What is Spectral Yield

I was reading a paper about channeling radiation and it had a graph of radiation with photon energy on the x- axis and spectral yield on the y-axis. I am unfamiliar with "spectral yield".

I looked in Jackson but with no luck. I goggled it and did not find a specific answer but I did find other papers that mentioned it. I believe it is # of photons/(# electrons * Solid Angle * photon energy)

I was wondering if someone can confirm this or point me in the right direction?

Thanks
Ben