Helium-3 detector operating mode

[Dan Knapp]

Helium-3 detector tubes can be operated in ionization chamber, proportional, or Geiger-Mueller mode depending upon the anode voltage. Miley and Murali, in their book Inertial Electrostatic Confinement (IEC) Fusion (Springer, 2014, p. 291) state that for IEC devices, “the ion chamber mode is generally employed to avoid current saturation effects.” Knoll (Radiation Detection and Measurement, 4th edition, Wiley, 2010, p.534) states ”… the usual implementation is as the fill gas in a proportional counter. Geiger mode operation is not attractive since there would be no capability to discriminate against pulses generated by gamma ray interactions.” Carl Willis, in his YouTube video on helium-3 detector setup (https://www.youtube.com/watch?v=skP3SWWR9Kw) operates the tube at 1400 volts, which would appear to be in the Geiger-Mueller range for the size tube shown in the video (ca. ½ in) at a usual He-3 pressure (ca. 4 atm). The Radiation Detection FAQ’s have several entries on helium-3 detectors, but none explicitly address this issue. The voltages stated in the FAQ’s (“within a few hundred volts of the maximum operating voltage”) would suggest Geiger-Mueller mode. The manufacturers (e.g. LND) often refer to the tubes as “proportional counter” tubes, but then include a spec on plateau flatness (typically 1% per 100 volts), which would imply that they are referring to the Geiger-Mueller plateau of the operation curve. Journal articles and even thesis documents don’t usually address this level of detail in detector operation. I had been under the impression that helium-3 tubes were generally operated in proportional mode, but none of the discussions I’ve read explicitly address determining that you’re in the proportional range. It would be of interest to hear from those using helium-3 detectors what mode they use and how they determine that they’re in that mode.

[Bruce Meagher]

Here’s my current understanding…

My He3 tubes are running in proportional mode. I know they are in their proportional region because I monitor the pulse height spectrum through a canberra 35+ MCA (see spectrum below). Of course this is the spectrum of the He3 capture reaction (He3 + n -> H3 + p + 765 keV), not the neutron energy spectrum. Changing the tube’s voltage in the proportional region just shifts the spectrum up or down. If the tubes were running in geiger mode the pulse height would essentially be centered around a single large value as the tube would avalanche irrespective of the energy deposited.

I think you are drawing the wrong conclusion about plateau flatness. This spec is referring to the counting plateau over the tube’s operating region (see figure below). As you increase the tube's voltage from 0V you start generating pulses as the charged particles from the neutron capture reaction (or a liberated electron from a gamma interaction) deposit their energy in the gas. The ion pairs generated by these charged particles are collected to produce a detectable event. At low voltages these ion pairs can recombine before being collected. When on the counting plateau the voltage is sufficient to detect all events (+/- some %) as related to the tube's sensitivity spec.

As a side note, from the pulse height spectrum, I set the lower discriminator just below the edge of the proton wall effect to practically discriminate all the gamma interactions.

He3 Spectrum

He3 spectrum.JPG (30.19 KiB) Viewed 3875 times

Counting Plateau.jpg (28.08 KiB) Viewed 3875 times

The operating voltage is tube dependent and many tubes want to operate below 1000V. The above figure from Crane & Baker is just an example.

[Richard Hull]

I operate in proportional mode as that is the mode demanded and quoted by the manufacturers of He3 neutron detectors. My 4 ATM Reuter & Stokes tube works at 1400 volts. 1" diameter X 24" long.

Richard Hull