the near x-ray/gamma-ray · pdf filehe x-ray/gamma-ray spectrometer on the near earth asteroid...

J. O. GOLDSTEN

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The NEAR X-Ray/Gamma-Ray Spectrometer

John O. Goldsten

he X-Ray/Gamma-Ray Spectrometer on the Near Earth Asteroid Rendezvous(NEAR) spacecraft will remotely sense X-ray and gamma-ray emissions from thesurface of near-Earth asteroid 433 Eros and look for characteristic “signatures” in thoseemissions to determine its elemental composition. During the planned 10 months oforbits about Eros beginning in early 1999, data returned from the instrument will beused to develop global maps of the asteroid, including the content and variations ofits surface composition, thereby providing important clues about its nature and origin.(Keywords: Gamma-ray, NEAR, Spectrometer, X-ray.)

INTRODUCTIONThe Near Earth Asteroid Rendezvous (NEAR)

mission was launched on 17 February 1996. NEAR isthe first spacecraft under NASA’s Discovery Program,an initiative for small, low-cost planetary missions. Asthe first to orbit an asteroid, NEAR promises to answerfundamental questions about the nature and origin ofnear-Earth objects.

Launched from a Delta II rocket, the 805-kg NEARspacecraft will follow a DV Earth-gravity–assisted tra-jectory and take approximately 3 years to reach 433Eros, one of the largest near-Earth asteroids.1,2 Thistrajectory also provided a flyby of main-belt asteroid253 Mathilde in June 1997.

During the planned 10 months at Eros, a suite ofremote sensing instruments will make detailed scienceobservations of the gross physical properties, surfacecomposition, and morphology of the asteroid. Fourmonths of the rendezvous are scheduled with orbitsas low as 35 km, corresponding to altitudes as low as15 km above the asteroid surface.

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The X-Ray/Gamma-Ray Spectrometer (XGRS),one of five major instruments on the NEAR spacecraft,is the primary experiment for determining the surfaceand near-surface elemental composition of Eros. Otherinstruments in the science payload include the Mul-tispectral Imager (MSI), Near-Infrared Spectrometer(NIS), Magnetometer, and NEAR Laser Rangefinder.The MSI will image the surface morphology of Eroswith spatial resolutions down to 5 m. The NIS will bethe primary instrument for measuring mineral abun-dances with a spatial resolution on the order of 300 m.The combined observations of the XGRS, MSI, andNIS should provide full coverage of Eros, yieldingglobal maps of its surface composition.

The XGRS is a complex instrument with littledesign heritage. To meet the strict schedule constraintsof the Discovery Program, the successful developmentof the XGRS required a focused team working closelywith other NEAR instrument teams. Formal work onXGRS began in December 1993. The fully tested and

NS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)

calibrated instrument was delivered to the NEARspacecraft in August 1995, a development time of lessthan 22 months.

X-RAY AND GAMMA-RAYSPECTROSCOPY: THEMEASUREMENT TECHNIQUE

The XGRS is a remote sensing instrument. Fromorbits of 35 to 100 km, it remotely senses the charac-teristic X-ray and gamma-ray emissions from the aster-oid surface. Remote sensing of this type is only possiblefor bodies with little or no atmosphere to absorb theseemissions. Orbital, rather than flyby, missions are pre-ferred for such measurements because of the typicallylong observation times required. The XGRS is de-signed to have sufficient sensitivity to distinguishbetween major meteorite types and will either identifythe type of meteorite to which Eros is linked or confirmthe absence of a relationship.3

X-ray spectroscopy from an orbiting spacecraft canbe used to identify elemental composition at the sur-face of an airless body in the following manner.4,5 X-rays from the Sun shining on Eros will produce signif-icant X-ray fluorescence from the elements containedin the very surface of the asteroid (the top 1 mm).When an element fluoresces, it emits X-rays with dis-crete energies that are unique or characteristic of thatelement (the strongest being Ka X-rays). The asteroid-pointing portion of the X-Ray Spectrometer (XRS)detects these characteristic X-rays in the 1- to 10-keVenergy range. From an analysis of the measured energyspectrum, elemental composition can be inferred.

Although the intensity of detected X-rays is a strongfunction of solar illumination and activity, the expect-ed returns should be sufficiently intense to measure thesurface abundances of elements Mg, Al, Si, Ca, and Fewith spatial resolutions down to 2 km. To ensure aproper quantitative analysis, XRS points two addition-al X-ray detectors sunward to directly measure theincident solar flux. XRS signal strength will, in gen-eral, improve as the Sun approaches solar maximumand will be particularly strong during the random solarflares that produce brief periods of high-level, high-energy solar X-rays.

