Cruise measurements taken with the near XGRS

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Pergamon www.elsevier.nl/locate/asr

CRUISE MEASUREMENTS

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Adv. Space Res. Vol. 24, No. 9, pp 1159-l 162, 1999 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-1177/99 $20.00 + 0.00

SO273-1177(99)00697-3

TAKEN WITH THE NEAR XGRS

R. Starr’, P. E. Clark’, L. G. Evans*, S. R. Floyd3, T. P. McClanahan3, J. I. Trombka3, W. V. Boynton4, J. Brtickner’, S. W. Squyres6, J. 0. Goldsten7, and R. L. McNutt, Jr.7

‘The Catholic University of America, Washington, DC 20064 USA Computer Sciences Corporation, Laurel, MD 20707 USA 3NASA/GSFC, Greenbelt, MD 20771, USA 4University of Arizona, Tucson, Arizona 85721 USA 5Max-Planck-Institut fur Chemie, Mainz, Germany 6Comell University, Ithaca, New York 14853 USA 7Johns Hopkins University Applied Physics Laboratory, Laurel, MD

S. Bailey4,

20723 USA

ABSTRACT The X-ray/Gamma-ray Spectrometer on the Near Earth Asteroid Rendezvous spacecraft will remotely detect characteristic x-ray and gamma-ray emissions from the surface of 433 Eros to develop global maps of the in the 1 to 10 keV range will elemental composition of the asteroid surface. Solar excited x-ray fluorescence be used to measure the surface abundances of Mg, Al, Si, Ca, Ti, and Fe with spatial resolutions down to 2 km. Gamma-ray emissions in the 0.1 to 10 MeV range will be used to measure cosmic-ray excited emissions from such elements as 0, Si, Fe, and H as well as naturally radioactive elements K, Th, and U to surface depths on the order of 10 cm. In-flight calibrations are essential to the understanding and analysis of data collected at Eros. 0 1999 COSPAR. Published by Elsevier Science Ltd. INTRODUCTION 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 promises to answer fundamental questions about the processes and conditions relevant to planetary formation. The X-ray/Gamma-ray Spectrometer (XGRS) will measure elemental abundances on the surface of Eros. The X-Ray Spectrometer (XRS) measures 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 (l-10 keV energy region) are detected with spatial resolution on the order of 2 km. The Gamma-Ray Spectrometer (GRS) has much coarser spatial resolution, but will measure sub-surface composition to depths on the order of 10 cm. Gamma rays are emitted by excited nuclei in the surface of solar system objects from radioactive decay (K, U, and Th) and by bombardment of cosmic rays and the secondary neutrons produced in the planet surface (e.g. H, 0, Si, The Mg, and Fe). The NEAR spacecraft will orbit Eros for about one year beginning in February 2000. combined observations of the XRS and GRS should provide full coverage of Eros enabling the development of global maps of Eros’s surface composition. INSTRUMENT

DESCRIPTION

X-Ray Suectrometer The X-Ray Spectrometer (XRS) consists of three identical gas-filled proportional counters designed and built by Metorex International Oy. Gas tubes of this type, shown in Figure 1, provide the large active area (25 cm’ each) required for remote sensing. The XRS achieves an energy resolution of 850 eV FWHM at 5.9 keV. The energy range of the detectors is 0.5 to 10 Fig. 1. X-ray proportional 1159

counter.

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keV. The energy resolution of the gas tubes is not sufficient to resolve the closely spaced Mg, Al, and Si lines. To separate these lines, the XRS uses balanced filters (Trombka et al., 1997). The two outer detectors have thin absorption filters, 8.5 pm thick, mounted externally. A Mg filter on one detector attenuates the Al line, and an Al filter on the other detector attenuates the Mg and Si lines. The very steep /, ,a,*,py, ,>w,:.. absorption edges of the filters make the separation of Fig. 2. Gamma-Ray Spectrometer. 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 center detector has no filter. The XRS employs rise-time discrimination circuitry to reject up to 70% of gamma-ray and charged particle background events (Goldsten ef al., 1997). 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 mm*. 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 mm’ (Clark, Trombka, and Floyd, 1995). The other is a new miniature x-ray detector developed and built by AMPTEK, Inc. The compact sensor consists of a Si-PIN photodiode mounted on a miniature thermo-electric cooler in a hermetic package 15 mm in diameter. Also mounted on the cooler are the input FET and RC feedback components to the charge-sensitive preamplifier, and an integrated circuit temperature monitor. A 75 pm 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. Gamma-Rav

