# Scattering Length

1. Oct 24, 2004

### manesh

what is the physical meaning of the scattering length of neutrons? What does mean -ve scattering length( for instance for 'H' !!!)??

Last edited: Oct 24, 2004
2. Oct 24, 2004

### marlon

mmm scattering length, perhaps i am having a language problem but to be honest i do not really know what you mean. The angle along which an incident particle is scattered off at some target particle gives you an idea of the strength of the interaction going on. Info on mass, charge and nuclear composition can be gained through such experiments.

In QFT things like scattering amplitude are used very often, but i do not know whether you are referring to this specific concept...

marlon

3. Oct 24, 2004

### Haelfix

I'm going to take a jab at this, but maybe what you want is the concept of a mean free path. Typically this is proportional to the avg statistical length a particle can traverse in a medium before its absorbed or emitted. Of course the constant of proportionality depends on what medium you are talking about (say lead)

There are other closely related lengths particle experimentalists use. Like the avg length before a hadronic shower, etc etc

4. Oct 26, 2004

### manesh

I am sorry..
I think you people didn't understand my question..
If you have a beam of nuetrons falling on a thinfilm the neutrons will get scattered. The strength of the scattering is proposional to the scattering length(the neutrons feels a Fermi potential, which is propotional to the scttering length of the neucleus, which is a constant for an atom). But the value of the scattering length for neutrons is different for the isotopes of the same element...for instance for H and for D the values are different. This difference is used in the isotopic substitution for contrast matching in neutron scattering experiments. ..more comments about scattering lengths are welcome..like the sign etc..
cheers

5. Oct 27, 2004

### nrqed

If I recall, the sign of the scattering length is related to the existence of bound states. A large positive scattering length signals the presence of a shallow (E near zero) bound state. A negative scattering length means that there is no bound state. The enrgy of the bound state can be found from the scattering length (it goes like $- {\hbar \over m^{} sl^2}$, if I recall correctly.

As for the meaning of the scattering length "ls" , if one considers low energy scattering, then the cross section (which will be isotropic, i.e. an S-wave) will be entirely determined by the energy of a shallow bound state. The cross section is essentially $4 \pi^{} sl^2$, iirc.

Anyway, I haven't looked at this concept in ages so take all this with a grain of salt!

Pat

6. May 21, 2009

### sarhas

Physical meaning of scattering length, (nothing to do with mean free path)

For low energies of the incident particle the details of the scattering potential are unimportant, only how the potential looks from far away. This is because at low energies the particle is not going to actually touch the object producing the scattering potential. The scattering length is a measure of how far from the potential the details become important.

This is similar to multipole expansion in electrodynamics. Two positive charges from far away will look like a single particle with twice the charge.

In regards to the other responses, with all due respect, if you don't think you have the answer you really shouldn't post, because the original person is going to have to look somewhere else anyways to be sure. At least try and look it up the answer first to at least know if your answer is not the correct one. Thanks.

7. May 21, 2009