Page 244 - Radiochemistry and nuclear chemistry

228 Radiochemistry alut Nuclear Chemistry

where 511 is the energy in keV equivalent of one electron mass.
The broadening of the photo pe~ has many causes such as inhomogeneities in the crystals
and variations in charge, or light, collection and noise in the preamplifier. However, for
scintillation detectors the main cause is found in the PMT where nonuniformity in the
photocathode, fluctuations in the high voltage imposed on each dynode, and statistical
variations in the small number of photoelectrons formed at the photocathode are all
contributing factors.
The resolution is the determining factor in the ability of the system to differentiate
between photo~s of v-rays of similar energy. Other features of the spectrum in Figure
8.19(b) are the backscatter peak and the X-ray peaks. The broad backscatter peak, located
between the X-rays and compton edge, arises by the absorption in the crystal of scattered
photons resulting from "t-ray absorption via Compton interactions in the material
surrounding the crystal. Obviously the magnitude of this peak is dependent on the distance
of this material from the crystal and on the nature and amount of the material. The X-ray
peaks (X in Figures 8.15 and 8.19(b)) are due to the absorption of the X-rays emitted in
the electronic rearrangement following the nuclear disintegration, following internal
conversion or after excitation of materials near the detector. Bursts of very low energy
electrons are generated thermally in a semiconductor or emitted spontaneously from the
photocathode in a PMT and are the cause of a noise peak at very low discriminator settings.
This limits the photon energies that can be studied to a minimum of some thousand electron
volts.
For y-energies above 1.02 MeV, pair production in the detector followed by annihilation
of the positron leads to the generation of pairs of 0.511 MeV photons. The probability
increases rapidly with energy above 1.02 MeV. One or both of these photons may escape
from the detector giving a deposited energy corresponding to the initial -y-energy less one
or two times 0.511 MeV. Positrons are also generated outside the detector and their
annihilation radiation reaches the detector. This gives rise to three smaller photo peaks
called escape peak, annihilation peak and double escape peak at the corresponding energies,
E. t-0.511 MeV, 0.511 MeV and E.y- 2 • 0.511 MeV, marked 1 e, A and 2e respectively in
Figure 8.15.
Coincident -y-rays emitted in rapid cascade during one decay may hit the detector at the
same time and register as one event with the combined energy. This gives two effects,
firstly the generation of a sum peak containing one count per coincidence and secondly
corresponding loss of one count in each of the peaks corresponding to the coincident
photons. Summing is strongly dependent on detector size and source to detector distance.
Finally the coincidence may be purely random at high count rates, giving random summing
between -y-rays of all occurring energies. In this case clear sum peaks will usually not be
seen (except for sources having a few intense -y-lines), but the corresponding counts are still
lost from respective photo peak.
The photo peaks from a well behaved detector are gaussians. Thus the FWHM value
determines the peak shape. The energy scale is calibrated against standard isotopes and is
normally almost linear. In most applications only the area below the photo peak but above
the underlying continuum is used as a measure of activity. The total efficiency (based on
photo peak area) varies usually with energy in the way illustrated by Figure 8.20.