Interpreting ionisation-chamber measurements as a function of wiggler field at a synchrotron source

Abstract

A comprehensive set of ionisation-chamber (IC) measurements have been collected with the primary white/pink x-ray beam at the Imaging and Medical Beamline (IMBL) of the Australian Synchrotron, as a function of the magnetic field in the superconducting multi-pole wiggler (SCMPW) insertion device. In addition to the standard in vacuo filters used for radiotherapy experiments at IMBL, six different ex vacuo filter sets, mounted on a rotation stage, were used to vary the x-ray beam quality. The ‘rotating-filter’ method introduced by Stevenson et al (2017 J. Synchrotron Radiat. 24 110–41) is then used to enable the SCMPW field to be determined for four nominal settings: 1.40, 2.00, 3.00 and 4.00 T. The ability to vary the SCMPW field at IMBL is a very powerful means of changing the intrinsic properties of the resulting x-ray beam in accord with the demands of a wide range of experimental studies in diverse areas. These include imaging/tomography, radiotherapy, dosimetry and high-energy diffraction. This makes a means of determining the wiggler field experimentally, most compelling. The analysis of the experimental results also provides a stringent test of the model developed to characterise the IMBL x-ray beam (Stevenson et al 2017 J. Synchrotron Radiat. 24 110–41). We will discuss both the utility/versatility and the deficiencies/limitations of this model. The model can be used to calculate, in addition to IC current, a variety of x-ray properties such as flux, beam quality, absorbed-dose rate and power density. In connection with the last of these properties, and by way of demonstration, key aspects associated with the front-end diamond filter will be provided.