Soectrometer

The NEAR GRS, shown in Figure 2, was designed and built by EMR Photoelectric, a division of Schlumberger. It employs a NaI(T1) scintillator situated within a thick cup shield fabricated from a single crystal of BGO (bismuth germanate). The NaI is a 2.54 cm x 7.62 cm right circular cylinder and couples to a 3.17 cm diameter metal ceramic photomultiplier tube (PMT). The BGO cup has outside dimensions of 8.9 cm x 14 cm and couples to a 7.6 cm diameter metal ceramic PMT. The BGO, acting as an anti-coincidence shield, reduces the cosmic-ray, Compton and pair-production background signal as well as providing direct, passive shielding from the local gamma-ray environment. This design approach eliminated the need for a boom and allowed the detector to be body mounted to the spacecraft. Although the energy resolution of a Nal is not nearly as good as a cooled Ge detector (Evans et al., 1996), the NEAR GRS detector operates at room temperature, is not subject to any serious radiation damage (important for long-duration missions such as NEAR), and given the integration times planned for the mission, meets the measurement requirements for the elements cited above. The measured energy resolutions for the NaI(T1) and BGO detectors are 8.7% and 14% FWHM, respectively at 662 keV. The cup design provides for a field-of-view of about 45 degrees. Since launch in February 1996 the 6000 hours. Detailed descriptions their operation, and the scientific XGRS can be found in Goldsten Trombka et al. (1997).

XGRS has operated for several extended of the instruments, 60x 10” goals of the NEAR 5 et al. (1997) and E Q

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The XRS response can be monitored in-flight by rotating “Fe radioactive sources into the field of view of the three asteroid pointing detectors. The response of the Al filtered detector is shown in Figure 3. In the solid curve the rise-time discrimination is turned off, while in the dashed curve it is enabled at energies above 2 keV. About 60% of the background is rejected in this example and losses in the 5.9 keV photopeak are only about 5%.

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Channel Number Fig. 3. Response of Al filtered x-ray proportional counter to Fe-55 source. The solid curve is rise-time discrimination off and the dashed curve is rise-time discrimination

The first was on

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---es40keV ---p > 100 keV Unfiltered Detector

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UTC Fig. 4. Calculated electron and proton fluxes encountered during Earth swing-by (left axis), and corresponding count rate in the unfiltered proportional counter (right axis).

due to the large solar particle events of November 1997, and the second occurred during the Earth Swing-By maneuver that occurred on 23 January 1998. In each case the response of the proportional counters was degraded, but recovered a few weeks later (Floyd et al., 1998). Figure 4 shows count rate data for the unfiltered proportional counter during the three hours surrounding the closest approach to the Earth during the swingAlso displayed are electron and by. All events above the lower level discriminator (-0.6 keV) are included. proton fluxes calculated using NASA’s AE8 code. For the ten minute period just prior to and fifteen minutes after closest approach (07:13 through 07:38 UTC) the high voltage for all detectors was commanded off to safing of the proportional counters due to avoid any damage due to the high count rates. Also, autonomous high particle fluxes caused the high voltages to cycle off two times before the detectors stayed on at about 30 The count rate in the proportional counter tracked changes in the electron minutes after closest approach. fluxes more closely than those of the protons, because of the greater flux of electrons that reached the NEAR spacecraft and interacted in the proportional counter. During passage through the outer radiation belts where only electrons contribute, the copper collimator as well as the Mg and Al filters were excited as shown in Figure 5. The K-alpha lines at 1.25, 1.49, and 8.03 keV from Mg, Al, and Cu, respectively are seen and serve as an additional confirmation of the energy scale of the detectors.

