[continues from 4.7.2]

4.8 Escape fractions

X-rays at energies harder than the Xe-L edge (4.78 keV) have a finite probability of giving rise to a fluorescence photon. This fluorescence photon may recombine (in this case a photon at its proper energy E is detected, although the Burst Length may be anomalous) or escape from the Be window (in this case a photon is detected at the residual energy).
The escape probability (escape fraction) depends on the geometry of the cell and on the gas filling pressure.
The effect is the presence of line-type features in the low energy spectrum (a line at E will give rise to a secondary peak at E-Efluor). The effect is complicated by the fact that the Xe L-edge has a fine structure (see simulated spectra). In the case of the MECS, four main escape peaks are detected; their description is given by:


		EFf(Ej) = H1f + H2f exp(-0.5 [(Ef-4.782)/H3f]2)

where Efluor is the energy of the considered fluorescence photon.

The relevant coefficients (in the order Efluor, H2,H3,H1) are kept in files m{1,2,3}_escape.coeff

Fig. 4.8-I : MECS escape fraction as function of energy.
Dotted/dashed lines indicate the four components, and the solid line the total escape fraction.
Note that currently the same coefficients are used for the 3 MECS units

Additional representations of simulated spectra showing the escape peaks can be obtained via the following form.


Select energy keV, representation and MECS unit

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