Remote gamma-ray spectrometry provides a com-plementary measure of near-surface elemental compo-sition.6–8 The Gamma-Ray Spectrometer (GRS) de-tects discrete-line gamma-ray emissions in the 0.1- to10-MeV energy range. In this range, spectroscopy canbe used to measure the abundance of O, Si, Fe, andH, which become excited or activated (radioactive)owing to continual bombardment by galactic cosmicrays. In addition, it can detect the presence of thenaturally radioactive elements K, Th, and U. Unlikethe low-energy X-rays, gamma rays are not as easilyabsorbed and therefore can escape from regions

JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1

THE NEAR X-RAY/GAMMA-RAY SPECTROMETER

beneath the top surface. This allows the GRS to probedeeper than the XRS to a surface depth on the orderof 10 cm.3

Solar illumination does not significantly affect thegamma-ray coverage of the asteroid, but the generallyweak gamma-ray emissions require the GRS to have awide field of view and to integrate for long periods oftime to achieve sufficient sensitivity to determine el-emental abundances quantitatively. Therefore, theGRS has very limited spatial resolution, perhaps only5 to 10 pixels, characterizing the entire asteroid.However, to the extent that X-rays and gamma rays canbe compared for the same element, differences inmeasured concentrations by the X-ray and gamma-rayexperiments should reveal the depth and extent of adust layer (regolith) on the asteroid.3

INSTRUMENT OVERVIEWThe philosophy of NEAR was to conceive a compre-

hensive science mission that could answer fundamentalquestions about the nature and origin of near-Earthasteroids while meeting the schedule and cost con-straints of the Discovery Initiative. To that end, thespacecraft has the necessary sophistication to achievethe mission goals (e.g., a three-axis–stabilized platform),yet also has the attributes of a simple, robust design withits fixed high-gain antenna and solar panels.

This philosophy translated to the science instru-ments as well. All instruments are fixed to the space-craft and rely on a passive thermal design. For the GRS,this setup excluded the use of an elaborate cryogeni-cally cooled detector mounted on a boom in themanner of Mars Observer.6 For the XRS, high cost anddevelopment time excluded the use of detectors madeof solid beryllium like those used on Apollo.9,10 Despitethese constraints, the XGRS contains a suitable com-plement of detectors that possess the reliability tosurvive a long-duration mission and provide sufficientsensitivity and resolution to measure and map thesurface composition of Eros.

The XGRS is partitioned as several assemblies ofdetectors, electronics, and a cabling harness. Thesecomponents are distributed over three deck surfaces ofthe NEAR spacecraft (Fig. 1). The asteroid-pointingX-ray and gamma-ray detector assemblies are mountednext to each other on the aft deck along with the otherscience instruments. Nearby are detector electronicsthat contain all of the analog signal processing func-tions. Behind the detector electronics package is adedicated data processing unit (DPU), which executesall instrument software, provides the digital interfaceto the spacecraft, and supplies instrument power andcontrol.

Two additional X-ray detectors sit atop the forwarddeck so they can monitor the solar X-ray flux. Knowl-edge of the input flux is required for quantitative

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Solar panel

Propulsionsubsystem

X-ray solarmonitors

Forwarddeck

Aft deck Gamma-RaySpectrometer

X-Ray Spectrometer

NEAR Laser RangefinderNear-Infrared Spectrograph

Multispectral Imager

Side panels

z

x 9 y 9

Figure 1. View of the NEAR spacecraft. The aft deck contains theinstrument palette and is almost completely in shadow. Thespacecraft z axis is directed through the high-gain antenna, whichis nominally pointed toward Earth. Instruments are oriented sothat their boresights are nearly coaligned with the x9 axis. The twoX-ray solar monitors (not visible) reside on the forward deck toalways face toward the Sun within 30°.

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analysis of the X-ray spectra from the asteroid.4 Thesolar monitors’ fields of view are normal to the solarpanels and are located above one corner of the deckso that they are not obscured by the high-gain anten-na. The spacecraft is generally oriented with its high-gain antenna directed toward Earth. With this orien-tation, the Sun is always within 30° of the central axisof the spacecraft.

For reliability, each of the seven XGRS detectorshas its own high-voltage power supply. The supplies aredivided into three stacks located on the inside surfaceof the aft deck. A separate electronics module tocontrol the high-voltage supplies is mounted atop thedetector electronics package.

The XGRS instrument (Fig. 2) has a total mass,including the DPU, of 27 kg and dissipates a maximumof 34 W. The power system allows power switching ofthe high-voltage subsystem, thermoelectric cooler, andoperational heaters.

