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17 August 2013

Homemade Spinthariscope

Recently I was lucky to stumble across a local online store which sells ion chamber module for homemade smoke alarm projects. Without much hesitation I bought two of these chambers knowing they contain tiny amount of radioactive materials.

Yes, you heard right. I was looking for an ionizing radiation source for some of my hobby experiments and Americium is the only radioactive man-made element (of the periodic table) entered the common household as ionization source inside smoke detector alarms.

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 ionizing source inside the chamber; and so I did.

The source was Americium-241 (the small embedded golden disc at the top of the plastic Petri dish), with major part of the radiation being alpha particles spitting out of the yellow metal (trivial part in gamma emission) with average energy of 4.5 million electron volts (MeV), factory spectroscopic evaluation full wave half maximum at 0.7 MeV. Because Americium-241 has a relatively short half-life of 432.6 years, each source contains less than 0.5 micrograms of americium dioxide has an activity of 0.8 microcuries which is equivalent to about 30,000 nuclear disintegration per second.

See, radioactive decay is a relatively energetic process. Literally chunks of matter in the form of alpha particles are being tossed out into the air from each decaying Americium atom (remember about 30,000 atoms are decaying at any moment) inside the golden alloy. After so, the fragmented Americium atom will become a Neptunium atom (also radioactive) which subsequently decay into other more stable elements. As a matter of fact, a 15 year old Americium source will contain about 5 percent Neptunium-237, which has a half-life of 2.14 million years.

Can we see this flying-in-the-air nuclear chunk? No. Even if we take this radioactive source and put it next to our eyes, we cannot see anything. No green glowing stuffs like we so often see in science fictions and cartoons. Radioactivity is invisible to our naked eyes but fortunately we have some other ways to make these energetic flying particles appear.

When alpha particle travel in air at great speed, in this case about 15,000 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 doesn't travel far, about 4 centimetres 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 physicist call "scintillation". A transparent material such as phosphor powder (the stuff you get from old TV screens) will emit flashes of light when it is being collided by energetic moving particles.

TV phosphor powder in plastic container

More than two years ago, a friend of mine gave me an old TV which I have dismantled to get its high voltage components. I also collected some phosphor materials by scraping off the powder inside the glass TV screen. I had to gently break the glass by covering the entire TV tube with a damp cloth to avoid vacuum implosion, a hit with a hammer near to the electron gun will crack the glass allowing air to enter the tube.

From a glance, it seems the grain size of the powder is quite coarse, and indeed they were flaked off from the glass surface. Among the materials that was scraped out, it is a mixture of three types of phosphor and there is no way to isolate them. The three types of powder corresponds to red, green and blue light emitted if energetic particles such as electrons collide on it. More specifically, typical TV phosphor components are denoted P22R, component: Y2O2S:Eu+Fe2O3 glows red at 611nm, P22G, component: ZnS:Cu,Al glows green at 530nm and P22B, component: ZnS:Ag glows blue at 450nm.

Using a cellophane tape, I stick some some phosphor material from the container making sure only the finest powder was glued on the tape, then the phosphor coated tape was mounted on a washer providing a hole for the alpha particles to strike from the bottom (refer to first photo on top, lower part of the Petri dish).

What I am making is essentially a spinthariscope, an early device to visualize radioactivity. It has only three components. A radioactive source (my Americium-241 alpha emitter), a phosphor screen for the alpha particles to scintillate on and lastly a magnifying glass powerful enough to collect the faint light coming out from the cellophane tape. Here's the schematic:

Before looking at the cellophane tape with a magnifying glass (binocular eyepiece), I had to test and see if the tape was scintillating. So I hooked up my DSLR, turned off the lights in my room, putting sensor sensitivity to the maximum and gave it a 25 second exposure, and amazing result shows:

The blue spot was a collective exposure of individual flashes happening on the phosphor coated tape. Of course, without a magnifying glass we cannot see those tiny individual flashes but as a whole, the blue spot corresponds to the surface of phosphor powder where it was radially exposed to an Americium alpha source directly below it. I was pretty excited to see the individual scintillations, and I was lucky yet again that my father have an old Russian binocular for me to disassemble.

The eyepiece of the binocular has short focal length and it is a compound lens which behaves like a convex high-power magnifying glass. This is ideal to be part of the spinthariscope component as the lens should be placed as close to the screen as possible to avoid losses of light and act as a "light gatherer".

So I cap on the lens and look through it. Nothing. I did not see anything at first but it was no surprise, because each scintillation only result in emission of a few light particles (i.e. photons) and therefore the light should be very faint. But when our eyes adapted to the dark, it has higher capability to detect those faint flashes of light and will usually take about 15 to 20 minutes to reach that kind of sensitivity in darkness.

I found a video in YouTube features what we are able to see through a spinthariscope. Please be noted that the video uses a movable alpha source, therefore the centre of the activity seems to move around the field-of-view.

It sparkled with beautiful flashes of light, where each spot represent one alpha particle striking through the phosphor powder on the cellophane tape. I here provide a simplified schematic of the mechanism:

The active area of the spinthariscope is this: radioisotope Americium-241 emits alpha particles travelling in straight line towards the phosphor coated tape. When an alpha particle collide with the phosphor materials, each atomic collision provides one photon (one quanta of light) and it is extremely faint. But I once read that human eyes are sensitive to even 6 photons per rod photoreceptor cell, (scientific citation here) I suppose each flash should compose of more than that amount of photons coming from each alpha particles, which implies it is a multiple collision event happening to one alpha particle. 

Enchanting views aside, light from scintillations in my spinthariscope is rather weak and it takes time for our eyes to adapt darkness for observation. Leslie Wright managed to use a photointensifier tube, something you can get from night-vision goggles to multiply the photons coming out from the scintillations and it was bright enough to be recorded with a compact camera.  

The next stop is for me is to take a controlled long exposure photograph in varying distance between the alpha source and the phosphored tape. Perhaps I am able to visualize the inverse-square law and do some elementary calculation based on brightness to validate it. 

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