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The GRS central detector assembly is based on a ruggedized thallium-activated [ Tl] sodium iodide

[ NaI] scintillator unit used in oilwell logging operations, designed and built by Schlumberger (below). NaI-based scintillators are widely used in downhole logging-tool applications to make measurements of density, natural radioactivity and elemental spectra. As an example, the EcoScope multifunction logging-while-drilling tool uses a NaI detector to make while-drilling spectroscopy measurements.2 8 use different materials.

Other logging tools

Interactions of gamma rays with solid materials depend on the energy of the gamma rays and on the density and the atomic number of the materials being investigated. These interactions can be classified by the level of energy absorbed by the substrate material.

At lower energy levels, the photoelectric effect, or Compton scattering, is prevalent. In this case, only a fraction of the gamma ray energy is deposited, and the rest leaves the material as low-energy photons. At higher

gamma ray energy levels, above 3 MeV, pair production becomes dominant.2 9

Identification of elemental compositions is performed primarily by measuring the charac- teristic photoelectric energy of individual nuclear varieties when excited by an external radiation source, such as solar wind or other cosmic rays. At higher energy levels the pair- production mechanism generates well-defined spectra. As such, the most accurate GRS measurements were made during periods of high solar-flare activity when gamma ray energy levels were at their highest. To

improve the elemental identification

capability of the GRS, an active detector cup shield was designed especially for NEAR. It was fabricated from a single bismuth germanate

[ BGO] crystal. The dense BGO cup acted as an active scintillator while providing direct and passive shielding from the local gamma ray environment and reducing unwanted back- ground signals.

The new design replaced the more expensive and less reliable long booms used in other missions to reduce unwanted signals from the activation of the spacecraft body itself by cosmic radiation. The GRS also provided sensitivity to the direction from which the gamma rays were coming.

Detour to a C-Type Asteroid

In early December 1993, NEAR mission managers at The Johns Hopkins University Applied Physics Laboratory reviewed a list of asteroids that might be in close proximity to NEAR’s flight path (next page, top). Asteroid 25 3 Mathilde was found to be within 0.015 AU, or about 2.25 million km

[ 1.4 million miles] , of NEAR’s planned orbital path. Engineers calculated that with slight changes in NEAR’s planned trajectory, it could encounter 25 3 Mathilde with only a 5 7 m/s

[ 187 ft/s] change in velocity, well within the spacecraft’s velocity margin.3 0

1985 , astronomical observations Aft deck Support

Although the dark asteroid was discovered in little was known about Mathilde. New from ground-based

telescopes showed it to be a C-type asteroid with an unusual rotation period of 15 days, almost an order of magnitude slower than most other known asteroid rotation periods.

Thermal spacers

NEAR encountered Mathilde on the way to Eros after five trajectory-correction maneuvers about 2 AU from the Sun.3 1 available power

from the spacecraft’s

At this distance, solar-

Teflon spacers Clamp

O ptical coupling

Small P M T

Spring B G O shield

N aI ( Tl) crystal


O ptical coupling

L arge P M T

G amma ray detector

powered system was down nearly 75 % . With this limited power, astronomers could use only the MSI to explore the surface of the asteroid, and radio-tracking data, before and after approach, to help determine the mass of the asteroid. During the flyby, Mathilde exerted a slight gravitational pull on the NEAR spacecraft. Because of Mathilde’s mass, the gravitational effects on NEAR’s path were detectable in the spacecraft’s radio-tracking data. Data from radio-tracking mass estimates

along with volume approximations helped scientists calculate the asteroid’s approximate density of 1.3 ± 0.3 g/cm3 [ 81.16 ± 18.73 lbm/ft3] . Because of the asteroid’s spectra, Mathilde was believed

to be similar in composition to Teflon wedge Spring > X GRS im aging sy stem s. The com b ined X -ray , gam m a ray spectrom eter sy stem ( X GRS) is show n

m ounted on the NEAR spacecraft ( top left) . Show n on the right side of the X RGS instrum ent is the gam m a ray spectrom eter. The assem b ly is m ounted to the aft deck of the NEAR spacecraft ( top right) . The sensor assem b ly ( b ottom left) contains the NaI( Tl) detector that is positioned w ithin the b ism uth germ anate ( BGO) cup shield to reduce unw anted b ack ground signals b y alm ost three orders of

m agnitude. The Schlum b erger photom ultiplier tub es ( PMTs) at each end convert the light output of the scintillation detectors into electrical signals. ( Im age and diagram courtesy of NASA/ J HUAPL. )

carbonaceous-chondrite meteorites. However, Mathilde’s density was half of that expected, implying either a high internal porosity or significant void space within the asteroid. Scientists imaged Mathilde over a 25 -minute

period during the spacecraft’s approach at a distance of 1,200 km [ 746 miles] and a speed of 9.93 km/s [ 22,213 mi/h] . A total of 5 34 images

5 2

Oilfield Review

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