Construction and Development of a BF3 Neutron Detector at Brookhaven National Laboratory

Most current radiation portal monitors (RPMs) use neutron detectorsbased upon 3He-filled gas proportional counters. 3He isin short supply in the world and continues to decline in availability.Concurrent with the decline in gas is a disproportionateincrease in the cost of available gas. It is therefore desirable to findsubstitutes for the 3He with technologies that will effect minimalchanges to currently deployed systems and provide equivalenteffectiveness in neutron detection. This project investigates thefeasibility of BF3 as a substitute for the 3He in configurations thatcan be readily installed in currently deployed systems.In response to this 3He shortage, the U.S. Department ofHomeland Security’s (DHS), Domestic Nuclear Detection Office(DNDO), Product Acquisition and Deployment Directorate(PADD) commissioned Brookhaven National Laboratory (BNL)to construct and test a boron tri-fluoride (BF3) based neutrondetection module (NDM). The NDM was required to meetspecific criteria as outlined in a DNDO Functional RequirementsDocument (FRD).1 The detector was to be built utilizing (asmuch as practicable) off the shelf components and have the sameexterior dimensions as current NDMs so that they can fit intoexisting portal monitor enclosures.The module was mounted in the standard Radiation PortalMonitor (RPM) NEMA enclosure inside the standard steelshroud, and shipped to the Nevada National Security Site (N2S2)for testing. Concurrently, a full-scale surrogate “BF3 detector,”fabricated with air replacing the BF3, was constructed for thepurpose of evaluating the ability of the design to survive beingdropped from a height that would be typical when performinga field replacement of an NDM on the tallest portal monitorconfiguration. This height corresponds to a condition such thatthe bottom of the NDM is at an elevation of 15 feet, or 457cm,above ground level.This paper discusses the design features of the detectorsystem, mitigation techniques developed to ameliorate thehazards posed by the BF3 gas, drop test results, and discussion ofneutron detection efficiency for the constructed detector system.BF3 detectors potentially have direct applications to internationalsafeguards where 3He neutron detectors are typically deployed.