An app note on Gamma-photon radiation detector (PDF!) by Maxim:
A PIN photodiode, four low-noise op amps and a comparator are used to detect individual photons of gamma radiation. The schematic, design considerations and component selection are discussed.
Here is an excellent tutorial on Op Amp gain & BW considerations in transimpedance amplifiers, which is directly related to this application note. Look for the .pdf download link at the bottom of the page.
http://e2e.ti.com/blogs_/b/precisionhub/archive/2014/05/08/part-iii-what-op-amp-bandwidth-do-i-need-transimpedance-amplifiers.aspx