12.2. Spectroscopy of Charged Particles 693
MCAOscilloscopeAmplifierPulserPreamplifierBias SupplyDetectorSourceVacuum
PumpFigure 12.2.1: Block diagram of a typical setup forα-particle spectroscopy.absorption of energy. This can only be done with gas filled detectors and carries
handling and operational difficulties. The other technique, as shown in Fig.12.2.1,
is to place the source and the detector in a vacuumed enclosure.
Let us now move on to the types of detectors suitable forα-spectroscopy. In
principle, any detector capable of detectingα-particles can be used. However the
most commonly used ones are the semiconductor detectors. In the early years of
α-spectroscopy, mostly gas filled detectors were used as the semiconductor detector
technology was in its infancy at that time. There are a few reasons behind preference
of semiconductor detectors over their gas filled counterparts, as described below.
Resolution:Resolution is a very important parameter as far as spectroscopy
is concerned. By resolution we mean how well the detector can differentiate
between two particles depositing unequal energies into its active volume. Note
that we are not talking about the electronics resolution yet. The resolution of
the detector is a separate issue and is concerned with how well the deposited
energy corresponds to the pulse height.
Linearity:It is an established fact that semiconductor detectors show better
linearity for particles of low to moderate energies.
Parasitic Absorption:A big issue with gas filled detectors is that they need
an enclosure to contain the filling gas. The enclosure is generally made of
a metal with an entrance window for the radiation. One can make such a
window very thin, of the order of a few microns, provided the gas is kept at
approximately normal temperature. However even in such thin windows there
is a high probability ofαabsorption. Of course this parasitic absorption leads
to signal loss and non-linearity in the response. A way around this problem is
to place the source inside the active volume of the detector, which is associated
with handling and operational difficulties. Semiconductor detectors do not
share this problem since they do not have any entrance windows. The incident
α-particles directly interact with the material in the active volume and deposit
their energies.The analog signal from the detector has to be amplified and shaped before being
processed by the pulse height analyzer. These analog units are generally installed
very close to the detector output. The output of the amplifier/shaper is a well shaped
pulse with height proportional to the energy deposited by the incidentα-particle.