Precise knowledge of the fast neutron spectrum and fluence is essential for several experimental endeavors requiring the low-background of the underground ground environment. These experiments include some of the most important directions in nuclear and particle physics that are under consideration for the Deep Underground Science and Engineering Laboratory (DUSEL) such as searches for WIMP dark matter, neutrinoless double beta decay, and solar neutrinos.
In recent years, the need for sensitive measurements of fast neutron fluences has outpaced the ability to perform such measurements. The current state of fast neutron measurement capability is inadequate to meet the needs of these experiments, and the development of this new technology would greatly advance the ability to characterize the fast neutron backgrounds.
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 was constructed in Russia and recently sent to NIST for testing. The detector was filled with liquid scintillator and tested with neutrons in the CNIF facility.
We are also working on a large volume detector to use in the underground environment where high efficiency is more important than energy resolution. A construction of a prototype detector was recently completed and assembled at Kimballton Underground Research Facility near Blacksburg, VA, as seen in figure 2. It consists of six He-3 tubes placed between two large blocks of plastic scintillator. It has been taking data on the fast neutron flux continuously since July of 2010. Using knowledge gained from this detector, we will design and construct the larger-volume detector that will have a greater sensitivity to the fast neutron flux. Upon completion, the detector would be moved to other underground laboratories to measure their fast neutron fluxes.
In addition to the application in the underground basic science community, an improved fast neutron detector has obvious application in the area of homeland security where the detection low fluence rates of fast neutrons from fissile material remains an outstanding problem. This innovation will address a critical national need and greatly improve the capability for rapid and accurate monitoring of contraband materials capable of causing catastrophic harm. The field of neutron dosimetry also requires the improved detection of higher energy neutrons. Existing spectrometers fail almost completely for determining neutron fields at medium and high-energy accelerator facilities, requiring multiple measurements with different detectors and complicated unfolding procedures. This need has only grown due to the increased use of 14 MeV neutron generators in interdiction and inspection technologies.
Figure 1: Final assembly of the 16-channel neutron spectrometer. The detector was recently assembled and tested in the CNIF.
Figure 2: Photograph of the enclosure containing the prototype fast neutron detector at the Kimballton Underground Research Facility (KURF).