1932 cloud chamber: how are the elements physically introduced?

January 29, 2017 10:10 AM

I am working on a project for which I need to understand how a cloud chamber works. Specifically, I am trying to understand how real physicists working in Berlin in 1932 used a Wilson cloud chamber to measure "the range of protons after collisions with neutrons", right after the discovery of the neutron.

Even more specifically, this is the experiment described in a book I am reading: "He proceeded to estimate the relative penetrating ability of the lithium, beryllium, and boron neutrons by measuring the range of protons after collisions with neutrons."

I have done extensive googling, and I understand that the cloud chamber contains a gaseous alcohol which allows a particle's path to be visible in the vapor. These vapor trails can be photographed and analyzed.

But, in the quotation above, how are the lithium/beryllium/boron neutrons physically introduced to the chamber? Is a solid block of boron just laid down in the middle of the vapour? Or is a tiny bit of it balanced on a needle? Are the neutrons somehow separated from the material before being put in the chamber? How are the protons launched at the sample? How do the protons enter the chamber?

Thank you!

posted by tessmartin to Science & Nature (9 answers total)

Was this work published? You can try Google Scholar with the scientists' names--the original paper is likely to have a "methods" section describing the apparatus in enough detail to reproduce it.

If I had to guess, I'd say this experiment works by putting some radioactive material in a cloud chamber. I have no idea how much--it depends on what fraction of the material is activated to the neutron-producing isotope. The material decays by neutron emission; the emitted neutron has an energy specific to the isotope that emitted it.

My guess is that this is not an alcohol cloud chamber but a hydrogen one. Some neutrons will collide elastically with hydrogen nuclei (protons). Those protons are charged, and their paths will show up in the cloud chamber. A magnetic field bends the paths allowing estimation of the momentum of the proton.
posted by lozierj at 10:49 AM on January 29, 2017

Some quick googling finds no evidence for hydrogen cloud chambers; I'm thinking of bubble chambers.

The hydrogen must be in the chamber somewhere--maybe in a vessel near or surrounding the source--or maybe introduced into the entire chamber. The original paper will have the details.
posted by lozierj at 10:55 AM on January 29, 2017

1935 paper based on the same work. Looks like a vaccum tube.

http://journals.aps.org/pr/abstract/10.1103/PhysRev.45.507 Paper on the source.
posted by nickggully at 10:56 AM on January 29, 2017

Response by poster: Thanks for the thoughts and links. Yes, the experiment I cite was published, but I can only find the paper online in German and can only access it for about $50. I will scrutinize the 1935 paper some more, but at first glance I am still unclear as to how the neutrons and protons enter the cloud chamber.
Is the material first made to produce neutrons outside of the cloud chamber, and these neutrons are introduced into the chamber? Or does it all happen inside the chamber? And then how are the protons introduced?
I bet you can tell I'm not a natural at this stuff, but I'm trying :)
posted by tessmartin at 11:05 AM on January 29, 2017

Best answer: Based on googling, you're talking about Lise Meitner verifying Chadwick's neutron experiments. Not sure how that he snuck in there.

Chadwick's original paper.
A cloud chamber can't detect neutrons, so an intermediary is required. Basically, an alpha source (polonium, etc) shoots alpha radiation at a light metal like lithium or beryllium. The metal emits neutrons, which hits the paraffin shell of the cloud chamber, causing the paraffin to emit a proton, which can be seen in the cloud chamber.
posted by zamboni at 11:06 AM on January 29, 2017

Response by poster: Oh, the paraffin makes up the shell of the cloud chamber?
posted by tessmartin at 11:10 AM on January 29, 2017 [1 favorite]

In this case, yes, so there's something for the neutron to interact with, causing a proton to be emitted.
posted by zamboni at 11:18 AM on January 29, 2017

There's a cloud chamber at the Griffith Observatory and you can call them to ask questions. The chamber is often under repair, I'm sure someone there probably works on it frequently and can answer your questions.
posted by jbenben at 11:39 AM on January 29, 2017

Response by poster: ok, thanks, this is all really helpful. I'm still struggling to find an actual photo of a cloud chamber with a shell of paraffin, or with some paraffin in front of it, but I'll keep hunting.
posted by tessmartin at 12:04 PM on January 29, 2017