Drm mod - Pseudo function for fine tuning RHESSI DRM parameters
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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 first parameter, a[0], changes the detector resolution. This parameter adjusts the bin size in energy (keV) of the model so it matches the data. It accomplishes this by adjusting the Full Width Half Mast (FWHM) fraction of the energy bin in the model. This is accomplished by adjusting the width of the energy bins in the model to match the energy bins in the data at the Fe line complex at approximately 6.7keV. In order to calibrate the small changes in the resolution and gain of the detector a fixed line needs to be used as a reference point, in the case of RHESSI we use the known Fe line. Note that the energy bins in the model are not changed, rather they are convolved with the Detector Response Matrix created by drm_mod to account for the resolution of the detectors along with other detector effects when going from the measured count rate spectrum to the photon spectrum. The default value of the parameter, a[0], is set to 1. This the generally accepted resolution of RHESSI in keV, hence without drm_mod the detector's are assumed to have a resolution of 1keV. In reality the resolution of each detector is slightly different and changes as a function of the time during the mission. In the relatively low energy domain where drm_mod adjusts the resolution is generally better than 1keV. In cases where drm_mod is used to fit the spectrum a value of a[0] between approximately ~.7keV and 1keV is expected. Any values outside of this range generally mean that the model being used is not correct for the data observed.
The second parameter, a[1], is the gain offset of the detector. This adjusts the model to fit the data by shifting the model in energy (keV). As with a[0], the FWHM fraction, the Fe line is used to measure the gain offset. Once again the gain offset is a detector effect that is taken into account in the Detector Response Matrix, the matrix recalculated by drm_mod. The gain offset is usually well determined. It generally only becomes necessary to use it as a fit parameter in regions of the flare where the count rate is high. As the count rate increases the gain will increase between 0.0keV and 0.3keV. There is no physical reason for the gain to be negative. The upper limit of approximately 0.3keV is arrived at by studies of the detectors during periods of high count rate. Once again if the gain offset, a[1], falls outside of these bounds when used as a free parameter the model should be reconsidered.
The third parameter, a[2], the measures the blanket coefficient. This varies the amount of attenuation at low energies due to the thermal blankets insulating the detectors. This parameter is always kept fixed. The effect is not well enough understood to let the value change.
- drm_mod can only be used to fit spectra for single detectors, when detectors are combined to create spectrum files it cannot be used.
- drm_mod is only useful in cases where the Fe line in visible in the spectrum at 6.7keV because it uses this line to adjust the value of FWHM fraction, a[0], and the gain offset, a[1].
- The FWHM fraction,, a[0], should only vary between approximately 0.7keV and 1keV, any larger or smaller values indicate a problem with the model
- The gain offset, a[1], should only vary between 0keV and 0.3keV and should only be used in instances of high count rate.
- the blanket coefficient, a[2], should always be kept fixed at a value of 1.
Further Explanation of the drm_mod parameters
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.
[STILL UNDER CONSTRUCTION]
