As part of our program in fast neutron technology, we are continuing our work in improving fast neutron detection and spectroscopy. The basic principle involved using a large volume of liquid scintillator to detector fast neutrons through their recoil interaction with protons in the scintillator. The neutrons thermalize and are captured, thus producing a signal indicating that the recoil event was due to a neutron. This capture serves to discriminate against background events. Figure 1 shows the final assembly of a 16-channel spectrometer constructed in collaboration with Russian researchers at the Institute for Nuclear Research. The size of the 16 segments was chosen so that a fast neutron interacts on average only once in a segment, thus allowing one to correct for the nonlinear light yield, which is the dominant cause of poor energy resolution. The detector constructed in Russia and recently sent to NIST for testing. The detector was filled with scintillator and tested with neutrons in the CNIF facility.
In collaboration with the University of Maryland, we are also working on a large volume detector to use in the underground environment where high efficiency is more important that energy resolution. A construction of a prototype detector (FaNS-1) was completed and assembled at Kimballton Underground Research Facility near Blacksburg, VA, as seen in the photograph. It consists of six He-3 tubes placed between six large blocks of plastic scintillator. It acquired data on the fast neutron flux from July of 2010 through July of 2012. Using knowledge gained from this detector, we designed and constructed the larger-volume detector (FaNS-2, shown in Figure 3) that has a greater sensitivity to the fast neutron flux. The detector is currently being calibrated at NIST after which it will be taken to a shallow underground location to measure the fast neutrons induced by cosmic rays.
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