A DEVICE TO MEASURE LOW LEVELS OF RADIOACTIVE CONTAMINANTS IN ULTRA-CLEAN MATERIALS

The purpose of this research was to develop a radiation detection device so sensitive that a decay rate of only one atom per 11.57 days per kilogram of material could be detected. Such a detector is needed for screening materials that will be used in exotic high energy physics experiments currently being planned for the near future. The research was performed deep underground at the Underground Mine State Park in Soudan, Minnesota. The overburden there is ~1800 meters water equivalent. The reason for performing the research at such depth was to vastly reduce the effects of cosmic radiation. The flux of muons and fast neutrons is about 100,000 times lower than at the surface. A small clean room quality lab building was constructed so that work could be performed in such a manner that radioactive contamination could be kept at a minimum. Glove boxes filled with dry nitrogen gas were used to further reduce contamination from dirt and also help reduce the concentration of the radioactive gas 222Ra and daughter radionuclides which are normally present in air. A massive lead shield (about 20 tons) was constructed in such a manner that an eight inch cube of space in the center was available for the sample and detector. The innermost 4" thick lead walls were made of ~460 year old lead previously used in double beta decay experiments and known to be virtually free of 210Pb. A one and one half inch thick shell of active plastic scintillator was imbedded in the center of the 16" thick lead walls, ceiling, and floor of the shield and is used to help reduce activity due to the few muons and fast neutrons seen at this depth. The thick lead shielding was necessary to shield the detector from gamma rays emitted by radionuclides in the rock walls of the mine. A sealable chamber was constructed and located on top of the shield that included a device for raising and lowering the detector and samples into and out of the center chamber of the shield. A plastic scintillator detector measuring 6"x6"x6" was fitted with wave length shifting fibers that allowed the light from ionizing radiation to be collected and transmitted outside the massive shield to photomultiplier tubes and electronics. The detector was calibrated for energy and detection efficiency and low resolution background spectra were collected. Results from these measurements show the figure of merit (using: efficiency/square root of background) for this plastic scintillation counting technique to be ~15 times better than for a 2 kg germanium detector for measuring surface contamination from atmospheric 222Rn daughters (210Pb, 210Bi, and 210Po). These daughter radionuclides are normally deposited everywhere onto all materials exposed to air. The results are encouraging and indicate that plastic scintillation counting techniques can be of benefit to the public by making available very sensitive counters for screening ultra-low background materials at an affordable cost. However, in order to reach the level required a multi element array of thin plastic scintillator sheets must be developed that will allow many thin samples to be counted at one time. In addition, more sophisticated light detection hardware, electronics, and computer software is needed