High local ionization density effects in x-ray excitations deduced from optical stimulation of trapped charge in Al2O3:C

X-rays are used extensively in luminescence studies. Recent studies have shown a strong sample and material dependence in the response of the optically stimulated luminescence (OSL) signal to x-rays, which cannot be accounted for by differences in the photoelectric cross-sections of these materials.

In this paper we give direct experimental evidence for a high local ionization density caused during low energy x-ray interactions; this is similar to the effect produced by high LET (linear energy transfer) radiations such as protons. This effect is deduced on the basis of optical stimulation of trapped charge in carbon doped aluminium oxide (Al2O3:C) after exposure to different radiations (90Sr/90Y beta, proton, 137Cs gamma and x-rays), and making a comparative analysis of the changes in the initial OSL decay rates.

The sensitive parameter λ (the decay constant) is>100% larger for irradiations carried out with x-rays in comparison to that for low LET radiation such as beta particles. We further show that the nature of the increase in λ with dose, λ(D), is a saturating exponential function plus a constant (λ), and that both λ(D) and λ are unique for a given radiation type or more specifically its LET.

Such behaviour is explained by superimposition of OSL from several first-order traps or by the presence of a competing trap in Al2O3:C. The understanding of x-ray interaction with OSL traps presented here explains the x-ray dose rate calibration problem in different materials, and opens up the area of studying ionization density effects in low energy x-ray interactions using the OSL of Al2O3:C.