urn:nasa:pds:context:instrument:xrs.near 1.0 1.7.0.0 Product_Context 2016-10-01 1.0 extracted metadata from PDS3 catalog and modified to comply with PDS4 Information Model urn:nasa:pds:context:instrument_host:spacecraft.near::1.0 instrument_to_instrument_host Adler, I., J.I. Trombka, J. Gerard, P. Lowman, R. Schamadebeck, H. Blodgett, E. Eller, L. Yin, R. Lamothe, P. Gorenstein, and P. Bjorkholm, Apollo 15 Geochemical X-Ray Fluorescence Experiment: Preliminary Report, Science, Vol. 175, pp. 436-440, 1972. reference.ADLERETAL1972A Adler, I., J. Gerard, J.I. Trombka, R. Schamadebeck, P. Lowman, H. Blodgett, L. Yin, E. Eller, and R. Lamothe, The Apollo 15 X-Ray Fluorescence Experiment, Proc. Lunar Sci Conf. 3rd, pp. 2157-2178, 1972. reference.ADLERETAL1972B Adler, I., J.I. Trombka, J. Gerard, P. Lowman, R. Schamadebeck, H. Blodgett, E. Eller, L. Yin, R. Lamothe, G. Osswald, P. Gorenstein, P. Bjorkholm, H. Gursky, and B. Harris, Apollo 16 Geochemical X-ray Fluorescence Experiment: Preliminary Report, Science, Vol. 177, pp. 256-259, 1972. reference.ADLERETAL1972C Bielefeld, M., R. Reedy, A. Metzger, J. Trombka, and J. Arnold, Proc. Lunar Sci. Conf. 7th, pp. 2661-2676, 1976. reference.BIELEFELDETAL1976 Bruckner, J., M. Koerfer, H. Waenke, A.N.F. Schroeder, D. Filges, P. Dragovitsch, P.A.J. Englert, R. Starr, J.I. Trombka, I. Taylor, D.M. Drake, and E.R. Schunk, IEEE Trans. Nucl. Sci., NS-38, pp. 209-217, 1991. reference.BRUCKNERETAL1991 Clark, B.E., J.F. Bell, F.P. Fanale, and D.J. O'Connor, Results of the Seven Color Asteroid Survey: Infrared spectral observations of ~50-km size S-, K-, and M-type asteroids, Icarus, Vol. 113, pp. 387-402, 1995. reference.CLARKETAL1995 Goldsten, J.O., Johns Hopkins APL Technical Digest, Vol. 19, pp. 126-135, 1998. reference.GOLDSTEN1998 Goldsten, J.O., R.L. McNutt, R.E. Gold, S.A. Gary, E. Fiore, S.E. Schneider, J.R. Hayes, J.I. Trombka, S.R. Floyd, W.V. Boynton, S. Bailey, J. Bruckner, S.W. Squyres, L.G. Evans, P.E. Clark, and R. Starr, Space Sciences Review, Vol. 82, pp. 169-216, 1997. reference.GOLDSTENETAL1997 Knoll, G.F., Radiation Detection And Measurement, Wiley & Sons, New York, 1989. reference.KNOLL1989 Starr, R., P.E. Clark, L.G. Evans, S.R. Floyd, T.P. McClanahan, J.I. Trombka, J.O. Goldsten, R.H. Maurer, R.L. McNutt, Jr., and D.R. Roth, Radiation effects in the Si-PIN detector on the Near Earth Asteroid Rendezvous Mission, Nuclear Instruments and Methods in Physics Research Section A, Vol. 428, pp. 209-215, 1999. reference.STARRETAL1999 Trombka, J., S. Floyd, W. Boynton, S. Bailey, J. Bruckner, S. Squyres, L. Evans, P. Clear, R. Starr, E. Fiore, R. Gold, J. Goldsten, and R. McNutt, Compositional Mapping With The Near X-ray/Gamma-ray Spectrometer, J. Geophys. Res., Vol. 102, pp. 23729-23750, 1997. reference.TROMBKAETAL1997 Yin, L, J. Trombka, I. Adler, and M. Bielefeld, X-ray Remote Sensing Techniques For Geochemical Analysis Of Planetary Surfaces, Remote Geochemical Analysis: Geochemical And Mineralogical Composition, C. Pieters and P. Englert (eds.), Cambridge University Press, New York, NY, USA, pp. 199-212, 1993. reference.YINETAL1993 XRAY SPECTROMETER Spectrometer not applicable not applicable Instrument Overview =================== The Near Earth Asteroid Rendezvous mission (NEAR) was successfully launched on 17 February 1996. NEAR was the first launch under NASA's Discovery Program, an initiative for small, low-cost planetary missions. As the first spacecraft to orbit an asteroid, the NEAR mission will address fundamental questions about the processes and conditions relevant to planetary formation. The X-ray/Gamma-ray Spectrometer (XGRS) is one of several instruments on-board the NEAR spacecraft that will study Eros during one year of orbital operations beginning in February 2000. The X-ray and gamma-ray spectrometers that comprise the XGRS are independent, but complementary experiments. In this document we focus on the X-Ray Spectrometer, its design, operation, calibration, and procedures for interpretation of the measurements to be made at Eros. History ======= In February 1996, the Near Earth Asteroid Rendezvous (NEAR) mission was launched into space on a planned three-year cruise to the near-Earth asteroid, 433 Eros. The NEAR spacecraft, the first in the Discovery mission program to be launched, was built by the John Hopkins University, Applied Physics Laboratory (APL). The spacecraft will orbit Eros for one year and carries a group of remote sensing instruments including an X-ray and gamma-ray spectrometer system (XGRS). Due to a problem with firing of the main rocket engine in December 1998, the rendezvous was delayed until