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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Microwatt wrote ...
made from scintillator screen+ photodiode+ transistor+ lm311 comparator?
This type of circuit is employed in many professional instruments using PIN photodiodes, both with and without a scintillator, but a few words of explanation:
At 10 keV detection efficiency approaches 100%, a figure which falls to about 1% at 150 keV.
There are two basic modes of operation:
A: DC Current Mode, where the current flowing through a PIN photodiode is proportional to the incident radiation.
B: Individual Photon Counting Pulse Mode - where the PIN photodiode is coupled into a charge integrating amplifier. Here we see a gamma photon strike on a PIN photodiode (top trace) with the resulting comparator output on the trace below:
Nowadays, specialised gamma ray detecting PIN photodiodes are available (at a breath-taking price!) from the likes of Centronic and Hamamatsu, BUT common glass lens type PIN photodiodes such as BPX65 were used in professional dosimeter designs until quite recently.*
For use below about 15 keV, the glass lens of BPX65 should be removed, so as not to block low energy photons.
PIN photodiodes have replaced GM tubes in many applications, and are used extensively in gamma ray spectrometers.
Don't underestimate the technical challenge of the charge integrating amplifier! The circuits look elementary, but bad layouts, poor component selection, and inadequate screening will catch the woolly-minded every time!
* See, for example, US Patent 5059801 assigned to the UK National Radiological Protection Board
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Proud Mary wrote ...
Microwatt wrote ...
made from scintillator screen+ photodiode+ transistor+ lm311 comparator?
This type of circuit is employed in many professional instruments using PIN photodiodes, both with and without a scintillator, but a few words of explanation:
At 10 keV detection efficiency approaches 100%, a figure which falls to about 1% at 150 keV.
There are two basic modes of operation:
A: DC Current Mode, where the current flowing through a PIN photodiode is proportional to the incident radiation.
B: Individual Photon Counting Pulse Mode - where the PIN photodiode is coupled into a charge integrating amplifier. Here we see a gamma photon strike on a PIN photodiode (top trace) with the resulting comparator output on the trace below:
Nowadays, specialised gamma ray detecting PIN photodiodes are available (at a breath-taking price!) from the likes of Centronic and Hamamatsu, BUT common glass lens type PIN photodiodes such as BPX65 were used in professional dosimeter designs until quite recently.*
For use below about 15 keV, the glass lens of BPX65 should be removed, so as not to block low energy photons.
PIN photodiodes have replaced GM tubes in many applications, and are used extensively in gamma ray spectrometers.
Don't underestimate the technical challenge of the charge integrating amplifier! The circuits look elementary, but bad layouts, poor component selection, and inadequate screening will catch the woolly-minded every time!
* See, for example, US Patent 5059801 assigned to the UK National Radiological Protection Board
If pin diodes can be used for cheap Geiger counters,
Would something like that work as a cheaper replacement than the BPX65? How sensitive is it to other forms of radiation, if at all?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Inducktion wrote ...
Proud Mary wrote ...
Microwatt wrote ...
made from scintillator screen+ photodiode+ transistor+ lm311 comparator?
This type of circuit is employed in many professional instruments using PIN photodiodes, both with and without a scintillator, but a few words of explanation:
At 10 keV detection efficiency approaches 100%, a figure which falls to about 1% at 150 keV.
There are two basic modes of operation:
A: DC Current Mode, where the current flowing through a PIN photodiode is proportional to the incident radiation.
B: Individual Photon Counting Pulse Mode - where the PIN photodiode is coupled into a charge integrating amplifier. Here we see a gamma photon strike on a PIN photodiode (top trace) with the resulting comparator output on the trace below:
Nowadays, specialised gamma ray detecting PIN photodiodes are available (at a breath-taking price!) from the likes of Centronic and Hamamatsu, BUT common glass lens type PIN photodiodes such as BPX65 were used in professional dosimeter designs until quite recently.*
For use below about 15 keV, the glass lens of BPX65 should be removed, so as not to block low energy photons.
PIN photodiodes have replaced GM tubes in many applications, and are used extensively in gamma ray spectrometers.
Don't underestimate the technical challenge of the charge integrating amplifier! The circuits look elementary, but bad layouts, poor component selection, and inadequate screening will catch the woolly-minded every time!
* See, for example, US Patent 5059801 assigned to the UK National Radiological Protection Board
If pin diodes can be used for cheap Geiger counters,
Would something like that work as a cheaper replacement than the BPX65? How sensitive is it to other forms of radiation, if at all?
I'm sure that a number of PIN photodiode types will work to some degree, but I wouldn't want to make guesses about the performance of parts with which I am not familiar. Small surface area to keep capacitance down to a bare minimum is essential for correct operation of the charge integrating preamplifier. BPX65 has a 1 mm.sq active area, and also has the advantage of metal can self-screening which becomes critical with ultra-high impedance electrometer inputs.
PIN photodiodes in pulse mode can be used for the direct detection of alpha particles.*
*See: Jpn. J. Appl. Phys. 47 (2008) pp. 1740-1741
Ultraviolet-Sensitive Windowless Silicon PIN Photodiodes for Alpha-Ray Spectrometry
Energy resolution measurements of ultraviolet-sensitive windowless silicon p–i–n photodiodes were carried out with alpha rays. A resolution of 12.6 keV in full width at half maximum for 5.486 MeV was achieved. The ultraviolet-sensitive silicon p–i–n photodiodes were found to be suitable to alpha-ray spectrometry.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Microwatt wrote ...
So why cannot a solar cell be used?
Solar cells can be used - and were used - as X-ray detectors in the Gy/hr regime (i.e. it would not be nearly sensitive enough to detect typical amateur sources such as mineral specimens of 100 μGy/hr at most for best pitchblende)
See below: Journal of Applied Physiology January 1963 vol. 18 no. 1 209
Note the meaty X-ray tube anode currents needed to bring the solar cell to life. Using the figures from the graph - 80kV, 500mA, 30 inches - the dose rate needed to produce 2V across the solar cell is a roasting 1100 Gy/hr.
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cheap geiger counter?
