Some sources on the Internet claim that it is impossible to detect elevated levels of radiation with unmodified mobile device. A series of tests described below show that iPod and similar devices can be successfully used for this task, given sufficiently high energy particles or high-energy quanta of radiation. These tests should convince that trueGeiger can actually work, under certain conditions, without even purchasing the app.
Test 1. Dental X-Ray Machine
The dental machine x-ray pulse is quite strong and is clearly visible in a video shot by mobile device without additional post processing or image analysis. Below are fragments of two still frames extracted from video shot with iPod. The first fragment is showing normal background noise of the sensor. The other fragment is from a movie obtained during dental x-ray exposure using the same device.
Warning: If you decide to reproduce this experiment, please consult with your dental office staff or look into radiological manual for safety guidelines.
Test 2. Ionizing Smoke Detector
Much weaker sources of radiation may be detected with the method used by trueGeiger though with less certainty and accuracy. One of the sources of radiation available in many households is an ionizing smoke detector (the other type of detectors uses photoelectric sensors and will not work for our purposes).
Note: Though smoke alarms do not present any significant radiation risk, we still would advice you against performing the described below experiment at home.
To test the method with low radiation levels we disassembled Kidde Ionization Smoke Alarm Model i9040 (available from Home Depot for less than $5) to extract metal holder with radioactive material (Americium-241, in the amount of 0.9 micro curie, as stated on the back sticker of the device).
Do not remove the radioactive material from the holder if you still decide to reproduce this experiment!
The image to the left shows ionization smoke detector used for our tests. Left: Original packaging (Home Depot item #21008051). Right: Ionization chamber cut out from the circuit board, with cover removed. The thin film of Americium-241 is in the center of the casing.
For the test, we took iPhone with screen-side camera and the radioactive sample from the disassembled smoke alarm to a dark room. When the camera was turned on and no additional radiation was present, the screen remained uniformly dark with some barely discernible low-level thermal noise shimmering pattern.
To achieve maximum exposure, we carefully aligned the radioactive source with the camera. It can be easily done when the camera is on by using feedback visible on the screen. Depending on the camera and achieved accuracy of the alignment with the source, you will start seeing spurious bright dots on the screen once in every few seconds. On most iPods they are very conspicuous and can’t be mistaken for normal background noise.
Note that Americium-241 is used in smoke detectors for its alpha-radioactivity. However alpha particles can be efficiently blocked even by a sheet of paper. Presumably, digital camera with its lens in place, is way less sensitive to alpha particles than to gamma and x-ray radiation, which are also emitted during alpha decay of Americium-241.
Check this source for Americium-241 safety guidelines and the penetrating power of its radiation. As always, we suggest not to trust everything you read on the Internet, make sure you have an independent peer-reviewed source of information!
Test 3. Cesium-137 Sample
A similar experiment can be performed with testing/calibration samples freely available on the Internet. We used Images Scientific Instruments to obtain 5 micro curie sample of Cesium-137.
The radiation from this source can be easily detected with both front and back cameras. Placing the source on iPod, as shown on another image below, gives highest readings with the main camera (i.e. the camera on the opposite side of the screen).
In a dark room hot pixels caused by the radiation source are quite conspicuous.
Left: iPod whose main camera covered with a paper patch;
Right: while the main camera is covered, it is exposed to the radiation from the sample as shown, i.e. from the front of the device.
Note: in this concrete series of tests we placed Cs-137 sample over the camera on the front side yet the main i.e. back camera was used to measure radiation level. That setup gives best detection rate.
Test 4. Airport security x-ray
A test with airport security x-ray produces results similar to dental x-ray but of much lesser intensity. The image on the left was extracted from a movie shot with main camera while iPod was placed flat on the tray. Screen-side camera produces similar image with more spots of larger size.
It is worth noting that the exposure time during this test was also much shorter – around 0.3-0.5 ms so only one or two frames from the movie show noticeable effect.
Test 5. In-flight cosmic rays detection
During a typical flight on a substantial altitude you can perform similar test: cover the screen-side camera with a paper sticker to block light and observe the effect of cosmic rays on the sensor. Depending on the plane altitude, latitude, time of day, solar activity and other factors (is there a supernova exploding nearby?) you may see conspicuous bright spots on the screen once in few minutes. The number of the detected particles is rather small since there are more cosmic rays in the low-energy part of the spectrum which is below the sensitivity threshold of an iPod or similar device. Hence trueGeiger wouldn’t detect any elevated radiation level during flight when using short sampling times. Some substantial statistics would be required for quantifiable detection of in-flight radiation.