TITLE:
Simulation of Gamma-Ray and Neutron Spectrometers for Microsatellite Missions
AUTHORS:
Masayuki Naito, Nobuyuki Hasebe, Junya Ishii, José A. Matias-Lopes, Valery V. Dmitrenko, Christian Wöhler, Kyeong Ja Kim
KEYWORDS:
Gamma-Ray Spectrometer, Neutron Spectrometer, Microsatellite. High Purity Germanium, CeBr3, LaBr3(Ce), CLYC
JOURNAL NAME:
Journal of Geoscience and Environment Protection,
Vol.6 No.8,
August
29,
2018
ABSTRACT: Microsatellites have recently opened windows of frequent and low cost missions
for planetary exploration. The performance of gamma-ray and neutron
spectrometers on future microsatellite missions is simulated to assess the possibility
of observation of hydrogen and major elements, given their concentration
on the observation target. The measured elemental abundance will
provide important geological constraints, and some of them may serve as
space resources. Four different types of target bodies with various hydrogen
concentrations in the range of 0 - 20,000 ppm are assumed as target compositions;
Earth’s core, C-type, S-type and Martian meteorites. Gamma-ray and
neutron emission rates show unique footprints that are related to the different
elemental compositions. The starting point is the solid angle subtended
between observation target and spectrometers that allow estimating the
gamma-ray and neutron count rates emitted by the celestial bodies. In this
work, three types of gamma-ray detectors; high-purity germanium (HPGe),
CeBr3 and LaBr3(Ce), a neutron spectrometer combining a lithium glass scintillator
with a boron loaded plastic scintillator and a dual mode spectrometer
Cs2LiYCl6(Ce) (CLYC) are simulated, focusing on their observation backgrounds
as a model case for microsatellite based measurements. The background
count level of both gamma-ray (except for the LaBr3 detector) and
neutron count rates was negligible under these particular conditions. The
gamma-ray detectors were compared by the figure of merit, which was determined
by their efficiency and energy resolution. It was found that each detector
has unique advantages. The HPGe detector has the highest figure of merit due to its excellent energy resolution, whereas the CLYC detector is low
in weight and power consumption due to its dual sensitivity to gamma-ray
and neutron. The CeBr3 detector is an intermediate choice. The neutron
count rates are calculated separately in three energy ranges, i.e. , thermal (500 keV), as a function of the
hydrogen concentration in the 0 - 20,000 ppm range. The thermal and epithermal
neutron count rates are found to decrease with hydrogen concentration,
while the fast neutron count rate increases with the target average atomic
mass. The optimal detector should be decided by the mission restraints on
mass, power consumption, and heat thermal design.