Physics and Engineering of Radiation Detection

12.2. Spectroscopy of Charged Particles 695

Table 12.2.1: Commonα-particle sources used for calibrations.

Source Isotope EnergyMeV Branching Ratio

Polonium^21084 Po 5.304 100%

Plutonium^23994 Pu 5.157 73.3%

5.144 15.1%

5.106 11.5%

Americium^24195 Am 5.486 84.5%

5.443 13%

53.882 16%

Curium^24496 Cm 5.805 76.4%

5.763 23.6%

unavoidable system offsets. It is therefore very important that the zero point energy
is included in the calibration curve. Some of the modern MCAs also enable the
user to reset the zeroth channel to correspond to the zero energy. Generally this is
done before the calibrations, which ensures that the zero point is also available to
determine the calibration curve.
We just mentioned that at least two sources are required for the calibrating the
instruments. However, it is also possible to calibrate the equipment with the help
of a source and a pulse generator. We will shortly see how it can be done.
A typical calibration setup is shown in Fig12.2.1. The pulser is shown connected
to the preamplifier by a dotted line because this is an optional instrument and not all
preamplifiers are equipped with a external charge injection circuitry. Since the range
ofα-particles in air is very small therefore the source must be kept in a vacuumed
container. The oscilloscope is used to look at individualα-particle pulses and helps
in adjusting the gain of the preamplifier. The following steps are typically taken to
perform the calibration using a source and a pulser.

Step-1:The source is placed in the enclosure, which is then pumped for high

vacuum.


Step-2:The bias supply is turned on and the high voltage to the detector is

slowly increased until the recommended value is reached.


Step-3: The gain of the preamplifier is varied until a good enough peak is

observable on the oscilloscope. The amplitude of the pulse is noted down.


Step-4:The MCA is reset and data acquisition started.