapping the distribution of hydrogen, both over the surface and as a function of depth in the upper few tens of centimeters. This information can be found in The Mars Odyssey Gamma-Ray Spectrometer Instrument Suite submitted to Space Science Reviews September 13, 2002 [BOYNTONETAL2004]. Scientific Objectives ===================== The chief scientific objectives are: - to quantitatively map the elemental abundances of the Martian surface, - to map the near surface hydrogen (and, by inference, water) and CO2 abundances and their stratigraphic distributions, and determine their seasonal variations, and - to determine the depth of the seasonal polar caps and their variation with time. The GRS will detect and count gamma rays and neutrons emitted from the Martian surface. By associating the energy of gamma rays with known nuclear transitions and determining the number of gamma rays emitted from a given portion of the Martian surface, it is possible to calculate the ratio of elemental abundances of the surface and discern their spatial distribution. By counting the number of neutrons and segregating those into thermal and epithermal energy bins it is possible to calculate the hydrogen abundance thus inferring the presence of water. These data permit a variety of Martian geoscience and life science problems to be addressed including the crust and mantle compositions, weathering processes, volcanism, and the volatile reservoirs and processes. Calibration =========== Calibration information for the GRS Gamma instrument is provided in the GRS Calibration Report (calib/grs_calibration_report.pdf). Calibration information for the neutron spectrometer is provided in the NS Calibration Summary (calib/ns_calibration_summary.pdf). A calibration report for the HEND is not yet available (Should be provided in the next PDS release). Operational Considerations and Operational Modes ================================================ The gamma-ray spectrometer data has been collected in different configurations. This data is only useful for science when the gamma sensor high voltage (HV) is ramped to greater than 3000 volts. This condition can only happen if the gamma sensor head anneal door is open and the detector is cold (below 143 degrees Kelvin). The gamma sensor is capable of generating data when the HV is not ramped, but it is only to verify that the electronics are functioning properly. Gamma sensor data has been taken in all mission phases thus far. During the mapping mission phase, data was taken with the GRS boom stowed and deployed. The GRS boom was deployed on June 4, 2002 which started the true mapping phase for the gamma sensor. The Neutron Spectrometer (NS) and High Energy Neutron Detector (HEND) data have also been taken in all mission phases: cruise, aerobraking, and mapping. These data are also only valid if the high voltage is on for the instruments. Detectors ========= The Gamma Sensor Head (GSH) consists of a high-purity germanium detector mounted in a passive cryogenic cooler, front stage electronics, and a pre-amplifier. The sensor-head must be isolated from all material that will interfere with the detection of gamma rays from the Martian surface. For this reason it must be isolated from the spacecraft by a boom, and the materials making up the sensor head are selected to avoid interference. The gamma-ray detector must be cooled to cryogenic temperatures to reduce the electronic noise induced by the thermal agitation of the germanium atoms. In addition, in the radiation flux of interplanetary space a warm detector will experience radiation damage. In order to remove the effects of radiation damage, the HP-GeD must be heated to temperatures of 105 degrees C and maintained at this temperature for a minimum of 48 hours and a maximum of 120 hours. The neutron spectrometer (NS) is comprised of the neutron sensor-head and its associated electronics. The neutron sensor-head is a four-section borated-plastic scintillator composed of BC454 coupled to photo-multiplier tubes (PMTs) and their associated high-voltage bleeder boards and pre-amplifiers. Each of the four segments is in the shape of a prism; mounted together the scintillators form a parallelepiped. One of the four segments must face in the direction of the spacecraft velocity vector; the opposite prism will then face in the negative velocity vector of the spacecraft, and the two remaining prisms will face towards Mars (nadir) and away from Mars (anti-nadir). This orientation together with coincident pulse logic allows separation of neutron radiation from other radiation and separation of neutron energies into broad energy bands. The neutron sensor-head must be separated from the spacecraft to reduce the effect of the flux of neutrons from the spacecraft. The required separation depends on the proximity and size of the neutron-moderating (hydrogen) materials in the spacecraft such as the fuel. The borated plastic scintillator has a heritage of space applications for detecting neutrons and uniquely identifying them as such in the presence of backgrounds of gamma rays and energetic charged particles that are generally encountered in space. Thermal and epithermal neutrons are identified by detection of a single interaction in one or two of the sensor segments that deposits an energy equal to the Q-value of the 10B(n,a)7Li* reaction. The signature of a fast neutron is the detection of a pair of interactions occurring within about 5 microseconds (the pulse-correlation time for fast neutrons interacting in BC454 is t = 2.2 microseconds) where the second interaction of the pair has the signature of a thermal or epithermal neutron just mentioned. If this condition is satisfied, then the pulse height of the first interaction gives the energy of the fast neutron. Separate determinations of the thermal, epithermal, and fast flux components of martian and spacecraft neutron flux spectra will be optimized if the sensor is oriented such that one of