Importance of thermal equilibrium in thermoluminescence measurements

Measurements of intensities, spectra, and yields of thermoluminescence from irradiated biological materials such as the aromatic amino-acids are complicated by difficulties in obtaining a suitable sample environment. A sample holder should be tissue equivalent for dose measurements, non-fluorescing and non-thermoluminescing, electrically and thermally conducting, structurally strong over a wide range of temperature, and capable of holding a high vacuum. After evaluating many materials with regard to the above requirements, a holder was designed and fabricated from boron carbide. Dose measurements in the holder using Fricke dosimeters and Victoreen r-meters indicated agreement with air dose values. All other requirements were satisfied by this holder. Thermoluminescence measurements, conducted in vacua ranging from 10^-6 to 50 Torr, showed light intensity depended not on the type but on the pressure of the gas. Sample temperature studies revealed that the sole function of the gas was to produce temperature equilibrium between the sample and the bottom of the sample holder, although the sample was almost completely surrounded by walls at the sample holder temperature. Study of the radiative heat transfer in the system explains this result and points to a possible source of error in thermoluminescent dosimetry.