Applications of the double moderator DePangher neutron dosimeter near accelerators

Abstract

A modified DePangher double-moderator BF 3 detector was used to measure stray fast neutron fields outside the biological shielding of a Cockcroft-Walton iongenerator, a cyclotron, and a linear accelerator. The instrument measures RBE dose rate between 0.1 and 5 MeV and average energy of the epicadmium neutron spectrum. Stray neutron fields of < 1 mrem/hr were detectable and “sky shine” from an accelerator without roof shielding was characterized. Such fields were not accurately measured with ordinary survey instruments. The average energy of stray neutrons outside accelerator shields was found to be approximately 0.8 MeV. Characteristics and limitations of the detector are discussed, including the use of gated pulse techniques for neutron measurement in high intensity pulsed gamma fields. The instrument was found to be useful in determing low intensity fast neutron dose rates that are often neglected or inadequately studied because of the lack of suitability of available survey meters.