Lanthanum as an internal flux monitor in rain scavenging studies involving indium tracer

In order to study the basic mechanisms of circulation in severe connective storms, it was decided to release indium-115 in the clouds for tagging the rain to be collected at the ground downstream following such a storm. For estimating very small amounts, of the order of a few picograms per ml of water, of indium in the collected rain samples by a most sensitive method such as neutron activation, it became necessary to have precise knowledge of the neutron flux in the reactor. This difficulty was circumvented by employing lanthanum as an internal flux monitor and iron as a scavenger for both indium and lanthanum in these rain samples