[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
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
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
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
are kept in files
Fig. 4.8-I : MECS escape fraction as function of energy.
Dotted/dashed lines indicate the four components, and the solid line the total
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.