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 re...
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



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    19 Discussions


    7 months ago

    Well, I have a webcam and tried it got nothing even tho I am using Uranium ore which emits gamma, alpha, etc. I have it in syrofoam case and it emitting 6 uS an Hr and 900 counts per minute. Ave 386 yikes.

    1 reply
    Higgs BosonSHOE0007

    Reply 6 months ago

    Well, 900 cpm is WAY less than a smoke detector Am-241 source, which I've measured to emit ~100k cpm with a mica windowed geiger counter. So you may be getting a counts, but at a very low rate (e.g. one every couple minutes or something) on the webcam.

    The webcam is also very insensitive to gammas, because the probability that a gamma will generate a photoelectron within the active region of one of the pixels (only a few tens of microns thick) is tiny. Betas have a similar issue, where their range is long enough in Si that even if they do pass through the active region, they'll only deposit a small amount of their energy, which may not be enough to create a hot pixel.

    Finally, if you really are getting ZERO counts, there might be a dead layer over the top of your particular model of webcam sensor that blocks all alphas, which only have a range of a few 10s of microns in Si. If there is anything at all between the Alphas and the webcam sensor (even dust), it could interfere with their measurement.


    1 year ago

    An idea build a Neutron gun with Alpha sources with Aluminum foil or gold.

    1 reply
    Higgs BosonSHOE0007

    Reply 1 year ago

    Generally Be is a better choice than gold or aluminum since its neutrons are more loosely bound, but even still the (alpha, n) reaction has a pretty small crossection. You would need an alpha source on the order of millicuries to really make a good neutron source. The lab I work at now has a 10mCi AmBe source, but making a source like that yourself would require MANY smoke detectors. I've seen people pull this off on youtube with some of the older smoke detectors that had much larger Am sources or the static eliminators that have gigantic (0.5-5mCi or so) Po-210 sources, but that's probably the only feasible DIY approach. That said, neutron sources can be a bit more dangerous than other sources (they have a higher weighting factor when determining equivalent dose) and can activate other non-radioactive elements. This would make for some fun experiments, but would require more care than most other check sources (not to mention the NRC would probably not be too happy to see someone making their own, unliscenced neutron source in their garage haha).


    4 years ago

    I've seen a lot of americum 241 experiments and often wondered if you could reproduce the effect of flashing lights experienced by astronauts when cosmic rays pass through the retina at the optic nerve junction. after seeing your instructable, i placed my phone camera directly over a sample, turned brightness up, and sure enough saw the little white specs flashing on my screen. My wife failed to see why this was so exciting, and reminded me once again what a nerd i am....LOL


    6 years ago on Step 2

    A follow-up project: Someone suggested you may be viewing 'noise' in the form of stray gamma rays or beta particles. Gamma rays are highly energetic x-rays, which like photons of light are uncharged. Beta particles are electrons, with negative charges. But alpha particles, which are the 'signal' of interest in this experiment, have a double positive charge. How can we tell these apart? A magnetic field of sufficient strength will deflect positive and negative charges in opposite directions. Gamma rays will pass straight through a magnetic field without changing direction. Go online or to your library and look up the average energy of the alpha particles emitted by the Americium. From that, can you design a magnet (permanent or electro-) whose field is in the right direction and strength to bend your alpha particles a convenient distance in a known direction in the webcam? Compare the position of the flashes with and without the magnetic field. How many are + charges or -charges? How many pass straight through? If you reverse the direction of the magnetic field, it will deflect the charged particles in the reverse direction. That way, the total deflections will be 2x as great as in the experiment with only one field direction. You may have to create a device to record the positions of the flashes to do this accurately. The device you will have built is called a mass spectrometer. With it, the energies of charged particles from different sources can be compared.

    1 reply
    Higgs Bosonharderm

    Reply 6 years ago on Step 2

    This sounds like a very interesting project! I will definitely do this when I get the time. thanks for the suggestion. One thing though is the flashes are not concentrated into one area, so It is likely a collimator would need to be made and then the magnetic field added. as there is very limited space in the webcam I am unsure it would all fit. If it doesn't maybe a alpha source in a vacuum chamber (to increase its range) and a alpha sensitive phosphor at the other end would be a better setup. I guess then it wouldn't be testing whether the webcam is seeing alpha or gamma though. Maybe a better way is to place something in front of the americium that would stop alpha but let gamma through (like a piece of paper or something) and see if the flashes stop (which they should).

    you could also try to see the alpha particles by the trails they will make. i have not done what i will describe here but i think it would work. get the americium alpha source( i've tried to extract nuclear energy from these before, search americium experiment on youtube) and place it in some sort of transparent chamber containing water droplets in a state where all they need to form a cloud is a nucleation site. trails will be formed where the particles provide nucleation sites.

    1 reply

    It sounds like you are describing a cloud chamber. Most of them use alcohol rather that water because it evaporates faster, and is generally easier to get in a state of super-saturation. But yeah placing the americium in a cloud chamber will work very well for seeing the tracks.


    6 years ago on Introduction

    Nice project.

    You are almost certainly not detecting Cherenkov radiation, which is the equivalent of a sonic boom for light. This requires highly relativistic particles, and heavy alphas are very non-relativistic. Most likely you are just seeing the alphas hitting the CMOS sensor directly. They produce electron-hole pairs as they lose their kinetic energy plowing through the silicon, and that mobile charge is what the sensor fundamentally detects.

    1 reply

    6 years ago on Introduction

    I would have to agree with simpleplan. As a suggestion for a project similar to this, I recommend building a cloud chamber. In a cloud chamber you can directly see particles traveling and interacting with the medium, and it is fairly inexpensive to build as well.

    1 reply
    Higgs Boson2gooey

    Reply 6 years ago on Introduction

    Oh yeah! I have had lots of fun with cloud chambers! I did this because I really wanted to make something with the webcam that has been sitting around my room for a really long time. Plus I didn't want to buy dry ice.

    Higgs Boson

    6 years ago on Introduction

    From what I have read, Americium does not emit any beta, and only a very small amount of gamma. From what I have read however, gamma cannot cause Cherenkov radiation, so the latter statement seems much more likely, however, after watching some videos i have taken in a very dark room, I noticed a few flashes every second, that were smaller so couldn't be seen well in the light.

    Your idea about a random number generator is good,
    Radioactive decay is often used for True Random Number Generators,
    due to the truly random nature of radioactive decay.

    The one problem though is generally the detector set up uses Geiger-Muller tubes/detectors, which are far from inexpensive.
    Your setup, though might take some Processing/ coding.... would be a much better setup.

    1 reply