Neutron Activation Analysis of Archaeological Pottery Samples of Large Size, Including Pieces of Low Symmetry Shape: How to Get Accurate Analytical Results in a Practical Way

The feasibility of the instrumental neutron activation analysis of entire pieces of archaeological pottery, using low thermal neutron fluxes, is examined and a new approach for the non-destructive analysis of entire pottery objects by INAA, using the conventional relative method, is described. The proposed method relies in the preparation of a comparison standard, which is a nominally identical replicate of the original object to be studied. INAA of small samples taken from that replicate allows determining its composition for the elements to be analyzed. Then the intact sample and intact standard are irradiated together with the neutrons from a nuclear reactor neutron beam, using a suitable turntable facility and monitored by neutron flux monitors. Finally, after proper decay times, the induced activities in sample, standard and flux monitors, are successively measured, by high-resolution gamma spectroscopy, using a high-efficiency germanium detector. In this way, several complicating effects such geometrical efficiency, neutron self-shielding and gamma ray attenuation are avoided and the need of complicated mathematical corrections is not needed. A potential advantage of the method is that it can be fully validated. Quantitative experiments using 7 - 13 hours of irradiation of pairs of 750 grams replicates, at low neutron fluxes of 3.9 x10⁶ n cm^-2 s^-1, followed by 100000 to 200000 seconds of counting in front of a 70% relative efficiency HPGe detector, led to recoveries between 90% and 110% for Sc and La. Another experiment, using pairs of replicates of small solid mud anthropomorphic objects, (weighing about 100 grams each), irradiated by 8 hours at a neutron flux of 10⁹ n cm^-2 s^-1, led to recoveries better than 90% and 110% for As, Ba, Ce, Co, Cr, Cs, Eu, Fe, Hf, La, Lu, Rb, Sb, Sc, Sm, Ta, Tb, Th, Yb and U, showing that the proposed method is suitable for LSNAA of entire pottery or mud archaeological objects. (author)