# Is a thermal neutron with very high speed effected by the gravity of a heavy nucleus

1. May 3, 2012

### 1907Quarter

just thinking cross sections here. a neutron that is freed as a result of a fission of pure U235. travels @c/10(fast!) will its trajectory be perturbed by the gravitational force emitted by a nearby nucleus, or do things on the quantum level work differently?

2. May 4, 2012

### M Quack

Re: is a thermal neutron with very high speed effected by the gravity of a heavy nucl

Neutrons are affected by gravity, but over much longer length scales. For ultracold neutrons that can give rise to interesting effects.

http://physicsworld.com/cws/article/news/2002/jan/17/neutrons-reveal-quantum-effects-of-gravity

The effect of gravity on neutrons is also a common problem in neutron small angle scattering.

How much neutrons are affected by the gravitational field of nearby nuclei I don't know. I guess someone will have to do the calculations (compare the kinetic energy of the neutron to the gravitational energy).

3. May 4, 2012

### mathman

Re: is a thermal neutron with very high speed effected by the gravity of a heavy nucl

The gravitational effects of the stuff in a reactor is extremely miniscule in the workings of a reactor. Even the earth's gravity will have very little effect, since the neutrons are moving quite fast and the distances involved are quite short.

4. May 4, 2012

### Astronuc

Staff Emeritus
Re: is a thermal neutron with very high speed effected by the gravity of a heavy nucl

One could calculate the force of gravity between a U235 nucleus and a neutron, and then compare to the nuclear force.

The gravitational effect is insignificant. The nuclear effects dwarf the effect of gravity.

A fast neutron in a LWR is more likely to interact with a proton in the cooling water than it is to be captured by a U235 or U238 atom. The slowing down of neutrons takes on the order of milliseconds. Control of the system is actually achieved by virture of delayed neutrons that come from certain radionuclides (e.g., Br-87, 88, 89, I-137, 138, Rb-93, 94) that emit neutrons seconds after the fission event from which they were created.

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