The NEAR spacecraft’s solid-state recorders anddownlink are limited resources, and their use and

Figure 2. Functional block diagram of the XGRS including the asteroid-pointing X-ray gas proportional counters,gamma-ray scintillation detectors (BGO and NaI) with photomultiplier tubes (PMT), shared preamp module, Si-PINphotodiode and gas proportional counter solar monitors with their associated preamps, and the central detectorelectronics box, which contains the pulse shapers and pulse-height analyzers (PHA). Spectra are formed in the dataprocessing unit (DPU), which also provides the spacecraft interface.

Thresholds

X-ray gasproportional

counter


counter


counter

X-raypreamp

(3 channels)

Gamma-raypreamp

(2 channels)

PMT Nal PMT

High-voltagecontrol

electronics(7 channels)

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Junction box

Detector electronics

X-ray PHA(2 channels)

X-ray PHA(2 channels)

Si-PIN PHA

X-ray shaper(4 channels)

Gamma-rayPHA

DPU interface

Bac

kpla

ne

Solid-statedetector

Si-PINpreamp

Steppermotor

DPU

Set points

Monitors

PHA data

Event rates

Cooler control

Monitors

Power

Active thermal control

Motor control and position indicators

Graded shield

Mgfoil

Alfoil

Calibrationrod (Fe-55)

Power/monitors

BGO

Gamma-rayscintillation detectors

X-raycollimator

X-ray solar monitors

AsteroidX-ray

detectors

Gas solarmonitorpreamp


counterMiniature

cooler


allocation are regularly negotiated throughout the mis-sion. Telemetry data rates for the XRS and GRS areindependently adjustable and may be raised or loweredby commanding shorter or longer integration periodsfrom 60 to 60,000 s. In this way, the XGRS can easilyrespond to changes in telemetry allocation. At Eros,the expected telemetry rates for the GRS and XRS willnominally be 70 and 180 bits/s, respectively. An addedsummary science mode provides a reduced set ofhighly compressed spectra from both experiments andcombines them into a single data stream with a telem-etry rate of only a few bits per second. This modeis particularly useful for checking the health andsafety of the instrument and for verifying its operatingconfiguration.

Both spectrometers are energy-dispersive ratherthan wavelength-dispersive systems. For this type ofinstrument (Fig. 3), a single incoming photon is ab-sorbed by the detector material (solid or gaseous) andproduces an output signal proportional to the energyabsorbed. The output signal is generally in the form ofa charge pulse, which may last from a few nanosecondsto several hundred nanoseconds. A charge-sensitivepreamplifier collects the charge from the detector andproduces a voltage step proportional to the totalamount of charge. A linear filter network then ampli-fies and shapes the preamplifier signal to reduce noiseand provide a smooth signal more suitable for “pulse-height” analysis, a process in which the peak value ofeach pulse is measured using an analog-to-digital con-verter. The DPU collects these measurements and bins

Figure 3. Generalized detection scheme for the XGRS. A singlephoton is absorbed in a detector and produces an electrical signalproportional to the energy. The electrical signal is amplified andshaped into a pseudo-Gaussian pulse. The peak value is capturedby a sample and hold (S/H) circuit and measured by an analog-to-digital (A/D) converter. Energy measurements are passed to adata processing unit (DPU) where they are histogrammed to forman energy spectrum. The discriminator is triggered whenever thesignal is above the electronics noise level. Pile-up rejectioncircuitry disallows analysis of photon events spaced too closely intime for an accurate measurement.

Detector

Preamp Shaper S/H +A/D DPU

Pile-uprejectioncircuitry

Dualdiscriminator

Energyspectrum

Preamp Energy

Timing

Photon



them according to pulse height or energy absorbed.After an appropriate accumulation time, in whichmany photons are processed, a complete pulse-heightdistribution or energy spectrum is accumulated andtelemetered to Earth. From an analysis of the teleme-tered spectrum, elemental composition can be in-ferred.

Timing circuits detect the arrival of photons when-ever the energy signals exceed a commandable thresh-old, which is generally set just above the noise levelof the electronics. The timing circuits also detect thenear-simultaneous arrival of photons (random, inde-pendent events) that generate a condition called pile-up and corrupt the energy measurements. Pile-up re-jection logic automatically discards these events toensure the highest-quality data.

This generalized detection scheme forms the basicapproach to the design of the XGRS. Additional tech-niques are employed to reduce unwanted backgroundsignals, enhance sensitivity, and provide long-termstability and in-flight calibration. Table 1 summarizesthe important characteristics and specifications of theinstrumentation.

INSTRUMENT DESIGN

Gamma-Ray SpectrometerAs noted previously, restrictions on mass, cost, and

mission duration precluded the use of the type ofgamma-ray detector on Mars Observer, which was acryogenically cooled, high-purity Ge detector mount-ed on an extended boom to remove it from the localbackground radiation of the spacecraft. Instead, theNEAR GRS employs an NaI(Tl) (thallium-activatedsodium iodide) scintillator mounted within a cupshield made of bismuth germanate (BGO). The denseBGO cup is itself an active scintillator and thus worksto reduce the Compton and pair-production contribu-tions to the unwanted background signal. In addition,it provides direct, passive shielding from the localgamma-ray environment. This design eliminates theneed for a boom and allows the detector to be body-mounted to the spacecraft.

