Pocket Size Spinthariscope




The spinthariscope was invented by William Crookes in 1903. While observing the apparently uniform fluorescence on a zinc sulfide screen created by the radioactive emissions (mostly alpha radiation) of a sample of radium bromide, he spilled some of the sample, and, owing to its extreme rarity and cost, he was eager to find and recover it.

Upon inspecting the zinc sulfide screen under a microscope, he noticed separate flashes of light created by individual alpha particle collisions with the screen. Crookes took his discovery a step further and invented a device specifically intended to view these scintillations.

It consisted of a small screen coated with zinc sulfide affixed to the end of a tube, with a tiny amount of radium salt suspended a short distance from the screen and a lens on the other end of the tube for viewing the screen. Crookes named his device from Greek σπινθήρ (spinth´ēr) "spark".


What's the difference between the upgraded, and regular version? The previous version used a reflection based fixed scintillation screen, while this version uses a transmission based adjustable scintillation screen. Compared to the previous version this one offers an increased scintillation rate, and the ability to set the distance between the source and the screen.

Step 1: Safety First

To build this device you'll need to deal with a radioactive substance: Americium 241.

In the process of radioactive decay, americium releases alpha particles and gamma rays. Alpha particles are relatively high energy particles, but travel only extremely short distances and do not penetrate the skin.

However, if americium is taken into the body and enters body tissues, alpha particles may produce damage to nearby cells.

The radiation from americium is the primary cause of adverse health effects from absorbed americium. Upon entering the body by any route of exposure, americium moves relatively rapidly through the body and is deposited on the surfaces of the bones where it remains for a long time.

The dose from this alpha and gamma radiation can cause changes in the genetic material of these cells that could result in health effects such as bone cancers.

Gamma rays can travel much greater distances and can penetrate the entire body. Since alpha particles do not penetrate the skin and the gamma rays released from americium sources are relatively low in energy, external exposure to americium is not usually considered to be a danger to your health.

Anyway you must use at least eye protection, dust mask and rubber gloves when dealing with Americium 241.

Step 2: The Legal Bit

Disassembling a smoke detector

Disassembling a smoke detector to obtain the americium source may be subject to laws in your country, and that proceeding could open you to legal liability.

You must check the laws in your country before disassembling a smoke detector.


State and local requirements for disposal of ionization smoke alarms vary. Some States conduct an annual roundup of ionization smoke detectors similar to that for hazardous household chemicals. Others allow ionization smoke detectors to be thrown out with ordinary trash but recommend that used smoke alarms be returned to the supplier. Some States require that used smoke detectors be returned to the supplier.

You must check with your local solid waste district, hazardous waste program, or health department to find out the procedures in your area.

Step 3: Materials Needed

To build this pocket size spinthariscope you will need:

...and a 3d printer. Or a friend with a 3d printer ;)

The only purpose of the ionization smoke alarm is to be disassembled to provide me the radioactive source. Now, I am not going to elaborate how ionization chamber works inside smoke alarms because it will digress from our current topic. What I was interested was extracting the radioactive source inside the smoke detector; and so I did.

Step 4: 3D Model

I designed a microscope extension to host both the radioactive source and the scintillation screen. You simply need to download the STL file and print your own extension.

Should you need to modify it just use the STEP file and import it in your design software.

Step 5: Assembly and Test

The assembly is pretty straightforward:

    • wear your gloves, eye protection and dust mask and remove the Americium 241 source from the smoke detector. You will need just the small button shown in the pictures (this instructable could help).
    • remove the light module from the microscope.
    • grab the 3D part you just printed.
    • glue the M8 nut in the hexagonal recess, it should be a tight fit so may need to push a little to get it in place.
    • glue the americium source on the tip of the M8 screw.
    • lay down some scotch tape with the sticky face facing upward, spread some ZnS powder on it and distribute it evenly.
    • cut the scotch tape to obtain a disc with a diameter of 15 mm
    • fit the disc into the 3D printed part with the sticky face (the face will not be so sticky now) in contact with the 3D printed part.
    • glue the microscope into the 3D printed part.
    • set the focus in order to get a clear view of the powder grains on the scotch tape
    • Fit the M8 screw in the M8 nut and gently screw it all in

    Put the spinthariscope in a dark place to let the powder go dark.

