The intensity of the 1001 keV -ray occurring in the decay of234mPa in equilibrium with natural uranium sources is found to be (0.818±0.030)%. This value is in agreement with a recent measurement employing depleted uranium sources, but does not agree with earlier reports employing radiochemically pure sources.
The presence of uranium in a sample enhances the true values of La, Ce, Nd, Sm determined by INAA if appropriate corrections
are not made for the interference. The enhancement of the true values comes about because the (n, γ) activation products of
these elements, viz.140La,141Ce,147Nd,153Sm, are also produced from the fission of235U (∼0.72% natural isotopic abundance) even when La, Ce, Nd, Sm are totally absent in the given sample. In a 5 hour irradiation
1 μg of U is found to be equal to 0.28 μg of Ce and 0.23 μg of Nd while the equivalent La is found to be dependent upon the
delay from end of irradiation to sample counting time. A numerical procedure is given to correct for these interferences.
Spectral interferences from fission and (n, γ) β products of uranium in the determination of other trace elements by INAA
is also investigated. Uranium is found to be determined best using the 278 keV gamma-ray of239Np.
A reactor-detector combination has been calibrated for routine determination of trace elements by instrumental neutron activation
analysis using the semi-absolute method with flux corrections. The reproducibility of the calibration constants is studied
using the standard rock AGV-1 for the activities140La,141Ce,153Sm,160Tb,152Eu,175Yb,177Lu,131Ba,60Co,51Cr,134Cs,181Hf,233Pa,46Sc, and182Ta. The results show that a calibration reproducibility with a relative precision of better than 5% can be achieved in many
of the cases without any special precautions. To study the applicability of the calibration constants, concentrations of the
corresponding elements have been determined in standard rocks, G-2, W-1 and GSP-1 and are compared with the recommended values.
The techniques employed to measure Th concentrations in acrylic (PMMA) samples at a level of a few tens of ppt are described. Measurements on eleven different acrylic samples yielded a mean Th concentration of 40±20 ppt.
Improvements in the detection limits of thorium and uranium in acrylics by neutron activation analysis are described. The average of results from 10 g samples was found to be higher than the results from 450 g samples taken from the same acrylic sheet. Using eighteen large (450 g) samples from a single sheet of acrylic gave an average value of 3.2±0.3 pg/g thorium and 1.1±0.7 pg/g uranium. Interpretation of the results is discussed. Epithermal neutron activation with Cd shielded irradiations did not improve the detection sensitivity for thorium.