You may not have realized it, but essentially any cell phone is capable of detecting ionizing gamma radiation out of the box (sort of). This detector happens to be the camera. Just as the camera responds to visible light, gamma rays can also produce "images" on your screen when they strike the camera (namely, tiny brief white flashes).
Today I'll go over how this works and how you can do it yourself using only a piece of tape. I used these detectors as crude ways to test my radiation sources before I had other detectors, and I believe that (other than perhaps a coffee can ion chamber) this is the easiest and cheapest way to get yourself a radiation detector and start measuring stuff (I've seen prepaid phones that work as detectors on sale for as little as $5 USD).
I had some luck with this phone last year trying to detect gamma rays
When radiation hits your camera
Gamma radiation is nothing more than very, very, very high energy/frequency electromagnetic radiation that originated from a nuclear process. Gamma photons typically have energies starting at a few thousand electronvolts (for reference, it takes about 13 electronvolts of energy to remove an electron from a hydrogen atom). As such, they are usually highly penetrating and should be shielded with lots of dense material, where they can be mitigated via the photoelectric effect, Compton scattering, or (for very high energy photons) Positron-Electron pair production. When a gamma photon strikes a digital camera sensor, the end result is that a lot of free electrons are produced by ionization (either from the gamma ray or secondary electrons).
Compton scattering, one of the ways gamma radiation interacts with matter.
But your digital camera sensor is reading out directly to the screen of your cell phone. So what do you see when a gamma photon hits your camera?
What gamma ray strikes look like
The answer is, not much. But you do see something, which is why cell phones can potentially be useful as radiation detectors.
To emphasize how easy it would be to miss a random gamma ray event on your screen, I went out and took a picture of one for you all. To detect gamma rays on your phone, you first need to cover up your camera completely. I used black electrical tape, but to be thorough you should also add a layer of aluminum foil to completely ensure that no light can enter the sensor. Then, I held a 1 microcurie (37,000 decay/second) Am-241 source extracted from a smoke alarm to the front of the covered camera.
Am-241 is an unstable isotope, decaying into Neptunium-237 with a half-life of over 400 years. Upon decay, an alpha particle with over 5 MeV of energy and a 59 keV gamma photon are released. It's this gamma photon that we are going to detect: The alpha particle won't even make it past the tape covering the lens, let alone the glass coverplate itself (for more on how alpha particles are absorbed, check out the parts of this paper, specifically the parts about interaction with matter)
Smoke alarm Am-241 sources are dirt cheap and work rather well as detector test sources, hence why I used it for this experiment.
The source looks like this: (The silver disc in the center is the actual Am-241 source)
What you can't see in this picture are thousands of alpha and gamma rays streaming out of the central silver disc, sourced in violent Am-241 decays. Covering up the camera lets you see some of the gamma rays, though. Here's my image of a gamma event on the camera sensor:
I've left this image full-resolution so you can see just how tiny these events actually on the screen and how easy they are to overlook if you don't know what to look for. Look at the left side of the picture. See the tiny, white speck? That's a couple pixels washed out by a gamma ray strike.
**EDIT: I've circled the gamma event for easier viewing.
This image was actually somewhat of a pain to take, as my phone disables burst shots in low light. So I used the very efficient, time-honored technique of spamming the camera button until I got a gamma event in the picture.
In this way, you can detect gamma radiation using just your phone and a piece of tape! You just have to look carefully. Only moderately radioactive sources will produce enough events to actually notice one go off, although I suppose if you started at the screen long enough you would see a background muon hit your camera and set off a pixel as shown above.
But to be honest, this isn't very useful on its own. After all, you can throw together a junkyard radiation detector for less than $20. That's where software additions come in.
Using the phone as a counting radiation detector
Several teams have developed software for phones that can pick up gamma interactions. Here's a paper on one if you are interested, and the subsequent article to go along with it. Awhile back before I had my current radiation detectors I played around with using phones as detectors, and found this app. I'm sure there are others like it on the IOS/Android application stores (but be careful, for every legitimate detector app there are 100 garbage hoax applications that pretend to detect radiation while doing nothing. If the app tells you to cover your camera, it's probably legitimate).
What this application does is automatically detect the gamma ray events that come in from the camera. This means it can pick up significantly more events than a patient human can. Unfortunately, this also means that it detects many, many false positives, and if you don't completely cover your camera lens with multiple layers you will find that your readings are extremely bad and full of false events. So cover your camera properly, or don't expect this to work.
This program will even give you CPM and approximate dosage readings, although I wouldn't expect the dosages to be very accurate unless you are using their calibration source.
Obtaining a safe radiation source to test your new detector
You'll also want a radiation source that you know works before trying this so that you can test that your phone detector is actually working. If you don't know where to get a radiation source and don't want to buy an overpriced source from a supplier or mess with potentially hazardous smoke alarms, I highly recommend negative ion stickers on Ebay.
These pseudo-scientific stickers/patches claim to block the (harmless) microwave radiation emitted by cellphones. The funny thing is that almost all of them contain Thorium for some reason, which releases actual harmful radiation... but in really small amounts. What this means is that the negative ion stickers, despite being a complete scam (if they really blocked your cell phone radiation you couldn't make calls!), are actually extremely good, cheap, and safe radiation detector test sources! I've found them for as little as $1.50 on Ebay.
The Quantum Biophysics Disc: 100% pseudoscience, 100% buzz words, 100% awesome and ironic cheap Thorium radiation source. Mine works great with both of my geiger counters. You get a nice mix of gamma and beta radiation, since the Thorium decay chain has many different isotopes decaying. Can you tell I love making fun of these things?
Other options include thorium lantern mantles/welding rods or small chunks of uranium ore, all of which can also be purchased legally on Ebay or other online sites.
Back to the detector!
Okay, that was a bit of a tangent. Now that you have your radiation source, have installed the application, and have completely and thoroughly covered up your camera lens, you can get to work detecting radiation.
You'll need to follow some calibration steps when you first open the program. Ensure your settings are properly set up for best results. I also found that this works better with some phones than others, and how well it worked didn't really seem to correlate with the quality of the phone. It's going to take a bit of fiddling to get this to work properly (at least, it did for me), but it should eventually work on most phones.
One last note: If it doesn't work well, try the front-facing camera! Sometimes you can get better results using this. Also, remember to take long time averaged exposures if you want to get accurate readings on weak sources (like potassium salt!)
Now you can detect gamma, high-enery XRay, and high-energy beta radiation with just your phone and some tape! Hopefully this inspires some of you to get into radiation detection in a safe and inexpensive way. This was for a time the only way I was able to detect radiation until I found a cheap CDV-700, so it holds a special place in my heart.
You can find the app's creator's website here with a FAQ section. I remember finding a table of which phones worked best, but the website is in German and I'm unable to locate it right now. It's somewhere here though.
Of course, if you don't want to use this application, you can still detect gammas by putting radiation sources on the covered lens and watching the screen.
Remember to stay safe with radiation.
Thanks for reading!
All images not credited are my own. You are welcome to use them with credit.