    Step 6: Conclusions

    Americium-241 emits mostly alpha particles (and a negligible portion of gamma emission) with average energy of 4.5 million electron volts (MeV). Each source contains less than 0.5 micrograms of americium dioxide with an activity of 0.8 microcuries which is equivalent to about 30000 nuclear disintegration per second.

    When alpha particle travel in air at great speed, in this case about 15000 kilometres per second (based on 4.5 MeV relativistic kinetic energy) it has high probability to collide with anything that comes in their way due to its relative huge size. Usually these particles don't travel far, about 4 centimeters at most in air and will lose their energy in scattering events. It is like smashing a cue ball into a pool table full of billiard balls - eventually the cue ball will stop as it deposits its kinetic energy into the surrounding ball during collisions.

    When collided in just the right way with the right materials, the passage of an alpha particle can be "seen". This is done with a process physicists call "scintillation".

    A suitable material such as ZnS:Ag or ZnS:Cu (zinc sulfide doped with silver or copper) will emit flashes of light when it is being collided by alpha particles.

    The light emitted is blue for ZnS:Ag and green for ZnS:Cu.

    I did not see anything at first but it was no surprise, because each scintillation is very faint. But when our eyes adapt to the dark, they have higher capability to detect those faint flashes of light and it will usually take about 15 to 20 minutes to reach that kind of sensitivity in darkness.

    Adjusting the position of the screw you can adjust the distance between the source and the screen. The number of scintillations will rapidly decrease if you increase the distance.

    In vacuum the scintillation density obeys an inverse square law – getting weaker with the square of distance, i.e. doubling the distance quarters the intensity. Tripling the distance leads to a ninth of the intensity. In air you need also to consider the scattering, so the scintillation density will get even weaker.

    So you did it! You juts gained a superpower! You can see the subatomic world with your bare eyes now!

    Have fun! :)


    I'd like to tanks all the users that commented this instructable for their contribute to make it better and safer!

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      32 Discussions


      3 months ago

      Cool be looking for that post.
      the nrc considered a teacher using the source in the classroom.

      In the case of misuse, the following scenarios are considered: (1) exposure to a teacher who
      removes an 241Am source from a smoke detector for use in classroom demonstrations about
      radioactivity, and (2) exposure to a person who removes an 241Am source from a smoke
      detector and subsequently swallows it by accident. To estimate the potential radiation dose to
      the teacher, it is assumed that the teacher stores the 241Am source in a convenient location in a
      classroom and is exposed at an average distance of about 1 meter from the source for
      1000 h/yr. The EDE rate at 1 meter from a 37-kBq (1-Ci) source of 241Am without regard for
      shielding by other materials is about 0.09 nSv/h (9 nrem/h) (see Section 2.15.3) and the annual
      EDE to the teacher could be 9×105
      mSv (0.009 mrem). Also, the teacher handles the 241Am
      source for 10 h/yr during classroom demonstrations at a nominal 1 cm distance (due to the size
      of the disk upon which the source is mounted) then the dose equivalent to a small area of skin
      on the hand is about 0.3 mSv (30 mrem), based on a calculation using photon fluence and
      tissue (skin) energy absorption. Assuming a 10 cm2
      exposed skin area out of a total skin area
      of 1.8×104
      (ICRP 26) and a skin weighting factor of 0.01 (ICRP 60), the calculated EDE
      would be less than 1×105
      mSv (< 0.001 mrem).

      so like about spending a day in Denver.

