ISO detector workshop

(Jan 14-16, 1998, ESAC, Madrid, Spain)

updated 7/Jan/98

Transient behaviour of the LW channel of ISOCAM
A. Abergel

The response of each CAM-LW pixel strongly depends on what has been observed before. This long-term transient response after changes in photon flux level is a well-known characteristics of extrinsic IR photoconductors working under low background conditions (the detector used in the LW channel of ISOCAM is a Gallium doped Silicon photoconductor hybridized by Indium bumps).

The pixel response after a change of the incident flux level can be separated at least in three phases:

For most of the observations, the CAM response is NEVER stabilized. Without any correction, this memory effect biases the photometry with a factor which can be as high as 40%. However, the response for a given history of input flux starting from the switching on of CAM is always the same. Therefore, a correction of these effects is theoretically possible. The limited number of frames per sky position or CVF spectral position delivered by ISOCAM generally does not allow any fitting with a physical model or a specific function. Therefore we have developed a method based on the inversion of a simplified empirical model of the response (Abergel et al. 1996 AA 315, L329). Far from perfect, this method is easily adaptable for other detectors, and allows a systematic correction of the ISOCAM data without any fitting, with a precision of typically 10%. This number is actually the ultimate photometric precision of ISOCAM. It should be possible to improve it significantly, by optimizing the model of the CAM response which is inverted for the correction. I will detail the work still to be done.

Long Term Behaviour of LWS Detectors
M. Burgdorf, T. Lim, D. Ewart, for the LIDT and LWS Consortium

The responsiviy of the LWS detectors are affected by three factors, a long term drift within a revolution, short term changes in responsivity due to individual particle hits and memory effects due to the illumination history of the individual detector. The application of calibration techniques to remove these effects and their effectiveness are described. The long term behaviour of the responsivity of the detectors throughout the mission is also described. A detailed analysis of the dark current and the noise of the doped-germanium detectors in the LWS is given. Particular emphasis is put on the investigation of trends both within a revolution and over the duration of the mission. The results obtained in flight are compared with pre-launch measurements in the laboratory.

Space Weather and the ISOPHOT Detectors
H. Castaneda et al.

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.

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.

PHT Latch-Up and Impact on Detectors
L. Cornwall

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.

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.

Solar Energetic Particle, Cosmic-Ray and Radiation-belt Environments
E.J. Daly (ESA/TOS-EMA), P. Buehler, L. Desorgher and A. Zehnder (PSI)

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.

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.

Plans for Far Infrared Imaging with SIRTF
C.W. Engelbracht, E.T. Young, G.H. Rieke, et al.

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).

Deglitching Methods by the ISOPHOT Int.Analysis
C. Gabriel, J. Acosta-Pulido, M. Haas

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.

Transient Modeling with ISOPHOT IA
C. Gabriel, J. Acosta-Pulido

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.

Non-Linearity in the response of LWS Detectors
S. Leeks, P. Clegg et al.

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.

Analysis of isocam glitch statistics and comparison with cosmic ray models and flight data
P. Nieminen

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.

Radiation-induced Effects in Extrinsic IR Detectors
M. Patrashin

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.

In-flight Performance of NICMOS Detectors
M. Rieke

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.

Overview of the Performance of LWS germanium photoconductors
B. Swinyard et al.

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.

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