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

Abstract

Purpose: 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. Methods: 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 cm À3 ) were examined for each performance area. Results: Heat transfer time constants were found to vary from 52 AE 2 to 117.4 AE 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 60 Co to 10 MV (58.4% ≤ %ddð10Þ x ≤ 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%. Conclusion: 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.

Absolute dosimetry of a 1.5 T MR‐guided accelerator‐based high‐energy photon beam in water and solid phantoms using Aerrow

“…Associated uncertainties for these factors represent the combined type A sampling uncertainties given by EGSnrc with a coverage factor, k = 1. When normalized to the dose conversion obtained for 60 Co under reference conditions with the probe in the parallel orientation, , a beam quality dependence factor of 1.006(1) is obtained for the MR‐linac ( B = 0) . Furthermore, the dose conversion factors for the setups involving the SWHE phantom were found to be equal to the equivalent in‐water setups to within 0.1 %.…”

“…This influencing quantity is assumed to have a rectangular probability distribution, with a width of 0.5%. The uncertainty associated with the magnetic field perturbation factor, k B , is 0.1% due to the MC‐related sampling uncertainty involved in its calculation . The dose perturbation/dose conversion refers to the type A statistical and type B systematic (e.g., photon interaction cross sections) uncertainty in the MC simulations used to calculate this quantity.…”

“…When normalized to the dose conversion obtained for 60 Co under reference conditions with the probe in the parallel orientation, ðf D core !D w w;core;CoÀ60 Þ MC ¼ 1:1156 6 ð Þ, a beam quality dependence factor of 1.006(1) is obtained for the MR-linac (B = 0). 56 Furthermore, the dose conversion factors for the setups involving the SWHE phantom were found to be equal to the equivalent in-water setups to within 0.1 %. The mass impurity correction, k imp , was calculated by first scoring the doses in the central graphite volume and the largest contributing impurity, the silica aerogel, in the parallel orientation.…”

“…. An expanded investigation of as a function of Aerrow orientation and silica aerogel density has been described by Bancheri et al…”

“…The uncertainty associated with the magnetic field perturbation factor, k B , is 0.1% due to the MC-related sampling uncertainty involved in its calculation. 56 The dose perturbation/dose conversion refers to the type A statistical and type B systematic (e.g., photon interaction cross sections) uncertainty in the MC simulations used to calculate this quantity. Volume averaging refers to the effect of the finite size of the core and the averaging of the dose that occurs over its extent due to radial nonuniformity.…”