Patent ID: 9329303
Filing Date: 2016-05-03
CPC Classification: G01V

Claim Text:
1. An apparatus for evaluating an earth formation intersected by a borehole, the apparatus comprising: a first radiation responsive element optically coupled to a first light responsive device and configured to absorb substantially all incident thermal neutrons and produce first light scintillations indicative of the absorbed thermal neutrons, the first light responsive device configured to produce an output in response to detection of the first light scintillations; and a second radiation responsive element optically coupled to a second light responsive device and configured to produce second light scintillations indicative of at least epithermal neutrons that traverse the first radiation responsive element, the second light responsive device configured to produce an output in response to detection of the second light scintillations; wherein the second radiation responsive element is disposed within the first radiation responsive element such that the first radiation responsive element and the second radiation responsive element detect neutrons at substantially the same depth along the wellbore; wherein the first radiation responsive element has a radial thickness selected to be sufficiently greater than the mean free path of thermal neutrons in the first radiation responsive element such that the first radiation responsive element absorbs substantially all incident thermal neutrons and produces first light scintillations indicative of the absorbed thermal neutrons while letting substantially all epithermal neutrons pass through; and wherein a count of the first light scintillations over a period of time following exposure of the earth formation to energy from a radiation source is at least 90 percent of a count of the thermal neutrons incident on the first radiation responsive element over the period of time.