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Constancy checks of a well‐type ionization chamber should be performed regularly as part of a quality assurance regime. The goal of this work was to test the feasibility of using a linear accelerator and an orthovoltage unit to check the constancy of a well‐type chamber's response to an external radiation source. The reproducibility, linearity with dose, variation with dose‐rate, and variation between energy‐matched units of the well‐type chamber response when exposed to 6 MV beams was examined. The robustness to errors in establishing the measurement conditions, including setting the source‐to‐surface distance and gantry angle, rotation of the chamber around the central axis of the beam, and the effect of changing the length of the chamber cable exposed to the field, were tested. The reproducibility and linearity with dose of the chamber response, and robustness to errors in establishing the measurement conditions for 100 kVp and 250 kVp beams from an orthovoltage unit, were also examined. The combined uncertainty, including contributions from errors in establishing the reference conditions, for well‐type chamber measurements using a 6 MV beam from a linear accelerator is 1.0%. The combined uncertainties for measurements using 100 and 250 kVp beams were 1.8% and 1.5%, respectively. When focus‐source distance errors were reduced to ≤1mm, the combined uncertainties for the 100 and 250 kVp beams were 1.2% and 1.1%, respectively, when the dose to the chamber was confined to the linear region of the dose‐response curve. The response of a well‐type chamber should remain constant to within 1.2% when exposed to a constant dose from an external beam unit, if reference conditions can be reproducibly established. However, the uncertainty for establishing reference conditions for output measurements for an orthovoltage unit can be reduced, which would justify a reduction of the tolerance for constancy measurements.PACS numbers: 87.55.Qr, 87.56.Fc

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Abstract. A 3D ultrasonic simulation study is presented, aimed at understanding the effect of element length for imaging small volumetric flaws with linear arrays in ultrasonically noisy materials. The geometry of a linear array can be described by the width, pitch and total number of the elements along with the length perpendicular to imaging plane. This paper is concerned with the latter parameter, which tends to be ignored in array optimization studies and is often chosen arbitrarily for industrial array inspections. A 3D analytical model based on imaging a point target is described, validated and used to make calculations of relative Signal-to-Noise Ratio (SNR) as a function of element length. SNR is found to be highly sensitive to element length with a 12dB variation observed over the length range investigated. It is then demonstrated that the optimal length can be predicted directly from the Point Spread Function (PSF) of the imaging system as well as the natural focal point of the array element from 2D beam profiles perpendicular to the imaging plane. This result suggests that the optimal length for any imaging position can be predicted without the need for a full 3D model and is independent of element pitch and the number of elements. Array element design guidelines are then described with respect to wavelength and extensions of these results are discussed for application to realistically-sized defects and coarse-grained materials.

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