The pixel response after a change of the incident flux level can be separated at least in three phases:
A clear correlation is established between the changes in the responsivity of
the far infrared detectors P3, C100, C200, and PHT-S with several geophysical
indeces. In particular, a correlation has been established between the
geomagnetic Ap index and the responsivity signals of the detectors. This
strongly suggests that the physical mechanism responsible for the changes in the
Ap index is also driving the changes in the responsivity of the detectors. The
effect of solar activity it seen clearly when the autocorrelation function of
the responsivity checks it is computed and it is found a peak around 27 days, as
it would be expected if they originate from recurrent disturbances in the Sun.
Finally, we discuss the effect on the operation of the instrument related with
the coronal mass ejections of January 6 and November 4 1997, the last one the
largest event observed during the mission, affecting the four instruments. A
detailed time analysis shows the effect of the change in time of the high energy
particle fluxes in the space on the glitch rate and responsivity of the
detectors.
A well known phenomenon of CMOS devices is the occurrence of high supply
currents after switch on, which is called `Latch up'. Since ISOPHOT has
been operating we have found that a similar effect, causing additional
currents in the Micro Ampere range, may be induced or may occur spontaneously
in some of the detector assemblies. In the case of PHT-P2, the effect is
induced in certain circumstances when the detector is switched from
`standby' to on. PHT-C100 and PHT-C200 both display spontaneous
latch-up effects, which may occur when another ISO instrument is prime.
The impact of these latch ups on science data is discussed. Also changes
to the logic for the switch on sequence for P2, to prevent latch ups are
described.
Data on the radiation environment during the ISO mission has been recorded
by the REM instrument on the STRV microsatellite in an equatorial
geostationary transfer orbit (GTO). These are presented, along with data
from other sources. The November 1997 Solar Energetic Proton Event is
discussed. Other recent experience on radiation background effects has come
from the Hipparcos satellite operation in its un-planned GTO orbit. These
data reveal much about the dynamics of the radiation background.
Space Weather and the ISOPHOT Detectors
Most of the changes observed in the ISOPHOT detectors at the responsivity checks
can be traced to variations in the conditions of the magnetosphere and the space
weather. We present in this work a complete study of the effects of the space
environment on the responsivity and in-orbit performance of the ISOPHOT
detectors during the ISO mission, and summarize the lessons learned in the
operation of the instrument that will be useful for the future space missions.
H. Castaneda et al.PHT Latch-Up and Impact on Detectors
The ISOPHOT detectors are operated with readout electronics located very
close to the detector pixels. The Cold Readout Electronics (CREs) were
fabricated in CMOS technology in specially designed chips capable of
working at cryogenic temperatures.
L. CornwallSolar Energetic Particle, Cosmic-Ray and
Radiation-belt Environments
A review is made of the main features of radiation environments of
detectors in space. These environments are described in terms of models of
particle fluxes which are functions of particle energy, solar cycle phase,
etc.. However, the models are often highly simplified or "worst-case",
limiting their applicability in background analyses.
E.J. Daly (ESA/TOS-EMA), P. Buehler, L. Desorgher and A. Zehnder (PSI)
Plans for Far Infrared Imaging with SIRTF
The Multiband Imaging Photometer for SIRTF (MIPS) is being designed to
obtain extremely sensitive images at 70 and 160 microns, using detector
arrays of Ge:Ga photoconductors. Although the array readouts are fabricated
on silicon, a design and fabrication process has been developed which makes
them dc-stable at the array operating temperature of 1.7K. The capacitive
trans-impedance amplifer (CTIA) readout circuit maintains the detector bias
as signals accumulate and virtually eliminates crosstalk between pixels due
to ionizing particle hits. Graded contacts on the detectors minimize hook
response and spontaneous spiking. Despite these desirable performance
features, the detectors are subject to shortcomings typical of this detector
type, such as long time constants due to dielectric relaxation and changes
of responsivity as a result of ionization damage by energetic
particles. Therefore, a high fidelity simulation of the far infrared sky is
being used as a testbed, so that signals can be extracted taking account of
realistic detector properties. Using this testbed, we demonstrate the
ability of a source fitting algorithm (DAOPHOT) to separate individual
galaxies reliably from infrared cirrus. We compare the performance of this
extraction approach with and without the effects of cosmic ray hits on the
detectors and of latent images due to the long detection time constants. We
discuss plans to update the simulation with laboratory test data to help
plan optimum ways to use MIPS to gain a very deep view of the far infrared
sky. Our goal is to detect sources reliably at 70 and 160 microns to
5-standard-deviation levels below 1 and 10 mJy respectively (assuming 2000
second integrations).
