Multi-Element Mercuric Iodide Detector Systems for X-Ray and Gamma-Ray Imaging
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MULTI-ELEMENT MERCURIC IODIDE DETECTOR SYSTEMS FOR X-RAY AND GAMMA-RAY IMAGING BRADLEY E. PATT' Xsirius, Inc., 4640
Admiralty
Way,
Suite
214,
Marina
del
Rey,
CA
90292
ABSTRACT The advancement of room-temperature mercuric iodide detector technology to the point of spectroscopy-grade energy resolution coupled with the advancement in long-term stability and the high efficiency of these detectors now make it's use in large scale array systems for x- and gamma-ray imaging applications viable. Camera design criterion differ depending upon the application. A HgI2 gamma-ray camera developed at EG&G Energy Measurements has been used for imaging with various apertures, and the energy dispersive imaging aspects have been examined. Concepts for applications such as medical diagnostic imaging and a satellite based telescopes for x- and gamma-ray mapping have been objectified. INTRODUCTION & BACKGROUND The area of x-ray and gamma-ray imaging has diverse applications including medical diagnostic imaging systems such as those used in x-ray radiography, x-ray computed tomography (CT), single-photon emission computed tomography (SPECT) and positron emission tomography (PET). In addition, x-ray and gamma-ray imaging systems are used in astronomy and astrophysics for mapping x-ray and gamma ray fields. Areas of current interest include solar-flare imaging, x-ray sky surveys, and surveys of various galactic regions from satellite borne instruments. Many types of image receptors are used or have been considered for modem x-ray and gamma-ray imaging systems. These include direct-exposure x-ray film, image plates, image intensifiers, gas-filled ionization chambers, scintillation detector systems, high purity germanium, and Si[Li] systems. The absorption efficiency, spatial resolution, energy resolution, and cooling requirements are compared in Table I. Table I: Comparison of the absorption efficiency, spatial resolution, energy resolution, and cooling requirements for candidate technologies for x-ray and gamma ray imaging. XR Film
Image Plates
Image Intens.
Ion. Chamb.
Scint.
Ave
Good
Poor
Good
Higi 2 / CdTe
HpGe
Si[Li]
Good
Poor
Good
Absorb. Effic.
Poor
Spatial Res.
Good
Good
Good
Good
Good
Good
Good
Good
Energy Res.
None
None
None
Poor
Poor
Good
Good
Good
No
No
No
No
No
Yes
Yes
No
Cry. Cooling Req'd
I
I
Author performed part of this work while at EG&G Energy Measurements, 130 Robin Hill Rd., Goleta CA 93117. Mat. Res. Soc. Symp. Proc. Vol. 302. @1993 Materials Research Society
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From the comparison in Table I we see that only materials such as mercuric iodide (HgI2) and cadmium telluride (CdTe) offer the possibility of good absorption efficiency, spatial resolution, and energy resolution without the need for cryogenic cooling. Scintillation detector systems, mainly NaI(TI) scintillators coupled to photomultiplier tubes (PMT's) have been the dominant technology for over 30 years. These systems have moderate energy and spatial resolution capability. The primary instrument based o
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