Exercise TIER calculation for SNM-11 tube

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Silviu Tamasdan
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Exercise TIER calculation for SNM-11 tube

Post by Silviu Tamasdan » Thu Nov 09, 2017 3:27 am

Like you know I don't have a neutron-producing fusor yet, but I do have a neutron source. So I did the following exercise to see if I can come up with a plausible TIER. In preparation for when I will have fusion neutrons.


Problem: I have no idea what the NVTH is for this tube. The only parameter clearly known is the tube sensitivity to neutrons: 0.8 pulses per neutron. I have seen this stated in many different ways in different places. 0.8pulses per neutron/cm^2, or 1.2 pulses per neutron/cm^2 etc. However the tube data sheet says clearly: 0.8 pulses per neutron. Not per area. So I will assume that this means plainly: if a neutron of the appropriate energy happens to intersect the tube anywhere, it has a 0.8 chance of producing a pulse. Or otherwise, 80% of the neutrons thermal will be counted.

Setup: 60-minute counts with the source located 11cm from the center of the tube, first with moderator, then without moderator.
The moderator, in keeping with my tradition, was composed of a 4-inch block of HDPE and 11 1-quart jugs of mineral oil. In an attempt to replicate the actual arrangement near a fusor, the source was outside the moderator and the tube inside.The HDPE block located directly between the source and the tube, and the oil surrounding the tube on the other 3 sides. The tube was suspended at an appropriate height above the work area in a lab clamp so I can remove the moderator from around it without changing its distance to the source. Pics below.

60 min count with moderator: 514 counts
60 min count without moderator: 29 counts
difference=485 counts in 60 min, or 8.08cpm.

factoring in the tube efficiency of 0.8: 485/0.8=606.25 actual neutrons in 60 min, or 10.1 neutrons/min.

cannot calculate TIER directly as NVTH is unknown.

But wait! I found this http://www.gammaspectacular.com/gamma-s ... eutron-150 and this http://www.gammaspectacular.com/phpBB3/ ... .php?t=113
The SNM-12 tubes used there are essentially the same as mine only smaller; the NVTH is not volume-dependent, so for the same technology it should be about the same. Thus the NVTH for my tube should be the same or very close, 0.15.

So, calculations.
TIER=4*pi*11cm^2(8.08/0.15)=3964 n/min or 66n/s

Now, my source is a Po/Be one, made with a Po-210 static eliminator that had an initial activity approx 5mCi. The exact date of manufacture is not given, but I acquired it on 10/12/17 and the date of manufacture is stamped "10/2017" Assuming the worst it could have been made on 10/1/2017.
A quick calculation gives a remanent activity of 4.11mCi as of today.

So, does a TIER of 3964n/min or 66n/s seem appropriate for this source? I think it is, and that would validate the methodology for calculating TIER for my planned fusion, whenever that may be.
counting without moderator; the angle of the picture is weird I agree, but the distance was 11cm. The source is inside the bubble wrap taped on top of the 2 white boxes.
counting with moderator
There _is_ madness to my method.

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Richard Hull
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Re: Exercise TIER calculation for SNM-11 tube

Post by Richard Hull » Fri Nov 10, 2017 8:24 am

I wish I could be of assistance here, but I am unfamiliar with the tube you are using. However, with that hot source and the tube in moderator I would hope you would get far more counts than you got. You are definitely counting neutrons, however try and increase thevoltage on the tube in 40 or 50 volt increments. Do not count for 60 minutes Count for 5 minutes. Keep checking your in and out of moderator counts. If the in-moderator counts go up over 5 minutes and the out of moderator counts stay close to the same as you up the voltage, keep increasing the voltage and or adjusting your level detection to keep the out of moderator count way down as your in-moderator count rises. You do not want to see your out of moderator counts shoot up all of a sudden.

At some point you will arrive at a useful voltage on the tube. At this point, stop and adjust only your level detection in the preamp. lower this detection level until the out of moderator counts starts to rise rapidly and then increase the detection level a bit. Hopefully you will arrive at a very low count out of moderator (background) and a very brisk count in-moderator. Regardless, when all is well, it must be night and day difference observed in and out of moderator counts/unit time. 10X or even a 100X differential.

Finally, do an in-moderator then out of moderator background check 20 minutes. In moderator should be a low count and out of moderator should be virtually zero counts.

Richard Hull
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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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