Neutron emission vs. flux (newbies)

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.
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Richard Hull
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Neutron emission vs. flux (newbies)

Post by Richard Hull »

This is basic and should help newbies.

When we speak of the neutron emission or, more correctly, "TOTAL ISOTROPIC NEUTRON EMISSION" from our fusors, we are looking at the core as a virtual point source of neutrons. We are treating it like a small arc lamp and counting every photon that radiates away during a specific time interval. (Total number of neutron bullets fired per unit time).

Needless to say, they move out in every direction just like light photons away from the arc. The total isotropic emission is a way of classifing just how good we are doing at our fusor effort and serves as a benchmark of performance. (How bright our fusor or fusion device is) The normal units are NEUTRONS/SECOND.

When discussing neutron dose or exposure or neutron fields, the term "NEUTRON FLUX" or simply "FLUX" is the only term ever used. Flux speaks of relative danger and is just not a good term to use when describing fusor or fusion performance. Flux, as is classically viewed, is a measure of the number of neutrons passing through a given area (square centimeter) in unit time (sec). Flux is given in neutrons per square centimeter per second.

Due to the inverse square law, the same neutron source will have different flux values based on differing distances from the source. If you know the flux and the distance from the source, you can back figure to obtain the istropic Neutron emission rates. It also follows that you can figure out the flux at any point from a fusor if you know the isotropic neutron emission rate.

The above distinctions are very important to keep in mind.
For purposes of comparison, a flux of 50n/sq cm/sec means nothing in terms of how well a fusor is performing unless the distance is stated.

Example:

If the flux above is taken at 8cm, (~3 inches), from the core where the outer casing is, the fusor is producing about 13,000 neutrons per second. However, if this flux is at 10 meters, (~ 33 feet), the device is cranking out 210,000,000 neutrons/sec. So flux is meaningless without a stated distance from the device in comparing fusion device outputs.

However, for safety purposes, the long distance, 50n/sq cm/sec, flux tells us that getting closer than 10 meters might be inadvisable. Due to the inverse sqare law, moving 10 times closer means 100 times the flux, or, 5000 n/sq cm/sec. This is a much nastier flux. So, at 1 meter your exposure to fast neutrons would constitute a long term health hazard.

So, brag about your fusion source's performance in neutrons/second - isotropic emission.

Protect yourself and discuss radiation protection with others by knowing the flux field you are in at a given distance.

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
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Re: Neutron emission vs. flux (newbies)

Post by Starfire »

How thick would the water shield have to be for the 50n/sq cm/sec, flux at 1 metre? Richard [ without the borax ]
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Richard Hull
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Re: Neutron emission vs. flux (newbies)

Post by Richard Hull »

If you are talking about stopping virtually all of the neutrons? A meter or two thick water wall should do it. (however, due to the exponential, you can never stop all of the neutrons!!) If you are talking about attenuating them severely (80% or more)...About 1 foot thick. If you are talking about cutting the flux in half and have the few that make it through be so thermalized that they would almost bounce off your skin, about 6" of water would do the trick. YOU CAN'T be sure about stopping every neutron!! You can easily thermalize them and stop or capture a few and the rest you will only scatter after thermalizing them. Paraffin would only need about 3-4" thickness.

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
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