Moderator for Neutron Detector

[guest]

I've just finished visiting the Mc Master-Carr web site, and have found that they offer ultra high molecular weight polyethylene rod at $36.12 /ft for 6" diameter. This is not too bad a price, and certainly preferable to trying to cast my own moderator or to try and assemble a stack of pierced disks. Strangely enough, the UHMW material is cheaper than lower density grades.
The idea behind this is to put together a moderated neutron detector using a gadolinium loaded silicone scintillator. Since the device will be used strictly as a neutron counter with a virtually monoenergetic neutron source, one can dispense with the elaborate sets of holes and cadmium shields used to straighten out the energy response of the detector. This is a technique used with standard neutron detectors used where there is a concern about the RBE of the incoming neutrons, not with counting the absolute neutron flux.
Since the material is translucent, an outer optical shield will be necessary. One could use PVC or black polyethylene pipe for this purpose. I plan to use a 1/16" lead shield to further reduce gamma sensitivity due to background radiation.

[DaveC]

What about Paraffin wax? It is not very different (other than average molecular weight) , from PE.

Dave Cooper

[DaveC]

What about Paraffin wax? It is not very different (other than average molecular weight) , from PE.

Dave Cooper

[guest]

Paraffin is not as dense as the UHMW polyethylene, but I suppose it would work, as long as you cast it in something strong like a section of PVC pipe. I favor the strength and heat resistance of the polyethylene.

[jim-frank]

How well would a Boron laden water solution do as a moderator? For example, boric acid or borax solution?

[guest]

Well, it's water, and you'd have to keep it contained. You'd need more of it, as it is not as dense as the high molecular weight polyethylene. Also, you don't want to borate the water at all, as this would tend to absorb the neutrons that you've gone to such great pains to moderate. Boron and cadmium are used selectively in commercial neutron counters to straighten out the response as a function of energy to make the instruments more useful in radiation safety monitoring. Since fusor hacks are looking for monoenergetic neutrons from the D-D reaction, this sort of expedient is not necessary or desirable, as it would tend to lower the efficiency of the detector.

[DaveC]

I think, UHMW PE is a little more dense than water... most of the PE ranges from about 0.92 to about 1.05 or a bit more.

Dave Cooepr

[Richard Hester]

I looked it up, and yes (I was surprised), polyethelyene is a little less dense than water. However, you can machine it, there is no containment problem, it won't melt on a hot day, and I would ship it anywhere with the utmost confidence in having it arrive undamaged. Perhaps this is why all the moderated neutron detectors I have seen use polyethylene and not paraffin or water. My finished detectors (I'm contemplating four types) are going to Ludlum in Texas for calibration, and I want them to arrive and come back in one piece.

[jim-frank]

I have to admit, I am a little confused about the absorption mechanism in Boron. I know that 10B has a rather large cross section, but is that measured for thermal neutrons? How does boron absorb fast neutrons? I'm rather new to the world of detecting the slippery little characters, and any advice from the 'old men' would be appreciated.
Thanks
Jim Frank

[guest]

It's pretty simple. You are using a chunk of material with high concentration of light nuclei to themalize incoming fast neutrons by repeated inelastic collisions. There is a detector in the center of the moderator that is sensitive to thermal neutrons. Ideally, a fair portion of the neutrons thermalized by the moderator will eventually trickle their way to the detector. The last thing you need is an additional loss mechanism that stands a good chance of eating up the neutrons as they are thermalized, without giving them a chance to reach the detector, which is what would hapen if you mized boron evenly with the moderator. Remember that the neutron thermalization process is a random one, and neutrons can become thermalized just about anywhere in the volume of the moderator, depending on the ratio of direct to grazing collisions they have with the moderating nuclei. Boron and cadmium shields are used in commercial neutron detectors, but they are very selectively applied, and the neutron absorbing material is not mixed evenly throughout the volume of the moderator. In the commercial detectors, these shields are used to even out the energy response of the detector so that the detector can be calibrated for the biological effect of the neutrons (RBE). We have no such concerns, as we are trying to measure a monoenergetic, very low level neutron source. Every lost count makes it that much harder to accurately measure the neutron flux without long counting times.
The reason why the moderated detector approach is attractive at all is that these detectors are pretty insensitive to gamma and other incedental radiation, so that any pulses you get above normal background are much more likely to be due to neutrons. The standard detector used in a moderated system is a BF3 or 3He proportional counter tube. 10B and 3He have large cross setions for thermal neutron absorbtion, and yield up charged particles, making the results easier (but not too easy ) to detect with a proportional counter setup.
Bicron does make boron-loaded plastic scintillators, but that is another detection mechanism, and another subject...