Gamma-Ray Attenuation in Basement Ceilings

Experiments have been performed which give further insight into the so-called “in-and-down” scattering problem. Experimental ceiling attenuation factors have been developed and compared with previously published results of radiation attenuation in basement ceilings.The experiments were performed on a full-scale structure with basement ceiling mass thicknesses of 12 and 0 psf at several solid angle fractions. Additional experiments were performed for several ceiling mass thicknesses at a solid angle fraction near unity. Generally, the experiments showed that previously published ceiling attenuation factors appear inadequate for large solid angle fractions and large overhead mass thicknesses. For small mass thicknesses and solid angle fractions below ≈0.8, the agreement between experiment and previously published results is reasonable. It is clear that ceiling attenuation factors should demonstrate a solid angle fraction dependence as well as a mass thickness dependence. However, these experiments show that there is no reason to believe that in the limit as the solid angle fraction approaches unity, the ceiling attenuation factor should approach old formulations which had no solid angle fraction dependence.It has also been shown in this work that ceiling attenuation factors may be experimentally determined solely from finite field data. There is no need to estimate far-field contributions or skyshine contributions in the determination of ceiling attenuation factors.