Neutron coincidence counting of plutonium and high-energy muon-induced time-correlated events in lead

Coincidence counting of time-correlated spontaneous fission neutrons from even-atomic-numbered Pu isotopes has been a well documented methodology for determining quantitatively the amount of Pu present in a waste container. Recently, a number of 55-gallon radioactive mixed waste drums were assayed on Pacific Northwest National Laboratory's (PNNL) segmented gamma scan neutron assay system (SGSNAS). The drums weighed 680, 750, and 529 pounds, respectively. A large portion of the drum weight consisted of potentially contaminated Pb bricks. The Pb brick waste was originally used as radiological shielding. The Pb brick shielding was loaded into 55-gallon drums during decontamination and hot cell clean out operations. The drums were then sent to PNNL's nondestructive assay facility for counting. In each case, the results were transuranic. No gamma ray lines were present to support a TRU result. However, the neutron coincidence reals were detected in numbers well above background levels. The third drum, weighing 529 pounds, was not part of a contaminated area clean out and therefore raised some serious questions. How could this possibly be TRU waste? The counting system was operating perfectly having recently passed the fourth cycle of the Waste Isolation Pilot Plant (WIPP) performance demonstration program. The SGSNAS system was rechecked using 241 AmLi and 252 Cf standards to test the system's singles and doubles (reals) response functions and gate timing. Having passed these tests, another test was implemented. Twenty clean, Pb bricks (500 pounds) were loaded into a new 55-gallon drum. Each Pb brick was 2'' high by 4'' wide by 8'' long and weighed approximately 25 pounds. The Pb stack consisted of 2 bricks per layer - ten layers (20'') high. The first 2000-second neutron count gave a Pu gram equivalent reals rate of 1.87 g. This paper will show the results of subsequent measurements, the Pu gram equivalents, and coincidences of the presumed muon-initiated events versus the amount of Pb present.