Neutrons_Setup.avi

A helium-3 proportional tube and NIM electronics are used to detect neutrons from a weak homemade AmBe neutron source. The video illustrates my preferred method for setting up the high-voltage bias, the amplifier gain, and the SCA lower-level discriminator in order to achieve sensitivity and selectivity for neutrons. I finish with a comment about the neutron energy response of the detector (it is sensitive to slow neutrons) versus the source spectrum (fast neutrons), and the need for a moderator to make this kind of detector sensitive to fast neutrons.

Basically, the procedure is as follows.

A. Begin with high voltage off, the amplifier gain at a minimum, the shaping time at 2-6 microseconds (for most neutron-detecting proportional tubes), the SCA's ULD at full scale, the LLD at zero.

B. Set high voltage to within the detector's proportional range, but not above the rated voltage of the cabling, connectors, or preamplifier. Always change the voltage slowly to avoid damage to the preamp.

C. In order to establish signals of an appropriate amplitude for the rest of the electronics chain, the amplifier gain will be increased until a strong gamma radiation response is observed. I place a gamma source (uranium ore) next to the tube to make this process faster and more definitive.

D. Pulse-height discrimination again non-neutron radiation is established by increasing the LLD on the SCA until gamma counts effectively cease. Now the detector is sensitive and selective for neutrons.