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Definiton of neutron flux
On 15:49, 26 June 2011 Belromain changed the definition used by e.g. NRC to some unfamiliar definition. I personally don't do research in fission, but fusion. Never the less, my understanding of the definition of neutron flux (or any flux) is particles/(area time). The flux is a local (defined in a point) quantity, so defining it via total distance travelled by particles within a volume and timespan is to me misleading.
Further more, it should be per volume, not per area!!
Answer concerning the definition of the neutron flux
The unit of the neutron flux is indeed per cm2 and per s, but defining it as the total number of neutrons crossing an area is a bit different and is called angular flux (defined for a given direction, tangential to the crossing area). The angular neutron flux is equal to the usual neutron flux only in the case of a monoangular neutron flux. In the general case, all possible area directions must be taken into account, by integrating the angular neutron flux over all directions to obtain the neutron flux. It can be shown that it is equivalent to the distance travelled by all neutrons per unit volume and unit time, with the volume tending to 0 to obtain a point flux.
The definition of the neutron flux as a distance travelled has the advantage of making a better link with the macroscopic cross section, which is the number of reactions per unit length travelled by neutrons. Multiplying the macroscopic cross section with the neutron flux gives hence the volumic reaction rate.
A more mathematical definition can also be found in the litterature (and in particular in the NRC website ), corresponding to the product of the neutron density with the neutron speed: Φ = n v. I made the choice of the physical interpretation for the sake of clarity and simplicity, but if having the mathematical definition can dispel doubts and is more familiar for some, it is worth presenting it. But in any case, the NRC does not definite the flux as the number of neutrons crossing an area.
Below are also listed some references presenting the neutron flux as the distance travelled per unit volume and per unit time:
"In physical terms, the quantity Phi is the total distance travelled in one second by all neutrons in 1 cm3, since it is obtained by multiplying the number of neutrons in that cm3 by the speed each is travelling."
"When the angular flux is integrated over all directions, one obtains the flux. This can be interpreted as the total distance travelled at time t by all neutrons, per cm3 per eV per s." (in this definition, the neutron flux is also per unit energy)
"In physical terms, the integrated flux is the total distance traveled in one second by all the particles in the one cubic centimeter volume, as it is obtained by multiplying the number of particles in that cubic centimeter by the speed of each one.This is equivalent to the total length of all the particle tracks laid down in one cubic centimeter in one second."
- "NRC website". Archived from the original
|url=(help) on 2013-01-06.
- "Canteach". Archived from the original
|url=(help) on 2013-01-06.
- Stammler, Abbate. Methods of Steady-state Reactor Physics in Nuclear Design.
- Hebert. Multigroup Neutron Transport and Diffusion Computations.
I am not a physicist. I am a curious layman. I arrived at this page by clicking a link on the Neutron Bomb page, where it indicates such a bomb could be used to disable launching ICBMs due to neutron flux. What I read here does not indicate such could happen. Either the directing page is wrong or there is something about neutron flux that I am missing here. — Preceding unsigned comment added by 126.96.36.199 (talk) 10:08, 12 April 2014 (UTC)