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ISGRI

Due to the charge loss in the CdTe crystal, for a given energy deposit, events which have interacted at different depths in the crystal get different measured amplitudes (PHA). The rise-time of the signal induced by these event is also different, and in general a deeper interaction induces a larger charge loss, at the same time as a longer rise-time. It is possible to correct this charge loss effect, by taking into account the rise-time information of the signal.

In OSA11, the reconstructed energy of each deposition is determined from a two-dimensional time-dependent look-up table (ISGR-RISE-MOD). In this table, for each pair of pulse height and rise time, a suitable energy value is provided (see Section [*]). This data structure is retrieved in the dal3ibis library, which performs the correction, and is linked in ibis_isgr_energy and ibis_comp_energy.

The ISGR-RISE-MOD table is composed of NUM_ENER lines, giving for each incident energy the correction factor corr for a given value of the rise-time.
Table: Content of ISGR-RISE-MOD Data Structure
Column Name Description
ENERGY Energy at which the gain-offset relationship is measured
CHANNEL Channel at which the gain-offset relationship is measured
CORR Rise-time correction for a given rise-time value (0-127)

Each ISGRI pixel has slightly different gain properties. The linear gain parameters, gain and offset, separately for pulse rise time and pulse height, were calibrated pre-launch, and values stored in the ISGR-OFFS-MOD table.

Table: Content of ISGR-OFFS-MOD Data Structure
Column Name Description
AGAIN Amplitude Gain
AOFFSET Amplitude Offset
RTGAIN Risetime Gain
RTOFFSET Risetime Offset
PIXTYPE Pixel type

Response of ISGRI pixels also depends on the detector temperature, which typically ranges from -5 to 10 deg C. The temperature sensor data is available from each of 8 modules of ISGRI (MDU, each with an associated electronic module MCE). A temperature-dependent correction to the gain is applied for each MDU since OSA10.

In addition to this correction, individual MDUs reveal various evolution patterns likely related to the changes of electronics. These variations result in changes to gain and offset, both for the rise time and pulse height. Moreover, some of the changes cannot be described as external product of two independent linear laws. Most notably MDU 6 diverged strongly from the nominal average detector evolution in 2012, resulting in apparent loss efficiency. To correct for these effects, a new structure is introduced in OSA11, ISGR-MCEC-MOD (MCE-level correction). The correction is applied in the library dal3xibis.

ISGRI detector pixel bias is only enabled during the part of the orbit when scientific observations are possible. The bias is re-enabled at the beginning of every orbit. Sometimes the bias is also reset during the scientific observations, up to several times per orbit. Frequent resets of the bias are typically caused by IREM crashes.

Every time the bias is switched-on, the detector response needs some time to stabilize. The fastest changes are associated with the change of the bias value and happen on a time scale of less than an hour, and are mostly irrelevant for the scientific data calibration. The changes associated with the field configuration in the pixels are much slower, and stabilize only after many hours or days. Moreover, properties of the post-switch-on response evolution caused by internal field stabilization (in other words, the detector polarization) strongly depend on the radiation dose received by the detector in the past. The amplitude of this evolution increased substantially during the mission lifetime. Moreover, since 2015 it has become more irregular.

In OSA11, a new structure is introduced to correct this intra-revolution change of the response by introducing a rapidly evolving correction to the PHA-RT - energy relation, ISGR-L2RE-MOD (L2RE stands for for LUT2 Rapid Evolution). Like other corrections related to energy reconstruction, in OSA11 this correction is performed in dal3ibis.


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Next: PICsIT Up: Calibration Corrections Previous: Calibration Corrections

2020-09-18