[continues from 4.7.2]
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.