FAQ - Build a HDPE block moderator for detection/activation

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Richard Hull
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FAQ - Build a HDPE block moderator for detection/activation

Post by Richard Hull »

The one thing we know is that while most fusors are anisotropic emitters of neutrons, even the "hot spots" around the fusor are minimal emitters of neutrons in the lowest end of fusor operation. (Under 30kv applied)

As there is no beaming either, the limited neutron emission at your hottest spot is still like a "pointish" source. The ideal is to not allow the inverse square law to limit your detection number or activation possibilities.

Fast neutrons, which are what D-D fusion is producing, will not count or activate. A moderator is needed to slow the neutrons to thermal velocities so they can be counted and used to activate various elements.

Thus, the true ideal is to locate your hot spot by whatever means available to you and place your moderator as close to this location as is possible. This means you should try and jam your moderator up against the fusor as close as possible. In many instances, the normal solid moderator, is most often the one that is used for detection with a BF3 of 3He tube placed near its central core, just will not fit jammed up against a fusor.

To obtain this result you might turn to a moderator assembled with small blocks of HDPE. In my case I had a local plastics company saw a 6 foot plank of 1 x 6-inch HDPE into (3) 1-inch strips ... 1 x 1 x 72 inch strips and (2) 1 x 1.5 x 72 inch strips. Taking these home, I used my chop saw to cut the blocks out of these strips to various lengths that I felt would suit the purpose of assembling a much more intimate to the fusor, and thus, more efficient activation "neutron oven". I cut out several blocks of the following lengths. 1", 1.5", 2", 3" 4" and 6" from both the 1X1 and 1X1.5 strips, and still have 2 long, uncut, 6 foot strips remaining!

Piling up the blocks appropriately has resulted in more activation counts at any operational fluence, compared to my older setup within my 3He detection moderator located near but not in any contact with the fusor.
I attach images that show my assembly. It must be remembered that you must place any detector or element to be activated at a point within the moderator that there is 32mm to 42 mm of HDPE between the shell of the fusor and the detector or element to obtain thermal neutrons. Also, the moderator needs to be extended outward at least 100mm above, below, and around the element or tube as a reflector.

Richard Hull
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I chose to bore out a 5/8-inch hole within one of the blocks 1x1x6 inch and embed the GM tube with rhodium wrap to allow for intimate stacking within the block moderator.
I chose to bore out a 5/8-inch hole within one of the blocks 1x1x6 inch and embed the GM tube with rhodium wrap to allow for intimate stacking within the block moderator.
A closeup image of the assembly during construction
A closeup image of the assembly during construction
The finished moderator with lead outer shield to keep GM background count low. As can be seen, I have plenty of blocks left over.<br />Plus, two 6-foot long uncut strips of HDPE yet to be cut into blocks!
The finished moderator with lead outer shield to keep GM background count low. As can be seen, I have plenty of blocks left over.
Plus, two 6-foot long uncut strips of HDPE yet to be cut into blocks!
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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