Ray-tracing Monte Carlo simulations of the ISOCAM LW CCD detector
of the two
detectors (Long-Wave, or LW) contains a 32 x 32 pixel Si:Ga CCD
with dimensions of 3.2 x 3.2 x 0.5 mm3. The sensitive
volume of this CCD is continually traversed by high-energy
cosmic rays (mainly protons, but also heavier ions)
penetrating the spacecraft, depositing charge along
their track, and occasionally creating secondary
particle showers both within and outside of the
Along with a number of other possible interference mechanisms,
these primary and secondary sources can briefly illuminate a pixel
or a group of pixels in the CCD, overwhelming the photon response
from the real observation target. As a consequence, spurious "glitches"
in the detector response are seen.
The simplest way to simulate this phenomenon is to assume an
isotropic source distribution for the incident cosmic particles,
ignore physical interactions in the spacecraft and in the instrument,
and follow the purely mathematical, straight tracks of the impinging
particles through the CCD. Such non-interactive rays will lead
to a certain track length distribution, that in turn can be translated
into an affected pixel number distribution using the so-called
"taxi metric". In this approach, it is as a first approximation
assumed that only those pixels directly coinciding with the track
are affected (in reality, especially for the heavier incident
ions, the amount of charge liberated is so large that it can spread
to the surrounding pixels). The simulated distribution can consequently
be compared with real glitch data.
We have done such a ray-tracing simulation using the
Monte Carlo package from CERN. The plot below shows a comparison
between the simulation results for 105 incident particle
entries (dashed line) and the CGLITCH glitch data set we obtained
from Vilspa (solid line).
Please note the following comments on these curves.
- There is difference in the low-number (1-3 pixels affected)
part of the distribution between the data and the simulation.
Possible, but at this point not certain, explanations for this
difference can be cosmic ray-induced low-energy secondary particles
not considered in the simulation; fake counts in the ISOCAM
deglitching algorithms; and/or counts of a-particles emitted
from the Thorium coating in the lens system of the instrument.
In decaying, Th232 produces a-particles of energy
~4 MeV, which will be stopped in less than 20 mm in Silicon.
Since one CCD pixel has a cross-sectional area of 10 x 10 mm2,
it is conceivable that an excess of glitches with a low number
of pixels affected could be produced via this mechanism.
- Based on the Monte Carlo simulation, the mean number of pixels
affected was calculated to be ~8.4, whereas the CGLITCH data
set yields a value of ~9.0. However, these figures cannot be
directly compared, since the simulation gives the number of
pixels affected for each individual ray/particle, whereas the
CGLITCH data contains the number of pixels affected per given
integration time. In that integration time more than one particle
may have penetrated the CCD, such that the total number of pixels
affected is in this case rather the sum of pixels affected per
each incident particle. Moreover, secondary particle production
has not been considered in the simulation. The normalisation
between the two curves is here chosen to be at 10 pixels affected.
- There is a knee in the simulated distribution that is not
visible in the CGLITCH data. The knee is a result of the fact
that the chord length distribution within a rectangular volume,
although being continuous, is not a smooth one. It is unclear
why this feature is not visible in the data: again, it is possible
that secondary particle production smoothes the distribution.
An important aspect is also the geometry of the local shielding
around the CCD. This may have an effect on the directionality
of the incoming radiation, and hence on the particle track length
- Since the simulated distribution is purely a mathematical
one, it ends at 64 pixels affected. This corresponds to the
maximum 2-dimensional particle track length from one corner
of the CCD to the diagonally opposite corner (the z-coordinate,
or the depth, of the entrance and exit points is not in this
- The measured distribution naturally does not stop at 64 pixels,
but continues up the maximum of 1024 pixels. There can be several
mechanisms responsible for cases where several hundreds of pixels
are simultaneously affected, most plausible one being a cosmic
ray-induced nuclear event in the immediate vicinity of the CCD,
particularly in the high-Z shielding surrounding the detector.
In such a case, an extensive shower of secondary particles can
be created, covering large portions, or the whole, of the CCD.
An energetic primary heavy ion may also deposit enough energy
to saturate the device. Note, however, that these events are
several orders of magnitude less frequent than the ones where
a few or some tens of pixels are involved.
For questions, comments, and further information, contact
Nieminen at ESA/ESTEC.