Subcritical Multiplication and Spontaneous Fission in Uranium

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.
3l
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Re: Subcritical Multiplication and Spontaneous Fission in Uranium

Post by 3l » Tue Dec 06, 2005 5:51 pm

Hi folks:

Think in terms of molar ratio. UO2 (1 part uranium atom and two parts Oxygen...hence 3 parts)
Only the uranium fissions the oxygen is inert or worst yet pure poison.
that is where the third comes from.
Then you adjust for the number of u235 vs u238 in the uranium pile itself.
grams or horseshoes it is a ratio.
that gives you the proper amounts when you mutiply by the molar gram weight of uo2 times the adjusted mole ratio (how much u235 exists).

Happy Fusoring!
Larry Leins
Fusor Tech

Alex Aitken
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Re: Subcritical Multiplication and Spontaneous Fission in Uranium

Post by Alex Aitken » Wed Dec 07, 2005 7:47 am

Thats a reasonably convicing rule of thumb, oxygen last I looked was pretty indifferent to neutrons, but it wouldnt hold up as an assumption for the experiment. For example if you replace the oxygen with boron you have a much more effective neutron sheild than multiplier, but the same molar ratio of impurities to U-235.

3l
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Re: Subcritical Multiplication and Spontaneous Fission in Uranium

Post by 3l » Wed Dec 07, 2005 11:37 pm

Hi Marvin:

I could do all the probalilities for you but the group would be lost...
so I left out the quantum field theory for clarity.

DNADA! :>)

Happy Fusoring!
Larry Leins
Fusor Tech

AllenWallace
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Re: Subcritical Multiplication and Spontaneous Fission in Uranium

Post by AllenWallace » Sat Dec 10, 2005 3:43 am

Carl,
Could you please redo some of your measurments with your apperatus
turned upside down? Specifically, cosmic rays can hit your U target and
create muons and neutrons. I'm not certian of the muon sensitivity of H3
detector tubes, but it is a known trick that you measure an increase of
neutron flux by shielding the detector with lead. By turning your apperatus
upside down, the cosmic rays will hopefully generate neutrons with paths
traveling away from the detector.

from http://www.ngdc.noaa.gov/stp/SOLAR/COSM ... osmic.html

"When the secondary cosmic rays interact in the monitor,
(actually in lead surrounding the counters) they cause nuclear
disintegrations... composed of ... neutrons..."


Also I'm confused about the statistics you are using... I might need a
refresher course! You state: "The count results shown below are averages
and standard deviations per three minute interval. Different numbers of
samples were used for different experiments. ....
4. Empty sample chamber, source out (measure B). Result: 13.80 +/- 0.31"

Can I assume that this measurement was about 42 counts over a 3 minute
period? Which is an average of about 14 counts per minute? I would have
then taken the square root of 42 and stated the total count was 42 with a 67%
chance that the count is +- 6.4 or 30% accurate? and that the 1 minute
average was 14 CPM +- 2.1 counts. Please explain. I suspect that I may be
incorreclty assigning units.

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Carl Willis
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Re: Subcritical Multiplication and Spontaneous Fission in Uranium

Post by Carl Willis » Tue Dec 13, 2005 10:57 pm

Hi Allen,

You raise an interesting point regarding the cosmic rays and their ability to cause spallation in high-Z nuclei (and maybe to a lesser extent produce photoneutrons and photofissions). The experiment had no method of removing the influence of these effects. I think the influence is small, but I really don't know. Testing with lead or bismuth samples would be a way to find out and I do plan to try that. There's unlikely to be a change caused by turning the equipment around--for one thing, the uranium, the tube, and the source all lie in the same horizontal plane (that schematic is a "top down" view). Second, the neutrons detected in the He-3 peak in the tube are virtually all thermalized, meaning they have scattered multiple times and lost all traces of whatever anisotropic angular distribution they may have had when born.

Figuring that the total sea-level secondary cosmic ray fluence rate (mostly muons) is about 0.03 / cm^2 / s, figuring that the uranium samples presented a physical cross-section of maybe 30 cm^2 tops, we might expect a current through the sample of 50 / min. The samples are quite thin in terms of the range of the cosmic rays, so maybe only 1% interact. Most of the interactions will be scattering, but in the neutron-producing reactions it may be possible for many neutrons to be released (if the nucleus is evaporated). I have no handy data on spallation rates in heavy elements caused by cosmogenic muons. A conservative guess (and it's a total guess) is that you get one neutron per interaction on average. Maybe 1 in 5 of these neutrons will be slowed down and reflected into the tube. Summing this all up, I guess that the effect of locally-produced spallation neutrons is only going to be less than about 1 count per 10 minutes.

I'll try to find more information and I'll try to get around to sticking a big ol' chunk of lead next to the He-3 tube to see what happens. Might be a while before I can do that though.

On your other question about the statistics: The 13.80 +/- 0.31 in the example you picked is per 3 minutes, not per minute (so it would be about 4.5 / min). You're trying to use that handy feature of the Poisson Distribution to say that the 1-sigma standard deviation is the square root of the count, which would be true if that number came from one count. It actually came from N=144 separate counts in that particular case, so the standard deviation is SDEV = sqrt(mean / N) = sqrt(13.80 / 144). You can see why this was a time-consuming experiment now! I didn't really have space in the first post for all the fine details so some were left out. I hope this clarification makes it more understandable.

-Carl
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

3l
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Re: Subcritical Multiplication and Spontaneous Fission in Uranium

Post by 3l » Fri Jan 27, 2006 4:25 pm

Hi Folks:

Carl's comic ray data is right in line with the pubs when you eliminate all the crazy stuff like spontanious fision,spallation and most importantly positron fission of uranium atoms. (1mev> gamma does the trick).

Happy Fusoring!
Larry Leins
Fusor Tech

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