nucreonics DATA SHEET no. 13 
Locating Compton Edges and 
Gamma-Ray Spectra 
Backscatter Peaks in Scintillation Spectra 
By B. CRASEMANN and H. EASTERDAY 
Department of Physics,* University of Oregon, Eugene, Oregon 
IN A SCINTILLATION SPECTRUM, each 
photopeak is accompanied by a charac- 
teristic Compton distribution and a 
backscatter peak. Figure 1 shows 
these features in the pulse-height 
distribution of 0.51-Mev annihilation 
radiation, obtained with a cylindri- 
cal 13g X 1-in. NalI(TI) crystal. This 
article reviews the behavior of these 
characteristic features. 
Compton Edge 
The Compton edge corresponds to 
the maximum energy that an incident 
photon can impart to an electron in the 
scintillation crystal by Compton scat- 
tering. Ifthescattered photon escapes 
from the crystal, a pulse is obtained 
whose height corresponds to the energy 
received by the electron in this process. 
The energy of the Compton edge, 
Ecomp, for an incident photon of energy 
E, is given by 
* This work was supported by the Na- 
tional Science Foundation. 
Backscatter Peak 
The backscatter peak is due to scat- 
tered photons from Compton processes 
in the material surrounding the scintil- 
lation crystal, mainly in the glass layers 
between the crystal and the cathode of 
the photomultiplier tube. After under- 
going Compton scattering through 
180 deg, an incident photon of energy 
E, will be left with an energy Ey, = 
Ey — Ecomp. This is the energy of the 
backscatter peak. 
Analyzing Spectra 
In the analysis of complex scintilla- 
tion spectra, care must be taken to 
identify backscatter peaks and Comp- 
ton electron distributions and to avoid 
confusing these with photoelectron 
peaks. If the energy of the incident 
gamma-ray is known, the backscatter 
peak often provides a useful additional 
calibration point. Repeated calcula- 
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Incident Photon Energy (Mev) 
FIG. 2, Compton-edge and backscatter energies vs incident photon energy 
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Energy (Mev) 
FIG. 1. Scintillation spectrum of 0.511- 
Mev annihilation radiation 
tion of Compton-edge and backscatter 
energies can be avoided by use of Fig. 
2, which gives these quantities for in- 
cident gammas up to 2 Mev. 
The Compton edge is more promi- 
nent in the spectrum of high-energy 
gamma rays since the cross section for 
Compton scattering decreases more 
slowly with energy than the cross sec- 
tion for the production of photoelec- 
trons, and the escape probability for 
scattered photons is larger. On the 
other hand, the backscatter peak be- 
comes less noticeable as the incident 
gamma energy increases: with increas- 
ing energy, the total Compton cross- 
section decreases, and the angular dis- 
tribution of the secondary photons 
from Compton events becomes more 
-peaked in the forward direction, so 
that backscattering becomes less. 
With larger scintillation crystals, the 
Compton edge becomes less pronounced 
since the escape probability for scat- 
tered photons is reduced. The back- 
scatter peak does not decrease with 
larger crystals; however, the effect can 
be diminished by the use of a crystal 
container of low atomic number and a 
thin glass cover. The number of 
Compton events in the photomultiplier 
face can be reduced drastically by 
allowing the radiation to enter the 
crystal parallel to the tube face. 
219 