The gamma-ray detector subassembly (Fig. 4) wasdesigned and built by EMR Photoelectric, a divisionof Schlumberger. The central detector assembly isbased on a ruggedized NaI(Tl) unit used in oil welllogging operations. The BGO cup was designed espe-cially for NEAR and is fabricated from a single crystal.The mass of the complete detector assembly is 6 kg.

Scintillation detectors produce a light intensityproportional to the energy absorbed with a wavelengththat is characteristic of the detector material. TheNaI(Tl) and BGO crystals each optically couple to aphotomultiplier tube (PMT) equipped with an opti-mized photocathode to convert its light output to

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13

Table 1. Instrument characteristics.

Component Specifications

GRSPrime detector NaI(Tl) scintillator, 2.54 × 7.62 cmShield detector BGOa scintillator cup, 8.9 × 14 cmEnergy range 0.1 to 10 MeV, 1024-channel spectraNaI(Tl) energy resolution 8.7% fwhmb @ 662 keVBGOa energy resolution 14% fwhm @ 662 keVField of view ≈50° fwhm @ 145 keVAnticoincidence background rejection >500:1 above 5 MeVCounting rate 10 kHz maximum (1 kHz nominal)Mass 6 kg

XRSX-ray detectors (3) Single-wire gas-filled proportional countersWindow 25-mm beryllium with 2-mm grid supportActive aperture area 25 cm2 per detectorEnergy range 0.5 to 10 keV, 256-channel spectraEnergy resolution 850 eV fwhm @ 5.9 keVCollimator Small-cell honeycomb, Be-Cu foilField of view 5° fwhm @ 5.9 keVRise time rejection of background >50% above 2 keV; >70% @ 6 keVCounting rate 10 kHz maximum (1 kHz nominal)In-flight calibration sources Fe-55; rotated into field of view by command

X-ray solar monitorsEnergy range 1 to 10 keV, 256-channel spectraField of view 60°Counting rate 10 kHz (strong solar flare)Redundant detectors

Gas-filled proportional counter Graded shield design; 1 mm2 equivalent areaSolid-state detector Si-PIN photodiode, 1.1 × 1.1 mm, mounted

on miniature thermoelectric coolerEnergy resolution Gas: 850 eV fwhm @ 5.9 keV

PIN: 600 eV fwhm @ 5.9 keV

a Bismuth germanate.bFull width at half maximum.

electrical signals and provide a signal gain of roughly100,000. Crystal and PMT combinations were care-fully chosen by EMR to achieve the best energyresolution.

The measured energy resolutions for the NaI(Tl)and BGO detectors mounted in the flight configura-tion are 8.7% and 14% full width at half maximum(fwhm), respectively, at an energy of 662 keV (thestandard gamma-ray calibration line of Cs-137). Thepresence of the small PMT between the NaI crystaland the incident gamma rays introduces a small but

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characterizable loss in sensitivity for the NaI detectorthat is a function of both energy and angle.3

The flight spare detector exhibits nearly identicalperformance and will be used extensively throughoutthe mission to further characterize detector perfor-mance and develop a detector model. Although theenergy resolution of a NaI(Tl) scintillator is not nearlyas good as that of a cooled Ge detector,8 the NEARdetector operates at room temperature, is not subjectto any serious radiation damage (important for long-duration missions such as NEAR), and, given the



integration times planned for the mission, meets themeasurement requirements for the elements citedpreviously.

Temperature and voltage stability are extremelyimportant to maintain system performance. The lightoutputs of the NaI(Tl) and BGO scintillators varysignificantly with temperature. Temperature fluctua-tions during asteroid operations would adversely affectdetector calibration, so the detector assembly is ther-mally isolated and wrapped with operational heaters tostabilize the gamma-ray detector temperature to within0.25°C. The signal gains of the PMTs are not partic-ularly sensitive to temperature, but are very sensitiveto voltage variations. The PMT high-voltage powersupplies must be stable to a fraction of a volt overtemperature so as not to adversely affect GRS calibra-tion. To meet this requirement, the XGRS uses anexternal feedback control system to produce ultra-stable high-voltage outputs from ordinary high-voltagepower supplies.

The cup shield acts as an active collimator. Particlesor gamma rays entering through the shield will produce

Figure 4. NEAR gamma-ray detector: (a) sensor assembly; (b)detector cross section showing central NaI(Tl) detector situatedwithin an active BGO cup shield to reduce unwanted backgroundsignals by almost 3 orders of magnitude. The photomultipliertubes (PMTs) at each end convert the light output of the scintilla-tion detectors into electrical signals.

