Apparent Neutron Emissions from Polyethylene Capsules during Neutron Activation and Delayed Neutron Counting

At Imperial College uranium is determined at very low levels in environmental samples by delayed neutron counting. High density polyethylene capsules are used for transporting the samples in a pneumatic transfer system, from the reactor, to the neutron detection rig. The detector has a background of 3.6 counts per minute, however the empty transport capsules produce 11.0 counts above the background. We have determined that neither uranium in the polyethylene or from contamination of the capsule nor ¹⁷N from the ¹⁷O(n,p)¹⁷N and ¹⁸O(n,d)¹⁷N reactions are the cause. This paper reports the results of the investigation into the source of the extra counts.