There is a lot of confusion about what can be measured and what cannot be measured with our RadioactivityCounter. This often depends on the phone model and sensitivity. Here some general questions and answer to help understanding what we are properties of our App and about radiation in general. If you have suggestions please mail us.
1. What is CPM0 ?
We call it the zero count. It is the background noise. But with our sensor (CMOS) it a sum of the background radiation and the CMOS sensor noise. The noise level (n=...) determines where we cut off the camera noise. But some sensors are excellent and above a certain level only radiation impacts are counted, some other sensor have counts even for very high n values. Try to keep it low by increasing then value for n. Should be around 1-10 CPM. And take a look at our tables for a comparison with our measurements. Now its important to get a precise CPM0 value. Start a measurement with a LOG file as long as possible (best over night with a loader) or at least 30 minutes. In the statistic you can see a mean value also on the main screen. Use the CPM0 value displayed including comma parts in the ADJUST menu (you can use the menu use for CPM0 in the statistic page). Also assign the background radiation value in the settings menu to your background radiation (around 70 nGy/h for Germany higher values depends on the location). In the adjust menu you can also see the calibration curve (after you entered the values from the table of you phone is listed). Around CPM0 +-50% we decided to compensate for the sensor noise, so any measurements won’t show exact results in this area for the moment.
2. I have a Tritium watch (H3) - I don’t see counts.
Tritium is a beta emitter with around 45 keV. This means most of the beta particles are absorbed in the glass around the radiator. Some gamma is emitted due to the so called bremsstrahlung, but it is so low that you won’nt be able to measure it.
3. I have an old smoke detector with AM241 (americium). I cannot detect radiation.
AM241 is a almost pure ALPHA radiator (90%). This can be absorbed by a sheet of paper. So ALPHA radiation cannot pass the plastic in a mobile phone or the cover of the CMOS sensor itself (unless all is removed !) . For ALPHA rays measurement is possible with MICA windows in GM tubes only. But there is a small amount of gamma rays, and if you make a long time measurement you can see a increase of the radiation with most of the phones (the mroe sensitive ones).
4. How can I measure Thorium Rods
Thorium rods were used for welding, usually contain around 2% TH231. Thorium is a ALPHA radiator. But due to the decay chain also emits some beta and some gamma rays. The beta rays are absorbed by most mobile phones, some can detect it (iPad 2 upcoming app for example). The rods are very thin around 2 mm. Also the CMOS sensor is small (usually 2mm x 2mm) so the number of rays hitting the sensor is not very much. To detect this radiation you need a long integration time a very good calibrated phone and one which can detect the beta rays (they are absorbed by thin layers of plastic or metalized parts). Best is to start a LOG file. The CPM0 should be around 1-5 CPM to have a chance to see the effect. Then use multiple rods to increase the chance to hit the sensor. Measure for at least 60 minutes. Use the LOG file in a spreadsheet application and compare the results to the pure background effect. At some intervals multiple counts can occur more often than with the pure background. Multiple counts are typical as we count for energy not only for hits (larger dots or lines appear).
5. I have a vintage radium clock, how can I measure and what is expected ?
Depends on the activity of the contained radium. RA226 is a ALPHA radiator (the reason to use it as a source for the illumination in reaction with a phosphorescence material). Due to the decay chain (RA226 (alpha) in 1600 years half life to RA222 (alpha) in 3.8 days to PO218 (alpha) in 3.1 min to Lead-214 (BETA) in 27 min to Bismuth 214 (beta) ....
So there is a chance to detect it with the beta emitted from Lead-214 and also some gamma rays (some chance on the way of the decay chain). For comparison with a GM Tube USE a 3mm COPPER to shield of the beta rays to see the remaining gamma radiation. You can also compare the beta and gamma radiation this way. If there is no difference, you phone is shielding the beta rays completely (possible on some constructions).
6. How about Thorium doped lenses in vintage cameras ?
The TH lenses contain up to 30 % TH-232. Its emitting mainly ALPHA rays but also lots of BETA rays and some small amount of gamma. You have to go close to the TH lenses. The camera case and also other non TH lenses might shield it completely. We will provide some video examples soon on how to measure for this.
7. I fly often - do I see the increased radiation.
We did some tests meanwhile and yes you can see it. In around 11km height the radiation is increased to around 10 µGy/h. You will see sometimes a lot of counts occuring in a short time. The particles hitting the athmosphere produce for example myons and neutrons with a high energy. The phone can detect those particles after our tests. Sun flares or cosmic gamma burst can increase the counts quickly (don’t be afraid if this happens). a A GM tube won’t show these impacts as it counts for hits not for energy of the particles and has a dead time gap usually to large to detect bursts of particles).
Example with the highest pulse (confirmed by satellite measurement class 5 cosmic event total duration of bursts within 10 min) height 7km:
11/21/2011 19:51:23 11548 7995 5839 1284,7 11548 0 40 discharging
8. How can I measure a increase of the background radiation or HOT-spots ?
You need a long integration time > 30 min and compare the LOG files.
9 How many hits can I get with a 1000 Bq activity source in theory ?
1 Bq means 1 decay per second, that is one particle flyes away per second in any direction. Lets estimate how many particles can be hit at maximum by a 1000 Bq source measured with a sensor of 2mm x 2mm like our CMOS sensor in 3 cm distance.
First calculate the area at the distance of 3cm:
A = 4 PI * r *r = 4 * 3.141592 * 30.0 mm * 30.0 mm = 11309 mm2
A 1000 Bq source has 0.0884 Hits/(s * mm2) (=1000Bq/11309mm2)
So the sensor with 2mm x 2mm area gets a maximum of 0.35 hits per second or 21.2 CPM at maximum. Usually most are passing through for gamma rays, only a small part activates the sensor which depends on material energy of the particle and many more. To calculate for how many µGy/h at this distance there are complicated formulas and tables and you need to know the type of source (energy distribution etc.).
WARNING : DON’T experiment with radiation sources this is dangerous !! Never dismantle any vintage or other objects containing radioactive sources - very dangerous to ingest material. Please have a look at the general available radiation protection information on handling radioactive materials.
(C) 2016 MultiMediaStudio Dipl. Ing. Rolf-Dieter Klein