PHITS simulations of the Protective curtain experiment onboard the Service module of ISS: Comparison with absorbed doses measured with TLDs

Artikel i vetenskaplig tidskrift, 2013

"Protective curtain" was the physical experiment onboard the International Space Station (ISS) aimed on radiation measurement of the dose - reducing effect of the additional shielding made of hygienic water-soaked wipes and towels placed on the wall in the crew cabin of the Service module Zvezda. The measurements were performed with 12 detector packages composed of thermoluminescent detectors (TLDs) and plastic nuclear track detectors (PNTDs) placed at the Protective curtain, so that they created pairs of shielded and unshielded detectors. We simulated the experiment by the general purpose 3D Monte Carlo Particle and Heavy Ion Transport code System (PHITS), as 10 cm-thick water-filled panels housed in a model of the Zvezda module. External radiation environment was modeled using the AP8MIN and ISO-15390 standard models for the trapped proton (TP) and galactic cosmic ray (GCR) spectra, respectively. The absorbed doses were calculated for all detector packages used in the experiment. Comparison of calculated results with experimental data (TLDs) showed good agreement for the total (TP+GCR) absorbed doses. Further, we analyzed the systematic uncertainty introduced by differences in the detector thicknesses used in the simulations from the ones used in the measurements. The reducing effect of the Protective curtain was studied by comparing the calculated absorbed doses in shielded and unshielded detectors separately for the TPs and GCRs. In case of TPs, the reducing effect was larger than 60% and 40% for pairs of detectors located at aluminum wall and at crew cabin window, respectively. In case of GCRs, small shielding effect was observed for detectors located behind the window but for those located behind the aluminum wall, the effect was even opposite: the absorbed doses in the unshielded detectors were about 10% lower than in the shielded ones. This result was confirmed by the depth-dose analysis using rectangular source emitting broad parallel incident particles impinging on the simple geometry composed of aluminum/glass box and water box of variable thickness simulating the spacecraft wall/window and Protective curtain, respectively. The additional dose in the shielded detectors is related to the secondary fragments known as the "wall effect". However, since GCR contributes by about 30% and 15% only to the total dose in water in shielded and unshielded detectors, respectively, the total shielding effect is high and the Protective curtain is very efficient when it is applied on a spacecraft at low-Earth orbits.