Positron

A genius

So this evening I find myself reading a physics paper written back in 1928 by Paul Dirac. The reason I am reading this paper is because I've been watching the 1981 Horizon program about Richard Feynman.

Initially I'd thought about doing a blog on Feynman, but there has been so much written about him that you can find it elsewhere. I'm sure he'd say talking about physics is more important.

Feynman talked about was his work on Quantum Electro Dynamics (for which he was awarded the Nobel Prize) that came out of him doing work on the Dirac Equation.

Now I vaguely remember the Dirac equation from days gone by and while I didn't remember exactly what it was I did remember that it was a biggie. Dirac had written a paper on the relativistic wave equation of the electron. Which is the paper that I have just read prior to writing this.

Two things have come out of me reading this paper. The first is that I am now aware just how rusty I have become at Physics! The second is on page 612 of the paper. This is one of those great moments in Physics. Dirac doesn't say that a new particle exists as such but he does point out that electrons can have either positive or negative charge. This is the start of a thought process that leads to him predicting the existence of an anti electron in a paper he published three years later in 1931. This later became known as the positron.

Now here is one of those many instances where we see just how difficult it can be matching theory and experiment. In 1929 Dmitri Skobeltsyn actually saw a positron using a Wilson cloud chamber (the particle detector of its day) as did Chung-Yao Chao, but his results were inconclusive and he did not follow it up. Had they been fully aware of Dirac's work, they may have realised what they were seeing, but no. So the prize for discovery went to Carl Anderson who discovered the positron on August 2 1932. He was also the person who came up with the name positron.

First image of a positron

How do we know they saw a positron and not something else? Take a look at the image on the right. This is the picture Anderson took of the positron. Can you see it? It's a bit like one of those scans you see at the hospital where you can't figure out what your looking at. Ok, here we go...

The thick line running from left to right is a piece of lead that seperates the top of the cloud chamber from the bottom. In the middle, looking a bit like a thin curved scratch is a line starting at the bottom (about 7 o clock) and curving off to the left at the top (about 11 o clock). Can you see it now? That is the IT! That is the track left by the positron.

The thing that looks a little bit like a hair on a lense is the first recognised proof of anti-matter. It won Carl Anderson the 1936 Nobel prize for physics.

How do we know we are moving from the bottom to the top and not the other way? the positron curves more at the top than the bottom. Which means it has lost energy by the time it reaches the top.

How do we know it as a positive charge and not a negative charge like the electron? The cloud chamber is  placed in a magnetic field. These fields cause positively charge particles like positrons and protons to curve in the opposite direction to negatively charged particles like electrons. In this case positively charged particles bend to the left, negatively charged to the right. So from the curve we know that is positively charged.

Taking a look at the amount of curve and the speed of the particle in the picture tells us that we have a particle that has the same mass to charge ratio as an electron.

This combination of evidence, the positive charge and the charge to mass ratio lead to the conclusion that the positron had been sighted.

This is the end of the post. I started with Feynman and ended with the positron, which is probably the way it should be. Especially as it was Feynman who later suggested that a positron may just be an electron travelling backwards in time! Blimey Richard, you know how to make my head spin.

What follows is further information on how the charge to mass ratio was found.

Charge to Mass Ratio

How do we know that it is a positron and not some other charged particle like a proton? Just over 30 years earlier. In 1897 J J Thompson had calculated the charge to mass ratio of the electron. Using Netwon's second law of motion, F=ma, and something called the Lorentz force law it is possible to get the charge to mass ratio, but how? The Lorentz force is giveb by the following equation

F = q[ E + (v x B) ]

now this may look a bit scarey and when we combine it with  F = ma, it can look even worse because we end up with

m a = q [ E + ( v x B) ] ,

where m is the mass of a particle and a is its acceleration. Divide both sides by q gives us

 (m / q) a = E + (v x B)

now here is the cool part. If we only apply a magnetic field, E =0 and B we know because we apply it our selves and have measured it. Then the equation becomes

( m / q) a = v x B .

( m / q) is the mass to charge ratio.  We get v and a from watching just how fast the particle is moving. So now we have the mass to charge ratio. When we compare this value with that J J Thompson got for the electron we find it is the same!