PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2010-05-27, SONC, initial release " RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = RL INSTRUMENT_ID = "APXS" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "ALPHA PARTICLE X-RAY SPECTROMETER" INSTRUMENT_TYPE = "SPECTROMETER" INSTRUMENT_DESC = " Instrument Overview =================== The Alpha Particle X-Ray Spectrometer (APXS) is an instrument that will provide information on the elemental composition of the comet surface. The instrument is based on the interaction of alpha particles and x-rays with matter. It operates in two modes: the alpha mode (Rutherford backscattering) and the x-ray mode for alpha-particle and x-ray induced x-ray spectroscopy. The APXS combines these methods in one single instrument while being low in mass and power consumption. Historically, there was also a proton mode that has been omitted in the new design in favor of an advanced x-ray mode. The acronym APXS stands for the old and new design. Using the x-ray mode, the Rosetta APXS is able to determine in-situ concentrations of elements from sodium to strontium at levels down to several hundred ppm. Light elements such as carbon, nitrogen and oxygen can be detected using the alpha mode, usually with sensitivity of 1 atom %. In addition, most of the elements are determined by both modes, albeit with different efficiency and accuracy. Hydrogen(and helium) cannot be measured at all by nature of the methods, although, water can still be inferred from any excess of oxygen (which APXS determines accurately) over what is required from the overall stoichiometry (assuming no other light elements in the sample). The Rosetta APXS consists of a sensor head, a deployment device and electronics. The sensor head contains Curium-244 alpha sources (with an activity of about 40 mCi at integration time and with a half-life of 18.11 years) that bombard the sample with alpha particles of an energy of about 5.5MeV and x-rays of about 14 to 21 keV (emitted by the Plutonium-240 daughter). The back-scattered alpha particles are measured by six alpha detectors and the x-ray radiation by one high-resolution x-ray detector (energy resolution, measured as full width at half maximum (FWHM), is 180 eV at 6.4 keV at temperatures below -40degC). Scientific Objectives ===================== The goal of the Rosetta APXS experiment is to determine the chemical composition of the cometary surface at the landing site and its potential alteration with time due to increase activity when the comet approaches the Sun. The data obtained will be used to characterize the surface of the comet, to obtain the gas/dust ratio, determine the chemical composition of the dust component, and to compare the dust with the composition of known meteorite types. These results will be brought into context with other measurements made on the lander and the orbiter to fully obtain a more complete picture of the present state of the comet, and to get insight into its evolution and origin. Calibration =========== In the laboratory, the instrument was calibrated in vacuum measuring a geological sample, called SSK-1 (already used for the MPF (Mars Pathfinder Rover Sojourner) and MER (Mars Exploration Rover) APXS) with known composition in a standard geometry (distance between sample surface and detector). The data from this sample are used to compare the performance and sensitivity of the Rosetta APXS with MPF APXS and MER APXS. This cross calibration mainly determines the energy scale, the detector efficiency and the quality of the alpha and the x-ray spectra ( endpoint sharpness in the alpha mode and FWHM of the x-ray lines in the x-ray mode). Using these instrument specific properties, the calibration of the Rosetta APXS can be achieved using the extensive MER calibration and future new calibration measurements tailored to cometary samples. Operational Considerations ========================== The APXS can store the current XRAM content if it is continuously powered with 5V on the KAL line. When it is powered OFF, the content gets lost and is restored with the PROM default values after the next power ON. Please note that after power ON, those values have to be uploaded if values different from the default ones need to be used. That is necessary for example in the case of changing the threshold in order to obtain response for low energies and in the case of the safe LG parameter to operate the deployment of the deployment device. Due to problems observed in previous payload checkouts and for security, the command for downloading the relevant spectra will be sent twice. In that way we assure that even in the case we may have some error with the download of the spectra, the probability of losing important science will be substantially reduced Detectors ========= Silicon drift detector for x-ray detection. 6 alpha solid state silicon detectors. Electronics =========== Details on the electronics of the instrument are described in the APXS ADP. Location ======== The sensor head is located on the outside of the lander, mounted in an opening of the balcony's floor. To bring the APXS in contact with the comet surface, it is moved by the deployment device that can lower and raise the sensor head by a command. Operational Modes ================= Normal mode - used to obtain energy spectra and during data transmission. Deployment mode - used to operate the deployment device. Subsystems ========== Not applicable Measured Parameters =================== Counts vs. energy spectra. 512 channels (1 to 14.5 KeV) for the X-ray mode and 256 channels (0.6 to 6 MeV) for the Alpha mode. " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID ="KLINGELHOFERETAL2007" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END