I don't think the steel or lead will really be relevant in this case, if it is like other fusors. If you need something structural to hold up the paraffin, steel would be fine. Lexan or aluminum or acrylic would be better, strictly speaking, because of long-lived activation concerns with the steel. The lead is doing little good for shielding secondary photons unless it is outside the neutron shielding.
However, in all the amateur fusors built to date, x-rays are the first shielding priority. If you have a limited amount of lead, my recommendation would be to apply it to stopping x-rays.
-Carl
FAQ - NEUTRON SAFETY
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Carl Willis
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Re: FAQ - NEUTRON SAFETY
Then there is water - a very good Neutron shield - Harwell has taken Neutron video photographs of the boiling kettle which shows the 'x-ray ' image through the metal with a blanked out section of the water at the bottom, untill the water starts to boil and the steam obscures the picture.
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Richard Hull
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Re: FAQ - NEUTRON SAFETY
No one produces enough neutrons to need a significant shield here. Runs are short at max energy and the isotropic yield is such that any incident flux is trivial based on time of exposure and frequency of exposure.
As Carl notes, x-rays are the #1 bad guy here.
For the uneasy guys who were both a belt and suspenders to hold up their pants, a simple 3 inch thick borated paraffin shield cast up in a wooden box is fine to kill the bulk of any fast neuts. What makes it through will be scattered some will even go back out the way they came in.
What is really scary is that such a shield might demand a 30 lb melt of paraffin and borax. The fire hazard associated with this might make a no shield neutron hazard in the 3 minute white hot grid run in a fusor look preferable.
Richard Hull
As Carl notes, x-rays are the #1 bad guy here.
For the uneasy guys who were both a belt and suspenders to hold up their pants, a simple 3 inch thick borated paraffin shield cast up in a wooden box is fine to kill the bulk of any fast neuts. What makes it through will be scattered some will even go back out the way they came in.
What is really scary is that such a shield might demand a 30 lb melt of paraffin and borax. The fire hazard associated with this might make a no shield neutron hazard in the 3 minute white hot grid run in a fusor look preferable.
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
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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longstreet
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Re: FAQ - NEUTRON SAFETY
Yes, paraffin can potentially be very scarry in it's liquid state. Though I can't imagine it being more dangerous than your average deep fry. At least you can't deep fry yourself with the paraffin.
Re: FAQ - NEUTRON SAFETY
"So Boron is a total stopper, but is far more suitable as a thermal neut. stopper due to the monsterous B10 cross section for same."
Richard,
You also mentioned in you first post of this thread that Hydrogen has a large cross section, and is the reason why water absorbs (slows down) neutrons so well.
Hydrogen is the smallest of atoms from the periodic table, and as such has the least number of sub-atomic particles in its nucleus, subsequently giving it the smallest nucleus.
Could you please clarify, either by link, or by elaboration what is meant by "large cross section"?
T.I.A.
Rog
Richard,
You also mentioned in you first post of this thread that Hydrogen has a large cross section, and is the reason why water absorbs (slows down) neutrons so well.
Hydrogen is the smallest of atoms from the periodic table, and as such has the least number of sub-atomic particles in its nucleus, subsequently giving it the smallest nucleus.
Could you please clarify, either by link, or by elaboration what is meant by "large cross section"?
T.I.A.
Rog
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Richard Hull
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Re: FAQ - NEUTRON SAFETY
Hydrogen is the number one moderator for fast neutrons on a real world useage basis! Water, paraffin and plastics are the hydrogenous materials of choice. (CHEAP per unit moderation volume)
The best stopper of thermal neuts would be gadolinium.
Hydrogen might have a small nucleus, but it also forms very tight molecules. Cross section is AN APPARENT, empirically measured, nuclear area presented to neutrons for a specific substance at a specific neutron energy. Most substances look really big to slow neutrons, but tiny to fast neutrons. It varies over a wide range. It is related to many factors. molecular structure of solids, crystallography of metals, nuclei size, etc. Cross section is a crap shoot and generalizations in guessing at it, one material to the next, will prove a snare to one's feet. Of course, once the cross section is measured, it is often easy to see why it is so. Hindsight is 20-20.
All the foregoing is why YOU MUST HAVE complete access to all relevant corss sectional charts related to any work or experiment. Cross sections vary like the wind even for a single substance for you have to also know what your neutron is doing. It is part of the equation!
Once a neutron hits thermal velocities the cross section tends to follow the 1/v rule. Up to neutron thermalization, cross sections are all over the place.
Resonances are really wild! take a look at what silver's cross section does just before neuts in it are thermallized.
Richard Hull
The best stopper of thermal neuts would be gadolinium.
Hydrogen might have a small nucleus, but it also forms very tight molecules. Cross section is AN APPARENT, empirically measured, nuclear area presented to neutrons for a specific substance at a specific neutron energy. Most substances look really big to slow neutrons, but tiny to fast neutrons. It varies over a wide range. It is related to many factors. molecular structure of solids, crystallography of metals, nuclei size, etc. Cross section is a crap shoot and generalizations in guessing at it, one material to the next, will prove a snare to one's feet. Of course, once the cross section is measured, it is often easy to see why it is so. Hindsight is 20-20.
All the foregoing is why YOU MUST HAVE complete access to all relevant corss sectional charts related to any work or experiment. Cross sections vary like the wind even for a single substance for you have to also know what your neutron is doing. It is part of the equation!
Once a neutron hits thermal velocities the cross section tends to follow the 1/v rule. Up to neutron thermalization, cross sections are all over the place.
Resonances are really wild! take a look at what silver's cross section does just before neuts in it are thermallized.
