CdZnTe gamma ray spectrometer for orbital planetary missions Page: 3 of 5
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CdZnTe GAMMA RAY SPECTROMETER FOR ORBITAL PLANETARY MISSIONS*
T. H. Prettyman, W. C. Feldman, S. A. Storms, K. R. Fuller, C. E. Moss, M. C. Browne,
D. J. Lawrence, and K. D. Ianakiev, Los Alamos National Laboratory; S. A. Soldner, eV Products
*This work was supported by NASA's Planetary Instrument Definition and Development Program
Knowledge of surface elemental composition is needed to understand the formation and evolution of planetary
bodies. Gamma rays and neutrons produced by the interaction of galactic cosmic rays with surface materials can be
detected from orbit and analyzed to determine composition. Using gamma ray spectroscopy, major rock forming
elements such as Fe, Ti, Al, Si, Mg, and Ca can be detected. The addition of neutron spectroscopy enables the
mapping of hydrogen and rare earth elements. Thermal and epithermal neutron data are also used to determine the
equilibrium density of neutrons in the surface, which is needed to convert gamma ray count rates to relative
elemental abundance. Fast neutron spectroscopy can be used to determine the flux of galactic cosmic rays, which is
needed for determining absolute elemental abundances from orbit.
The first mission to fly both gamma ray and neutron spectrometers was Lunar Prospector, which gathered
data over the entire surface of the moon. The gamma ray spectrometer (GRS) on Lunar Prospector was a BGO
detector, which had a pulse height resolution of 13% full width at half maximum (FWHM) at 662 keV. The GRS
was deployed on a boom to minimize the detection of background gamma rays produced in the spacecraft. Maps of
thorium with 100 km spatial resolution have been constructed using GRS data. Accurate maps of Fe and Ti have
been constructed by analyzing the high-energy portion of the pulse height spectrum (above 5 MeV). For BGO,
deconvolution is required to determine peak areas for gamma rays from elements other than Fe and Th. Because the
background continuum caused by scattering of gamma rays in the surface and by high-energy nuclear reactions is
not well defined, accurate estimates of the abundance of some elements are difficult to obtain.
New planetary science missions are being planned to explore Mars, Mercury, the asteroid belt, and the
outer planets. Based on the experience obtained by the Lunar Prospector program, most of these missions will
include both neutron and gamma ray spectrometers. However, due to the cost and risk associated with boom
deployment, spectrometers will be mounted on the deck of the spacecraft for some missions. In addition,
scintillation detectors will be favored for gamma ray spectroscopy because their performance is well understood and
they are relatively inexpensive to implement.
Significant improvements in the pulse height resolution relative to scintillation detectors can be made
through the use of a new room temperature detection technology. CdZnTe, a wide band-gap semiconductor, can be
used to make gamma ray spectrometers for planetary science. Coplanar grid CdZnTe detectors have the best peak
shape and pulse height resolution (better than 3% FWHM at 662 keV) and can perform gamma ray spectroscopy up
to 10 MeV. The size of coplanar grid detectors that can be manufactured routinely is about 1 cm3. For an orbiting
instrument, a CdZnTe detector at least 16 cm3 in size is needed. Consequently, methods to combine signals from
multiple detectors to make a large-volume spectrometer have been developed.
A conceptual design for a multi-element CdZnTe detector for planetary science is shown in Fig. 1. The
CdZnTe detector consists of a 4x4 array of 1-cm3 coplanar grid detectors. Detectors for the array can be
manufactured in - 1 year at a cost of $150K. The array is shielded from the spacecraft by a BGO detector. In
practice, the CdZnTe and BGO detectors will be surrounded by a boron-loaded plastic anticoincidence shield, which
will reject cosmic ray events and acquire fast and epithermal neutron data. Signals from individual CdZnTe
detectors and the BGO detector are combined in a field programmable gate array to produce pulse height spectra and
list mode data that are transmitted back to Earth.
We have simulated three types of spectra that could be used to determine elemental composition: an
accepted spectrum, which is formed by events with interactions that occur only in the CdZnTe array; a pair
spectrum, formed by events in which an interaction is detected in the CdZnTe array coincident with an interaction in
the BGO that produces a pulse within a window about 511 keV; and a telescope spectrum, in which the sum of the
pulse height in the array and BGO detector is recorded for events in which the pulse height in the BGO is larger than
the pulse height for the array. Each of these acquisition modes is designed to suppress the response of the
spectrometer to gamma rays generated in the spacecraft. The accepted spectrum is the most effective of the three,
providing a factor-of-five suppression at 3 MeV for a 6-cm thick BGO crystal.
Simulated pulse height spectra for the three acquisition modes are shown in Fig. 2. An average lunar
gamma ray spectrum, including lines and continuum from the lunar surface, was used in the simulation. Acquisition
was assumed to take place at low altitude (<100 km). The accepted spectrum shows well-resolved peaks for six
major elements at energies below 3 MeV. The double escape peak that appears in the pair spectrum at 5400 keV
combines capture gamma rays from Ti and Ca. Ti can be determined independently using the 6761 keV capture
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Feldman, W. C. (William C.); Storms, S. A. (Steven A.); Fuller, K. R. (Kenneth R.); Moss, C. E. (Calvin E.); Browne, M. C. (Michael C.); Lawrence, David J. (David Jeffery), et al. CdZnTe gamma ray spectrometer for orbital planetary missions, article, January 1, 2001; United States. (https://digital.library.unt.edu/ark:/67531/metadc934418/m1/3/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.