Experimental determination of detection limits for performing neutron activation analysis for gold in the field

Measurements are presented of gold concentration in rock/soil samples by de layed neutron activation analysis using a device and method that are potent ially field portable. The device consists of a polyethylene moderator and C f-252 as the source of neutrons for activating the samples and a high-purit y germanium detector to measure the 412-keV gamma-ray emissions from activa ted gold. This information is used to extract the gold concentration in the sample. Two types of samples were investigated: (a) pure SiO2 doped with a known amount of gold chloride and (b) U.S. Geological Survey standards. Th e former types were used to evaluate optimum device performance and to cali brate the device and method. The latter types were used to show typical sys tem performance for the intended application (field exploration for gold de posits). It was found that the device was capable of determining gold conce ntrations to similar to 10 ppb with a turnaround time (the sum of irradiati on decay, and counting times) of similar to 10 days. For samples where the gold concentration was much higher (i.e., gold ore), turnaround times are s imilar to 2 days and could be shortened further by sacrificing accuracy (e. g., lessening irradiation, decay, and counting times) or by augmenting sour ce strength.