isotropic neutron measurement

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Jake Wells
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isotropic neutron measurement

Post by Jake Wells »

I saw Robert Tubbs fusor 5 first neutrons video about 2:52 minutes in I saw he was getting 250 CPM on his neutron probe he said it was 3 feet away from the plasma. He multiplied 250 by 50 to get 12500 n/s. My question why did he multiply his neutron count by 50? and for Richards video 5:27 minuets in he said that he was getting 300000 CPM on his probe so he got 300000 n/s. my question is what are you multiplying your count rate on your neutron detector to find out how many neutrons a second you are getting isotropically?
Roberts fusor http://www.youtube.com/watch?v=PtqRrVTjHCg
Richards fusor http://www.youtube.com/watch?v=FndkBps4EAM
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JakeJHecla
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Re: isotropic neutron measurement

Post by JakeJHecla »

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

TIER= Total isotropic emission rate (n/s)
r= distance from fusor (cm)
cps= counts per second measured (most meters read in cpm, you have to convert)
nvth= cps in a field of 1 n/cm^2.s (varies based on the tube)
Jake Wells
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Re: isotropic neutron measurement

Post by Jake Wells »

why did he multiply by fifty as you said nvth= cps in a field of 1 n/cm^2.s (varies based on the tube). 1 neutron for every centimeter. the tube is 3 feet from the grid he multiplied by fifty shouldn't he have multiplied by 91.44 because 3 feet=91cm. I am still confused on the math can you please go into a little more detail.
“The day science begins to study non-physical phenomena, it will make more progress in one decade than in all the previous centuries of its existence.”
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Richard Hull
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Re: isotropic neutron measurement

Post by Richard Hull »

Before you go to far with my video, you are assuming I am reading counts per minute or counts per second from my digital readout in my verbal call out in the video. I am not!

I, too, have a multiplicaation factor of 20! Where did it come from? I have a ful explaination of this in a FAQ and one of my old posts on neutron counting. READ IT.

My call out was based on CPM on the digital counter read out X 20. Thus, during a 1 minute count, If I see the counter pass 10,000 counts while still running I know that I am doing over 200,000 isotropic neutrons per second. If it is still counting at 15,000 counts I know I have crossed the 300,000n/s line and if it halts a 16,205 counts after one minute I know I am actually producing about 324,000 n/s

One must calibrate a neutron counter..... all counters...... to get a simple constant "K" to get an easy multiplier for what a digital counter yields in CPM with a rigidly and forever fixed position neutron detector to yield an isotropic emission rate in neutrons per second. If your counter moves or is portable you have to do some fancy math every time to get a new K factor, (bummer, but doable)

The upshot is I used about 25, 10 minute calibrated BTI bubble detector counts over about 15 different days and runs to meltdown into an average CPM/mrem count on my rigidly fixed location, 3He counter. From here it was a simple matter to convert to neutrons per Square CM per second flux and then to isotropic emission in neutrons per second. Only simple algebra was needed.

All this effort and tedious work, sadly, results in never better than +/- 10% accuracy. (if that) However, for all the probable error, it is much better than a poke in the eye with a sharp stick.

Note**** This was original post's question would have better posted in the neutron and radiation detection forum as it was not a fusion theory question.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Jake Wells
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Re: isotropic neutron measurement

Post by Jake Wells »

I have read FAQ: NEUTRON COUNTING..... and FAQ - Neutron flux vs. emission I am still confused to get your neutrons a second do you multiply your count rate by 4*pi*and the radius of your fusor to get your counts per second. or do you find the area of your neutron tube then multiply it by your counts per minute. I have read the faqs and I am still confused. What did he multiply his count number to get neutrons a second? what is the multiplication factor?
“The day science begins to study non-physical phenomena, it will make more progress in one decade than in all the previous centuries of its existence.”
― Nikola Tesla
JakeJHecla
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Re: isotropic neutron measurement

Post by JakeJHecla »

JakeWells, I don't know what the specific number Robert was using, and it doesn't matter.

