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4. THEORY OF OPERATION
4.1. What You Will Learn in this Chapter:
Briefly review a high purity germanium (HPGe) gamma-ray spectroscopy system.
Learn how a shaping amplifier controls the tradeoffs between energy resolution and data rate
in an HPGe system.
Compare analog and digital spectroscopy systems.
4.2. Review of the HPGe System
An HPGe detector is a semiconductor diode. Its operation is similar to that of a photodiode
except that it is a few thousand times larger. The detector is operated under reverse bias
(typically a few thousand volts) and at about the temperature of liquid nitrogen. An evacuated
cryostat provides the clean vacuum and low temperature needed by the detector.
When the detector absorbs a gamma-ray photon, electron-hole pairs are created. The resulting
charge pulse is integrated by a charge-sensitive preamplifier to produce a voltage step
proportional to the energy of the gamma ray.
The signal produced by the HPGe detector is very small. A 1-MeV gamma-ray photon produces
a signal of about one-tenth of a volt out of the preamplifier. This small signal must be amplified,
filtered to produce the best signal-to-noise ratio, and measured to a precision of one part in
16,000 in a few microseconds. The amplitude measurement is used to increment the appropriate
element (channel) of an array in data memory. The result of many such measurements is the
spectrum of gamma-ray energies seen by the detector.
4.3. The Shaping Amplifier
The signal produced by the preamplifier must be further processed before being digitized. Three
problems must be addressed:
1. The signals are not referenced to any particular voltage level (such as the system ground) but
instead have a random DC component.
2. Improvements in signal-to-noise ratio can be made by appropriate filtering.
3. The signals are not matched to the range of typical analog-to-digital circuits (ADCs).
The classic solution to these problems is performed by a device called the Shaping Amplifier.
The dc component is removed and low-frequency noise reduced by electronically differentiating
(high-pass filtering) the signal. High-frequency noise is reduced by multiple integrations (low-
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