Gain stabilization for radioactivity well logging apparatus

Gain stabilization is provided for well logging apparatus of the type having a scintillation crystal optically coupled to a photomultiplier in a sonde to detect radiation in the borehole and formation elements in response to neutron bombardment. The gain stabilization apparatus includes a light emitting diode driven by a pulser to furnish regular scintillations to the photomultiplier, resulting in a stabilization pulse which is furnished to a spectrum stabilizer at the surface. The light emitting diode is provided with means for mounting the LED between the scintillation crystal and the photomultiplier, and for providing optical coupling such that the scintillations from the LED are sensed by and pass through the same system as the operative scintillations from the detector crystal. The mounting means may be provided with a groove for locating temperature-compensating elements. (author)