February 2000. The NEAR spacecraft will be the first spacecraft to orbit a body as small as Eros, which is estimated to be 33 km X 13 km X 13 km. From telescopic, radar, and other observations, it has been inferred that Eros is an S-type asteroid, one of the most common type of near-Earth asteroids. It is not known whether S-type asteroids come from differentiated or undifferentiated parent bodies. One of the prime mission science objectives is to obtain global elemental composition maps. These elemental composition results are needed with sufficient accuracy to enable comparison with major meteorite types. The results would also be used to assess the compositional heterogeneity of the asteroid and help to answer questions about differentiation. The selection of X-ray and gamma-ray spectrometers for this mission was based on their ability to produce global elemental composition maps of the asteroid. Gamma-ray measurements can determine the abundance of elements such as O, Si, Fe, Ti, Mg, K, Th, and U depending on the actual composition. X-Ray Spectrometer Overview =========================== The X-Ray Spectrometer (XRS) will measure characteristic X-ray emissions induced in the surface of the asteroid by the incident solar flux. The K-alpha lines for the elements Mg, Al, Si, Ca, Ti, and Fe will be detected with spatial resolution on the order of 3 km when counting statistics are not a limiting factor. These measurements can be used to obtain both qualitative and quantitative information on elemental composition. The X-ray spectrometer on the NEAR mission is a non-dispersive spectroscopic system. In this approach, the incoming X-ray photon is absorbed by the detector material and a signal proportional to the absorbed energy is measured by the detector as a voltage pulse at the detector output. An analog to digital conversion is then performed and the count is binned by 'pulse height' or energy loss and a spectrum is obtained and telemetered to Earth. From an analysis of the pulse height spectrum, elemental composition can be inferred. The choice among various types of X-ray detectors was strongly influenced by the constraints of the mission. For the NEAR mission, the detectors were chosen for the sensitivity in the energy regions of scientific interest, while also being consistent with the cost, mass, power and reliability constraints of the mission. The most prominent fluorescent lines for the major elements Mg, Al, Si, Ca, Ti, and Fe are the K-alpha lines (1-10 keV). The strength of these emissions from planetary surfaces is strongly dependent on the chemical composition of the surface as well as on the incident solar spectrum, but are of sufficient intensity to allow orbital measurement by detectors like those on the NEAR spacecraft. In addition to line fluorescence, solar X-rays also can be coherently and incoherently scattered from a planetary surface, contributing an unwanted background signal. Astronomical X-ray sky sources, which could be sources of background, are eliminated at Eros, because the XRS is collimated to a 5-degree field of view and the asteroid completely fills the field of view when the spacecraft is below 100-km altitude. The solar flux from 1 to 10 keV, the energy region of interest, can be modeled with several prominent lines superposed on a continuum described by a fourth to sixth order power law (depending on the level of solar activity). In modeling the solar output for the NEAR mission the best estimates of solar output anticipated near solar maximum have been used and range from approximately B1 to M1 levels. The solar intensity decreases by three to four orders of magnitude from 1 to 10 keV. Fluorescent lines as well as the scatter-induced background, therefore, have greater intensity at lower energies. As the level of solar activity increases, relatively more output occurs at higher energies, the slope of the spectrum becomes less steep, and the overall magnitude of the X-ray flux increases. This process is called hardening. Solar output is highly variable, and can typically change by an order of magnitude or more within minutes. Higher solar activity will yield better statistics, shorter integration times, and hence higher resolution maps, especially for heavier elements such as Fe. Because of its variability, the Sun's output must be monitored in order to be able to obtain quantitative results. An introduction to X-ray remote sensing techniques for geochemical analysis can be found in [YINETAL1993]. Detectors --------- To detect X-rays in the 1 to 10 keV region, either solid state or gas proportional counter detectors can be used. Cryogenically