4 ms·

> Gamma neutron sources inevitably have two problems. First, the gamma emitter is short-lived. Sb-124 has a 60 day half-life which meant that it had to be peri

by rdtsc 9y ago

> Gamma neutron sources inevitably have two problems. First, the gamma emitter is short-lived. Sb-124 has a 60 day half-life which meant that it had to be periodically reactivated in a reactor. Second, the gamma source presents an exposure hazard that requires lead shielding. A family member was working in a facility selling Californium-252, a strong neutron emitter. So gamma sources might be awkward to work with but these things are not exactly fun to work with either. It's probably the most expensive commercially sold substance on earth ($20M+ / gram). The half life is only 2 years. Now that's a nice revenue stream, well as long as Oak Ridge keeps its reactor going and there are enough customers out there. Also due to being a strong neutron emitter, it has a ridiculous amount of shielding. Lead won't work for it like it does for gamma rays, but things like polyethylene work well. So there would be lots of it, even for a tiny amount like a tiny fraction of a gram.

lostmsu 9y ago
> A family member was working in a facility selling Californium-252 Did he sell to government, or basically anybody could buy isotopes just like that?
- rdtsc 9y ago
  Bought from government (Oak Ridge) and sold to customers. Some are governments some are companies which use it for diagnostics like say detect structural issues in concrete. I think there are medical uses as well. As far as who can buy it and what the approval process is? I imagine there are controls in place. I never inquired about that.
  - badmin_ 9y ago
    Depending on the amount, a license would be required. Here's a link to current limitations: https://www.nrc.gov/reading-rm/doc-collections/cfr/part030/part030-0071.html https://www.nrc.gov/reading-rm/doc-collections/cfr/part030/p... Link regarding exempt amounts:https://www.nrc.gov/reading-rm/doc-collections/cfr/part030/part030-0018.html https://www.nrc.gov/reading-rm/doc-collections/cfr/part030/p...
    - rdtsc 9y ago
      Ah interesting. Apparently there is no exempt quantity of Cf-252.
      - badmin_ 9y ago
        Well I just found out that I was grossly underestimating the danger of handling Cf-252, given its half-life. Look what ORNL used to transport 1g: https://en.m.wikipedia.org/wiki/Californium#/media/File%3ACfShield.JPG https://en.m.wikipedia.org/wiki/Californium#/media/File%3ACf...
        rdtsc 9y ago
        Yeah. A single fission event can generate 2-4 neutrons, so even a tiny spec can be pretty dangerous. Here are the drums used for shipping much smaller amounts: http://www.frontier-cf252.com/custom-type-a-packaging-radioactive-materials.html http://www.frontier-cf252.com/custom-type-a-packaging-radioa...
pmiller2 9y ago
What makes polyethylene a good neutron shielding and lead not?
- rdtsc 9y ago
  Not a physicist but I was explained it was like this: Anything that has a mass close to the mass of the neutron would work better because we'd want to have a lot of elastic collisions to slow neutrons down. So something with lots of hydrogen would work, since the nucleus of hydrogen is a single proton so about the same mass as a neutron. It would mean water works. But so does polyethylene and it can be molded into various shapes and doesn't leak like water. Another way to think about it is lead is heavy nucleus, imagine it like a bowling ball. A neutron on other hand are more like tennis balls. A fast flying tennis ball hitting a bunch of bowling balls will not slow down too much and will keep bouncing around at at high speed. Now imagine this fast tennis ball hitting lots other tennis balls. Then the collisions are more elastic and it will slow down the neutron quicker.
  - terminado 9y ago
    And this is exactly the sort of principle that helps to distinguish between the mechanical aspects of quantum physics, and the much-hyped angular momentum spin uncertainty aspects. People make so much hay about the curious non-locality and uncertainty of spin, and tend to get tunnel vision about how nothing makes sense anymore, leaving out the broader strokes which have pretty reasonable underlying principles.
- InclinedPlane 9y ago
  Let's say you have a neutron shooting through some solid matter, you have a couple things that can happen. One is that the neutron can just not get close to any nuclei. This depends on the density and thickness of the matter the neutrons pass through. Another is that the neutron can come close to a nucleus and scatter off of it. Or, a neutron can come close to a nucleus and either be absorbed by it or cause some neutronic nuclear reaction (such as neutron induced fission). There are two super interesting things that are relevant to neutron scattering off of Hydrogen nuclei (protons). One is that due to quantum effects the de Broglie wavelength of the proton is larger than for other nuclei (partly due to the standard inverse relationship between energy and wavelength). This means that protons are a much larger "target" than more massive nuclei in terms of neutron scattering, which is a bit opposite of what you might imagine from a non-quantum perspective. Additionally, protons and neutrons are nearly the same exact mass, within single digit percentage points. Now, let's say you take a tennis ball and you throw it at a wall, it'll bounce off the wall at nearly the same speed, right? And almost exactly the same thing will happen if you throw it at a bowling ball. This is just the way momentum and kinetic energy works. But if you throw a tennis ball at another tennis ball and you hit it dead on then you'll transfer all of the momentum from the first ball to the second, just as in a Newton's Cradle or in a game of billiards. Those two facts together mean that Hydrogen nuclei are super good at slowing down neutrons. They are much larger targets to the neutrons and when neutrons scatter off of them the neutrons tend to lose a lot of their momentum. So it doesn't take many scatterings before the neutrons are no longer at ionizing energy levels and aren't dangerous to humans (they will decay with a half-life of several minutes into protons and electrons (and neutrinos)). Meanwhile, those protons will get a ton of momentum foisted onto them, but they are charged particles moving through atomic matter chock-a-block with other charged particles so they don't get far. The net result is that neutron radiation is blocked. Polyethylene is basically (CH2)n so it contains lots of Hydrogen atoms (water is also a good neutron moderator/blocker). Additionally, Carbon is also one of the better neutron moderators, so the combination is pretty effective. On the other hand, lead is just a bunch of heavy nuclei. Which aren't actually that "big" from a neutron's perspective due to quantum effects, and most of the time when a neutron hits a lead nucleus it's just going to ricochet and maintain almost 100% of its original speed/energy, so you need a lot of lead to make a difference.

