The award extends liquid Noble Gas (LNG) detector technology from the low rate, low radiation background environments as has been developed for astrophysical dark matter searches, to the high rate, high radiation environment found near spent fuel assemblies. LNG gamma ray detection technology will alleviate many of the count rate and radiation damage problems which crystal detectors encounter in high radiation environments.
The development of LNG gamma ray detectors fits DNDO's strategic planning which intends an integrated, mutually enhancing infrastructure for detection, coordination, and response, as safe guards measures used for Special Nuclear Materials (SNM) accounting is a block to one of the pathways to Nuclear Terrorism. The award will provide an excellent training of graduate students in both physics and nuclear engineering.
There are a number of problems that demand the use of radiation detectors, specifically gamma ray detectors capability of operating in very high radiation fields. One can compare this to the problem of a cell tower used in the communications industry. The tower must be capable of handling the many calls passing through the tower, especially in a busy city. For radiation detectors there is the problem of actively interrogating cargo containers for Portal Security. The faster the detector can operate the more quickly cargo can be cleared and the more information about the cargo can be collected. For worldwide nuclear safeguards, spent nuclear fuel or reprocessing of spend fuel, must be well monitored and characterized over its life as inaccurate accounting of Special Nuclear Materials is a pathway to Nuclear Terrorism. In this case the detectors must be able to width stand high doses of radiation to be practical. In addition there can be emergency management problems. An example are those caused by the Japanese earth quark that greatly damaged the Fukushima reactor complex. The very high levels of radiation in the immediate area blinded radiation detectors so that a clear understanding of what was going on was difficult to obtain. To solve these problems our research group, some of its members shown in Figure 1 with our first instrument, studied the possibility of using the low cost Noble Gas krypton as a gamma ray detector media for work in very high radiation fields. We proposed to extend the present Liquid Noble Gas detector technology from the low rate, low radiation, low background environments as has been developed for astrophysical dark matter searches, to the high rate, high radiation environment found characteristic of the detection problems discussed above. While Krypton is low cost, it has a major problem. Krypton is extracted commercially from the atmosphere as a byproduct of liquid oxygen production. Atmospheric krypton is contaminated with radioactive Krypton released during reprocessing of spent nuclear fuel (the primary source) and from nuclear weapons testing. Consequently, the concentration of atmospheric krypton has been increasing at 1.9% per year. Our studies show that Krypton detectors, as depicted in Figure 2 below, could operate at very high rates because the scintillation light it produces when it interacts with gamma ray radiation has a very short lifetime. The light is completely gone in about 60 billionths of a second. This allows for millions of gamma rays to be detected each second, even in the presence of the "polluting" radioactive krypton, if the detector dimensions are careful selected and the electronic pulses the detector produces are analyzed to pull apart signals close to each other in time. While the radioactive krypton does reduce the detector’s performance, the fast scintillation light it produces allows accurate measurements of gamma ray energy for elemental analysis. Because of the lower costs of Krypton in comparison to Xenon, which would be the alternative, more detectors can be deployed, helping further to reduce costs. In addition if the liquid is lost for any reason it can quickly be replaced at lower costs in comparison to Xenon. The lower cost of the working media also simplifies the overall system design and costs.