its prism segments faces in the direction of the spacecraft velocity vector (frontward), one faces in the opposite direction (backward), one faces downward toward the martian surface, and one faces upward from Mars. This configuration has the advantage that it allows discrimination between neutrons originating in Mars from those originating in the spacecraft, thereby reducing (but not eliminating) the need for separation from hydrogenous spacecraft materials. Given this orientation, all three neutron flux components from both the spacecraft and Mars can be measured and separately identified. Specifically, the difference in counting rates of the front- and back-facing BC454 prism segments provides a measure of thermal neutrons from Mars alone. Contamination by thermals from the spacecraft, and all epithermal neutrons, is reduced by virtue of the motion of the sensor relative to Mars. The motion of the sensor relative to Mars ensures that the front-facing segment selectively scoops up the thermals from Mars while the back-facing segment outruns them. The fact that all four prism segments are stationary relative to the spacecraft ensures that the ratio of spacecraft contributions to each of them is fixed by geometry, which can be calibrated during cruise. A variant of the same technique can be used to separate martian and spacecraft epithermal neutrons. In this case the upward-facing segment is shielded from Mars by the other three segments. In consequence, the counting rate of the upward facing prism can be used to subtract the spacecraft contribution to the counting rate of the backward-facing prism to provide a measure of the martian epithermal flux alone. This result is possible because, as just mentioned, the backward facing prism will outrun the thermal neutrons coming from Mars, thus making it sensitive only to martian epithermal and fast neutrons. However, as discussed previously, the special properties of BC454 can be used to separate the epithermal and fast neutron events electronically. In addition, the normalized difference between single counts having the signature of a thermal or epithermal neutron registered in the downward- and upward-facing prisms will be used to provide a check on the combined thermal and epithermal neutron fluxes from Mars determined using the frontward-, backward- and upward-facing prisms, respectively. All four prism segments can be used to provide separate measures of martian and spacecraft fast neutrons. As for the epithermals, but to a lesser extent, this separation is facilitated by the absorption properties of the geometric arrangement of all four BC454 segments relative to the surface of Mars and the spacecraft, respectively. It is only feasible for this kind of an arrangement because the hydrogen moderator in the sensor is contained in an active element of the detector (the BC454 scintillator) as opposed to a passive element such as is provided by a polyethylene (or equivalent) moderator that is placed around a thermal neutron sensor. The High Energy Neutron Detector (HEND) is a self-contained experiment which relies on the GRS for power and command and data handling. HEND detects neutrons and soft gamma rays. Neutrons are detected in thermal (0.01 eV - 1 eV), epithermal (1 eV - 1 keV), fast (1 keV - 1 MeV), and high-energy (1 MeV - 10 MeV) energy ranges. Provisions have been made to separate detected neutrons from gamma-rays and also provide anti-coincidence between protons and high-energy neutrons. HEND allows count rates from 0.01 per second for high-energy neutrons up to 104 for gamma rays. The energy resolution for neutrons and gamma-rays is 20% or better. A low background count rate due to the HEND is achieved through materials selection, active anti-coincidence shielding, and amplitude thresholding. Data is synchronized with the GRS so that all GRS data can be registered with respect to time. HEND also monitors the gamma-ray flux from solar flares and bursts with 0.25 - 1 second time resolution. The high-energy neutron detector is comprised of three 3He sensors, a Stilben scintillator detector, and associated electronics. All HEND electronics are housed in the HEND sensor-head except for those circuits provided by the UA in the CEB to support the HEND RS-422 interface. HEND has a separate spacecraft power switch provided through the GRS CEB. Electronics =========== The remainder of the gamma-ray sensor electronics consists of the gamma analog processing system which provides peak-hold and 14-bit analog to digital conversion of the pulse as well as level discriminators and other instrument control functions plus the high-voltage bias supply to provide the bias voltage for the germanium detector. These electronics together with housekeeping electronics are housed in the MSO GRS Central Electronics Box (CEB). The remainder of the NS electronics consists of the neutron analog chain which shapes the pulses resulting from detection and provides 8-bit analog to digital conversion of the pulse as well as level discriminators and other instrument control functions. These electronics are housed in the CEB. The digital electronics process the output of the ADCs from the gamma-ray and neutron detectors. It also processes all housekeeping data. It receives and processes all instrument commands. It provides the interface with the spacecraft command and data handling system and provides the interface with all other components of the MSO GRS. It includes low-voltage power supplies for the MSO GRS including specially filtered low-voltage power supplies for the GRS analog electronics chain. Operational Modes ================= Gamma-ray spectrometer 1. Door open, closed 2. Boom deployed, undeployed 3. High voltage up, down Neutron Spectrometer (NS) 1. High voltage up, down High Energy Neutron Detector (HEND) 1. High voltage up, down " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "BOYNTONETAL2004" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END