# In the Pound Rebka Experiment, how were the gamma rays generated?

Lest my question be dismissed as poorly-worded, let me first say that I do understand the concept of how a gamma ray is emitted when an electron in an excited (higher energy) state transitions to a lower state. Also, I'm aware that they used $^{57}\text{Fe}$ as its photon energy (and thus gamma ray frequency) were/are well-known to an accurate value.

I've read the Wikipedia page and this Physics SE question/answer, but I still can't figure out...

# How did they get the $^{57}\text{Fe}$ electrons all excited to begin with?

Certainly the electrons in the iron samples don't maintain their excited state unless one keeps adding some external energy, correct?

Related side question: How difficult was it to get "enriched" $^{57}\text{Fe}$? And, just how well-enriched were the samples?

Facetious side-question: How loud could one hear the rumbling bass notes across the Harvard Campus while Dr. Pound and his student were running the experiment? :)

## 1 Answer

The photons in question were not atomic photons derived from relaxing an excitation of the electronic states of the atom. They were nuclear photons derived relaxing an excited nuclear states. The excited state of Fe-57 is generated by the electron-capture decay of Cobalt-57

Colbalt-57 can be produced in quantities sufficient for use in this kind of experiment by putting a sample containing Cobalt-56 in a environment with high neutron flux such as near an operating nuclear reactor or a intense neutron source. Co-57 has a half life around nine months so it is well suited to this kind of application (long enough for the rate to be treated as constant over a few days, but short enough to get significant rate).

• thank you for the quick answer... Plus, you automatically "answered" my side question. Now I feel a little embarrassed about confusing electron vs. nuclear photons /gamma rays. But, in my defense, the word "cobalt" is NOT FOUND on either of the two pages I linked. :-\ Apr 9 '17 at 17:36