One of the small, unassuming—and much cooler than you might expect—exhibits at Pacific Science Center is the Particle Aquarium, also known as a cloud chamber or cosmic ray aquarium.

In Short

The particle aquarium creates a supersaturated environment of alcohol vapor. Charged particles (from space as cosmic rays or from radioactive decay closer at hand) pass through this supersaturated field. They leave behind a trail of ions, and those ions are nucleation sites, places where the alcohol vapor can condense into liquid droplets, which makes a trail of tiny droplets of alcohol. You see those trails, so you’re seeing the path of a cosmic ray from space.

As many cosmic rays as you see in the chamber, there are more than that there, and just that many are going through your body too at this very moment. You can also identify characteristics of the particle by examining the shape of the path.

How it Works

Look closely.

At the top is a place where alcohol is dripped into a small tray that runs around the edge. This is where the alcohol is heated until it evaporates, filling the entire chamber with alcohol vapor. At the bottom of the chamber is a chilled plate. As the vapor falls towards the plate it cools, creating a supersaturated layer of vapor near the bottom of the chamber. This layer is primed and ready to condense, it just needs a little push.

Any kind of push will create condensation in the supersaturated layer. As a charged particle passes through air it leaves a trail of ions in its wake. This is not to say ions are falling off the charged particle—no, it is interacting with the molecules in the air, turning them into ions.

Those ions cause the supersaturated alcohol vapor to condense, which shows the trail of the charged particle. The droplets fall to the cooled plate, and the track disappears.

Charged Particles and Ions

Remember back in chemistry when you learned that an atom has protons and neutrons in the nucleus, and is surrounded by a cloud of electrons? You also learned the protons (positive charge) and electrons (negative charge) tend to be in balance with each other, giving you a neutral atom. Depending on your teacher she might have called this a “happy atom.” Well, knock one of those electrons away—because it’s way more complicated to knock off a proton, we call that fission—and you create a positive ion, or what your teacher might have called an “unhappy atom.” You can do the opposite too—add an electron—and you have a negative ion.

Well, charged particles are, by their very definition, charged. Positive charges and negative charges attract, negative and negative repel, and positive and positive repel. So, if you get a charged particle near a neutral air molecule you could very well affect the outer electrons for that molecule. (And yes, I switched from using atoms to molecules – they work the same way in this case)

Cosmic Rays and Radioactive Decay

There are all kinds of cosmic rays in space (mostly from the Sun) and a good number of those make it down through our atmosphere to us. Not nearly all of them though, our atmosphere protects us. Here are three major types of rays:

• Alpha radiation is Helium nucleii (Heliums without electrons)—they have a positive charge, and are fairly heavy as particles go.
• Beta radiation is either electrons or positrons. When they’re electrons they have a negative charge, when positrons they’ll have a positive charge. These are fairly lightweight.
• Gamma radiation is photons. They’re charge-less and very lightweight.

Watch for

• Thickness: heavier particles can make thicker lines
• Speed: faster particles travel straighter
• Deflection: a ray’s interaction with other particles, electrons, etc will cause deflection
• Simultaneous events: probably from the same source
• Cascade events: one ray/particle interacts with something, causing several rays/particles to be released making their own tracks.
• Wandering: often caused by a low-energy ray making many interactions
• Curvature: in the presence of a magnetic field, positively charged particles will curve one direction and negatively charged particles will curve the opposite way.

Experiments to Think About

Both of these mean acquiring items which are generally not that safe to carry around unprotected, and sometimes require special licenses to have in your possession.

• Introduce a strong magnetic field. The path of a charged particle changes in the presence of a magnetic field.
• Introduce a radioactive source. Radioactive sources create charged particles.

Vocabulary

Supersaturated—when mixing sugar in your tea, you can only dissolve so much. If you heat up your tea you can dissolve more. If you then let your tea cool off, under ideal conditions, the sugar stays dissolved, even though you couldn’t have dissolved that much sugar in cold tea. This is a supersaturated solution – it’s more saturated than you could make it normally. This is why you should either add sugar to your iced tea before you cool it – or you have to add sugar syrup.

Nucleation site—if you then drop a sugar crystal or some such into the cold, supersaturated tea a the sugar will start to come out of solution and grow more sugar crystals on that first one. The first crystal (which can sometimes just be a rough spot, a spoon, a string, or a piece of sand) is the nucleation site. It acts as a nucleus for the new crystals.

Ion—an atom with too many or too few electrons.

Reservations

I need to do more research, there are two major problems with this article. First – I use the words particle and ray almost interchangeably, I should sort that out. Second – I would like to clarify how to identify types of radiation, and the likelihood the rays you’re seeing are from space versus from radioactive elements in the Earth. I’ll keep working on it, but this should definitely get you started on figuring out how awesome the particle aquarium is. Oh, and I want to stick in some pictures.

Want More?

Andy Foland’s Cloud Chamber
Cloud Chambers
More Cloud Chambers
A Great Lesson Plan (College Level)
A Place to Practice Identifing Tracks

~ A l i c e !

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