Fluke Biomedical 943-35 - Manual (Page 13)

Theory of Operation
Introduction
3
3-1
Section 3
Theory of Operation
3.1 Introduction
A brief review of gamma radiation is necessary to understand the absorption processes that are inherent
to the operation in the spectrometer mode. This consideration is necessary because in practical
applications, not all of the energy of the incident photon is given up in every case. There are three
absorption processes: Compton scatter, pair production, and photoelectric process.
Compton scatter is a process where penetrating photons are scattered by the electrons that are in the
crystal. In this process, part of the energy is given up to the electron and part is retained by the photon.
The ratio of retained energy to that given off is dependent on the angle of collision. The electron will give
rise to secondary ionization of somewhat less total energy than the energy of the incident photon. The
lower energy output is generally referred to as Compton smear or scatter, most common at intermediate
photon energies.
Pair production is a process where the photon gives up energy to create an electron-positron pair. This
requires minimum energy of 1.02 MeV since it is a transformation of kinetic energy into mass. Photo
energy in excess of 1.02 MeV appears as kinetic energy of the electron and positron. The energy is
absorbed by the Nal crystal through ionization. The positron, being unstable in the presence of electrons,
will annihilate, causing two photons with 0.51 MeV each. Either of these photons may be absorbed
through the photoelectric process, through Compton scatter, or escape the crystal. In practice, pair
production becomes evident with incident photons above 2 MeV.
In the photoelectric process, all of the photon's energy is given up to one electron within the Nal crystal.
The charged electron will have a short ionization path within the crystal resulting in a light output that is
directly proportional to the incident radiation energy. This process permits using the detector to identify
the energy level of the incident gamma ray. With many isotopes, isotopic identification can be performed
using information derived from the gamma ray.
Photons derived from various sources, whether radioactive or visible light, impinge on the metal shield at
the front of the detector. Any photons containing energy less than a specific value (approximately 20 keV)
are highly attenuated by the shield. The remaining high-energy photons penetrate the shield and interact
with the Nal crystal, causing a pulse of light to be produced. The amount of light emitted is proportional to
the energy of the absorbed photon. The crystal is optically coupled to a 10-stage photomultiplier tube
(PMT). The light produced from the crystal is seen by the photocathode in the PMT. The cathode is
excited by the light and emits electrons from its surface. A string of 10 Dynodes in the PMT, at
sequentially increasing potentials, causes a cascade effect that delivers a shower of electrons at the
anode of the PMT. This appears as a negative pulse, proportional to the energy of the original photon. By
varying the PMT high voltage, the gain of the detector and output pulse height may be set to achieve a
specific output, based upon the isotope and activity of the incident radiation.