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Old 10-10-2019, 10:41 AM
brossa is offline
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What happens to thermalized neutrons?


If I spray a bunch of high-energy neutrons into a huge tank of water, the neutrons bounce off the hydrogen and oxygen nuclei in the water. This slows the neutrons down and heats the water. At some stage in the process the neutrons slow down to the point that they are more likely to be captured by a target nucleus rather than bounce off, depending upon the capture cross-section of the target nucleus.

So what happens to the neutrons that I spray into the water tank? Are they all ultimately absorbed by hydrogen or oxygen nuclei, making deuterium/tritium and oxygen 17/18/19? Is it more likely that they would decay into protons/electrons/antineutrinos before being captured? Do they just bop around indefinitely? For every neutron added to the tank, should I expect a transmutation event?
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Old 10-10-2019, 01:33 PM
Frankenstein Monster is offline
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The neutron scattering cross section of Hydrogen is much larger than the absorption cross section. So the neutrons will bounce around a lot but given enough water (a few meters thick) all of the neutrons will eventually be absorbed, turning a Hydrogen into a Deuterium.

The ratio for Deuterium is even more lopsided (AAUI) but once there's really alot of it then some will turn into Tritium.

Don't know what the tritium does. I would guess it decays into something that then again absorbs neutrons.

The neutron absorption cross sections for Oxygen is smaller so less of the oxygen (relative to hydrogen) will turn into its absorption product (O16 -> O17 -> O18 -> ?)

As long as there is any hydrogen or other neutron absorbing material left absolutely no neutrons will decay to protons (let alone hydrogen atoms) (it's something on the order of 10^-1000 so really zero). Wiki: Free neutron decay

If after a REALLY long time truly ALL of the hydrogen and oxygen have turned into something that does not absorb neutrons, then you would start seeing neutron decays.
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Old 10-10-2019, 06:04 PM
brossa is offline
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What is the timespan between neutron injection and absorption? Are the neutrons all thermalized and absorbed within a characteristic amount of time, or is it more like a half-life where some of them can persist for a while?
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Old 10-11-2019, 06:11 AM
Frankenstein Monster is offline
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According to this calculation the mean free path between thermal neutron captures in water is about half a meter. Given that the typical speed of thermal neutrons is 2km/sec then the typical capture time would be 250 microseconds.

Yes these are stochastic processes so there is a distribution of capture times for the neutron population.

The thermal neutron energy distribution itself is a Maxwell-Boltzmann distribution so there's a range of speeds and energies in those neutrons.

Last edited by Frankenstein Monster; 10-11-2019 at 06:12 AM.
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Old 10-11-2019, 06:42 AM
Frankenstein Monster is offline
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On second thought my earlier post can't be quite right. "Absolutely no neutrons??" Given a mean free neutron life of 880 seconds and a mean capture time of 0.00025 seconds (and a mean scattering time of a microsecond) there should be a very very small, but not really zero fraction of neutron decays.

Those neutrons should all end up as a tiny amount of Hydrogen gas in the water.
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