Parraphene facts

[3l]

Hi Folks:

I just ran into an item which I had not considered at all.
Borated paraphene.
I know the effectiveness of regular paraphene for moderating fast neutrons.
Can someone speak to this ethyl version?

Happy Fusoring!
Larry Leins
Fusor Tech

[Richard Hester]

That's borated paraffin in American-speak. The stuff is meant as a neutron shield. The paraffin (both the hydrogern and carbon component, but especially the hydrogen) moderates fast neutrons into slow neutrons, the boron component absorbs the slow neutrons. You still need to shield against hydrogen capture radiation and any gammas given off by the n, 10B reaction, but that's relatively straightforward, and probably only a real issue if the neutron flux is intense.
An easy way to borate paraffin would be to melt it and mix in finely powdered boric acid (roach powder). Load the paraffin with as much boric acid as it can keep in suspension. All the constituents of boric acid are relatively low atomic weight, so they can do a bit of neutron moderation themselves....

[bwsparxz]

I made some already. After I seen United Nuclear selling it for $2 a block. Looks no more that a pound. I made it much cheaper.

[Richard Hester]

How much boric acid were you able to load into a pound of paraffin?

[Richard Hull]

I made this stuff back in 1999. I used a 50-50 mix, by mass. It is rather tough to intermix as it is not miscible in the true sense. The problem is that the Borax wants to float. Still, with a little more effort than it takes to say, it can be done, though not fun.

I found it more profitable to pour a layer of about 1" of paraffin, let it harden and then cast the mix over the top of this to a depth of 3-4". In use, place the paraffin side towards the neut source.

Ricahrd Hull

[KJNW]

Hi Richard, I notice that in this particular example a 50 / 50 mix is cited. I believe I have also seen a 70 / 30 number inferred in another thread with regards to paraffin and boron. The commercial stuff is polyethylene with 5% boron. There is also a 2% variety of the commercial stuff. My question is; "Since we know we are trying to stop a 2.45 MEV Neutron, how do we determine an optimum ratio for paraffin and boron?" Is there any empirical research available by which this ratio can be determined with some degree of certainty?
Carl G.

[Carl Willis]

Hi Carl,

A radiation transport modeling code could be used in a straightforward manner to reliably predict a ratio of borax:plastic (or boric oxide, boric acid, boron carbide, etc.) that is optimized with respect to some dosimetric quantity, assuming some fixed source, target (human), and shielding geometry. I use MCNPX extensively for shielding analysis, but I believe anyone can freely download and install GEANT or FLUKA to do many of the same things. I suspect commercial shielding is formulated based more on what's mechanically practical (what will mix nicely, stick together, resist water, support its own weight) than on some definition of what is optimal.

Designing neutron biological shielding is a coupled neutron and photon problem. When neutrons are captured on boron, 400-keV gamma rays are released. Hydrogen capture produces 2-MeV gammas. Behind any meaningfully-effective neutron shield, most of the dose will actually be due to secondary gamma rays. It's not uncommon to plan heterogeneous shields in which a layer of neutron absorber is surrounded by a gamma absorber.

-Carl Willis

[Richard Hull]

Carl is correct. I have a number of ideas on shielding in a simple FAQ, search them out.

I now tend to favor a shield I hope to make that doesn't involve a mix. 2-3 inches of just paraffin plate followed by a box or two thickness of Borax and a final plate of lead would be ideal in many ways for a simple amateur "shadow cone pattern" shield at our level of neutron production.

My choise would be a 1/8" sheet of lead as first intercept to stop x-rays followed by a layer of paraffin and then the borax. This would be practical. However a 1/2" thick lead plate facing the observer would aid in, but not stop all of the H and B gammas.

There will be scatter from the paraffin, of course, but such scatter is in the 2pi plane facing the fusor some of that scatter will be lower RBE thermals. Again, our stuff is intrinsically a very weak neutron emitter even at 10e6 n/s isotropic. This is a level achieved by few fusioneers,thus far.

A shadow cone shield need not be big or ungainly. If I ever feel I really need major protection, my shadow cone shield will be movable and suspended from the ceiling on tracks allowing it to be moved for servicing the fusor without heavy lifting. Note: The key advantage of a shadow cone shield is its small size and need to be hard up against the fusor body when operating inorder to cast a shielding shadow cone over inhabited zones.

Shielding is an issue that is based on the temerity of the user and decisions should be related to output of the device, frequency and length of exposure.

Richard Hull

[Doug Coulter]

Thanks for giving the facts on the produced X ray/gamma energies Carl -- useful information you don't see all over the place (at least not in my library). Add the numbers for Cd-113+n and we'd have it all in one spot. Would the number I just googled up as 558kv be correct?

Anecdotally, I found this out the hard way when trying to make a directional neutron "camera" of one pixel. I used borated wax and Cd in a misguided attempt to actually stop the neutrons, in front of a plastic scint/phototube system that had been shown to have decent neutron sensitivity with fast neutrons.

Boy did that ever *not* work! No directionality whatsoever, due to the gammas off the neutron absorption processes, and that with a goodly bit of lead between all that and the tube. I'm going to take that all apart again and just use a pure moderator for the next try, along with pure lead for the gammas and X rays the fusor makes....

[billwcf]

Here is an expensive alternative. Do I hear "group buy"? I don't want to cut it.
http://www.drct.com/dss/shielding/Neutron-g-shield.htm Approximately $900 plus shipping for the 1" thickness. Plus lead. -bill

[Carl Willis]

Hi Doug,

Yes, 558 keV is the strongest gamma ray from thermal neutron capture on Cd-113. The NNDC's "CapGam" utility can be used to find out what the capture gamma spectra are like for a wide range of nuclides:

http://www.nndc.bnl.gov/capgam/

-Carl