Support

Thermalspacers

Gamma-raydetector

Connector

Clamp

Teflonspacers

Aftdeck

Outboard

(a)

29.4 cm

8.90-cmdia.

Spring

Teflon wedge Spring

LargePMT

Opticalcoupling

Opticalcoupling

SmallPMT

Nal(Tl)crystal

BGOshield

(b)


output signals in the shield detector that are coinci-dent with signals produced in the central detector. Ananticoincidence system detects and rejects theseevents. The cup design confines the field of view toabout 45° at lower energies. At higher energies, thefield of view is somewhat larger.3

With this detector configuration, much of the com-position signal is lost at higher energies owing to pairproduction. One or both of the gamma rays producedby annihilation will probably escape the small centralcrystal and be absorbed in the shield. This energy lossappears in the raw NaI spectrum as secondary peaksshifted down in energy from the photopeak by either0.511 or 1.022 MeV, named the first and second escapepeaks, respectively. In the anticoincidence spectrum,these escape peaks are suppressed because of eventrejection by the anticoincidence system.

For many gamma rays of interest, more events arecontained in the first and second escape peaks than inthe photopeak itself, and rejection of these eventswould cause an unacceptable loss in sensitivity. Torecover this information, the GRS watches for anycoincident events with shield energies falling near0.511 or 1.022 MeV and produces two additionalspectra containing only these types of events. Al-though these coincidence modes are somewhat vulner-able to false detections arising from galactic cosmicrays and Compton scattering, extensive testing andcalibration have demonstrated the overall improve-ment in performance using this new technique.3

Extensive ground calibrations were performed onthe GRS using various radioactive sources.3 For in-flight energy calibration, the GRS relies on prominentspectral lines such as the 511-keV annihilation lineand others generated by the activation of the NaI(Tl)and BGO crystals due to galactic cosmic-ray bombard-ment. Periodic instrument calibrations obtained dur-ing the cruise to Eros and in orbit will be validatedusing ground-based measurements on the flight sparedetectors and will be compared with measurementsfrom the other NEAR instruments and Earth-basedobservations. Figure 5 shows pulse-height spectra ob-tained in flight from the GRS.

X-Ray SpectrometerThe XRS consists of three identical gas-filled pro-

portional counters designed and built by MetorexInternational Oy of Finland. Gas tubes of this typeprovide the large active area and therefore the sensi-tivity required for remote sensing. Similar detectorshave been flown on lunar orbital missions and mostrecently on Apollo missions 15 and 16.4,10,11 TheNEAR XRS uses an updated design and achieves anenergy resolution of 850 eV fwhm at 5.9 keV. Theenergy range of the detectors extends from 0.5 to 10

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J. O. GOLDSTEN

keV. Figure 6 shows an exploded view of the majormechanical assemblies of the X-ray sensor.

The gas proportional counters are similar to thegamma-ray detectors in that they detect individualphotons. An incoming X-ray photon penetrates thethin window of a sealed gas-filled chamber and inter-acts with the gas, producing an energetic photoelec-tron (the photoelectric effect). The kinetic energy ofthe electron is then progressively absorbed in the gas,leaving an ionization trail. The free electrons from theionization are attracted to a thin wire anode at highpotential stretched down the center of the chamber.In the region of high electric field strength, very closeto the wire, the electrons reach sufficient energy toliberate even more electrons in a stable multiplicationeffect that provides signal gain. This signal gain helpsto overcome the preamplifier noise such that theenergy resolution of the gas tubes is primarily limitedby the Poisson statistics of the ionization process.

Occasionally, an incoming X-ray photon with suf-ficient energy will interact with an inner-shell electronof the gas (primarily argon for the XRS). In this case,the resulting photoelectron is less energetic than ex-pected owing to the significant energy required forionization. Eventually, a nearby free electron will “fall”into the ionized atom, resulting in the emission of anX-ray with an energy characteristic to argon (3 keV).Usually, this argon X-ray is quickly reabsorbed in thegas, and so the total energy measured by the detectorremains unchanged. However, if the argon X-ray es-capes the gas tube without being absorbed, the mea-sured energy will appear lower by the escaped amount.This overall effect produces “escape” peaks in thespectra that are downshifted in energy by 3 keV fromthe primary peaks, and with intensities roughly 10%

Figure 5 Background spectra obtained in flight with the NEARNaI detector. The accumulation time is approximately 38 days.The anticoincidence spectrum reveals structure that cannot beobserved in the raw spectrum.

