Density effects of silica aerogel insulation on the performance of a graphite probe calorimeter

With the introduction of a novel graphite probe calorimeter, called the Aerrow, various thermal insulating materials are being explored to further improve the device. Silica-based aerogels are proving to be an optimal material due to their low densities, small thermal conductivities, rigidity, and machinability. The aim of this work is to determine how various silica aerogel densities affect the Aerrow's performance.

Performance concerns three areas: heat transfer from the core, the Aerrow's beam quality dependence, and the effects of an applied magnetic field on its measurement of absorbed dose to water. A numerical heat transfer study was done to determine heat transfer time constants. The EGSnrc radiation transport toolkit was used to determine absorbed dose conversion factors which are used to quantify the Aerrow's beam quality dependence. Dose conversion factors for Cobalt-60 and two clinical photon beams (6 and 10 MV) were determined. Magnetic field perturbation factors are used to characterize the Aerrow's performance under an applied magnetic field. EGSnrc with the magnetic field transport algorithm was used to determine these perturbations for a 1.5 T MR-linac. Several aerogel densities (0.01-0.55 g  $$ ) were examined for each performance area.

Heat transfer time constants were found to vary from 52 ± 2 to 117.4 ± 0.4 s. The time constants decreased with increasing aerogel density. The Aerrow's beam quality dependence varied between 0.5% and 1%, decreasing with increasing aerogel density. Beam quality dependence was determined in the range of $$ Co to 10 MV (58.4%  ≤  $$  ≤ 73.5%). Under an applied magnetic field, perturbations were smallest when the Aerrow was parallel to the field. Perturbations varied more so when the Aerrow was perpendicular to the magnetic field and increased with increasing aerogel density. In all cases, perturbations were less than 0.6% from unity with a relative uncertainty of 0.1%.

Silica-based aerogels demonstrate an improved performance over thermal insulation used in previous iterations of the Aerrow. With it, the Aerrow has shown to be robust in several areas. If heat transfer can be properly corrected for in the dose determination and the parallel orientation is used under a magnetic field, then the high density aerogel is possibly more preferable.