Quality factor measurements at NTF

The dose equivalent rate in the radiation field outside of the polydoor at the Neutron Therapy Facility has been measured, using a Chipmunk, assuming a quality factor (QF) of 5, to be 25 mrem/hr. This kind of dose rate if true introduced occupancy restrictions and NTF is operating under an exemption. Based on the previous CR-39 studies of the neutron field around NTF,and the amount of shielding around the NTF, it was difficult to believe that a significant neutron field exists in this area, and contributes to the measured dose rate. If the field was mostly due to gamma rays the QF setting on the Chipmunk could be reliably set to a value of one. One method of obtaining a qualitative understanding of the relative abundance of neutron and gamma contribution to the absorbed doses, is to measure the quality factor for the field. This was determined using a recombination chamber. The recombination chamber is a gas filled ion chamber that can measure the average quality factor of a radiation field of unknown composition and energy spectrum. To use the recombination chamber in an unknown field, one needs to measured a calibration curve using radiation fields of known quality factor. The individual neutron and gamma components of the radiation field were also determined in these studies by use of an Andersson-Braun counter to measure the dose equivalent rate due to neutrons, and a Cutup ion chamber to measure the gamma dose rate. The neutron dose equivalent rate in this area of NTF has been estimated by Vylet and is consistent with the present measurements