Initial measurement of site boundary neutron dose and comparison with calculations

For most accelerators adequate side shielding can be provided at minimal cost to meet the most aggressive radiation protection regulations and, further, the likely requirement to increase shielding thickness still more at a later date can be done usually by heaping more earth or applying local shielding at minimal expense and inconvenience. This moderately happy state of affairs does not unfortunately hold true with roof shielding. The cost of roof shielding is largely predicated on the roof span and the necessary structural engineering requirements for its support. These measures can be extremely expensive and where one is dealing with the rather extensive unsupported spans typical of experimental halls devoted to experiments with high energy electron beams; it is necessary to specify the roof thickness as carefully as possible with the constant concern that adding more earth later is not likely to be possible without rebuilding the hall. Because of the nature of roof skyshine, and for most high energy accelerator facilities neutron skyshine, the effect of the radiation is likely to extend to the facility fence-line where one is concerned about the exposure of the general population. Very properly the dose limit for the general population is set at a rather low value (1 mSv y-1) and in order for the Jefferson Lab (JLab) to ensure strict compliance with this limit they have a design goal for the fence line of 0.1 mSv y-1. However, because natural neutron backgrounds are low (30--40 microSv y-1) and the methods of detection and measurement permit rejection of background interference from photons, they can measure the JLab produced neutron radiation with good sensitivity and precision