      1 reply

      3 months ago on Step 6

      I speak as a physics teacher (secondary school, UK) who has looked into the safety of smoke detectors both for use as a demonstration in class, and so as to give advice at CPD workshops I run for the Institute of Physics.

      Please, NEVER dismantle a smoke detector. Every syllabus I have seen states that alpha particles cannot go through paper, so you might assume that they will never penetrate their metal ionisation chamber within the smoke alarm, nor the plastic body of the smoke alarm itself. However, if you hold a school Geiger counter near a smoke detector, you WILL detect radiation, above background level, coming from it. If it can penetrate the ionisation chamber and the body of the smoke alarm, it can certainly penetrate your gloves!

      CLEAPPS (Consortium of Local Education Authorities for the Provision of Science Services) publication L93: "Ionising Radiations and Radioactive Substances" advises that "The plastic cover of the smoke alarm may be opened for observation, to detect ionising radiation and to insert the battery. Under no circumstances should the metal ionisation chamber be opened."

      This is a very interesting Instructable, but my advice is to look at these things on the web, and not to do them yourself.

      3 replies

      Reply 2 months ago

      I believe what you are detecting when you hold your Geiger counter near an intact (closed) smoke alarm, are the low intensity gamma rays emitted by the Americium. The alpha particles are stopped inside. The hazard would be if Americium particles were dislodged from the metal base while it is being disassembled. I had a A.C. Gilbert spinthariscope, 60 years ago, with radium in it. I'm still here. Sunlight causes cancer with great certainty. This project, probably will not, if it is done carefully.


      Reply 3 months ago

      I just found another possibility regarding your geiger counter and higher readings than background radiation. The long and the skinny is that geiger counters are not useful for accurately measuring low dose sources of radiation. here's the quote and the link to the page. "... when it comes to understanding some of the limitations of Geiger counters – it helps to understand that alpha particles have a lot more energy than do beta particles. It also helps to understand that radiation dose is a measure of the amount of energy that’s deposited by radiation in an object – more energy means more dose. So looking at this graph, we can see that high-energy radiation hitting a detector leaves a bigger signal than does low-energy radiation. Now look to the right, in the Geiger-Muller region – in this region the high-energy radiation produces exactly the same signal as the low-energy radiation. So we can’t tell the difference between high-energy and low-energy radiation, which means that we can’t necessarily tell how much radiation dose a person was exposed to if we’re just measuring with a Geiger counter. This is one reason that we can’t always use a Geiger counter to measure radiation dose rate accurately. If a Geiger counter, for example, is calibrated to measure radiation dose rate from the radionuclide Cs-137 it will be right on the money as long as you’re trying to measure radiation dose from this nuclide. But what if you’re trying to measure radiation from cobalt-60 (Co-60)? Well, then you’re in a bit of trouble – radiation from Co-60 is twice the energy as radiation from Cs-137 so whatever your detector reads will be only half the actual radiation dose rate. On the other hand, a lot of radionuclides are lower-energy; in this case, your meter is going to read a higher dose-rate than is actually the case. The bottom line is that a Geiger counter will only give an accurate radiation dose-rate reading if it’s measuring the same radioactive material it was calibrated with. This is why Geiger counters aren’t always the best instruments to use to measure radiation dose rates." link here: http://www.ntanet.net/how-do-geiger-counters-work


