Physics and Engineering of Radiation Detection

12.1. Spectroscopy of Photons 677

e+

e+

e+

e

γ

γ

e

e

γ

e

γ

γ

Double Escape

e

No Escape

Single Escape

γ

γ

Figure 12.1.4: Energy depo-

sition of high energy photons

above pair production threshold

in a detector.

spectrum (see Fig.12.1.3). The single escape peak can be easily identified since

it appears at an energy ofEγ− 0. 511 MeV. Just like single escape peak, a dou-

ble escape peak is also sometimes visible in the spectrum. This corresponds to

the escape of both annihilation photons from the detector. The double escape

peak appears atEγ− 0. 511 MeV. It could happen that none of the annihila-

tion photons escapes the detector. In this case the full energy carried by the

incident photon gets deposited in the detector. The height of the output pulse

then corresponds to the full energy peak of the spectrum (Eγin Fig.12.1.3).

Shield Scattering Peaks:These are the peaks that appear when photons

hit the shielding material and are scattered into to the detector. Since these

photons lose most of their energy during the collision, these peaks appear at

the lower end of the spectrum. A very commonly seen shield scattering peak

is the so calledbackscatter peak. This peak corresponds to the deposition of

energy by photons that are backscattered from the shield into the detector.

The backscatter peak is generally fairly broad due to scattering of photons at

different angles that are close to 180^0.

X-Ray Peak: Whenγ-ray photons interact with the highZmaterial of the

shield, they can produce x-ray photons. The energy of these photons is sub-

stantially lower than that of the originalγ-ray photons. Therefore, as shown

in Fig.12.1.3, the energy deposited by them in the detector produces a peak at

the lower end of the energy spectrum.

12.1.BCalibration

Calibrating the apparatus is the first step for any spectroscopic measurement. Here
by calibration we mean determination of the relationship between MCA channels