Gamma tracers are commonly measured using either solid scintillation detectors or semiconductor detectors.

Solid scintillation detectors: These are of different types, but the most generally applicable is the detector based on a single crystal of sodium iodide doped with traces of thallium, the so-called NaI(Tl) detector. The crystal is optically coupled to a PMT. Interaction of a gamma photon with the scintillation crystal results in the emission of light, which is detected by the PMT.

The light output is proportional to the gamma energy. The electronic system associated with the PMT analyses the pulses according to pulse amplitude (energy) and stores the results in a multichannel analyser. Thus, energy and intensity are recorded, and the result is the gamma energy spectrum of the radiation source.

The NaI(Tl) detector has a high intrinsic efficiency but limited energy resolution. The scintillation crystals are provided in different sizes. The efficiency for high gamma energies increases with detector volume.

Common counting equipment has cylindrical crystal sizes of 50 mm x 50 mm to 125 mm x 125 mm (height x diameter): the larger the crystal, the higher the price. The detectors can be made quite rugged and are suitable in field instrumentation.

Semiconductor detectors: Today, these are mainly based on high purity germanium crystals, so-called HPGe detectors, where a semiconductor junction is created by suitable elemental dopants on the crystal surface. A gamma ray interacting with the detector will result in an excitation of electrons from the valence band to the conduction band in the crystal, and a small electrical pulse is created in a high voltage field. The pulse height is proportional to the gamma energy. The pulses are sorted and stored in a multichannel analyser.

The intrinsic efficiency of semiconductor detectors has, for many years, been lower than that of NaI(Tl) detectors. At present, it is, however, possible to purchase detectors with efficiencies >100% relative to that of a 75 mm x 75 mm NaI(Tl) detector, but prices are very high. The main advantage of an HPGe detector is, however, its excellent energy resolution. This property may be indispensable for the analysis of complex radiation sources. HPGe detectors need cooling to the temperature of liquid N2 during operation and are not generally practicable as field instrumentation.

In general, gamma tracer detection requires little sample preparation except for the extreme low energy emitters (i. e.125I). There are several ways to reduce the minimum detectable concentration in gamma detection:

• Increase the intrinsic detector efficiency: This is a matter of cost.

• Increase counting sample volume (constant activity concentration in the sample leads to higher total activity in the sample): There is a practical limit to the sample size.

• Optimize the counting geometry by shaping the counting sample: For a given radionuclide, a selected detection set-up and a certain sample volume, there is an optimum shape of the sample volumes. For practical reasons these are most often cylindrical shapes.

• Enrich the tracer from a large to a smaller sample volume (increased total activity for a better sample counting geometry): This requires sample treatment either by liquid evaporation or by chemical separation. Sample treatment time and cost increase.

• Reduce the background level by effective detector shielding: This is most often done by passive shielding with lead walls (5-10 cm thickness) around the detector and sample.

A typical counting set-up for a NaI(Tl) detector is shown in Fig. 22. A 1000 mL Marinelli sample container, 75 mm x 75 mm NaI(Tl) detector, Pb shield (5-10 cm), a Sn (or Cd) screen to filter Pb X rays generated by the sample activity in the Pb shield, Cu filter screen to filter away Sn (or Cd) X rays generated by the Pb X rays in the Sn (or Cd) screen.

• With NaI(Tl) detector based analytical equipment, detection limits of <0.2 Bq/L can be obtained using Marinelli beakers and reasonable counting times for common radionuclides such as 22Na, 60Co and 125I.

• For HPGe detectors, the corresponding detection limits are <0.1 Bq/L.