by rdtsc 9y ago

lostmsu 9y ago

> A family member was working in a facility selling Californium-252 Did he sell to government, or basically anybody could buy isotopes just like that?

rdtsc 9y ago
Bought from government (Oak Ridge) and sold to customers. Some are governments some are companies which use it for diagnostics like say detect structural issues in concrete. I think there are medical uses as well. As far as who can buy it and what the approval process is? I imagine there are controls in place. I never inquired about that.
- badmin_ 9y ago
  Depending on the amount, a license would be required. Here's a link to current limitations: https://www.nrc.gov/reading-rm/doc-collections/cfr/part030/part030-0071.html https://www.nrc.gov/reading-rm/doc-collections/cfr/part030/p... Link regarding exempt amounts:https://www.nrc.gov/reading-rm/doc-collections/cfr/part030/part030-0018.html https://www.nrc.gov/reading-rm/doc-collections/cfr/part030/p...
  - rdtsc 9y ago
    Ah interesting. Apparently there is no exempt quantity of Cf-252.
    - badmin_ 9y ago
      Well I just found out that I was grossly underestimating the danger of handling Cf-252, given its half-life. Look what ORNL used to transport 1g: https://en.m.wikipedia.org/wiki/Californium#/media/File%3ACfShield.JPG https://en.m.wikipedia.org/wiki/Californium#/media/File%3ACf...
      - rdtsc 9y ago
        Yeah. A single fission event can generate 2-4 neutrons, so even a tiny spec can be pretty dangerous. Here are the drums used for shipping much smaller amounts: http://www.frontier-cf252.com/custom-type-a-packaging-radioactive-materials.html http://www.frontier-cf252.com/custom-type-a-packaging-radioa...

pmiller2 9y ago

What makes polyethylene a good neutron shielding and lead not?

rdtsc 9y ago
Not a physicist but I was explained it was like this: Anything that has a mass close to the mass of the neutron would work better because we'd want to have a lot of elastic collisions to slow neutrons down. So something with lots of hydrogen would work, since the nucleus of hydrogen is a single proton so about the same mass as a neutron. It would mean water works. But so does polyethylene and it can be molded into various shapes and doesn't leak like water. Another way to think about it is lead is heavy nucleus, imagine it like a bowling ball. A neutron on other hand are more like tennis balls. A fast flying tennis ball hitting a bunch of bowling balls will not slow down too much and will keep bouncing around at at high speed. Now imagine this fast tennis ball hitting lots other tennis balls. Then the collisions are more elastic and it will slow down the neutron quicker.
- terminado 9y ago
  And this is exactly the sort of principle that helps to distinguish between the mechanical aspects of quantum physics, and the much-hyped angular momentum spin uncertainty aspects. People make so much hay about the curious non-locality and uncertainty of spin, and tend to get tunnel vision about how nothing makes sense anymore, leaving out the broader strokes which have pretty reasonable underlying principles.
InclinedPlane 9y ago
Let's say you have a neutron shooting through some solid matter, you have a couple things that can happen. One is that the neutron can just not get close to any nuclei. This depends on the density and thickness of the matter the neutrons pass through. Another is that the neutron can come close to a nucleus and scatter off of it. Or, a neutron can come close to a nucleus and either be absorbed by it or cause some neutronic nuclear reaction (such as neutron induced fission). There are two super interesting things that are relevant to neutron scattering off of Hydrogen nuclei (protons). One is that due to quantum effects the de Broglie wavelength of the proton is larger than for other nuclei (partly due to the standard inverse relationship between energy and wavelength). This means that protons are a much larger "target" than more massive nuclei in terms of neutron scattering, which is a bit opposite of what you might imagine from a non-quantum perspective. Additionally, protons and neutrons are nearly the same exact mass, within single digit percentage points. Now, let's say you take a tennis ball and you throw it at a wall, it'll bounce off the wall at nearly the same speed, right? And almost exactly the same thing will happen if you throw it at a bowling ball. This is just the way momentum and kinetic energy works. But if you throw a tennis ball at another tennis ball and you hit it dead on then you'll transfer all of the momentum from the first ball to the second, just as in a Newton's Cradle or in a game of billiards. Those two facts together mean that Hydrogen nuclei are super good at slowing down neutrons. They are much larger targets to the neutrons and when neutrons scatter off of them the neutrons tend to lose a lot of their momentum. So it doesn't take many scatterings before the neutrons are no longer at ionizing energy levels and aren't dangerous to humans (they will decay with a half-life of several minutes into protons and electrons (and neutrinos)). Meanwhile, those protons will get a ton of momentum foisted onto them, but they are charged particles moving through atomic matter chock-a-block with other charged particles so they don't get far. The net result is that neutron radiation is blocked. Polyethylene is basically (CH2)n so it contains lots of Hydrogen atoms (water is also a good neutron moderator/blocker). Additionally, Carbon is also one of the better neutron moderators, so the combination is pretty effective. On the other hand, lead is just a bunch of heavy nuclei. Which aren't actually that "big" from a neutron's perspective due to quantum effects, and most of the time when a neutron hits a lead nucleus it's just going to ricochet and maintain almost 100% of its original speed/energy, so you need a lot of lead to make a difference.