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
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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Carl Willis
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Re: FAQ - NEUTRON SAFETY
Hi Roger,
Cross-section, as used here, refers to a measure of the probability of interaction of a neutron with a nucleus. Interaction rate in a medium depends on the neutron flux, the atomic density of the material, the area, the thickness, and the cross-section, which ends up having units of area. It can be interpreted as the effective cross-sectional area of a nucleus. Sometimes it is close to the physical cross-sectional area, other times it is not.
Hydrogen attains a higher binding energy per nucleon as a result of capturing a neutron via the strong nuclear force to form deuterium. This is behind why the proton has an apparent cross-section to a thermal neutron of 3.3E-25 square centimeters, while having a physical cross-section that is closer to 8E-27 sq. cm. For other nuclei the discrepancy can be even larger.
-Carl
Cross-section, as used here, refers to a measure of the probability of interaction of a neutron with a nucleus. Interaction rate in a medium depends on the neutron flux, the atomic density of the material, the area, the thickness, and the cross-section, which ends up having units of area. It can be interpreted as the effective cross-sectional area of a nucleus. Sometimes it is close to the physical cross-sectional area, other times it is not.
Hydrogen attains a higher binding energy per nucleon as a result of capturing a neutron via the strong nuclear force to form deuterium. This is behind why the proton has an apparent cross-section to a thermal neutron of 3.3E-25 square centimeters, while having a physical cross-section that is closer to 8E-27 sq. cm. For other nuclei the discrepancy can be even larger.
-Carl
Re: FAQ - NEUTRON SAFETY
Water is a really good neutron moderator and deflector.
About 1/4" of water should get your neutrons down to thermal speeds. So if you build a 1" thick water tank and then put your borax behind it you should have a nice shield with out the fire hazard.
However you exchange that for a water hazard. Fore!
About 1/4" of water should get your neutrons down to thermal speeds. So if you build a 1" thick water tank and then put your borax behind it you should have a nice shield with out the fire hazard.
However you exchange that for a water hazard. Fore!
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Steven Sesselmann
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Re: FAQ - NEUTRON SAFETY
1/4 inch of water sounds like an awfully thin barrier to thermalise 4.3 mev neutrons.
I am no expert on neutron moderation, but I would have guessed that these little neutral particles would speed through an inch of water with a low probability of hitting anything at all.
Does anyone have some hard data on this?
Steven
I am no expert on neutron moderation, but I would have guessed that these little neutral particles would speed through an inch of water with a low probability of hitting anything at all.
Does anyone have some hard data on this?
Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
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Carl Willis
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Re: FAQ - NEUTRON SAFETY
Hi Steven
See attached: ENDF VI/B cross-sections in H-1 for radiative capture (top) and total (bottom). The difference between these values is almost entirely due to elastic scattering. At high energies, elastic scattering dominates. At low energies, radiative capture dominates.
Analytical solutions of the energy-dependent neutron flux as a function of distance from the source in a material are possible--sort of. You have to account for the downscattering of neutrons into lower energy groups, which of course have higher probability of capture.
But some simple calculations are illustrative of quantitatively how good a moderator water is.
1. The atom density (n) of H-1 in water is 6.7E+22 atom / cm^3.
2. The elastic scattering cross-section (s, approximated by bottom graph) at 2.5 MeV is about 2.5 b (or 2.5E-24 cm^2).
3. The macroscopic cross-section is S = ns, or (6.7E+22)*(2.5E-24) = 0.167 events / cm. (The inverse of S is called the "mean free path." It's about 6 cm.)
4. How thick must your water barrier be to attenuate the unscattered flux (I) to a tenth of its original value? I = (I0) exp (-St), where t is the thickness, I0 the original value of the unscattered flux. So
-ln (0.1) / S = t = 13.8 cm.
From this example, we get the idea that you must use at least 13.8 cm of water to reduce your flux by an order of magnitude. Realistically, you will need more because once-scattered neutrons do not disappear, but simply are scattered into lower-energy groups that still have penetrating power. The actual thickness of water needed to meet some shielding goal is, of course, dependent on the specific goals of the shielding. Usually a code like MCNP is used to do shielding analysis.
Hope this helps.
-Carl
See attached: ENDF VI/B cross-sections in H-1 for radiative capture (top) and total (bottom). The difference between these values is almost entirely due to elastic scattering. At high energies, elastic scattering dominates. At low energies, radiative capture dominates.
Analytical solutions of the energy-dependent neutron flux as a function of distance from the source in a material are possible--sort of. You have to account for the downscattering of neutrons into lower energy groups, which of course have higher probability of capture.
But some simple calculations are illustrative of quantitatively how good a moderator water is.
1. The atom density (n) of H-1 in water is 6.7E+22 atom / cm^3.
2. The elastic scattering cross-section (s, approximated by bottom graph) at 2.5 MeV is about 2.5 b (or 2.5E-24 cm^2).
3. The macroscopic cross-section is S = ns, or (6.7E+22)*(2.5E-24) = 0.167 events / cm. (The inverse of S is called the "mean free path." It's about 6 cm.)
4. How thick must your water barrier be to attenuate the unscattered flux (I) to a tenth of its original value? I = (I0) exp (-St), where t is the thickness, I0 the original value of the unscattered flux. So
-ln (0.1) / S = t = 13.8 cm.
From this example, we get the idea that you must use at least 13.8 cm of water to reduce your flux by an order of magnitude. Realistically, you will need more because once-scattered neutrons do not disappear, but simply are scattered into lower-energy groups that still have penetrating power. The actual thickness of water needed to meet some shielding goal is, of course, dependent on the specific goals of the shielding. Usually a code like MCNP is used to do shielding analysis.
Hope this helps.
-Carl
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