Introduction: How to See Alpha Particles

About: I'm Ben. I'm currently weaseling my way through undergrad at MIT where I'm majoring in physics and nuclear science and engineering. I made this account back in middle school (hence the cheesy name), and I real…
I've been looking for a simple experiment I could do involving physics that I already had all of the materials for. After a bit of searching online I came across a project on using a webcam and smoke detector, and of course I had to build it. basically you will be able to see small white dots flash on the screen where alpha particles were. Although you do not technically see the alpha particles directly, what you do see is the indirect effect of the alpha particles. Originally I thought that this may be an effect of Cherenkov radiation, but since alphas cannot go fast enough to cause this, something else is going on. The sensor on the camera can detect single high energy photons. Americium does emmit small amounts of gamma, but mostly it is an alpha emitter. What may be happening is alpha particles are hitting the CMOS and interacting with it and possibly creating very small amounts of Bremsstrahlung radiation which the sensor is able to see. This is a very fun and relatively easy project

Learning Goal: The goal of this project was to learn about radioactive decay, and how it interacts with different mediums.

Now before I continue, a brief note on safety.


A small radio active source extracted from a smoke detector is used. It should not be too much of a concern, but it is not to be taken lightly. The sample is made is made from Americium-241, which is a strong alpha emmiter. At atmospheric pressures alpha particles can only travel a few centimeters, but do take caution. 

Okay that being said I take no responsibility something bad happens doing this project. So long as you use common sense it should be fine.

Judges see last step for contest info.

Look closely at the video and you should see the small white flashes, the tell-tale sign of alpha particles interacting with the CMOS.

Step 1: Materials

I was able to get everything I needed for this project for under about $20, so this project is really accessible to anyone who wants to explore this fascinating topic. It would make a great science fair project, or could just be a fun project.

You will need:
-A webcam:
I got mine from amazon for about $5 (not including shipping). It works great and at that price, how could I pass it up. I'll leave a link to where you can get one. If you already have a cam you want to use, make sure it has 1.7 megapixel resolution or better (otherwise the small flashes of light will not be visible), but other than that almost any web cam will work. Mine had a CMOS visual sensor in it, but CCD will also work just as well. Before you buy your webcam make sure it is compatible with your computer and operating system.

-Smoke detector:
You can find these at pretty much any home improvement store. To make sure it is the kind that actually contains Americium check the label on the back. It usually says right on it that it contains "0.9 microcuries of Americium 241 a radioactive element..." but as long as it says it is an ionization type smoke detector it should work. These are $10-$15 at home.

-Video capture software:
My PC didn't come loaded with a capture software on it compatible with my webcam, so I downloaded Debut Video Capture software. I had the trial run for a while before it devolved to the freeware, for this project the free version will be just fine.

(you will also need a computer but I am guessing you already have one since you are reading this)

this is not very important because everything should just fit back in the webcam housing, but I made a case out of a margarine container to make it look better. 

Step 2: Dissect Your Smoke Detector

A while ago one of our smoke detectors stopped working. There was a short in it somewhere which caused it to go off in the middle of the night one night. Needless to say it was removed very swiftly, and was laying in our basement until I rediscovered it, and decided to use it for this project. Unfortunately I do not have pictures of the dismantleing process, but it is relatively simple.

Pretty much all smoke detectors are like this. If there are screws unscrew them. If not pry it apart with a flat head screwdriver. Once inside you can cut wires to the battery and piezo buzzer, and remove the PCB, you are looking for a bulky aluminum case. It is pretty obvious. This case is connected to the board by two solder joints, so fire up your iron and desolder the two joints until the case falls off. You should see a plastic case stuck to the board, with a metal plat on top. That plate should be connected to a chip underneath, so desolder that and pry the metal plate off. look on the back of the PCB for three plastic bits poking through some holes going to the plastic case, and break them off. Remove the case, and there should be a small metal button in a hole in the center of the plastic. use pliers to pull it out from the back side of the plastic case. This is the radioactive material so be careful with it. try to avoid touching it directly, and keep it in a clearly labeled metal case until you use it. ALWAYS WASH YOUR HANDS WELL AFTER HANDLING THE SAMPLE EVEN IF YOU DON'T DIRECTLY TOUCH IT!

for more detailed instructions on smoke detector dismantling go to this instructable:

Step 3: The Webcam

Almost no modifications need to be done to the webcam of this project. You won't be able to put the sample behind the camera lens sense it will block the alpha radiation (paper is all it takes to stop alpha particles), so I mounted it on the other side of the lens. On my cam the lens just screwed right off, allowing me to easily glue it onto the back side of the lens. Be sure that you glue the sample with the Americium foil facing the CMOS (the foil side has a slight indent). Apply glue (Elmer's will work fine) to the back side of the lens, and place the back side (not foil side) of the sample onto this. I placed a notecard on top of this to stop the alpha radiation as it dried. Once the glue has dried, screw the lens back on. the sample glued on the lens was enough to block all of the outside light, and the seal between the lens and PCB was good enough to block light from the LEDs. If you have power LEDs or something inside creating too much light, you can desolder them add some jumpers and reroute them outside the enclosure, if you need to block more outside light you can add an enclosure to help block more. I technically didn't need to, but I wanted to make mine look a lot cooler.

Step 4: Enclosure

If you are having trouble getting it dark enough, you should probably put the whole setup in a cardboard box and close it up when it is running. That is really all it should take. 

I wanted to make it look cooler though, so I decided to make an enclosure out of a margarine container I painted black. After adding the radioactive sign, it looked really cool, and once that was finished, I was completely done with the construction.

Please note that you do not have to make an enclosure it just makes it look better.

Step 5: Plug It In

Plug in your camera, and open your video capture software. If your integrated webcam turns on, go to options and select the proper webcam. Once everything is in order, look closely at the screen, and look for small dots a few pixels big flashing white or blue for a moment on the screen. You have just seen high energy photons created by radiation due to the alpha decay from the americium. To me the fact you can see the direct evidence of a single radiated particle is just absolutely amazing. You could use this project in a science fair, or for educational purposes. You could even try as the original project suggested to make a true random number generator by assigning a certain number to each pixel, and running the device for a period of time and averaging the pixel numbers that were activated. I do not have enough programming experience to take it this far, but if you do I highly recommend trying it, and posting some pictures in the comments.

Step 6: Contest Entry

The Hands On Learning Contest:
School has been good for learning many things, but I have always had to pursue my interests outside of school. In science classes, you sometimes do labs, but most of the time you read a textbook, and take notes on lectures. there is a big difference between reading about an experiment, and actually doing it. This is just one example of me teaching myself concepts in a hands on manner that is never taught in the schools in my area. Doing these things can go a very long way in solidifying your understanding of many subjects, and on top of that can be a lot of fun.

Result: I was a finalist!

Hack it Contest:
By hacking common household materials, you can make a very cool experiment. what more is there to say?

Result: I was a first prize winner!
Off the grid contest:
This is a cool USB powered device, which is easy to make, very cheap, but with very large implications.

Result: Nothing.

Hands-on Learning Contest

Finalist in the
Hands-on Learning Contest

Hack It! Contest

First Prize in the
Hack It! Contest