A ²D(γ,n)¹H Tritiated Water Detector with ²³²U/²²⁸Th Source

A long-lived gamma source, containing 370 MBq of ²³²U (72 yr) and 110 MBq of ²²⁸Th (1.9 yr), was developed as a permanent replacement for neutron-activated ²⁴Na (15-h) sources that have been used in a ²D(γ,n)¹H tritiated water detector. The ²²⁸Th, which will build up to 340 MBq in 10 yr and then decay in equilibrium with ²³²U, emits copious 2614-keV gamma rays for the (γ, n) process. Competing (α, n) backgrounds are reduced by placing the uranium/thorium source in a gold matrix to provide threshold and coulomb barriers against the reaction. The detection limits (∼0.0l-ml D₂O) for typical stainless steel pipes (∼2-cm diam) measured with the uranium/thorium source are almost as low as those for comparable ²⁴Na sources, which do not produce (α,n) backgrounds. This minor sensitivity loss can be offset by longer counting times, if necessary. Lower personnel exposure is realized with the new source. Overall, the practical advantages of using a longer lived source strongly favor the use of ²³²U/²²⁸Th instead of ²⁴Na.