Here's how this works:

1) Imagine your neutron source as an infinitesimally small point shooting out neutrons in all directions. Imagine that there are N of them being emitted per second.
2) Now back up to an arbitrary distance away. Your point source is still emitting N neutrons per second, but you're only seeing a small fraction. Why? The flux is spread across a sphere with a radius R (your distance to the source). If you were to take a one-second snapshot and count the neutrons that pass through this imaginary sphere, you would count N of them. However, the flux density (neutrons per square centimeter per second, n/cm^2.s) is now N divided by the area of the sphere. The formula for the area of a sphere, as you should know, is 4*pi*R^2.
3) Now you have a detector, and it's ticking away in front of you. Its calibration sheet should have a number on it labeled NVTH (Bob's He-3 tube I estimate has an NVTH around 50-70). This indicates how many counts per second it would make in a field of 1 neutron per square centimeter per second (thermal neutrons).
4) Now you have all the info you need to get TIER. If your tube is reading 100cps and it has an NVTH of 50, you know the field at your distance is 2 n/cm^2.s. Therefore, if you are 100cm away, you have 4*pi*(100cm^2)*(2n/cm^2.s) = ~250,000n/s.
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Re: isotropic neutron measurement

Post by Jake Wells »

4*pi*(100cm^2)*(2n/cm^2.s) = ~250,000n/s.
1) 100cm^2= 10000cm
2) 10000cm*2(your neutrons per square cm per second) =20000n/s reading on the tube
3) 20000n/s(4)*(PI)= 251,327n/s
Is my math correct?

Ok so can I give an example to see if I am right. lets say jack (a fictional character) is getting 60000 CPM off his fusor. he would divide 60000 cpm by 60 to get CPS or neutrons a second. he got 1000 n/s and has a he3 tube that has a NVTH of 50. he would divide his counts per second buy his NVTH to get 20 n/cm^2.s his probe is 50 cm away. he plugs in the numbers to get 4*PI*(50cm^2)*(20 n/cm^2.s) to get a neutron count of ~628318 n/s is my example problem correct? if its not can you please explain to me what I am doing wrong.
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JakeJHecla
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Re: isotropic neutron measurement

Post by JakeJHecla »

Yes, your math is right, though your units are a little screwy (though not unforgivably so) . As for my example, I rounded off the number I provided because the additional values weren't significant figures in the calculation and I'm not a mental math wizard.
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Re: isotropic neutron measurement

Post by Jake Wells »

So Roberts counter was reading 250 CPM 250/60s=4.16 n/s. his tube had NVTH of 50. 4.16/50=.083n/cm^2.s his tube was 91.44cm away.
1) 4*(PI)*(91.44cm^2)*(.083n/cm^2.s)
2) 91.44^2=8361.27cm
3) 8361.27*.083n/cm^2.s=639.89n/s
4) 639.89n/s*(4)*(PI)=~8720n/s
Robert said he got 12500n/s what did I do wrong in my calculation?
“The day science begins to study non-physical phenomena, it will make more progress in one decade than in all the previous centuries of its existence.”
― Nikola Tesla
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Re: isotropic neutron measurement

Post by JakeJHecla »

I estimated the NVTH at ~50 from what I've seen out of that tube (It was used to verify 1st neutrons on my system), but could be considerably higher or lower. Further, I don't know the specifics of his calibration off hand.
Jake Wells
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Re: isotropic neutron measurement

Post by Jake Wells »

thank you for helping me on the math I really appreciate it.

-Jake Wells
“The day science begins to study non-physical phenomena, it will make more progress in one decade than in all the previous centuries of its existence.”
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Richard Hull
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Re: isotropic neutron measurement

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Before one gets to full of oneself with precise math calculations, There are about 10 or more issues that throw flies into the ointment. As such, you must realize it is all highly imprecise as assumptions are made throughout the process that just aren't that well controlled or even controlable.

Is the fusor external neutron emission uniform? Not really
Is it a stable emitter over time? Absolutely not.
Is the fusor a point source? It is NOT!
Is the detector fixed forever? In most cases, it is not.
Is the final count rate at the end of a timed run huge or small? If huge, you are better off and, within the above issues, your reduction more reliable. 20-30 counts over 10 minutes will pin things down poorly; while 16,340 counts over the same period will be far more valuable. This has to do with detector efficiency.

I have said this many times........You will never know the exact emission to better than 10-20%. However, you can gauge improvements in performance, run-to-run, rather easily, to within 1%!

You might say....Why bother? Again, it is all better than a poke in the eye with a sharp stick.

I hated casting these pearls of widom here.............It is the wrong forum! Folks wanting to know about neutron counting and neutron numbers will not expect this here, but search in the proper forum "neutron and radiation detection", missing all of this!

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Jake Wells
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Re: isotropic neutron measurement

Post by Jake Wells »

I know that my TIER neutron rate will never be precise but car my calculations in the right ball park?
This is my last question any more will be put in the neutron radiation column. Sorry for the mix-up.
“The day science begins to study non-physical phenomena, it will make more progress in one decade than in all the previous centuries of its existence.”
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Re: isotropic neutron measurement

Post by Richard Hull »

Yes, they can be in the right ball park, provided you try and control the fusion rate over the timed period, (maintain a rather steady voltage, current and D2 pressure), and develop a good detection scheme.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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