Re: Wanted - a simple way to detect x-rays

["Robert Nowotny" <r.nowotny@bmtp.akh-wien.ac.at>]

Hello,

> Does any one have a simple electronic circuit using off the self
> diodes to detect x-rays? I also thought a photodiode and a phosphor
> might work.

it depends on what you want to detect.

To simply detect x-rays in radiography you could connect a photodiode
(e.g. BPW34 with around 300kOhm in parallel) to an oscilloscope (typ.
1 MOhm // a few pF input impedance) without any other circuitry and
view the photovoltaic response. You will not get much more information
than 'existance' and duration of the x-ray pulse.

Measurement of ionization current could be also done with a 
PIN-photodiode and an op amp in a current-to-voltage circuit. Samples
for and discussion of such circuits (noise, bandwidth) is available
amongst others in application notes and data sheets from Burr-Brown
(http://www.burr-brown.com) or Analog Devices (http://www.analog.com). E.g. 
You could ask your local representative for the 'Applications 
Handbook' from Burr-Brown in which this type of opamp circuit is 
discussed. For a BPW34 (about 7 mm2 sensitive area and 0.3 mm 
thickness of the depletion layer at around 30 V bias) you could 
expect something like 100 nA per R/s.

Besides direct ionization in the photodiode you could also 
use scintillators but the respose of Si-diodes in the blue and 
UV-region drops fast, so one should rather go for green or even red
emitters.

For the detection of single x-ray photons PIN-photodiodes used at room
temperature are suitable with some limitations for energy dispersive
measurements (low counting efficiency, resolution above 2 keV at 60
keV photon energy). Then one has to use common electronics for photon
spectrometry (charge sensitive preamp, shaping amp, multichannel
analyser, etc.).

Regards,

Robert Nowotny
---------------------------------------------------
Univ.Doz. Dr. Robert Nowotny<R.Nowotny@BMTP.AKH-Wien.ac.at>
Institut f. Biomed. Technik und Physik, Univ. Wien
AKH-4L, Waehringer Guertel 18-20, A-1090 Wien, Austria
Phone: +43-1-40400-1988; Fax: +43-1-40400-3988