Comments for "Photodiode Gamma Ray Detector"

30th November 2014 12:26

David Mooney wrote ...

Hi, Alan,

When building your circuit, I used the FET (a K596) from a cheap Nexxtech microphone for PCs, and the circuit detects americium. In the store, I used the compass app of an iPhone to weed out the dynamic mics, which will not contain FETs.

27th November 2014 02:23

David Mooney wrote ...

Hi, Alan,

I found the following an interesting backgrounder to duplicating your circuit:

Johansen GA, Jackson P. Radioisotope Gauges for Industrial Process Measurements. Chichester, England; Wiley: 2004.

9th November 2014 05:18

David_Mooney wrote ...

Hi, Alan,

How did you make such a convenient Americium test source safely?

13th September 2014 22:26

vbc wrote ...

hi Alan,

good work i must say. I will drop a hint that power diode 1N4001-8 also work well as detector(with bit lower detection efficiency). Can you try this with your amp and let us know how good ? maybe you may need one more amplification stage.

best wishes,


25th February 2014 00:02

Giang Nguyen wrote ...

Hi Alan, can you give me the schematic or layout of the circut? I'm researching the cheap radiantion dectector circuit like that. Ty a lot!!!!!

13th December 2013 02:16

Giang Nguyen wrote ...

Hello Alan!

I'm reasearching for the radiantion detector circuit using the bpw34. However, i dont understand how to reckon the radiation doses by counting pulses which are born by the interaction between the radiantion and bpw34. For example, with GM- Tube SBM-20 we'll have the dose = (conversion coefficient*pulses). Looking forwards to seeing your reply as soon as possible.

Thanks a lot

8th September 2013 23:18

QUTE10 wrote ...

Any progress on the enhanced circuit diagram alan still cant get mine quite right the elektor one works fine but for some reason the front end on my version of your circuit is deaf maybe my fets are fried but just cant make it work like yours any help would be appreciated as i really think your ciecuit holds the most promise for size and sensitivity.

3rd July 2013 08:06

David wrote ...


First of all, great work, I love your website!

I'm curious as to how much mechanical shock affects your circuit? The ones I have developed thus far appear to be very sensitive, much more sensitive to shock than to photons.



15th June 2013 21:13

Leon Harris wrote ...

This is great!

How about covering the sensor with a little bit of zinc sulfide (silver doped) plastic, to sensitise it to alphas and betas?

I am thinking "glow in the dark" plastic.


I can feel a cosmic muon hodoscope coming on .....

4th June 2013 20:05

Mark Magness wrote...

Nice project!

I work in the engineering section for a Cancer Treatment Centre and we have been developing a dosimetry device for our Clinical Linear Accelerators using these BPW34's. They seem to work well after we blast 'em with radiation for a while to desensitise their response.

4th June 2013 17:42

Alan Yates wrote...


Using HOPG for the window is an interesting idea...

Not sure a laser diode would work well for a detector though, it is very small, can't be very sensitive?



4th June 2013 16:53

BotherSaidPooh wrote...

Hi, I have some ideas to add.

1) I have some JFETage here from old scopes if anyone would like them.

2) Any JFET should work, a 2N3819 or J309 etc.

3) Re. end window, I've had success using pyrographite (the levitating kind) on ebay as this is non restricted and freely available.

One little 10mm2 slab can make a dozen sensors.

4) Re. diodes, a nice diode can be had from a demised green laser pointer and its basically

identical to the quoted diode plus the plastic window is a LOT thinner.

5) Re. diodes (again), gut an old laser diode with the sensor, this is exposed and also sensitive in this way.

4th February 2013 07:46

austen wrote ...

Any chance of an updated schematic alan, for 2013 with the enhancements you mention (temperature stabilised and ac coupled).we await your pleasure.

23rd December 2012 08:02

austen wrote ...

Alan please could you tell us what resistor lies above the pot , I have built a version of the circuit using three bpw34,s in parallel and while I can get the thing to pick up back ground hum from the bench supply I can get nowhere near the noise floor never mind init.Please help cos its doing my nut in now, I,ve built the elektor one up successfully but wanted a more compact one for vaseline glass prospecting in the local antiques stores.

23rd November 2012 14:38

Ray wrote...

Hey Alan

Nice simple design. One thing that concerned me about your jfet stage is that the gate is always positive with respect to the source and could weakly bias on the diode junction defeating the high z which normally is only limited by the leakage current of the diode junction under negative gate bias. Having said that if you can detect Americium - it must be okay! Any thoughts?

Your circuit inspired me to design my own detector with a huge and expensive 10mmx10mm photodiode. I used an op amp approach rather than the discret fet. you can check it out at

5th November 2012 01:06

Qute10 wrote ...