During cruise, the gain in the GRS central NaI detector has slowly decreased due to aging of the photomultiplier tube (PMT), while the BGO has been much more stable. The NaI PMT is an older design and is more than that of the BGO. During 40 daycof continuous sensitive to the aging effects of the space environment operation in the fall of 1996 the BGO gain remained nearly constant, while the NaI gain decreased by about 7000 I I r I I 18%. Since the gamma-ray fluxes from the surface of Mg Eros are such that long accumulation times will be ---Al MgKa(1.25keV) required for elemental abundance analysis, gain correction is essential. Figure 6 shows the NaI anticoincidence spectrum with and without gain correction for the 40 days of summed data. The improved definition of background gamma-ray lines in the gain corrected spectrum is evident. Though this aging effect is irreversible, it does not pose a problem for the GRS experiment since the effect can be off-set by increasing the PMT high voltage. Activation of the BGO shield during passage through 250 150 200 0 50 100 the Earth’s radiation belts was observed after the January 23 swing-by. Analysis of the decay of several Channel Number of these lines is a useful check on the identification Fig. 5. Mg (solid) and Al (dashed) filtered proportional and understanding of the background that must be counters. Excitation of fluorescent lines during Earth removed for quantitative abundance measurements. A swing-by is seen.

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Nal AC Raw --.-- Nal AC Adjusted (x2)

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Energy (keV) Fig. 6. Raw (solid) and gain adjusted (dashed) Nal spectra summed over about 40 days. Several background lines have been identified. The gain adjusted spectrum has been multiplied by two for cl&y.

sample of one such measurement is shown in Fipre 7, where the decay of a line at 1077 keV from Ga is shown. This line is a spallation product, most likely due to 72Ge(p,cx)68Ga. The measured half-life, 3241 +751 s (l-o) is in good agreement with the actual value of 4080 s. 5-

CONCLUSION The extensive periods of detector on-time prior to orbital operations at Eros have been essential to our understanding of the operation and response of the XGRS. These measurements have given us increased confidence in our ability to detect and analyze the xray and gamma-ray emissions from the surface of Eros and accomplish our goal of mapping its surface and understanding its origins.

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66Ga- 1077 keV T,,2= 32415751 s (Measured) T,, = 4080 s (Actual) ’ ’ ’ ’ ’ ’ ’ 2000

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Time (s) Fig. 7. Decay of Ga-68 1077-keV line vs. time.

ACKNOWLEDGMENTS The authors wish to thank B. J. Anderson of the NEAR Magnetometer tions. We are also grateful to the NEAR mission operations team at the ratory whose cooperation and support along with that of the program Scientist, A. F. Cheng, has made possible the collection of thousands of

team for performing the AE8 calculaJohns Hopkins Applied Physics Labomanager, T. B. Coughlin and Project hours of cruise data.

REFERENCES Clark, P. E., J. I. Trombka, and S. Floyd, Lunar Planet. Sci. XXVI, 253 (1995). Evans, L. G.,. R. Starr, J. I Trombka, J. Bruckner, S. H. Bailey, J. 0. Goldsten, and R. L. McNutt, Jr., Preprints of the Second IAA International Conference on Low Cost Planetary Missions, IAA-L-0905P, Laurel, Maryland, USA, April 16-19 (1996). Floyd, S. R., J. I. Trombka, P. E. Clark, R. Starr, H. W. Leidecker, J. 0. Goldsten, and D. R. Roth, Nucl. Inst, and Meth. In Phys. Res. A 422 577 (1998). Goldsten, J. O., R. L. McNutt, Jr., 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 Sci. Rev. 82, 169 (1997). Trombka, J. I., W. V. Boynton, J. Bruckner, S. W. Squyres, P. E. Clark, R. Starr, L. G. Evans, S. R. Floyd, E. Fiore, R. E. Gold, J. 0. Goldsten, R. L. McNutt, Jr., and S. H. Bailey, J. Geophys. Res. 102, 23729 (1997).