During last decade several new
technologies for radiation measurements were developed and significant
progress was achieved in miniaturisation and power reduction. Together with
modern simulation tools such novel devices will allow for implementation in
space simple but powerful detection systems capable to meet wide range of
requirements. The study of these detectors was also motivated by lack of
measurements and difficulties in radiation belts modelling still persisting
after over 40 years from their discovery. Current data does not cover the
whole range of particle energies and their space distribution. In particular
the low energy electrons (up to few hundred keV) are still short of data. Such
electrons can provide new facts about acceleration processes in the
magnetosphere. They can also cause background increase in the X-ray
observations or create radiation hazard for spacecraft instruments. Therefore
new instruments optimised for space radiation measurements need to be
developed and flown.
After an extended inquiry of suitable
detector technologies the microstrip detector was chosen as the most promising
candidate. In the selection procedure a set of requirements posed on the
radiation monitor in space has been defined and applied. Modern detection
technologies like Silicon microstrip detectors are good candidates for space
applications as radiation monitors. Such miniaturized instruments can be
easily modified to various particle types and energy ranges and adapted to
different and harsh space environments. The potential of the microstrip based
device for space measurements has been demonstrated in course of this work
with the help of already existing detector models and extensive computer
simulations.
The candidate detector - Small
Particle Monitor (SPM) - was defined using as an example the MYTHEN microstrip
system developed at PSI. The model detector is a Si semiconductor consisting
of 128 strips, 50 mm
wide and 8 mm long. Their thickness of 300 mm
has the advantage of being fully depleted even with standard spacecraft bus
voltages. Moreover, the electrons with energies up to about 250 keV deposit
their all energy inside of the waver. In the first step of the study such
components as sensor dead layers, collimator geometry and veto detector were
defined and optimized. Several assumptions were also made on the whole device
like the housing shape, dimensions and thickness, total mass and on number of
printed boards and connectors. Prototype design has dimensions of L=5, W=5 and
H=3 cm and full weight of 150 g. Power consumption is estimated to be about
200 mW. Detector response toward different radiation sources was modelled
using GEANT4 packet. Analysis of the electron responses demonstrates detector
ability for its space use. The data for mono-energetic electrons reveal the
low energy threshold of 8 keV and the peak to tail ratio reaching a factor of
4. The response matrix is almost flat up to ca 200 keV with the maximum around
150 keV. At energies larger than 200 keV, the response matrix values quickly
decrease. Therefore measurements at higher energies shall be correspondingly
longer.
Spectral contamination by higher
energy electrons (as well as heavier particles) could be only partially
prevented by the use of a veto detector. Considering electron spectra and
fluxes in space it will not pose any problems. The high energy particle
intensities are orders of magnitude lower and they cause only small
contamination below few hundred keV. Simulations did not indicate any troubles
with enhanced levels of secondary radiation like bremsstrahlung. This
conclusion however, was drawn using only results from the whole particle
monitor but did not include any secondary radiation generated by spacecraft
materials in the monitor vicinity.
It was also found that single
electrons can cause multiple events as they cross through several strips.
Number of such events depends on the strip width and strongly increases with
the electron energy. In order to keep their level low the strip size should be
modified by increasing the strip widths or by clipping several strips
together. The suggested width of the detector strip pitch is equal to 250 mm.
In order to keep the detector area unchanged one might reduce the strip length
accordingly. In addition, the collimator opening geometry should prevent
detection of particles coming at large angles.
Performance of the monitor in orbit
was simulated for both electrons and background particles: protons and X-rays,
taking their characteristic energy spectra in space. Static radiation belt
models AE8MIN and AP8MIN (NASA) and CREME Cosmic Ray model from the SPENVIS
suite were used for this purpose. Results were obtained for LEO (500 km, 51°),
polar (700 km, 89°),
GEO (36000 km, 0°)
and GTO (300-36000 km, 31°)
orbits. They all show smooth electron responses with acceptable counting
rates. Thus, even for the highest anticipated fluxes the detector rates should
be still tolerable by existing read-out electronics.
Simulations based on the static models
show that for regions with higher electron flux concentrations like electron
belts the spectral contamination with orbit averaged proton fluxes is on the
level of few percent. Thus the detector is very well suited for measurements
in predominantly electron environments. Background sources like diffuse X-ray
photons or Cosmic Rays have levels of only few counts per second and will not
mess up with higher rate detections of electrons. However, bigger background
separation difficulties may arise in regions dominated by protons. In the
current monitor design the low energy responses for all particles look
similar. Therefore one must apply special measures for better discrimination
between different particles and in order to get rid of the unwanted proton
events. The first method takes advantage of differences the multi-strip
response distributions between protons and electrons. As protons deposit their
whole energy practically always in only one strip, this information can used
to determine their contamination level. Another method utilises difference in
the stopping power between protons and electrons of the same energies. It
implies covering of several strips with a thin layer of absorber. Protons with
energies up to 1000 keV will be fully stopped in only 10 mm
thick Al foil (the same layer barely stops 35 keV electrons). It will reduce
proton fluxes by even three orders of magnitude. Other methods are also
possible but they are either technically difficult like depletion layer
changes with the applied voltage or often not allowed onboard like sweeping
out unwanted particles by using magnetic fields.
Detector response simulations were
complemented and verified by irradiating a test microstrip detector with low
energy photons using secondary X-ray sources. The prototype developed for PSI
X-ray crystallography was already equipped with the readout chip designed in
radiation hard technology. Measurements showed a good agreement with the
simulations and allowed for confirmation of the detector performance
requirements: noise level (5s)
»
6 keV, energy resolution »
3 keV and maximum read-out rates »
1 MHz/strip.
Current monitor design can be also
easily adapted to measure other particles and energy ranges. Using already
existing GEANT4 simulation tools such a study is readily possible.
Overall results from this study imply that the
proposed detector fulfils initial conditions put on its space implementation
and is a good candidate for future measurements on satellites. Present
development of the small particle monitor can be continued with more refined
modelling. Further, detailed requirements studies including their experimental
testing can also be performed in the laboratory using already existing
detector and readout-chip prototypes.
