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Computed Tomography (CT) high-resolution imaging have been used to investigate if there is a significant change in the crystal-to-window distance, i.e. the air gap thickness, in a small n-type detector cooled to 77 K, and in a medium sized p-type HPGe detector when cooled to 100 K. The findings were compared to detector dimension data made available by the manufacturer. The air gap thickness increased by (0.38±0.07) mm for the n-type detector and by (0.40±0.15) mm for the p-type detector when the detectors were cooled to 77 resp. 100 K compared to at room temperature. Monte Carlo calculations indicate that these differences have a significant impact on the efficiency in close geometries (<5 cm). In the energy range of 40–700 keV with a source placed directly on endcap, the change in detector efficiency with temperature is 1.9–2.9% for the n-type detector and 0.3–2.1% for the p-type detector. The measured air gap thickness when cooling the detector was 1.1 mm thicker than manufacturer data for the n-type detector and 0.2 mm thicker for the p-type detector. In the energy range of 40–700 keV and with a source on endcap, this result in a change in detector efficiency of 5.2–7.1% for the n-type detector and 0.2–1.0% for the p-type detector, i.e. the detector efficiency is overestimated using data available by the manufacturer. [ABSTRACT FROM AUTHOR]

Abstract: This paper extends the work presented in a previous publication (Hansford, 2011 , subsequently referred to as Paper 1) to optimise the performance of a simple reflection-mode X-ray diffraction instrument with respect to the geometrical configuration. Both papers focus on the maximisation or minimisation of quantifiable characteristics of the X-ray beam incident on the sample in order to optimise the peak heights and widths in the resulting diffractogram. The reason for this approach is mathematical tractability, and its success relies on the relationship between the incident beam characteristics and the diffractogram properties. Part of Paper 1 was devoted to the minimisation of the equatorial divergence angle of the X-ray beam, while keeping the total X-ray flux constant, in order to minimise the diffractogram peak widths. While equatorial divergence and peak widths are certainly linked, their relationship is complex and depends quite significantly on other geometric parameters. An alternative optimisation approach is taken in this paper, focusing on the maximisation of the X-ray flux and minimisation of the incident beam axial divergence, while equatorial divergence is kept constant to ensure at least reasonable resolution. The rationale for this approach is that the incident beam flux and the axial divergence angle are more directly linked to diffractogram peak heights and peak asymmetry, respectively. Families of candidate geometric solutions result from this approach which is regarded as advantageous because there is no unique optimised solution and it allows the parameter space to be fully explored. [Copyright &y& Elsevier]

All available measured data for the response characteristics of the Leake counter have been gathered together. These data, augmented by previously unpublished work, have been compared to Monte Carlo simulations of the instrument''s response characteristics in the energy range from thermal to 20 MeV. A response function has been derived, which is recommended as the best currently available for the instrument. Folding this function with workplace energy distributions has enabled an assessment of the impact of this new response function to be made. Similar work, which will be published separately, has been carried out for the NM2 and the Studsvik 2202D neutron area survey instruments. [Copyright &y& Elsevier]