Abstract: An apparatus to survey subsurface formations includes a sonde which is configured to move through a borehole. The sonde includes a neutron generator that emits neutrons into the subsurface formations. A crystal detector connected to the sonde monitors the neutron output of the neutron generator. The crystal detector can comprise an yttrium silicate crystal, an yttrium aluminate crystal, a lanthanum phosphate crystal, or a lutetium phosphate crystal. A lanthanum phosphate crystal or a lutetium phosphate crystal can also be used to detect photons coming from the subsurface formations. The sonde pressure casing can be fabricated from borated stainless steel to shield the internals of the sonde from thermal neutrons.