X-ray now use a charged plate to image.  But I wonder if you couldn't go 'retro'  by using a rare earth screen material that glows in violet coupled with a PM to measure radiation.  The older style x-ray cassettes used rare earth screens to lower radiation exposure to patients.  The exposure could be reduced as much as 1200% compared to directly exposing film.  There are various types or sensitivity of screen material available depending on how 'detailed' you desired your image tp be.  I wonder if such material could be switched out to measure different intensity of radiation.  I have noted that two micro-curie of Amercium-241 on the more sensitive screen material glows a nice violet with the lights off.  By the way the Amercium  was aquired by deconstruction of an old smoke detector.

Hello Robert, What about CZT detectors? Would this be feasible as DIY?

Hi Robert, I am very interested in your project.  A couple of years ago I built a simple detector published by Elektor magazine using the BPW34F.  I had trouble with it breaking into self-oscillation when I hit a launch deadline and dropped the project.  This year I moved to building a Scintillation-PMT unit instead (project on Hackaday.io).  In response to discussion there I looked into a SiPM but found the cost too high for my tiny hobbyist budget.  For that reason I opted to go with a traditional PMT.
I am not knowledgeable enough yet to understand how to accurately capture the gamma energy levels from the pulses, so I have not finalized any design for the detector electronics.
I have two questions for you: Is there a way to use your approach to measure the pulse energy (relative, not absolute)?  And can you point me to any references on the subject of using the PMT pulse to measure the gamma energy (eV) ?

I wish you the best in your project and its continued success.

I understand, the Pin Photo-diodes have their problems with noise and feedback and SiPM costs and scintillators also, I'm working on a cost-effective solution "coming soon", well sort of, as its taking time due to new job and lower budgets.   Have a look at the cosmic pi project it uses a lower cost  SiPM  - http://cosmicpi.org/2016/08/02/alpha-design-for-front-end-released/  For your traditional PMT/scintillator stuff check out http://www.gammaspectacular.com/ - suplus parts are the quickest way in try  http://www.sovtube.com/33-scintillators-/ 

Hi Robert. Is the polarization of the photodiode in the schematics +24 V?  Should it not be -24V?

Oops - sorry placed the diode incorrectly in the circuit, I've now corrected it :P  Yes you are correct it would need to be -24V and it would work as  I started that way but then found I had a inverted output, also +24 regulators are more easier to come by than -24V regulators.   

Why not use a large single crystal silicon solar cell in a light tight enclosure? Or better yet, a semiconductor with higher density, and therefore higher gamma efficiency, like CIGS, Cadmium Telluride, or Gallium Arsenide? There is no reason the junction needs to be sensitive to optical wavelengths. Stacking the devices gives a larger interaction volume, and a higher chance of gamma ray detection.

I really do wish it was that simple, but alas there is a lot more involved.  There is much difference between a gamma ray and optical light as there is to radio waves. So the method of measurement is as vastly different.  Although both a solar cell and photo diode are essentially a pn junction there construction and the way they are used is vastly different almost opposite.  A photodiode is designed to maximize photo current, minimise dark current, noise and capacitance to make it sufficiently fast. While a solar cells are optimized for the maximum conversion of incident light into electrical energy.   Further to increase sensitivity to the fast pulses of ionising radiation the electrical method of detection is not by amplifying anything coming out the cell, but rather applying a reverse bias to the junction and measuring impedance change, so one has to employ all kinds of jiggery pokery electronics to get it to work.