Bureaucratic FormalitiesMeaningful Muon Rate Comparison

Med Webster

QuarkNet 2013Vanderbilt University

Procedures

1) Sign the Quarknet time sheet – Preferably at the end of your last day this year.

2) Verify that I have correct information about you. Get the half page sheet and note corrections.

3) On Friday check about travel forms for travel reimbursement.

4) Get a VU parking pass if you will need one Friday.

Quoting Uncertainty - Statistics

No measurement has any significance unless it is associated with an uncertainty.

Starting point – A measurement which is a simple counting of random events has an uncertainty which is the square root of the number of counts.

If our counters record 100 counts, the proper way to report that is 100 +/- 10 counts. That means that if I repeat the exact experiment, I probably will get a result which is between 90 and 110. I am very unlikely to get a result which is less than 70 or greater than 130.

Rate is more useful than counts. How do I get the rate from the number of counts?

If that run was 2 hours long, What is the rate?

100 counts/(2 Hours) = 50 counts/hr

And for a the error on the rate estimate:

10 counts/(2 hr) = 5 counts/hr

And the rate is to be quoted as

50 +/- 5 counts/hr

Bruno Rossi and physics in the ’30s and ‘40s

Atoms, molecules, electrons, and the nucleus were pretty firmly established.

Einstein had his nobel prize (for the photoelectric effect and Brownian Motion – NOT for relativity).

The leading theoretical physicists were convinced about relativity, but experimental physicists less so. E=mc^2 OK, but time dilation and space contraction were questioned.

The world was in turmoil leading to WW II – We discussed this at length two years ago in the Oak Ridge, atomic bomb context. People with Jewish connections were in particular trouble. Physicists helped one another and probably fared better than most.

Rossi was a promising, but not yet well known, young, Italian physicist. Now that I have said the thing it would have been immodest for him to say, it is best to let him tell his own story in his reminiscences given at a Fermilab conference in 1980. Please read it despite the bad reproduction of the figures. Read it as a story, not bothering particularly with the equations.

Initial Cosmic Rays and Showers

Cosmic Rays come from outer space, are mainly protons (hydrogen nuclei) and the ones responsible for the things we measure have very large energies.

They hit air nuclei high in the upper atmosphere and make more particles, mainly pi mesons, sharing energy. The new particles repeat the process making an avalanche or shower.

The shower terminates when energy has been shared so many ways that individual particles do not have enough energy for particle formation.

8 July 2013 Vanderbilt Quarknet 8

Some Pions decay into Muons

A pion is a strongly interacting particle like a proton and usually interacts (makes more particles) if it has enough energy and hits a nucleus. A few pions decay into a muon and a neutrino before they happen to hit a nucleus.

Unlike a pion or proton, a muon does not interact strongly and can go right through many nuclei without interacting. The showers peak at about 60,000 feet and it is primarily the muons which reach sea level, where they comprise 95% of the cosmic radiation.

Matter is made of electrons and nuclei.

When a heavy particle hits a much lighter particle, the light paraticle is batted out of the way and the heavy particle goes on almost as if nothing had happened. But it does lose a little energy, just like friction.

Most of the charged particles we will meet are much heavier than the electron (200 x for muon).

When a pion hits a nucleus, it probably showers: hadron – strongly interacting particle

When a muon hits a nucleus, it just goes right on through it: lepton – weakly interacting particle. The next slide shows how this friction effect removes the lower energy muons from the detected radiation.

Not hit a nucleus is dotted line

Both pions and electrons loose energy to this friction, removing the low energy particles. Pions are also removed by hitting a nucleus – the steeper curve. There is a significant fraction of pions in the CR at Mt. Evans altitude but only a very small fraction of pions at Mt. Mitchell.

This is the background needed to understand several aspects of our procedure. Some questions:

Why did I do the first part of the experiment at my house instead of at Stevenson at Vandy? Which is more similar to Mt. Mitchell for cosmic rays?(See next slide)

Why put the 2 slabs in for both Nashville and Mt. Mitchell? Hint look at flux vs. bricks plot.

Why put 21 additional slabs in for Mt. Mitchell but not for Vandy. Hint How much additional air is above the counter at Nashville than at Mt. Mitchell. Air pressure is 0.8 atmospheres at Mt. Mitchell and nearly 1.0 atmospheres at Nashville.

Procedural Questions

Why do we record 3-fold coincidences. Why do we get any 3-folds?

Why record data every hour or so instead of just at the end of the experiment?