Neutron Flux Calculation

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Garrett Young
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Neutron Flux Calculation

Post by Garrett Young » Wed Feb 15, 2017 5:23 am

In the following setup:

SNM-42 Corona Tube
- Neutron sensitivity, pulses per neutron/ cm2 - 0.8
- 122 cm^2 surface area
- Positioned 50 cm from Chamber Center
- 300 cpm above background

How do I calculate the neutron flux per second?

TIER=(4*pi*r^2)*(cps/nvth)

31416*((300/60)/0.8)

196350 n/sec ?

Edit:

Does the physical surface area/cross section of the detector tube not factor into the equation because that's accounted for in the neutron sensitivity number?
Last edited by Garrett Young on Wed Feb 15, 2017 2:34 pm, edited 1 time in total.
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Richard Hull
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Re: Neutron Flux Calculation

Post by Richard Hull » Wed Feb 15, 2017 10:30 am

Question....... How do you know it is counting neutrons. Just because you have a neutron detector tube, this has no relevance to whether it is actually counting neutrons. No real data here regarding where the neutrons are supposedly coming from or the operational conditions of the source.

No idicated calibration reference.

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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Garrett Young
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Re: Neutron Flux Calculation

Post by Garrett Young » Wed Feb 15, 2017 2:27 pm

I was asking the question to clarify the calculation not prove a particular neutron flux.

However ...
No detection when outside of the moderator (7.5cm HDPE) or with non-deuterium working gas.

I'm not applying to the neutron club with this limited information just asking a question.
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Alex Aitken
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Re: Neutron Flux Calculation

Post by Alex Aitken » Wed Feb 15, 2017 3:33 pm

Richard has a recent reply in the new users area about this. The tube detects slow neutrons, so you need a calibration between detections in the exact setup you have (tube, moderator, bias, threshold) and the fast neutrons of the same energy that cause it. When you have a fast neutron flux reading you can then scale up for surface of a sphere.

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Garrett Young
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Re: Neutron Flux Calculation

Post by Garrett Young » Wed Feb 15, 2017 4:06 pm

Is it safe to say that the translation factor between fast and detected thermalized neutrons would always scale up the detected count?
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Richard Hull
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Re: Neutron Flux Calculation

Post by Richard Hull » Wed Feb 15, 2017 7:16 pm

Yes, the tube and its specified NV number are related at time of specification with the assumption of a uniform flux over and through its entire volume and crossection. The calcs appear correct in an idealized situation. NV numbers always climb with increased detection volume, cross section and pressure of a given gas fill. The NV number relies on complete moderation of fast neutrons and the manufacturer often supplies moderator specs for fast neutron detection to jibe with their stated NV numbers

It is rare to have an idealized situation. This is why a decent calibration will back up a theoretical calulation, as in the comparison of a bubble detector versus electronic neutron detection.

As noted, zero above background counts outside the moderator vs. increased counts within would indicate neutrons are being counted. This gives the ultimate proof that only fast neutrons are being produced or are present.

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.

Dan Tibbets
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Re: Neutron Flux Calculation

Post by Dan Tibbets » Fri Feb 17, 2017 1:15 am

This site is presumably useful.

http://www.gammaspectacular.com/fusion_calculator.html

Of course the counter, weather bubble or electronic counter needs a calibration.
I suppose with some work, you could use silver activation to help calibrate a counter, but the variables- silver purity, etc. introduces uncertainity. I understand commercially calibrated neutron counters carry significant increased cost. I don't know if a bubble detector could be used to calibreate an electronic counter- moderator setup.

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Richard Hull
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Re: Neutron Flux Calculation

Post by Richard Hull » Fri Feb 17, 2017 10:32 pm

A freshly binned BTI bubble detector can readily be used to calibrate to within 20% accuracy the isotropic emission of a fusor and thereby, an electronic counter with tube in a moderator.

I have done this in the past and it is easy, though tedious, temporally. I did a complete and thorough exposition in a past FAQ with an example.

Key points to such calibration..........

1. You may never-ever move you detector-moderator unit following calibration in it physical postional relationship to the fusor.
2. You must use a digital, totalizing rate meter counter with timer to count pulses from the detector.
3. You must take data over many days and operating sessions to get statistically significant data for reduction using the bubble detector.

Again, all of this already exists in the FAQs.

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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