ORTEC Choice of Detector Shielding

Choice of Detector Shielding

For low-level counting of samples, shielding of the detector to reduce ambient background radiation is essential. Many materials are used in shield designs, lead being the most common because of its high atomic number and density. Pre-World War II steel is used in some designs.

Figure 32. Half-Thickness Values vs. Energy
for Commonly Used Shielding Materials.

Figure 33. Photon Attenuation as a
Function of Pb Shield Thickness for Various Energies.

The thickness chosen for the principal shielding material depends on the required attenuation of gamma rays of a specific energy. For environmental applications covering the energy range from 0 to 2 MeV, 4 in. of lead is sufficient. Figure 32 shows the half-thickness values vs energy for commonly used shielding materials. For lead, the half-thickness for a 1 MeV gamma ray is 0.85 cm, which means that a 1-MeV gamma ray passing through 10 cm (4 in.) of lead will be attenuated by a factor of 3200; a 2-MeV gamma ray, by a factor of 175. These attenuation factors are adequate for most applications. Still greater thicknesses provide additional attenuation of gamma rays, but also increase the probability of undesired cosmic-ray interaction within the shield. Beyond 4 to 5 in. of lead, the background will actually increase because of this effect. Figure 33, showing the fraction remaining of photons incident on the shield, leads to the same conclusions.

Figure 32 also shows that the half-thickness does not increase significantly at energies above 2 MeV, so that the conclusions for shield thickness also apply to prompt gamma measurements up to 10 MeV.

For some applications, the reduction of the lead K x ray is desirable. A graded-Z shield may be used for this purpose. The graded-Z shield works by providing materials with decreasing atomic numbers toward the detector in order to absorb the lead x ray photoelectrically and emit a secondary x ray of lower energy. Typical graded-Z shields use lead-cadmium-copper as the shielding materials. The required thickness of the cadmium and copper may be determined by examining Fig. 32 and noting the half-thickness values in those materials at the energies of interest.

For example, 0.3 mm is the half-thickness of cadmium at 80 keV (Pb K x rays); therefore, 10 half-thicknesses (3 mm) would reduce the peak by a factor of 1000. The copper is used to absorb the cadmium x rays at 22 keV and emit lower-energy x rays at about 3 keV. The half-thickness of copper for 20-keV photons is 0.03 mm and for 80-keV photons is 1 mm; so 0.3 mm would attenuate the 22-keV photons by a factor of 1000, but would also provide an additional 20% attenuation at 80 keV. Commercially available graded-Z liner thicknesses vary, but typical specifications are 0.02 in. (0.5 mm) for the Cd liner and 0.62 in. (1.57 cm) for the Cu liner. These dimensions would result in a 100-fold decrease in the Pb x rays and essentially complete attenuation (2 X 10¹⁶) for the Cd x rays.

Other materials, such as Al and Lucite, are sometimes used in graded-Z shields used with x-ray detectors. Use of the graded-Z shield will result in higher backscatter effects within the shield and may actually reduce the detection limit for nuclides with principal peaks within the backscatter peak energies.

For the primary shielding material, both the graded-Z liner and the support structure should be constructed from materials with concentrations of radioactive nuclides as low as possible.

For even lower-level measurements, active shielding methods provide additional enhancement of signal-to-background ratio. These methods should be considered if passive shielding alone will not provide the required measurement sensitivity.