Functional Principles
20
User Manual
In the X-ray tube, where fast moving electrons are used to produce the continuous excitation
spectrum for the measurements, some electrons will by collision transfer energy to inner shell
electrons bound to the target material atoms and ionize the atom. Subsequently a relaxation
process will take place that leads to the emission of characteristic X-rays, as described above. An
X-ray tube spectrum is therefore always composed of the continuous bremsstrahlung spectrum and
of the target materials characteristic fluorescence lines. Within the sample in turn, where a
continuous excitation gives rise mainly to a characteristic line spectrum the photo-electrons
produced during the initial energy transfer are fast-moving and can, hence, produce
bremsstrahlung radiation in the sample.
The X-rays produced in the sample are emitted isotropically. Only a small fraction of them is
emitted in the direction of the detector and can be detected.
3.2 Detection of X-ray Quanta
There are several quite different concepts for detecting X-rays. They all are however driven by the
same basic necessity to transform photons into a signal which can be processed by electronics, i.e.
a voltage or current. Ideally not only the number but also the energy of the incident photon can be
derived from this electric signal.
In the M1 ORA/MISTRAL two different types of detectors are available:
Proportional counter detector (PCD),
Silicon drift detector (SDD).
The proportional counter detector is a cylindrical capacitor filled with gas, usually Argon, and with
an entrance window which is transparent for X-rays. The incident photon ionizes multiple Ar atoms
on its way through the detector. The produced electrons and ions are accelerated in the strong
electric field between the electrodes. Close to the anode, which is a thin wire in the center of the
detector, the field becomes very strong and the electrons gain enough energy to ionize secondary
Ar atoms whose electrons themselves can ionize further atoms. The resulting avalanche creates
enough free charges to form an easily detectable current. Since the number of initially ionized Ar
atoms is proportional to the energy of the detected photon the intensity of the current is
proportional to the incident photon energy. However, with the intermediate avalanche effect being a
highly statistical process the energy resolution of a proportional counter is limited.
Silicon drift detectors are solid state detectors; the incident photon is absorbed in a silicon crystal
and electron-hole-pairs are created. Again, the number of created charge carriers is proportional to
the incident photon energy. The electrons are collected in an electric field and guided to the anode
where they are detected as a current pulse. In contrast to the proportional counter the electrons
detected are all produced by the incident X-ray quant. There is no additional statistical process
involved. This yields a much better energy resolution than for the PC detectors but the current
created by X-rays with energies of ~ 10 keV is still only in the femtoampere regime and needs
higher effort for signal amplification and shaping.