C.W. Engelbracht, E.T. Young, G.H. Rieke, et al.Deglitching Methods by the ISOPHOT Int.Analysis
The ISOPHOT detectors are continuously affected by high energy particles hits,
with an averaged frequency of an event every 7 seconds as registered by the
detector. The net effect is observed as a disturbance in the readout voltages,
which affects the signal to noise ratio and degrades the photometric accuracy
level. Due to the highly redundant readout method of ISOPHOT it is relatively
simple to identify hits with enough energy to produce any disturbance in the
readout. The most energetic particles can produce a momentary change in the
detector response, which lasts for a certain time and appears as a tail in the
signal sequence. Three different methods are implemented into PIA to cope with
this problem. All three are based in statistical analysis, identifying deviant
points. Two algorithms operate at the readout voltage level, either flagging the
affected data for avoiding its further use, or trying to reconstruct the
affected parts to minimize the loss of information. A second detection can be
done at the signal level, after integration ramp fitting. In this paper we will
present the algorithms together with a standard set of parameters which provide
very good results for the majority of ISOPHOT data. PIA provides a simple
interface to perform an individual treatment of the data, optimizing the
algorithm parameters.
C. Gabriel, J. Acosta-Pulido, M. Haas Transient Modeling with ISOPHOT IA
One of the most difficult problems we are confronting in the ISOPHOT
calibration, is given by the long stabilization times, which produce detector
transient responses. Examples of this behaviour will be shown for the different
subinstruments. Several physical and empirical transient models have been
proposed and implemented within the ISOPHOT Interactive Analisis for correcting
the signal drifts. The results achieved so far, the difficulties found and the
data reduction strategies implemented in PIA will be summarized by this paper.
C. Gabriel, J. Acosta-PulidoNon-Linearity in the response of LWS Detectors
The Long Wavelength Spectrometer on ISO has 10 detectors connected to
intergrating amplifiers. The integration ramps - the gate voltages as a
function of time - have to be converted into the flux of the source. For low
photocurrents, the ramps produced by the detectors are linear and can be
calibrated using the standard LWS calibration source, Uranus. Higher
photocurrents, however, produce non-linear ramps which must be
calibrated. The method of converting these ramps into photocurrents is
discussed, as well as the correction for non-linearity applied to
observations with 1/4 and 1/2 second ramps. This correction is made using
models and observations of Saturn and Mars. The data for the two planets
seem to indicate that, when observing very strong (Saturn strength) sources,
the detectors may be entering a different regime possibly due to photon
curing of the detectors.
S. Leeks, P. Clegg et al.Analysis of isocam glitch statistics and
comparison with cosmic ray models and flight data
Analysis of ISOCAM long-term glitch statistics is presented along with a
comparison to environmental data from the IMP-8 spacecraft and two high
latitude neutron monitors. Ray-tracing simulations of the ISOCAM LW CCD using
the GEANT code are combined with IMP-8 results, as well as CREME96 and CHIME
cosmic ray model predictions, to yield estimates of the CCD affected pixel
number distribution.
P. NieminenRadiation-induced Effects in Extrinsic IR
Detectors
Experimental results and a model of the low-dose radiation effects in
extrinsic infrared detectors will be presented. The effects will be
illustrated by the examples from the ground based tests, such as
100 MeV protons irradiation of Si:Ga and gamma-irradiation of stressed
Ge:Ga photoconductors. The effects are explained in terms of a change
of the effective compensation ratio of the samples due to recharging
of the donor and acceptor impurities by electrons and holes generated
under the irradiation, as well as taking into account that the heavily
doped near contact layers block minority carriers inside the semiconductor
bulk and prevent their sweepout from the detector. Some results of the
mathematical modelling will be also discussed.
M. PatrashinIn-flight Performance of NICMOS Detectors
The HgCdTe photodiode arrays flown on NICMOS were thoroughly characterized
before launch, and therefore a detailed comparison with the performance
during flight is possible. A suite of groundbased irradiation testing is
also available for comparative studies. The in-flight DC stability, dark
current, radiometric performance, and response to cosmic rays of the NICMOS
detectors will be presented. Data for the first 6 months of flight
operation will be examined for any possible performance changes induced by
the flight environment.
M. RiekeOverview of the Performance of LWS germanium
photoconductors
The Long Wavelength Spectrometer on ISO utilises three types of doped
germanium photo-conductors for the detection of radiation in the 45-198
micron waveband;: Ge:Be, Ge:Ga(unstressed) and Ge:Ga(stressed). The
instrument has been in almost continuous operation in-orbit for two years,
with this perspective some important conclusions on the design and operation
of doped germanium photo-conductors for detection in the FIR can now be
drawn. This paper reviews the conditions encountered in the space
environment, the affect these had on the detector performance and the
choices made for the operating parameters of the detectors. The methods for
curing the affects of the ionising radiation dose and for correcting for
drifts in detector responsivity are also briefly reviewed together with the
derived instrument NEP's and responsivities.
B. Swinyard et al.