Experimental Determination of Detection Limits for Performing Neutron Activation Analysis for Gold in the Field

AbstractMeasurements are presented of gold concentration in rock/soil samples by delayed neutron activation analysis using a device and method that are potentially field portable. The device consists of a polyethylene moderator and 252Cf as the source of neutrons for activating the samples and a high-purity germanium detector to measure the 412-keV gamma-ray emissions from activated gold. This information is used to extract the gold concentration in the sample. Two types of samples were investigated: (a) pure SiO2doped with a known amount of gold chloride and (b) U.S. Geological Survey standards. The former types were used to evaluate optimum device performance and to calibrate the device and method. The latter types were used to show typical system performance for the intended application (field exploration for gold deposits). It was found that the device was capable of determining gold concentrations to ~10 ppb with a turnaround time (the sum of irradiation, decay, and counting times) of ~10 days. For samples where the gold concentration was much higher (i.e., gold ore), turnaround times are ~2 days and could be shortened further by sacrificing accuracy (e.g., lessening irradiation, decay, and counting times) or by augmenting source strength.