FAQ- Isotropic emission rates from simple neutron counter data - conversion factor

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Richard Hull
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FAQ- Isotropic emission rates from simple neutron counter data - conversion factor

Post by Richard Hull » Tue Sep 06, 2011 6:00 pm

The rather mouthy title means that you can derive your isotroic emission rate in neutrons per second from the CPM data taken by an electronic neutron detector and counter by empirically deriving a conversion factor. The accuracy is +/- 10% or so and has many caveats and gottcha's, but it allows for fast conversion of CPM counts to total emission rates from your fusor. In spite of all this is it much better than a poke in the eye with a sharp stick.

First, you must have the following items to begin with or you you can't benefit right now from this posting.

1. Stable and rigidly fixed electronic neutron detector. (It must never move relative to your fixed fusor chamber or be altered or adjusted significantly in any way.)
2. A newish, calibrated neutron dosimeter BTI bubble detector
3. A working fusor that fuses; preferably one producing at least 250k n/s

The bubble detector is great, but has a limited lifespan. It has been "binned" and carries a rating in bubbles/mr dose.

Set up your fusor to run near its maximum level of operation. Keep the BTI bubble detector away from the fusor and zero'd (top screwed down tightly.).

Start fusing until you feel your audible clicking or count rate on the neutron detector is about the best you can achieve in stable operation. Have a stop watch handy.

Place the BTI at some distance X from the fusor. Record the temperature and loosen the BTI compression cap to activate the BTI. At the same time start the stop watch and reset the electronic counter to zero, letting it start counting from this point in time.

The stop watch should now be running, the counter counting and the BTI collecting bubbles. Go back to shepherding the fusor for stable opeartion and run until the bubble detector collects a good number of bubbles. If you have a 30 bub/mrem rated bubble detector, then 30-40 bubbles should do. Now, turn off the high voltage to the fusor and stop the stop watch and inhibit or stop the electronic counter.

Record the number of bubbles, distance of the detector from the fusor, temperature, the electronic count and the time over the run.

Use the FAQ at:

viewtopic.php?f=13&t=5728&hilit=FAQ#p34172

to compute and determine the average isotropic emission rate of your fusor over the run, correcting for temperature of the bubble detector.

Let us say you log about 390,000 n/s using the FAQ above from your bubble detector and run time data. Let us say you ran for 15 minutes and the electronic counter logged 119,800 counts over your timed run. That works out to 119800/15 = 7986.6 cpm average over the period. Our conversion factor is 390000/7986.6 = 48.83 as we are conservative, lets round down to just 48. Log this factor in your notebook.

If, on a future run two years later, long after your BTI is loaded with bubbles and is worthless, you can record a counter based cpm reading of,say, 5943cpm and just multiply by 48 to show you produced on the order of 48 X 5943 = 285,264n/s isotropic during that run.

The key gotcha's are that you must be using the same fusor and your neutron detector has not been moved or altered in any significant way since you derived the conversion factor, originally.

My conversion factor for fusor IV and my fixed 3He detector is 20.69 but I use a simple conversion factor of 20. To home in on this factor, I used several recorded runs over several months with the BTI to arrive at a total summed emission rate for all runs and the summed CPM counter data that resulted from all those runs to figure my factor hoping this would allow for a bit better statisitcal average to base the final conversion factor upon. This conversion factor frees you of having to constantly buy or replace BTI detectors after your original first buy.

This method will work with any form of true electronic neutron detection system, but demands an intial bubble detector buy. It also assumes that over 10-20 minute data collection runs, the electronic detection system accumulates at least a thousand or more counts. A good neutron scintillator built around the Bicron BC-720 scintillator, a boron lined tube, BF3 or 3He detector will all easily fall into this range.

We need not struggle on the math, decimal point wise, as it is all a bit sloppy and +/-10% is a hoped for accuracy when all is said and done.

You could, in theory, use this electronic factor to calibrate silver activation efforts, but the further down the chain you go, the less robust the data derivation gets and the wider the range of error

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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Doug Coulter
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Re: FAQ- Isotropic emission rates from simple neutron counter data - conversion factor

Post by Doug Coulter » Tue Sep 06, 2011 9:53 pm

As a contrarian, I'd suggest going the other way -- using the silver, which is dead reliable compared to a BTI, to calibrate the emi and other garbage sensitive electronic counters. But that's me, and that requires a known "neutron oven" geometry and spacing. I know the issues with both electronic counters and nearly-failed BTI's have confused me, but silver always tells the truth. And if all you're after is "is this run better or worse than some other run" silver is the one I trust the most. Simply stated, it can't be activated by EMI (but a counter can be), and temperature is utterly don't-care for silver (but not for BTIs). The rest, not so reliable as an activation.

Richard is right on about not moving your electronic counter. I'd add -- moving any other thing that acts like a moderator changes the readings too. Just walking near it during a run does it noticeably.
Why guess when you can know? Measure!

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