Physics and Engineering of Radiation Detection

676 Chapter 12. Radiation Spectroscopy

Eγ−1.02 MeV Eγ−511 keV Eγ−255 keV Eγ

Backscatter

Noise

x−Ray

Single Escape

Double Escape

Compton Continuum

Compton Edge

Counts (arbitrary normalization)

Energy (not to scale)

Figure 12.1.3: Typicalγ-ray pulse height spectrum for a shielded setup of

source and detector.

The highest energy peak and the Compton edge are sketched in Fig.12.1.3, which
represents a typical energy spectrum of a radioactive source. The other notable
features of the spectrum are:

Compton Continuum:The curve below the Compton edge is called Comp-

ton continuum. This corresponds to the distribution of energy between incident

photons and the scattered electrons during Compton scattering at different an-

gles. Since Compton scattering is possible at all angles therefore this continuum

can extend up to the beginning of the spectrum.

Noise Peak: Most MCAs allow the user to gate off the noise counts from

the spectrum. This is done be selecting a threshold pulse height below which

no events are recorded and displayed. However in some situations one might

be interested in keeping this information in the data stream. The noise peak

shown in Fig.12.1.3 refers to this case.

Escape Peaks: If the energy of the incident photon is greater than the

threshold for pair production, that isEgamma> 1. 02 MeV, it can produce

an electron-positron pair in the detector as well as in the shield. A pair pro-

duction event in the detector is shown in Fig.12.1.4. The positron thus pro-

duced has very short half life and quickly combines with a nearby electron to

produce photons, a process known as annihilation. Most of the annihilation

events produce two photons traveling in opposite directions. Now, as shown in

Fig.12.1.4, it can happen that one or both of these photons escape the detector

volume without depositing any appreciable energy. If one photon escapes and

the other deposits most of its energy, it leads to asingle escape peakin the