      Reply 3 months ago

      Interesting. here's what the Canadian Government has to say about Radiation from smoke detectors. "The radiation source in ionization chamber smoke detectors is sandwiched between metal foils, which keep the radioactive material well contained.
      The tiny amount of radiation that can be measured outside the unit does not pose any health risk. In fact, the average annual radiation dose a person receives from a smoke detector is 0.01 percent of the dose they receive from natural background radiation."
      I would pose that if you have detected amounts of radiation from an Ionization chamber smoke detector, I would report it immediately to your government for them to take action. My government strictly regulates all radioactive materials and perform over a hundred tests on these devices to make sure they are safe. the detector you tested should be reported immediately it might be a breech of the regulations the UK has on them. either that, or the Geiger counter you used is out of calibration, and they need to be calibrated. Also, Geiger counters have a limited service life and it is not recommended to use them after that date. myself, having used several geiger counters at University (Nuclear Medicine), it is highly likely that the latter is the case rather than the former, Since Americium is so expensive and labor intensive to retrieve from Nuclear reactors, I don't think someone is going to deliberately nor accidentally put too much Americium in a source for a smoke detector. Most School Geiger counters don't need calibration since the lessons taught are of a empirical nature, meaning it's just good enough to show that there is radiation coming from whatever it is they are testing. Things like that are heavily regulated and when rules a breached, it means laws have been broken and usually prison time enues, so no one is going to be negligent when metering out minute amounts of radioactive Americium 241. Most people I know also do not wish to harm others and when in such positions of employ, take extra precaution when working. Much like Nurses, doctors, Surgeons etc. do when they work to improve people's health.



      3 months ago

      Do you think that if you attach a webcam to the microscope it would show the scintillations? That would make it easier to show what happens to the students. Thanks for the instructable. No smoke alarms here in Brazil, but I do have an Americium source at school, maybe I will try this sometime.

      5 replies
      Peter Balchalmateus

      Reply 3 months ago

      Remove the lens and filter from the webcam and just put the Americium near the ccd sensor. No need for ZnS. If you google for 'Americium webcam', you'll find examples.


      Reply 3 months ago

      Hi almateus, the new version is more suitable to be used with a USB microscope. The scintillations are brighter using the scotch tape screen, but I don't know if a normal webcam is sensitive enough. Putting the scotch tape directly in contact with the CCD (powder side facing outwards) should do the job though...


      Reply 3 months ago

      The webcam on a microscope is an awesome idea! Or, go to ebay and search for "USB Microscope" You can get a 1.3 megapixel USB microscope with > 100x magnification for just over $10 to $12 (including shipping.)


      Reply 3 months ago

      Thanks for reminding me that I have one of those already. It basically is a webcam with a lens in front of it. Don´t know if it can magnify 100X, or if the sensor is sensitive enough for low light conditions, but it is worth a try.


      Reply 3 months ago

      Olá, no Mercado Livre você acha alguns modelos, Não sei se usam o Americium. Grato


      3 months ago

      Well, Amercium 241 is cheap to get and you can get old smoke alarms on ebay for that or a smoke detector. I like this type of project. I bet other ore like Pitchblade which is 80 percent UO2 would not work since it not powerful enough alpha particles It 8000 cpm were Alpha can be 100,000 CPM in alpha.


      Question 3 months ago

      What's the difference between the upgraded, and regular version?

      1 answer

      Answer 3 months ago

      Hi RobertS770, the previous version used a reflection based fixed scintillation screen, while this version uses a transmission based adjustable scintillation screen. Compared to the previous version this one offers an increased scintillation rate, and the ability to set the distance between the source and the screen.


      3 months ago

      I LOVE this instructable. It's very cool. My wife teaches elementary school, I wonder if it would be considered safe enough for her to bring into the school to give a demonstration if I build this.
      Thanks again for this instructables.

      2 replies

      Reply 3 months ago

      Hi Tercero, Your wife and you probably realize this already, but considering how some people are these days, I would suggest that she run this experiment by the Principal, write up exactly what she will be doing and how the ingredients work and have the parents sign off on letting the children "participate". Accidents do happen and some kids just have an overwhelming need to touch and know why. There is always the one or two kids that have earplugs in when the word dangerous is said. Kids sometimes have no concept of danger (we were all there once :) ) and this can go terribly wrong. Good luck.


      Reply 3 months ago

      I finished building this, and instead used an adapter for my Nikon Z6 FX to take some pretty amazing shots.
      She showed the results to her class and explained what exactly they were looking at.
      Pretty amazing.