Nuclear Friday: Seeing The Light

You may not know this, but there are (or used to be) quite accessible places where members of the general public could look at uranium fission with their own eyes. I think most of my readers can guess that I have been to such a place given my obsessions. Back in the eighties I was in my high school’s science and engineering club and through that club I got to take a trip to College Station, Texas. The Texas A&M Nuclear science center is where I saw the light. There is absolutely nothing like it. Take a look for yourself, this is the very reactor I saw thirty years ago:

The video does not do it justice, nor do any photos you will find online. The blue  Cherenkov radiation is best seen with your own eyes. The light is produced when beta particles moving faster than the speed of light in water disrupt the electromagnetic fields between water molecules. While the speed of light in a vacuum is a constant, light travels slower in other media. In water, light travels at around .75 times the speed of light in vacuum. Beta particles are slightly faster than that. And that’s really fast.

When I saw this reactor it might have been running a little hotter than the 300W between pulses in this video. Or perhaps not, the camera in this video is close to the reactor core and may be surrounded by leaded crystal shielding. I was looking through the full depth of water and could see the blue glow clearly with a low level of background lighting.

And yes, this was completely safe. We had film badges that would turn dark if we were exposed to radiation. We also had to stand on a machine and put our hands into a box that scanned our hands and feet to see if we had picked up any radioactive material. This was necessary because this reactor was used to teach students how to make radioisotopes used in cancer treatment.

There are types of Cherenkov radiation other than this blue light. Cosmic rays of incredible power interact with the upper atmosphere producing exotic particles with high velocities. Since the speed of light in air is higher than in water, you can get Cherenkov radiation only at higher velocities than beta particles travel. The radiation is in the ultraviolet range, but we have detected it.

What is a Beta Particle? 

A beta particle is simply an electron free from any atom moving at a goodly fraction of the speed of light. They were named before it was really clear the electron even existed, so that’s why they have a different name. Beta particles usually come from neutron decay by the weak nuclear force. She does a good job of explaining balanced nuclear chemistry in this video. The release of beta particles in the reactor core is a bit different as it’s from fission by-products, but the basic principles still apply

What does the “$” sign mean in the video? 

Woah, this is getting a bit beyond my knowledge of nuclear physics. Basically, the dollar sign represents how many more neutrons a system is going above the minimum neutron emission required to maintain a sustained chain reaction. $Zero is perfect minimum balance, $3.50 is pretty damn high. A few bucks more and you’ve got a bomb, but reactors can’t go there. Canadian Manhattan Project physicist Louis Slotin called it a dollar because that’s the only key he wasn’t using on his typewriter. Unlike most Canadians working on the bomb, Slotin was not part of the British Tube Alloys project. He had moved to Chicago to work with Fermi and was recruited from there.

A Canadian Casualty:

Slotin would become the second direct victim of the Demon Core. This was the third plutonium core produced in 1945 and was intended to be part of a bomb similar to Fat Man and dropped on Japan. Truman cancelled that mission and ordered that the core not be made into a bomb. So yeah, you got physicists with a fully formed core, they’re gonna do weird things. Even though this core had already killed a physicist Slotin was allowed to mess around with it. He died 9 days later from acute radiation sickness.

Research Links: 

Randall Munroe of XKCD explains how water is pretty good radiation shielding.

Dr Manhattan from The Watchmen is the color of Cherenkov radiation.

And this radioactive contaminant is relevant these days.

[Important Update, late May 2016] 

It is widely believed that the Demon Core that killed Slotin and others was part of the bombs at the Crossroads tests. I have recently discovered from this source that the Demon core was reprocessed and mixed with new plutonium from Hanford to make a new core for Crossroads.



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