The BC-720 is not the only way to see neutrons with a PMT.
I was stunned to learn that Joe Zambelli was using a common piece of scintillation plastic like the BC-400 (Anthracene loaded polystyrene) for his neutron counter. This plastic is normally a gamma detector! It will actually also give off light with most any charged particle making it into the slug of material.
He also shocked me when he used the output of the PMT Raw! (No discrimination attempted)
This means he would be detecting 100% of anything that made the thing flash and at any energy.
A bit of thought and consideration led to the realization that this would be an OK detector for neutrons provided a known and calibrated fast neutron source was used for a standard to calibrate his instrument.
A run with the fusor off produced a torrent of counts in his 2" system of about 1,100 cpm. Thus, he established some sort of relative background count here. With the fusor in operation, this figure went into the tens of thousands/minute. This differental was not due to neutrons at the rate of one count per neutron. Instead, the protons produced in the medium gave multiple scintillations per partcle due to multiple energy releasing collsions as it gave up its energy willy-nelly throughout the medium.
There is probably some statistical averaging going on here so that using a good fast neutron source of known flux, one can arrive at a related measure for an unknwon neutron source of similar energy fast neuts.
This being the assumption, Joe took his homemade instrument to a local college which had a neutron source (Am-Be) and cal'd his instrument to it.
When I arrived with my BF3 counter in hand and heard his tale of how he worked his instrument up, I was somewhat incredulous. I quietely took readings with my instrument and he with his, with his fusor running. Surprisingly, we were within about 20% of one another. (good for absolute neutron metrology).
So, the common stuff offered cheaply by O.E. Technology should to OK for a crude neutron counter provided one has a cal source available.
Certainly, even uncal'd, the thing would be a superb indicator of differential performance, heralding improved performance as the fusor is modified or operated under different regimes.
I thought I would pass this along for the impoverished or inveterate cheapskates looking to break into neutron meterology. Remember, this thing detects most all neutrons and not just fasts, so you can't even discriminate to that level as with the BC-720 Therefore, it has severe limitations in the neutron arena.
This baby detects cosmic rays, beta, gamma and X-rays inaddition to most all neutrons. So we are looking to pull the neutrons out of the noise. Only a good cal'd fast neutron source can give us a base level to even attempt any effort at establishing absolute neutron count levels.
Richard Hull
Created on Monday, January 15, 2001 10:41 AM EDT by Richard Hull