Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: Homemade neutron detector?
Date: Jan 26, 11:52 am
Poster: Richard Hull

On Jan 26, 11:52 am, Richard Hull wrote:

>Measuring neutrons accurately is not an easy task even with fairly expensive equipment. As much art as science, it seems.

I have noted this in many past posts. It ain't like getting a geiger counter which detects virtually 100% of all alpha and beta that enter the chamber. RH

>With that in mind, has anyone constructed or even thought about a homemade neutron detector that would employ, say a normal gamma detector placed adjacent to a large container full of perhaps boric acid? In effect you could have a giant BF3 tube.

Good thinking, but there are some serious problems here. (or it would have been done years ago)

The alpha reaction is very short ranged in air about 2cm at max energy and the alpha is not necessarily at the full energy you note making the bulk of them even shorter ranged. The gamma thoughts are equally good, but the efficiency is way, way down below a common BF3 tube.

There are boron tubes using a thin film of pure B10 lining the entire walls of a normal counter tube. The tube is then filled with a much more useful detection gas (argon and quencher) at a much lower pressure and then the alpha reaction within the boron film which proceeds in that gas is very strong due to gas amplification. Unfortunately, they are rare and very expense. (As if BF3 tubes aren't expensive enough!)

The better BF3 tubes are about 25 percent efficient provided the moderator is calculated for the best response of a particular energy neutron.

The proton recoil system of the Bicron BC-720 has abysmal efficiency (~0.6%), but will not detect virtually any neutrons below 1mev and is vertually gamma insensitive. This allows for a much cleaner playing field albeit with bottomed out detection levels of at least 10,000neuts/sec virtually point blank in the 2" diameter.

The BF3 tube requires the unhandy large poly thermalizer which is averaged for a full spectrum of neuts. This places you much farther away from the chamber where the neuts are made and thus intercepting a smaller "solid angle".

True, neutron counting is a black art.

Now to what our good friend Jim Lux was talking about with the Am241-Be source idea. Sorry Charlie, been there, done that.

I have four nice very old industrial smoke detector Am241 strips of 15uc each! I put all four together and then covered them with finely powered elemental Be .9999 pure. The counters (both the survey meters) went from about 1 count every ten minutes to 1 count every 3 minutes with the source taped to the PE moderator!!! This makes sense, too.

Lets do the math.

A 1 curie Am241-Be source made by Isotope Products is warranted to produce 2.2X10e5 neuts per second.(got th' data sheet) This is about twice that of the best fusor III run to date. Now lets figure out what we might expect from my homemade 60uc source. It would be reasonable to assume that an amateur construction would not equal and certainly do no better than a pro source. Given this, and the best case scenario, we might hope for 12-13 neutrons per second isotropic emission from my "HOT" source at 4cm which is the closest I can get to the detector (BF3 tube) or about .06 neutrons per second hiting the 1sqcm tube end (optimum angle in a long BF3 tube for ~20% effeciency) From all this 1 neutron might enter the tube every 16 seconds with near perfect effeciency (20%) in a tube moderated for a specifc energy neutron we might expect a single recorded count every 5 X 16 or 80 seconds. My detection every 180 seconds speaks of the less than ideal detection conditions of my particular survey meter and, or, the skill of my source assembly, accuracy to the assumed (4) 15uc Am241 sources, etc. The math works out close enough for government work. (lotsa' slop in there)

Sooooo.... No! An Americium-Beryllium source of the homemade type, even a rather rich one, 60 times hotter than Jim proposed, is of little value.

Keep the good thoughts coming. The very thing that make neutrons great for research (their neutrality) makes them very tough to detect, unless you have a lethal blast of them.

Richard Hull