Raw pulse-height spectrum

Spectrum w/anticoincidence discrimination

Spectrum w/1.22-MeV coincidenceSpectrum w/0.51-MeV coincidence

Hydrogen (2.22 MeV)

Carbon (4.44 MeV)Oxygen (6.13 MeV)

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Bi210 (2.6 MeV)

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of the primary peaks. Of course, this effect does notoccur for emission lines with energies below 3 keV.

Flight Detectors

The flight detectors (Fig. 7) each have a rectangularaperture with an active area of 25 cm2. The windowmaterial is beryllium foil, 25 mm thick, supported byan external grid made of solid beryllium. The detectorhousing is stainless steel with a beryllium liner toabsorb fluorescence X-rays arising from the housingitself. The chamber is filled with a gas mixture of 90%argon and 10% methane to an absolute pressure slight-ly more than 1 atm. The anode wire is gold-platedtungsten with a diameter of 13 mm.

Unlike the gamma-ray detectors, the X-ray gastubes are not particularly sensitive to temperature andmay be operated over a wide temperature range, since

Figure 6 . Exploded view of the NEAR X-ray sensor assembly.

Calibration rodassembly

Collimatorassembly

Filter assembliesX-ray detectors

Support structure

Interface to aft deck

Preamp and thermal conductor


the multiplication effect depends more on the numberof gas molecules than the gas pressure. However, likethe gamma-ray PMTs, the gain in the gas tubes issensitive to voltage variations. To maintain calibra-tion, the XRS shares with the GRS the special feed-back control system that maintains ultrastable high-voltage outputs.

The gas proportional counters exhibit a high back-ground from cosmic rays because of their relativelylarge size (compared with a solid-state detector, forexample). To reduce this background, the XRS em-ploys rise-time discrimination circuitry to reject up to70% of these background events. The cosmic raysusually interact at the walls of the detectors and leavelong ionization trails that exhibit characteristicallylonger charge-collection times. The performance ofrise-time rejection is a function of energy and gainslittle below 2 keV (Mg, Al, Si emission lines occur at1.25, 1.49, and 1.74 keV, respectively), but significant-ly improves the signal-to-background ratio at higherenergies, especially for weak emission lines such as Fe(6.4 keV).

X-Ray Filters

The energy resolution of the gas tubes is not suffi-cient to resolve the closely spaced Mg, Al, and Si lines.To separate these lines, the XRS uses balanced filters,exploiting the small energy difference between the KaX-ray lines and the K-absorption edge in Mg and Al.3

An external Mg filter, 8.5 mm thick, on one of theouter detectors attenuates the Al line, and a similar Alfilter on the other outer detector attenuates the Mgand Si lines. The very steep absorption edges of thefilters make the separation of the lower-energy linespossible. At higher energies, the filters are essentiallytransparent and the Ca and Fe lines are resolved di-rectly by the detectors. The center detector has nofilter.

High-voltageconnector

Anode wire Berylliumgrid support

16.6 cm4.7-cm

dia.

Figure 7 . Outline (top) and cross-sectional view (bottom) of aNEAR gas-filled proportional X-ray detector.



X-Ray Collimator

The XRS requires a collimator to provide spatialresolution and eliminate unwanted X-ray sky back-ground. Since the pure beryllium machined collimatorused on the Apollo missions was too massive andcostly, a compact, honeycomb structure fabricatedfrom Be-Cu foil was substituted for the XRS. Copperis an acceptable material because characteristic Cu X-rays fall above and below the energy line emissionsfrom the asteroid surface (beryllium X-rays all liebelow the energy threshold).

To make the collimator, over 50 Be-Cu foils werefirst corrugated and then hand-assembled using anepoxy containing only light elements. The honey-comb section was then mounted in a sheet-metalframe. The collimator achieves better than 99% trans-mission, and its small-cell design eliminates the needfor precise alignment with the detectors.

X-Ray Calibration Rod

To enable calibration verification of the asteroid-pointing detectors, a Vespel rod containing three Fe-55 radioactive sources spans the X-ray detector assem-bly near the base of the collimator. The bottom edgeof the collimator is beveled to allow the rod to rotatethe sources into view of the detectors for periodic in-flight calibration without sacrificing active area. Anouter copper tube shields the sources when not in use.A stepper motor mechanism at one end directly drivesthe rod, and a pair of optical reference holes report itsposition.

The radioactive source holders are embedded axiallyin the rod, one at each detector location and at 45°from each other so that only one detector can be incalibration at a time. Pinholes in the outer copper tuberestrict the photon rate and also act as crude collima-tors to minimize source cross talk between detectorsand ensure maximum science return in the event of afailed mechanism. An in-flight pulse-height spectrumof the Mg-filtered proportional counter with the Fe-55source in the field of view is shown in Fig. 8.

