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Proud Mary

Wed Feb 24 2016, 07:23PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I guess this novel detector (link to ebay below) has a PIN photodiode in it. I don't have an iPhone so won't be buying one of these but if it works the price is attractive, especially for someone just starting out. I think you'd get bored with it quite quickly, unless you live in Japan, where Fukushima is still bleeding away into the environment. The capture area of PIN photodiodes is very small indeed but if it works as advertised you'd still get a good signal by pushing the detector right up to a piece of uraninite/pitchblende (say 30-100Î¼Gy/hr depending on quality).

Dragon64

Thu Feb 25 2016, 06:56AM

Registered Member #1438 Joined: Sat Apr 12 2008, 12:57AM
Location: Canada
Posts: 218

Welcome back Stella!

I bought one of these detectors seeing how much cheaper these are compared to tube counters and got some interesting results contrary my preconception as some trash.

The app (Smart Geiger for Android) these detectors use seems to have a permanent radiation of 0.10 uSv/hr @ background which makes me believe that it was programmed to always be at 0.10 uSv/hr at no activity of the detector.

Here is a list of conditions that resulted in unintended counts by my detector:

- Sensitive to vibrations at the top of the device (false counts when tapping the top of the counter)
- Sensitive to phone being plugged in for charging (false counts only during the action of plugging in or out)
- Sensitive to magnetic fields (false counts detected when near magnetic fields)
- Sensitive to high voltage (false counts detected when subjected to a nearby high voltage power supply)

Here are some measurements of the FTLab detector compared to my other detectors with a small Am-241 source. I do not have any gamma emitters I could test with and these measurements were taken roughly and serves as a rough comparison.

FTLab Detector (Photodiode)
@20cm = 0.1 uSv/h
@10cm = 0.1 uSv/h
@1 cm = ~0.24 uSv/h

Soeks Defender (SBM 20-1 Tube)
@20cm = ~0.15 uSv/h
@10cm = ~0.14 uSv/h
@1cm = ~1.60-1.66 uSv/h

GMC-320+ (M4011 Tube)
@20cm = ~0.11 uSv/h
@10cm = ~0.12-0.14 uSv/h
@1 cm = ~0.88-0.94 uSv/h

Arduino Geiger Counter (SBT-11A Tube)
@20cm = ~0.1-0.2 uSv/h
@10cm = ~0.2-0.3 uSv/h
@1cm = ~27-30 uSv/h

Arduino Geiger Counter (SI-8B Tube)
@20cm = ~6.0-7.0 uSv/h
@10cm = ~12-13 uSv/h
@1cm = ~45-39 uSv/h

Like what Stella said, this would probably be a fairly usable to detect higher than normal radiation levels but probably not too useful for background. I wouldn't also take the readings as anything very accurate but it is usable.

For the most part however, another detector app called Radioactivity Counter (Rolf-Dieter Klein) does a better job with options for calibration and greater sensitivity (as it uses the CCD of your phone). The only downside for using the Rolf-Dieter app is that you're damaging the CCD while it's being exposed (which doesn't matter if you use your unused phone

Wastrel

Thu Feb 25 2016, 02:01PM

Registered Member #4095 Joined: Thu Sept 15 2011, 03:19PM
Location: England.
Posts: 122

Am-241 produces some gamma and some X rays, this will be what the detectors are measuring unless you have a thin window tube.

Proud Mary

Thu Feb 25 2016, 03:36PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hi there, Dragon!

I thought about that 0.l ÂµSv/hr figure when I was out today, and of course you are right - that's a typical UK natural background radiation level unless you live in high radon area like parts of Cornwall, Devon, and Scotland where there are uranium deposits underground.

I guess you could ID radioactive ores like pitchblende with the iPhone detector, as you suggest, but there's not much else you could do with it. Really it's not much better than Cold War era military/civil defence GM detectors which are calibrated for disaster levels of radiation, and don't register anything at all on natural background.

If you buy GM tubes new from the manufacturer they sometimes come with a calibration curve - an xy graph -referencing the Co-60 specific gamma ray constant. Using this graph, you enter your counts per minute (cpm) in one axis, and read off your Gy/min on the other. Clearly, GM tube calibration by this means is only accurate with 1.3 MeV Co-60 gammas, but can still be acceptable for energies down to about 150 keV, below which the energy response curve of the tubes becomes very non-linear giving a huge over-response unless an energy compensating filter
is used. Feeble low energy X-rays barely able to seep through the glass can produce dramatic swings on some GM counters so people run off shrieking and expecting doom!

