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Improved NaI(Tl) and CsI(Tl)Detector Arrays for Environmental Airborne Gamma Spectrometry: Technical Progress Report

Airborne Gamma Spectrometry (AGS) is a well recognised method for mapping the distribution of natural and anthropogenic radionuclides in the environment using high-sensitivity γ-ray spectrometers mounted on aircraft. Such systems usually use large volume thallium doped sodium iodide (NaI(Tl)) spectrometers, conventionally consisting of a number of 10×10×40 cm (4000 cm 3) crystals each viewed by individual photomultiplier tubes. The energy resolution of such arrays is a significant factor affecting the sensitivity of the system to individual radionuclides. The higher energy γ-ray lines from the natural series are relatively well resolved and relatively easily measured, however, radionuclides of environmental relevance tend to have lower energy γ-ray lines which typically may be poorly resolved from other γ-rays with similar energies. In particular situations (such as early post-accident measurements of radioactive contamination) where complex mixtures of radionuclides exist these γ-rays may be impossible to resolve. Methods of improving the response of such detectors have been investigated; specifically whether gating information and modifications to pulse analysis could improve the composite resolution of NaI(Tl) detector arrays and permit investigation of angular distribution and self-collimation effects, and whether the use of CsI(Tl) as an alternative scintillant could result in detector systems with improved resolution and efficiency compared to NaI(Tl) and how such a detector system might be produced. To improve detector response a new second stage amplification system, a three stage shaping amplifier with a differentiation followed by two integrating stages, was developed. A prototype has considerably greater gain than the single amplifier previously used with improved resolution. Replacing the existing summing amplifier with a summing and shaping amplifier is straight forward and is expected to enhance the resolution of the array. A circuit to provide gating signals for individual NaI(Tl) crystals was also developed, and could be used to generate signals to identify which detector(s) register individual events, from which angular distribution and self-collimation information could be extracted from the array potentially yielding additional information on source distribution. Two methods of combining such gating systems with the three stage amplifier and an SAR ADC (evaluated separately) have been identified. Implementation of these developments would significantly enhance performance of the high volume NaI arrays in current use for AGS surveys. It is known that CsI(Tl) produces more photons per MeV of incident radiation than NaI(Tl), with peak light emission at longer wavelengths where it is well matched to Si PIN photodiodes. Small CsI(Tl)-photodiode detectors have been …