Abstract: A logging device is disclosed for utilization in the detection of low energy photons. The logging device includes a sonde suitable for insertion into a wellbore. An insulating storage vessel is disposed within the sonde and serves to insulate a photon detection device and its associated electronic circuitry. The insulating storage vessel is constructed utilizing an inner and outer cylindrical shell. Each shell is constructed utilizing a low thermal conductive metallic material such as stainless steel or titanium and each shell includes at least a portion constructed utilizing a low density material having a high photon transmission property wherein low energy photons may be detected by the photon detection device within the insulating storage vessel. In a preferred embodiment of the present invention, an aperture is provided in the sonde which includes a plug of low density, low atomic number material wherein low energy photons may penetrate the sonde.