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.