Drm mod - Pseudo function for fine tuning RHESSI DRM parameters
From RHESSI Wiki
Contents |
Detector Response Matrix Modification
Introduction
drm_mod adjusts the detector response matrix on the fly as intervals are fit to account for changes in the detector resolution and gain. It can only be used with spectrum files for single detectors.
Parameters
drm_mod - always returns a value of 0. Parameters are varied during fit and used in apply_drm method to compute drm on the fly.
- a[0] - FWHM fraction
- a[1] - Gain offset
- a[2] - Blanket Coefficient - default is 1, multiplies xsec
Fitting with drm_mod
This function recalculates the RHESSI Detector Response Matrix (DRM) to account for distortion of the solar spectrum by detector effects. As stated, the DRM is recalculated for each fit interval when drm_mod is used as part of the model, however the new DRM does not include the complete list of possible interactions. For most cases the recalculated DRM is sufficient to account for detector interactions.
The drm_mod model component is used mainly in cases where the bin size of the data is different from the model and/or there has been a gain change between the data and the model. The model tries to fit the data using the Fe line complex at ~6.7keV, because of this it should only be used in cases where the iron line is clearly visible in the spectrum and the data is being fitted from at least ~6keV. Another requirement for using drm_mod is the spectrum files must be generated for each detector individually. This is necessary due to the model component trying to match the bin size of the data at the Fe line. When detectors are combined to form spectral files the bins are changed because the energy edges from the individual detectors are different, this causes smearing of the counts across bins making it difficult to adjust the bin size of the model to match the data in a meaningful way.
There are three parameters in the drm_mod function which can be varied to allow for a better fit:
- A0 – Full Width Half Max (FWHM) Fraction
- A1 – Gain Offset
- A2 – Blanket Coefficient
Let’s look at each components function.
(A0), the first component is a measure of detector resolution. Resolution is the sum of a constant term (electronic resolution), a term proportional to the square root of energy (natural statistics of germanium electron/hole pairs), and a term proportional to energy (trapping).
The values of A, B, and C for each detector segment are read from a table of parameters vs dates (hessi_resol_vs_time.1000) and interpolated to the requested date. The units of the resolution are FWHM in keV of a gaussian line. The figure below shows the resolution for 21Apr2002 for an energy range of 3 – 160 keV, using the front segment of detector four.
