Exercise TIER calculation for SNM-11 tube
Posted: Wed Nov 08, 2017 10:27 pm
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.
TIER=4*pi*r^2*(cps/NVTH)
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.
Results:
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.
TIER=4*pi*r^2*(cps/NVTH)
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.
Results:
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.