detectors so that the likelihood of two photons striking the detector at the
same time is reduced. In the case of high-activity samples, the level of activ-
ity has to be reduced either by dilution or allowing to decay, in order to
reduce the sum peak.
Liquid Scintillation Counters
Low-energy b−-particles are normally absorbed within the source and in the
window and walls of the detectors, and therefore b−-emitters are difficult to
detect in gas or solid detectors. For this reason,b−-emitting radionuclides
are counted using the liquid scintillation technique in which the radioactive
sample is mixed with a scintillating material. A sample vial containing the
liquid scintillator and the radioactive sample of interest is placed between
two PM tubes connected in coincidence (Fig. 8.6). Each PM tube receives
the light photons emitted by the interaction of the by b−-particle with the
scintillator and converts them into a pulse, which is further amplified by an
amplifier. The two amplified pulses are then delivered to the coincidence
circuit that contains a PHA to analyze the pulse height for acceptance. A
count is registered in the scaler if an event is recorded in both PM tubes
simultaneously. Such coincidence counting reduces the background counts
due to noise, including terrestrial and cosmic radiations, radioactive
patients, etc.
The liquid scintillation solution is prepared by dissolving a primary scin-
tillating solute or fluor and often a secondary fluor in a solvent. The radioac-
tive sample is added to and thoroughly mixed with the scintillating solution
Liquid Scintillation Counters 93Fig. 8.6. A schematic diagram of a liquid scintillation counting system. Light
photons emitted from the sample strike the two photomultiplier tubes to produce
pulses. Only coincident pulses are counted.