Chemistry Reference
In-Depth Information
The output pulses of the main amplifier are transferred to an ADC. The
analog information of the pulse amplitudes is converted here into digital form.
The resultant number serves as an address for the connected MCA. It sorts the
different pulses according to their address and counts their numbers, which are
stored in the MCA. The different“channels”correspond to small energy
ranges. Usually, an MCA contains 2048 up to 8192 channels that are assigned to
consecutive increments of 5 eV up to 40 eV. A total range of 10, 20, 40, or
80 keV is indicated for the energy of X-ray photons. The storage capacity of
today's MCA is nearly unlimited. Usually, it is chosen to be 2
32
, that is, 4
×
10
9
counts per channel.
The digital form of an MCA's content is well adapted to computer operation.
The raw data can easily be processed by a dedicated minicomputer or a
personal computer (PC). The relevant contents can be shown as an energy-
dispersive spectrum and can be displayed on a monitor and plotted by a printer.
The spectrum is usually represented as a histogram indicating counts versus
energy and can already be observed during the measurement. The computer
can control the output devices and also a device called a timer, which is used to
start and to stop the measurement or data collection in a preset time. In most
systems, the timer is based on a“live time”clock. It stops any further pulse
counting during the dead time of the system when an input pulse is still being
processed.
3.8.2PerformanceCharacteristics
The performance of an EDS with semiconductor detector and electronic pulse
processor and of a WDS with a gas-filled or scintillation detector is character-
ized by four main features: the spectral efficiency of the detector, the spectral
resolution of the system, the input-output yield of the processor, and the
troublesome interference phenomena caused by neighboring lines, by sum
peaks, and by escape peaks resulting from the detector.
3.8.2.1DetectorEfficiency
The efficiency of a detector is defined as the percentage of detected photons
with respect to all incident photons. For a semiconductor detector, the effi-
ciency is nearly 100% for photons with energies between 5 and 10 keV but is
reduced for lower and higher energies by several absorption effects, which are
described by the formula [70]
ε
exp
μ
=
ρ
Be
ρ
Be
t
Be
μ
=
ρ
Au
ρ
Au
t
Au
μ
=
ρ
Si
ρ
Si
t
Si
1
exp
τ
=
ρ
Si
ρ
Si
d
Si
(3.24)
where (
μ
/
ρ
) is the total mass-absorption coefficient;
ρ
is the density of the
respective layer;
t
is its thickness; (
τ
/
ρ
) is the photoelectric mass-absorption
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