FAQ- The ideal moderator for neutron detector tube

[Richard Hull]

Real experimental data always beats rule of thumb.

I enclose an image related to moderator thickness designed to test the thermal neutron fluence from fast neutrons from D-D fusion in HDPE.
Virtually all here use HDPE as a moderator for proportional tubes to obtain maximum counts of moderator thermal neutrons at a given surrounding blanket. This is also the ideal distance within HDPE for maximum activation of materials placed within a moderator.

I have seen tables where paraffin is almost identical in its action to HDPE. Thus, you might use this graph for such wax moderators as well.

We see that peak thermalization occurs at just about 4 cm. However, there is a very good response from 3 to nearly 6 cm. anything less than or more than is a fall off in thermal fluence for detection and activation.

Based on an HDPE cylinder with the knowledge of varying tube diameters from 1.5cm to larger tubes of 6.35cm, an ideal moderator diameter with relative bore hole for the tubes would be from 10 cm to 15 cm in diameter respectively. The length of the moderator cylinder is usually made to totally enclose the length of tube, or at least the tube's rated active length. (manufacture's spec)

Activation

Activation is a function of time and flux. Thus, a larger moderator will, over a given period of time, have the fully thermalized flux for over most of its volume add to the overall intercepted flux coming back to the object being activated.

Richard Hull

[JoeBallantyne]

The following data is for paraffin wax moderators. The image is a capture of one of the figures in the attached paper. The 252Cf graph is closer to what the graph would be for 2.45MeV D-D neutrons, as 252Cf kicks out ~2.1MeV neutrons vs the ~4MeV neutrons from the AmBe source.

Bottom line, a thickness of 5.5cm - 6cm of paraffin is optimal for moderating fusor neutrons.

24.Optimizationofmoderatorassemblyforneutronflux.pdf

(723.21 KiB) Downloaded 188 times

Joe.

[Richard Hull]

Thanks for the paraffin data. Paraffin is the moderator in the early Eberline PNC-1 instruments in the early 60's, using the small Nancy Woods BF3 tubes with a 60cm gas fill pressure. This was the one and, to my knowledge, only,neutron counter over the entire Farnsworth effort. While not very sensitive for the starting amateur effort, Jim Kovalchick and I own this type of counter and at nearly 1 million n/s, isotropic, they will climb to over 1000 cpm. My large 3He tube in HDPE will count about 125,000 cpm at the megamark.

Paraffin was dropped in manufactured systems due to it being a fire hazard in large facilities, and far less thickness was needed as HDPE became common on the market in the late 60's on. The earliest use of HDPE was in the classic "remball" detector, again, using the ubiquitous Nancy Woods small BF3 tube as in the PNC-1. The 1970s "snoopy" was a giant cylindrical piece of HDPE with electronics riding on the back of this barrel shaped moderator. The military adopted this monster with the famous "AN" (Army/Navy) prefix moniker.

Richard Hull

[Jim Kovalchick]

Good post Richard. I wouldl like to add that when it comes to using a moderator to detect neutrons versus simply maximizing the total number of thermal neutrons for something like activation, there are situational based considerations. For example, if you want to use a thermal based tube for fast neutron detection, there may be a need to surround the detector with a thermal neutron absorbing material like cadmium foil. This is generally true when you are going for a consistent calibration and the detector may be moved into a different geometry from one messurement to the next. This is why the PNC-1 paraffin is enclosed in a neutron absorbing box.

[Frank Sanns]

How does the detector size and moderator size correlate?

An ideal situation would be a material that slows down neutrons without scattering them. A 1" detector tube would only need a 1" plate in front of it to slow down the neutrons.

It would be easy then to calculate the flux passing through that 1" plane. But alas such a moderator does not exist or is not in the public domain.

The solution then is to use a mass of moderator that will maximize the counts on a detector or maximize the flux for activation. But what then is the effective size of full detection area. The moderator has a much larger physical (not nuclear) cross sectional area than the detector tube. It intercepts many times the number of neutrons than the detector tube itself does. Of course it scatters them in all directions but what is the true diameter for calculation of accurate neutron counts?

[Jim Kovalchick]

Frank,
Scattering plays a big role by reflection. As you know, that's why the detector is surrounded.

The answers to your questions are in MCNP.

Jim