cooled Si(Li) detectors have good energy resolution in this energy region, but are ruled out by cost and mass limitations. Recently, room temperature solid state detectors have been developed with reasonably good energy resolution, but only for detectors of limited size (~1 cm2). Also, these detectors have no flight heritage. The only detectors that satisfied all of the measurement and resource requirements for the NEAR mission are gas proportional counters. The asteroid-pointing detector package includes three large-area (25 cm2) sealed gas proportional counters with thin (25 ?m) Be windows. The large area provides the necessary sensitivity to achieve the desired spatial resolution and the Be windows absorb the lower energy X-rays (below 1 keV) which would otherwise dominate the detector count rate. The fill gas is P-10 (90% argon and 10% methane). The Be window is supported by a rectangular Be support structure. The detector housing is steel with a Be liner to absorb Fe line emission from the housing [GOLDSTENETAL1997]. The sealed gas proportional counters chosen for this experiment are improved versions of instruments previously flown on Apollo 15 and 16. The energy resolution of current gas proportional counters is improved over those of the Apollo days, but is still not sufficient to resolve the low energy Mg, Al, and Si lines. As with the Apollo missions it is necessary to use balanced filters to resolve these closely spaced lines [ADLERETAL1972A], [ADLERETAL1972B] and [ADLERETAL1972C]. Two of the detectors have thin absorption filters, 8.5 m thick, mounted externally. A Mg filter on one detector attenuates the Al and Si lines, and an Al filter on the other detector attenuates the Si line. The very steep absorption edges of the filters make the separation of the lower energy lines possible. At higher energies, the filters are essentially transparent and the Ca and Fe lines are resolved directly by the detectors. The third detector has no filter. The energy resolution of these detectors is about 14.2% at 5.9 keV [TROMBKAETAL1997]. Two sunward-pointing X-ray detectors positioned on the forward deck of the spacecraft monitor the incident solar flux. The solar monitors experience very strong X-ray emissions directly from the sun, especially during solar flares, so the active area for a solar monitor needs to be only about 1 mm2. One monitor is a proportional counter identical to the three asteroid pointing detectors, but with a specially designed graded shield that reduces its effective area to about 1 mm2 (Clark, Trombka, and Floyd, 1995 [CLARKETAL1995]). The other solar monitor is a small Si-PIN photodiode. This solid-state detector is mounted on a miniature thermo-electric cooler in a hermetic package 15-mm in diameter. A 76 m thick Be window rejects the intense solar flux below 1 keV. The Si-PIN solar monitor achieves an energy resolution of 600 eV FWHM at 5.9 keV. The accumulation times or integration periods for the XRS measurement can be adjusted by ground command and may vary from 1 to 65535 seconds. The default for orbital operations is 100 seconds. During each integration period the XRS collects four 256-channel pulse height spectra: one for each of the three asteroid pointing detectors and one from either the proportional counter or the PIN solar monitor. The two solar monitors may both be powered on at the same time, but telemetry limitations forced a design that allows only one of the solar monitors to be pulse height analyzed at a time. Additional details of the detector design can be found in [GOLDSTENETAL1997]. A collimator is used to restrict the X-ray spectrometer field of view to about 5 degrees. In a 50-km orbit (on average about 40 km from the surface of the asteroid) this results in a spatial resolution of about 3 km. The collimator is also useful in reducing the cosmic X-ray background. The collimator uses a honeycomb design made of copper with 3% Be. The K, L and M X-ray lines excited in the collimator by solar X-rays, cosmic rays and asteroidal X-rays do not interfere with the surface line emissions. Calibration Sources ------------------- Three Fe-55 sources, mounted on a calibration rod, can be rotated, one at a time, into the field of view of the three asteroid pointing detectors to establish the energy calibration of the XRS. Knowledge of the energy calibration of the XRS and how it changes over time is necessary to sum spectra obtained over the same region of the asteroid, but collected at different times during the mission. The statistics in any one spectrum (typically about a 100-s accumulation) are insufficient to perform detailed analysis.