We are using activation of things that are easy to activate and short lived, eg silver and indium, to measure the neutron outputs from our fusors. Medium slow (not quite thermal) neutrons do that best. HDPE is a far better moderator than wax (or any other substance we know of) for this. The proposed counter is to be a standard so we can compare this indirect measurement of neutron flux. The "oven" we suggest is the most efficient at activating materials that have resonances just off thermal, like the above-mentioned substances (and several others that aren't as sensitive). There might be some people in a more advanced state making radioisotopes (some are, but they tend to have to own cyrogenic super sensitive gear to even be sure they did it, fusors make many orders magnitude lower neutron flux than a fission reactor -- 106 total neutrons/second *total* vs a fission reactor at 1016 neutrons per second per CC), but we're just doing it to measure neutrons, because every other way in existence suffers from being too expensive, too error prone, or too hard to do, and not repeatable run to run or across labs, all of which this solves neatly. This is described in the other posts above at length, please do read the other posts first.
Turns out that about 1.5" thick HDPE is just right for going from 2.5 mev neutrons to few-eV neutrons, which is where a resonance exists for activating these easy to activate, short half life materials, which makes the sample reusable in short order, and hot enough with relatively few neutrons to measure at all, without having to have to make so much radiation in the lab you put yourself in serious danger doing it. So, that's the design linked above. We put some more HDPE behind the sample to reflect some of the neutrons that missed back into the sample, it adds a little sensitivity to the process, not a heck of a lot, but some. You use short half live things, because due to fast decay, they count fast after activation with minimal neutrons needed to get a usable reading. Long half life things, like say gold, aren't so good as you don't get many counts back for a given neutron flux on them -- they barely come out of the background cosmic rays unless you really pushed a lot of neutrons into them. And you then have to wait weeks before you can do another experiment. Where say, silver is nice and loud. A good 5 min fusor run here gets it to about 1000cpm on a counter such as the one proposed, completely swamping the lab background count rate of maybe 90 cpm max, while not exposing me to lethal levels of neutrons and other radiation just to take the measurement.