Solar Monitors

The two sunward-pointing X-ray detectors posi-tioned on the forward deck of the spacecraft monitorthe incident solar flux. These monitors must possessa wide 60° field of view to observe the Sun contin-uously throughout the mission. Unlike the asteroid-pointing detectors, which must collect the weak emis-sions from the asteroid surface, the solar monitorsexperience the very strong X-ray emissions comingdirectly from the Sun, especially during solar flares.Therefore, the active area for a solar monitor is only1 mm2 compared with the 2500-mm2 active area of asingle gas tube.

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Figure 8. Pulse-height spectrum of the Mg-filtered proportional counter with the Fe-55source in the field of view. The fit to the spectrum (red curve) is accomplished by includingthe Ka (5.89-keV) and Kb (6.54-keV) emission lines in both the full energy and escapepeaks (blue curves) plus a polynomial for the background (green curve). The fit for the full-energy Ka line gives a fwhm of 840 eV.

The allowance for a small active area permitted theuse of a new miniature X-ray detector developed andbuilt by AMPTEK, Inc. (Fig. 9). The compact sensorconsists of a Si-PIN photodiode mounted on a min-iature thermoelectric cooler inside a hermetic packagethat is only 15 mm in diameter. Also mounted on thecooler are the input field-effect transistor and feedbackcomponents of the charge-sensitive preamplifier aswell as an integrated circuit temperature monitor. A76-mm beryllium window rejects the intense solar fluxbelow ~1 keV. The Si-PIN solar monitor achieves anenergy resolution of 600 eV fwhm at 5.9 keV.

Early tests indicated that exposure to ionizing radi-ation could produce significant damage in the detector.However, subsequent testing demonstrated that thedetector would anneal almost completely when heatedto 100°C for 24 hours. To address the possibility ofradiation damage during the mission, the XGRS con-tains circuitry to monitor the leakage current of theSi-PIN detector and, if necessary, to reverse the oper-ation of the thermoelectric cooler to provide in-flightannealing.

Since reliance on this new Si-PIN X-ray detectorfor such a critical function as the solar monitor wasconsidered too risky, a gas proportional counter, iden-tical to the asteroid-viewing ones, is also mounted onthe sunward deck. In this case, the large active areaof the gas tube needed to be severely restricted. Ratherthan using a simple pinhole, the gas tube is equippedwith a specially designed graded shield made fromseveral layers of progressively denser materials, eachwith a unique aperture size, to enhance detector

Figure 9 . Miniatureflux.

rely on prominenflight calibration.obtained in flighdetectors.

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Energy (keV)

Temperaturemonitor

Field-effecttransistor

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X-ray detector used to monitor solar X-ray

sensitivity at higher energies. Atlow energies, the shield has anequivalent aperture size of roughly1 mm2. With increasing energy,the higher-density layers becomeincreasingly transparent, openingup the effective area of the detec-tor and increasing its sensitivity.Such a modified energy response isbetter matched to the solar spec-trum that follows a power law andmay drop 4 orders of magnitude inintensity from 1 to 10 keV.

Telemetry data may be selectedfrom either solar monitor, or a“toggle mode” can be selected toalternate the data return fromeach. In this mode, the instrumentprovides nearly continuous obser-vations from both solar monitorsand allows close cross-calibrationbetween the two detectors with-out increasing the overall instru-ment data rate. The solar monitors

t line emissions from the Sun for in- Figure 10 shows pulse-height spectrat with both the gas tube and Si-PIN

Beryllium window

Feedbackcapacitor resistor

PIN photodiode(1.1 3 1.1 mm)

Hybrid package(15-mm dia.)

Stud

Peltier cooler

HNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998)


Figure 10. A comparison of solar spectra from the gas tube andsolid-state solar monitors. The bottom two traces show spectraduring a quiescent period. Note the significantly lower backgroundin the solid-state detector (green trace). The upper two tracesshow spectra during a solar flare. The predominant line near 6.6keV indicates strong Fe X-ray emissions, nearly 4 orders higherthan during quiescent conditions. The gas tube spectrum (redtrace) clearly shows the effect of the graded shield in “flattening”the detector response to solar input.

SUMMARYThe unique measurements provided by the NEAR

XGRS will make significant contributions to our un-derstanding of the composition and evolution of 433Eros and to near-Earth asteroids in general. By remote-ly sensing characteristic X-ray and gamma-ray emis-sions, the instrument will provide the first elementalabundance map of a small body. These measurements,which are sufficient to distinguish between majormeteorite types, will establish whether there is a chem-ical relationship between meteorites and the asteroidsconsidered to be their parent bodies.

