Head in the Clouds

Let's switch gears a bit to the lowest levels of the atom. We are going to observe cosmic rays (high energy particles from outer-space) as well as radiation from a mineral that I collected once.

Cosmic rays bombard us all day, we just do not see them. The come from our sun, our galaxy, and distant galaxies at the speed of light. These particular high-energy particles have a mass (unlike photons). Most are nuclei of far-away atoms, usually protons (+), and a few are electrons (-). The technical details of cosmic rays are complicated and still being researched. I won't even attempt to describe them further.

We will also observe radiation from a terrestrial mineral source. In this case it is a small rock that I found which has some very small microlite crystals. While pure microlite itself is not radioactive, it tends to contain some traces of uranium and thorium. I identified these crystals with the help of a geiger counter, which "clicks" when an energized particle is released from the mineral and makes its way into its tube.

Radiation.... In Chemistry, what do we mean by that? There are many types of radiation. We are talking about particles being ejected from an atom as it decays. Alpha particles are stopped by a sheet of paper or a few centimeters of air. They are basically two protons and two neutrons (or a helium nucleus). They have a positive charge. Beta particles penetrate paper but not sheet aluminum. They can be positive (positrons) or negative (electrons, in which case a neutron becomes a proton). Gamma particles can pass through lead. They are photons that have no mass or charge. 

We don't need to understand all of this, but you can research further. The main point is that energy particles that are not blocked by the atmosphere constantly bombard us from outer space (cosmic rays), and radioactive materials also emit energy particles (radioactive decay).

Now, we cannot see these particles, but if we create an observable cloud of frozen alcohol crystals, we can see the ionization of those crystals by particles that pass through them. Ionization is the acquisition of a new charge (positive or negative) by an atom or molecule. In this case the energy causes the frozen crystals to dissipate, breaking their chemical bonds, and leaving an observable trail that is much like the contrails of an airplane in the atmosphere.

This experiment required obtaining two tricky things: 90% or greater alcohol (this has been nearly unobtainable since the pandemic started), and dry ice which is frozen carbon dioxide (CO2). Dry ice is not that hard to find but does sublimate, so you have to buy just-in-time. We'll conduct this experiment at night so buy it that day and keep in a cooler. Note that 70% alcohol won't work well, as I learned the hard way.

What is sublimation? It is a frozen solid evaporating directly to a gas without going through a liquid phase. That is why it gives off fog that is popular at Halloween. Blowing on it or heating it in any way increases the fog. Dropping it in water causes the water to "boil" as carbon dioxide gas bubbles are released. Frankly, playing with dry ice is a good chemistry experiment in and of itself. 

The main safety issue here is to not burn yourself by touching (including eating and licking) the dry ice. Use tongs or gloves to handle. Also, the radioactive source (optional) should not be *too* radioactive. This is both for safety reasons and to prevent an "unseeable" number of particles. My sample was chosen because it is small (thus the name microlite) only mildly radioactive.

With this background, on to the experiment!

I used a spaghetti sauce jar (empty and cleaned, of course) with lid. The main thing is to use a container that can easily be seen through, or else you will not be able to see the particle activity inside. I cut a sponge to fit snuggly in the bottom of the jar (it has to stay in place when soaked and upside down). Also cut a circle of black construction paper that fits inside the bottom of the lid. 

Now, soak the sponge with 90% isopropyl alcohol and pour out any excess. Turn upside down. Put the black paper in the lid and close it up Now place the jar (still upside down) directly onto dry ice. Note you will wants a cutting board or something below the dry ice to absorb the shocking cold. You don't want to break a granite counter top. Since the ice never becomes a liquid, you don't have to worry about it melting and making a mess.

Make sure it is night and all lights are off now.  It can be day if you are in a fully dark room. The point is you need no light to see better. 

Wait about 20-30 minutes for the cloud to form inside the jar between the dry ice and the sponge. Use a flashlight to look down at the black paper. You are ready when you see (look carefully) tiny crystals of "ice" in a cloud against the black background. If you don't look at the black background, you likely won't see the tiny clear-white crystals.

Ok, now hold your view on the cloud and wait. Give or take some time, about every 30 seconds or so you will see a contrail form. It may be long and straight or short and squiggly. Those are cosmic rays penetrating the glass and ionizing the cloud in its path. Pretty cool. Sorry, I could get no picture of this.

Now, if you have a radiation source, repeat the experiment with the source on the black construction paper. Every now and them you will see little contrails that appear to be emitted from the source. Mine were short in this case.

Short fat tracks are caused by alpha particles. Long straight tracks are caused by murons. Squiggly lines are caused by electrons and positrons. If your track forks, you have witnessed your particle decay further.

Thanks for reading,

Paul

p.s. It has come to my attention that some people would like to repeat these experiments without the hassle of finding the chemicals and/or having to buy too much, etc. I will try to keep an Ebay store alive (seller: cinnabarminerals) that offers low cost and related material for the purpose of enabling education. I will title offerings "Brave New Chemist Science Pack - (related Blog post name)." You can always comment and ask where to buy certain equipment too.