Can i bump joes comment as i too would like to know this...

What is the value of the resistor in the schematic above the potentiometer?

14th September 2012 17:01

Joe wrote ...

What is the value of the resistor in the schematic above the potentiometer?

13th August 2012 11:22

Alan Yates wrote...


It is sensitive enough to tell you if you are in immediate danger from strong sources of gamma radiation (only). It will not detect trace contamination at levels near the noise, it just isn't sensitive enough.



9th August 2012 10:52

mark wrote ...

Great project! A quick question...As a stand alone unit could your device detect radiation similar to what was released during the Tsunami event in Japan? If so I'll be building one soon

9th July 2012 15:16

Sayan wrote...

Thanks for the suggestions - I found a few PN4117A on eBay. I will try them out and let you know how it works out.

Thanks !

3rd July 2012 03:07

kiranvarma wrote...

Nice project! you designed a costlier Gamma ray detector by using simple components. I must try it. very neat and clean explanation. Thank you Alan's

8th June 2012 11:14

Alan Yates wrote...


You can still find them, in particular I got some from element 14. But if you can't find them I can send you some. It is there aren't a lot of substitutes, but you can try JFETs with the smallest Idss and gate leakage current you can find.

An alternative is to use opamps, such as the LMC622 which have excellent input leakages.



5th June 2012 01:41

Sayan wrote...


The PN4117A part has been discontinued. Please let me know if there some alternatives I can look at.

Thanks !


12th May 2012 03:23

w5cdt wrote...

Nice Job. I'll be building one soon.


12th January 2012 14:22

Kevin wrote ...

Is the resistor going into the wiper of the 100K Trim-pot supposed to have a value?


25th October 2011 21:01

Alan Yates wrote...


I think the tally counter uses a similar or perhaps identical chip to most pedometers. All the cheap ones from china look to be quite similar. There is typically two and sometimes three capacitors, an encapsulated die mounted right on the board, the LCD, some kind of switch, and an LR55 battery.

The debounce time may be set by one of those caps, but fiddling with them doesn't seem to make much difference. Removing one of them completely stops the unit from working and is probably the oscillator. Worthy of more study anyway.



25th October 2011 20:55

Alan Yates wrote...


Yes, you could use the typical "charge sensitive amplifier" topology to null much of the capacitance of the diode, but I think it will still have poorer signal to noise ratio.

Worth a try though, it would improve the bandwidth of the device if nothing else, increasing the maximum count rate. The circuit is a bit of a hack, it could be much better.



23rd October 2011 11:44

Ted Carron wrote ...

Dear Sir,

I noted your reply to the person who asked if you had tried low cost solar cells as a large area detector.

You remarked that the extra capacitance reduces the amplitude. That made good sense to me. I went and looked at your circuit again. If I correctly understand it, you are using the diode current to develope a voltage pulse across the 10meg, and then amplifing that with the FET. Would it be possible to route the current into the virtual ground connection of an op-amp instead of the 10meg ? As a current output would that help alliviate the problem of the extra C ?


Ted Carron

18th October 2011 06:10

aRV k7hkl wrote ...

While looking at your latest Ray Detector effort I noticed your "Talley Counter" and it occurred to me that the cheap "Dollar Store" type pedometer might be modifiable to make an inexpensive Talley Counter. Some time ago I tried converting one to a frequency counter only to find that they don't count reliable above a few hundred KHz, but as a Talley Counter they might be adequate. If it works then you have another source of "Cheap Instrumentation" for your workshop.

16th October 2011 15:58

Alan Yates wrote...


Yes, I tried using small solar cells and larger die transistors. The larger capacitance of the big devices reduces the amplitude of the pulses observed and increases the relative noise level, making the device much less sensitive to lower energy radiation.

The calculator solar cells are too noisy to make a good detector unfortunately. You can parallel several BPW34s to make a bigger detector, I have not investigated the trade-off point where you need a seperate buffer for N photodiodes to make it worthwhile... A useful experiment I must undertake.



15th October 2011 05:14

Zilvinas wrote ...

Hi Alan,

Nice work!

Have you considered/tried to use solar cell from cheap calculators instead of pin diode?

73 LY2SS

12th October 2011 15:15

Alan Yates wrote...


Ginger beer, Bundaberg specifically.

All these sources are quite weak. You'd need to physically eat them or strap them to your body for an extended period of time to do any damage. The most dangerous is the nitrate salts of thorium and uranium, mainly from their chemical toxicity if you were to swallow them.



11th October 2011 18:39

peter marks wrote...


Not sick of your radiation detectors at all.

What beer are you drinking there?

These radiation sources you have... are they likely to cause you any harm?


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