So it's best not to expect a bunch of different GM tubes and counters all to agree on the dose rate from a given source at a given distance. If they roughly agree you are doing OK.

I wonder which photodiode they have put in the iPhone detector. Something inexpensive I expect.

Bjørn

Thu Feb 25 2016, 05:24PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Americium-241 decay into neptunium-237 and so on fast enough that samples from used smoke detectors can have have 10-20% non americium content. All my samples emit beta in addition to alpha and gamma.

johnf

Thu Feb 25 2016, 06:58PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Pin diodes are ideal for measuring Am radiation as the Am big peak is at 59keV. The stopping power of silicon detectors is 30keV or below this is why they are used for low energy detection.
As for gammas and x-rays.
Lets get it right they are the same the difference is how they are produced jiggle a nucleus hard enough you get a gamma. Jiggle the electron cloud hard enough you get an x-ray
And as for Am241 turning into Np237 you would be hard pushed to find the daughter product as Am241 half life is 432 years.

So this detector will give false readings ie low for energetic sources like Co60 1.1MeV because of the lack of stopping (Braking)

Dragon64

Thu Feb 25 2016, 08:24PM

Registered Member #1438 Joined: Sat Apr 12 2008, 12:57AM
Location: Canada
Posts: 218

Proud Mary wrote ...

If you buy GM tubes new from the manufacturer they sometimes come with a calibration curve - an xy graph -referencing the Co-60 specific gamma ray constant

I've had difficulty finding calibration curves for those old Russian tubes such as the SI-8B and the SBT-11A.

Wastrel wrote ...

Am-241 produces some gamma and some X rays, this will be what the detectors are measuring unless you have a thin window tube.

Forgot to take that into account though the thin mica plates in the pancakes should be fairly transparent to alpha radiation.

BjÃ¸rn wrote ...

Americium-241 decay into neptunium-237 and so on fast enough that samples from used smoke detectors can have have 10-20% non americium content. All my samples emit beta in addition to alpha and gamma.

Assuming these sources mostly americium to begin with and that they are no older than 20 years, wouldn't it be closer to ~3%? non americium content? (I don't know the time it takes for americium to go from being coated on those small discs to when they go out for sale)

Conundrum

Thu Feb 25 2016, 08:34PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Hi Stella, I found an old (1970s) vintage smoke alarm here.
It uses discrete JFET as the detector, but with what appears to be an open can 241Am element as the source.
I can't tell for sure but it might be one of the older ones as its silvered in a different way to the others I found.

Any ideas what they are using as a window material on these?
My own detectors use PG as its a lot more transparent to alphas and low energy betas and in fact someone
published a PhD thesis on using graphene as an ion transparent material.

,d.d2s
Its possible that they use an HV850 as the +/-55v source for the diode(s) so that is another one of my ideas circa 2012.
edit: maybe handy for making very small sensors based on single chip PMT arrays.

Proud Mary

Thu Feb 25 2016, 11:18PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hi AndrÃ©,

I'm afraid I don't have any hands on experience of smoke detectors or their alpha sources.

I use a small piece of pitchblende if I need a check source, because as a geological specimen it is exempt from regulation in the UK.

Great to see you!

Dragon64 wrote ...

I've had difficulty finding calibration curves for those old Russian tubes such as the SI-8B and the SBT-11A.

I've never seen a calibration curve come with a Cold War era Russian GM tube either - perhaps because the data sheets were created for the servicing technician, rather than the equipment designer, a function which would have been centralised under the Soviet system as I understand it

Conundrum

Fri Feb 26 2016, 06:49PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Sounds possible.
It is interesting to note that some of these old tubes are actually *more* sensitive with age, I do not know why.
It only applies if the tube is new and unused, old tubes do not show this effect.

An anecdotal-though-probably-true note is that in ze olden dayz (ie 1950's), they used to reduce sensitivity on some counters using a rheostat to fine tune the tube voltage.
These were very effective however prone to drift and the regulator tubes also wore out with age resulting in eventual tube damage due to avalanche.

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