Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

440 CHAPTER 9
(b) The energy of the Compton electron (Compton edge) is equal to the difference
between the energy of the incident photon and the scattered photon:
E Compton electron = E − E ′ = 0.661 − 0.185 = 0.476 MeV.
In pair production, a flicker of light representing the original quantum energy
minus 1.02 MeV is produced as the positron and negatron simultaneously dissipate
their energies in the crystal. After losing its energy, the positron combines with an
electron, thus annihilating the two particles and producing two photons of 0.51 MeV.
Depending on the time sequence, the crystal size, and the geometric location of the
initial interaction, we may have two pulses representing 0.51 MeV each—one light
pulse representing 1.02 MeV, or one light pulse representing the total energy of the
original photon.
Nuclear Spectroscopy
Nuclear spectroscopy is the analysis of radiation sources or radioisotopes by measuring
the energy distribution of the source. A spectrometer is an instrument that
separates the output pulses from a detector, usually a scintillation detector or a
semiconductor detector, according to size. Since the size distribution is proportional
to the energy of the detected radiation, the output of the spectrometer
provides detailed information that is useful in identifying unknown radioisotopes
and in counting one isotope in the presence of others. This technique has found
widespread application in X-ray and gamma-ray analysis using NaI(TI) scintillation
detectors and HPGe (high-purity germanium) semiconductor detectors, in beta
analysis using liquid scintillation detectors or plastic scintillation detectors, and in
alpha analysis using semiconductor detectors. Nuclear spectrometers are available
in two types, either a single-channel instrument or a multichannel analyzer (MCA).
The essentials of a single-channel spectrometer consist of the detector, a linear amplifier,
a pulse-height selector, and a readout device, such as a scaler or a ratemeter
(Fig. 9-10). The pulse height selector is an electronic “slit,” which may be adjusted
to pass pulses whose amplitude lies between any two desired limits of maximum and
minimum. The output from the pulse-height analyzer is a logic pulse to a scaler or to
a count ratemeter. The main use of the single-channel analyzer is to discriminate between
a desired radiation and other radiations that may be considered noise. Thus,
the single-channel spectrometer is used to measure one radiation in the presence
of another or to optimize the signal-to-noise ratio when a low-activity source is being
measured in the presence of a significant background.
An MCA (Fig. 9-11) has an analog-to-digital converter (ADC) instead of a pulseheight
selector to sort all the output pulses from the detector according to height.
The MCA also has a computer memory for storing the information from the ADC
or from another source. This feature allows automated data-processing operations
such as background subtraction and spectrum stripping. Spectrum stripping is a
technique for analysis of compound spectra that is based on sequential subtraction
of known gamma-ray spectra of individual isotopes from the compound spectrum