# Hi, quick problem regarding emission of gamma ray.

1. Nov 10, 2004

### teclo

A Fe (57) atom is in an excited state 14.4 keV above the ground state. The nucleus decays to the ground state with the emission of a gamm ray. What's the recoil speed of the nucleus?

I'm not sure how to set this up. I thought a photon would have no mass, therefor no momentum. If so I couldn't do this with conservation of momenumtum or energy. I must be missing something, anyone have any ideas?

2. Nov 10, 2004

### teclo

The closest thing my book has to a simliar example is dealing with the Rutherford experiement.

K(1) + 0 = K(3) + K(4)

so i would think that the initial energy of the atom equals the energy of the photon plus the energy of the atom after emission. I'm not sure about how to setup something like this though.

3. Nov 10, 2004

### Staff: Mentor

Despite being massless, photons do have momentum: p = E/c.

4. Nov 10, 2004

### ZapperZ

Staff Emeritus
A photon DOES have a momentum.

http://scienceworld.wolfram.com/physics/Photon.html

Even the classical version of light as EM radiation has something called "radiation pressure".

So, knowing this, can you now do the problem?

Zz.

5. Nov 10, 2004

### teclo

yes, acutally i worked the problem out earlier. i forgot that p of the photon is E/c.

with that it was rather easy to approximate the recoil velocity of the nucleus using conservation of momentum. if reference frame observes initial excited nucleus as v of 0

0 = mv + E/c

i was making this harder than it really was