Functional principles
M4 TORNADO
21
Analog voltage pulses of the
signal electronics
Digitized values of pulse heights
Channel allocation
Digital spectrum as distribution of
probabilities of incident energies
Fig. 3 Emergence of a digital X-ray spectrum
The allocation of the channel corresponding to the fitting energy range can be executed by the
software based on an energy/channel calibration. First of all, the voltage pulses of the
spectroscopic amplifier are digitalized in an analogue/digital converter and then allocated to the
according channels by means of a multi-channel analyzer (MCA). As soon as a detected X-ray
quantum is allocated to a certain channel, the counter of this channel is incremented by one. The
spectrum displayed by the software shows how many X-ray quanta were counted in a certain
energy range (channel). In the magnified section of last figure, the channel structure of the
spectrum can be recognized.
An examination of the signal processing reveals that the signal is just processed sequentially. But
due to the fast process, (a few 10 000 cps can be accumulated) it seems that the spectrum is
growing simultaneously. X-ray quanta, which are detected with extremely short time differences,
are excluded from registration. During signal processing the signal channel is closed. This time is
accumulated and called dead time.
The registration of an X-ray spectrum is a temporary accumulation process, which is subject to
statistical laws. It cannot be compared with a current measurement of a voltage or a current as
known from electrical engineering. The quality of the spectrum and therefore the analytical results
strongly depend on the fluorescence intensity and the time of measurement. Measurement times
between 10 s and 180 s are common.
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