A comparison of skyshine computational methods

Abstract

A variety of methods employing radiation transport and point-kernel codes have been used to model two skyshine problems. The first problem is a 1 MeV point source of photons on the surface of the earth inside a 2 m tall and 1 m radius silo having black walls. The skyshine radiation downfield from the point source was estimated with and without a 30-cm-thick concrete lid on the silo. The second benchmark problem is to estimate the skyshine radiation downfield from 12 cylindrical canisters emplaced in a low-level radioactive waste trench. The canisters are filled with ion-exchange resin with a representative radionuclide loading, largely 60Co, 134Cs and 137Cs. The solution methods include use of the MCNP code to solve the problem by directly employing variance reduction techniques, the single-scatter point kernel code GGG-GP, the QADMOD-GP point kernel code, the COHORT Monte Carlo code, the NAC International version of the SKYSHINE-III code, the KSU hybrid method and the associated KSU skyshine codes.

Skyshine Contribution to Gamma Ray Background Between 0 and 4 MeV

“…To separate the cosmic components, both primary and secondary, from the terrestrial and skyshine contributions, measurements were made on the Columbia River at a sufficient distance from land so that air scatter (skyshine) from ground sources and direct terrestrial radiation were negligible. It is reported that skyshine falls off to <0.01% at about 600 m from a source [8]. The boat was anchored in ~10 m of water at a position that was >1000 m laterally from land and any structures (location shown in Figure 6).…”

“…Skyshine from terrestrial sources has been considered a negligible contributor in most detection applications. However, skyshine from man-made sources on the ground, including accelerators [6], medical sources [7] and industrial sources [8] has disrupted specialized detection efforts in the surrounding area. This has been a recent concern in areas such as the ongoing effort to detect illicit nuclear sources at US border crossings and seaports.…”

Shielding of skyshine interference with radiation detection systems

“…These industrial sources of skyshine radiation can, however, dramatically affect sensitive radiation-detection instrumentation. Published results on skyshine show that the intensity of skyshine drops off exponentially with distance [3]. Additionally, the skyshine spectrum is dramatically shifted to lower energies from photon scattering, with spectra following an exponential drop in that intensity with energy.…”

Skyshine interference with radiation detection systems

“…This is not true for sensitive radiation detection instrumentation. Published results on skyshine show that the intensity of skyshine drops ofT slowly: 10% at --100 m, 1% at --200 m, 0.1% at 400 m, and 0.01% at --600 m [2].…”