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Under ideal conditions, the discrimination for an He3 detector should be set at about 20% of the peak neutron pulse amplitude. The peak neutron pulse height represents a neutron event where the proton and triton tracks are at maximum or near maximum length. Less optimized neutron events, like those occurring near the detector wall, will produce pulses of lesser magnitude than the peak pulse, down to slightly more than 20% of the peak pulse height. Gamma ray interaction pulses are normally well below 20% (usually well below 10%) of the neutron peak pulse height. You can see all this quite clearly by watching the pulses on an ocilloscope connected to pin one of the 4024 in a COSMOS tool. If a peak neutron pulse is say three volts, the gamma ray pulses will be down below .6 volts (with a gamma check source held next to the He3 detector), ranging down to pulses indistinguishable from noise.
So 20% of the neutron peak pulse height is a good place to set the neutron discrimination; you will get all or nearly all of the neutrons, and none or nearly none of the gamma rays, in an ideal environment. In a hot well environment, set the discriminator at about 50% of the peak neutron pulse height. For shallower cool wells, 30% may be a good compromise setting.
The discriminator setting for He3 neutron detectors is not critical. No noticeable difference in log "character" will be seen with settings as widely spread as 20% and 50% of neutron peak pulse height.
Happy calibrating!
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