The design approach of the XGRS enables theinstrument to achieve major science objectives in amanner unlike previous instruments, which relied onmassive booms, cryogenically cooled detectors, or

105

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0 1 2 3 4 5 6 7 8 9 10

Energy (keV)

Sol

ar fl

ux (

coun

ts)

Gas tube

Gas tube

PIN

PIN

expensive beryllium structures. In this way, the XGRSdesign is consistent with the demanding goals ofNASA’s Discovery Program to further our knowledgeof planetary science using innovative, low-costmissions.

REFERENCES1Farquhar, R. W., Dunham, D. W., and McAdams, J. V., “NEAR Mission

Overview and Trajectory Design,” J. Astronaut. Sci. 43(4), 353–371 (Oct–Dec 1995).

2Santo, A. G., Lee, S. C., and Gold, R. E., “NEAR Spacecraft andInstrumentation,” J. Astronaut. Sci. 43(4), 373–397 (Oct–Dec 1995).

3Trombka, J. I., Boynton, W. V., Brückner, J., Squyres, S. W., Clark, P. E.,et al., “Compositional Mapping with the NEAR X-Ray/Gamma-Ray Spec-trometer,” J. Geophys. Res. 102, 23,729–23,750 (1997).

4Clark, P. E., and Trombka, J. I., “Remote X-Ray Spectrometry for NEARand Future Missions: Modeling and Analyzing X-Ray Production fromSource to Surface, J. Geophys. Res. 102(E7), 16,361–16,384 (1997).

5Mandel’shtam, S., Tindo, I. P., Cheremukhin, G. S., Sorokin, L. S., andDmitriev, A. B., “Lunar X Rays and the Cosmic X-Ray BackgroundMeasured by the Lunar Satellite Luna-12,” Cosmic Res. 6, 100–106 (1968).

6Evans, L. G., Reedy, R. C., and Trombka, J. I., “Introduction to PlanetaryRemote Sensing Gamma Ray Spectroscopy,” in Remote Geochemical Analysis:Elemental and Mineralogical Composition, C. Pieters and P. Englert (eds.),Press Syndicate of University of Cambridge, Cambridge, England (1993).

7Boynton, W. V., Trombka, J. I., Feldman, W. C., Arnold, J. R., Englert,P. A. J., et al., “Science Applications of the Mars Observer Gamma-RaySpectrometer,” J. Geophys. Res. 97, 7681–7698 (1992).

8Egiazarov, B. G., Kononov, B. N., Kurochkin, S. S., Surkov, Yu. A., andShekhovtsov, N. A., “The Gamma Spectrometer on Luna-10 for the Studyof Lunar Rocks,” Cosmic Res. 6, 223–228 (1968).

9Evans, L. G., Starr R., Trombka, J. I., Brückner, J., Bailey, S. H., et al.,“Performance of the Gamma-Ray Detector System for the NEAR Missionand Application to Future Missions,” in Proc. 2nd IAA InternationalConference on Low-Cost Planetary Missions, IAA-L-0905P, Laurel, MD(1996).

10 Adler, I., Trombka, J., Gerard, J., Lowman, P., Schmadebeck, R., et al.,“Apollo 15 Geochemical X-Ray Fluorescence Experiment: PreliminaryReport,” Science 175, 436–440 (1972).

11 Adler, I., Trombka, J., Gerard, J., Lowman, P., Schmadebeck, R., et al.,“Apollo 16 Geochemical X-Ray Fluorescence Experiment: PreliminaryReport,” Science 177, 256–259 (1972).

ACKNOWLEDGMENTS: The author thanks all XGRS team members for theiroutstanding performance in developing this instrument and, in particular, APL’sTechnical Services Department for the tremendous support in meeting the verydemanding schedule dictated by the NEAR program. Many thanks also go to JoanButtermore and Ray Thompson for their fine work in assembling the instrument,Paul Bierman for his dedicated effort on the X-ray collimator, Eric Fiore for hiswork with the X-ray filters, Dennis Fort for his extensive work on rise-timediscrimination, Dave Lohr for the design of the X-ray preamps, John Boldt andPeter Eisenreich for providing a terrific DPU, Susan Schneider and John Hayes fordeveloping uncompromisingly sophisticated and complex flight software, AlbertChacos for providing superb instrument GSE, and, finally, Rob Gold and HugoDarlington for their general expertise in scientific instruments and helpful nature.This work was supported under contract N00039-95-C-0002 with the U.S. Navy.

JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 19, NUMBER 2 (1998) 135

THE AUTHOR

JOHN O. GOLDSTEN received a B.S. from The University of Pennsylvania in1982 and an M.S. from The Johns Hopkins University in 1986, both inelectrical engineering. He joined APL in 1982, and is currently a member of theSenior Staff in the Space Department’s Space Sciences Instrumentation Group.He has worked on several space-flight instruments including X-ray and gamma-ray spectrometers, ultraviolet spectrographic imagers, and energetic particledetectors. His e-mail address is [email protected].

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