Relative response of alanine and filtered alanine electron paramagnetic resonance dosimetry in mixed neutron-gamma fields

Abstract

An electron paramagnetic resonance (EPR) spectroscopy procedure was investigated for use in novel mixed neutron and gamma radiation fields. L-α-alanine ("alanine") is an amino acid that develops stable unpaired electrons when exposed to ionizing radiation. The number of unpaired electrons, measured via EPR, is proportional to the absorbed dose in tissue from photons. The EPR signal dependence on neutrons across different spectra, especially in specific moderated experimental conditions, is less well characterized than for gamma dosimetry. Alanine was exposed to gamma and neutron sources, a variety of neutron spectra, and escalating doses to develop calibration curves for sources at Colorado State University and determine the specific neutron source sensitivity. Neutron exposures have lower cumulative signals than gamma exposures for the same dose and alanine mass, with sensitivities ranging from 0.056 ±0.085 for the Bare PuBe source to 0.245 ±0.212 for the bare 252Cf. These values are on the low end of literature ranges or below literature sensitivity values for other neutron sources, leading to a hypothesis of a low dose-rate effect in alanine that should be investigated further. Thermal-neutron sensitive filters of cadmium and gadolinium metal foils were added to alanine dosimeters to investigate the signal response. A comparison of the bare- versus filtered signal response in the known mixed fields showed that the filters did not consistently increase the thermal-neutron sensitivity of alanine. At low total doses, where the gamma contribution to the total dose was very low, the cadmium and gadolinium foils increased the alanine response to the field. When the gamma dose contribution rose to a source-specific point, photon interactions in the foils decreased the photon fluence in the alanine pellets, leading to a decreased dosimeter response. This dual-effect of the thermal-neutron sensitive foils is a novel finding, and should be investigated with source specific gamma spectra. While a filtered-alanine dosimeter is not usable at the current stage, bare alanine calibration curves for the neutron sources at CSU were created and available for subsequent irradiations in various fields at unknown doses.