Cosmic raysand how a Bristol Physicist won the Nobel Prize

Dr Helen Heath

1788

http://books.google.co.uk/books?id=by5EAAAAcAAJ

100 years of Cosmic Rays

NASA TV

What are cosmic rays

• Primary Cosmic rays• 85% protons• 12% helium nuclei (alpha particles)• 3% heavier nuclei• 2% electrons

http://www.phys.uu.nl/~thooft/quarks.gif

Baryons

Mesons

Particle Energy(MeV) v year

1900 1920 1940 1960 1980 2000 20201

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First Cy-clotron

BevatronAntiproton

LHC

Mass of lightest “new particles”

Natural Radioactivty

Cecil Frank Powell

• Nobel Prize 1950

“for his development of the photographic method of studying nuclear processes and his discoveries regarding mesons made with this method.

The Start of Big Physics?

• “These developments in international collaboration formed an essential background to the setting up of large international laboratories such as CERN”

D.H.Perkins

40 years of Particle Physics Conference

Bristol July 1987

Sub-atomic particles

• Nucleus• Protons • Neutrons

• Electrons• First antimatter particle – positron• Photons

Yukawa

• Proposed a field theory of nuclear forces.• Field theory requires field quanta.

• We can estimate the mass of such a quanta from the Uncertainty Principle• E t ≈ h (h=6.63x10-34Js)• t ≈10-15/c ≈ 0.3x10-22s• E ≈ 2.2x10-11J• E = mc2 m=0.24 x 10-27 kg=0.14mp

Mesons

• If Yukawa’s theory was correct • there must be a field particle

• The hunt was on for a meson• Today we use meson to mean a combination

of a quark and an antiquark• Originally it was a particle with a mass

between the proton and electron• Mesos – Intermediate

(Modern Aside)

• Mass is unexplained • Proposed Higgs

Mechanism• Requires Higgs Field• ..and therefore the

Higgs boson

Experimental techniques

• Cloud Chambers• Bubble Chambers• Emulsions• Solid detectors

Cloud Chamber

• The discovery of the positron• Carl D. Anderson – Physical Review 1933

Bubble Chamber

©CERN photo

Cecil Powell

• Cecil Powell started an autobiography which can be found at• http://www.phy.bris.ac.uk/history.html

• He succeeded academically winning a scholarship to Judd School and then moving on to Sidney Sussex Cambridge

• From an early age he was an enthusiastic, if not always, talented investigator.

After Cambridge

• Nearly became a teacher

• Research student at Cambridge • Supervised by Wilson

• Recruited by Tyndall to Bristol in 1928

The emulsion technique

• Cloud chambers and bubble chambers need to be photographed

• Emulsions are continuously active• Emulsions are also rather portable

• Important for early observatories & balloons

http://www.imcce.fr/phemu03/Promenade/pages5/545.html

Photographic Emulsion

• Grains of silver bromide suspended in gelatine.

• Light causes changes to the silver bromide.

• Developer changes the affected grains to silver

• Fixer removes the remaining silver bromide

Powell’s research group

The pion

• In 1936 C. Anderson and S Neddermeyer observed negatively charged particles with mass intermediate between that of the proton and the electron• Initially called mesotrons• Renamed the mu-meson in 1947

• Now mesons are a subset of hadrons • The mu meson is a lepton

Measuring energies

• Energy loss in the Emulsion is approximately continuous.• The range of a particle depends on its energy

Double meson events

• Seen in events from the Jungfrau Joch• The second meson has a range of 600 m

Interpretation

• In Double meson events one meson decays to another

• - -

• Since the pion has stopped all the kinetic energy of the decay products comes from the change in mass• Pion mass =139.6 MeV/c2

• Muon mass = 105.7 MeV/c2

After the Nobel Prize• Powell continued

to work with emulsions

• Left is the first example of a Kaon decay to three pions

• Work switched to balloons rather than mountains.

Modern Cosmic Ray Work

LHC Energies

Open Question

• Origin of very energetic cosmic rays

• GZK –Cutoff

(Griesen-Zatsepin-Kuzmin)

Cronin, J. W., 1999, Cosmic Rays: the most energetic particles in the universe, RvMA 71, 165–172

Sparse Array – Large area

HiSPARC project High School Project

on Astrophysics Research with Cosmics

>100 detector across

The Netherlands

HiSPARC – school detector

Students build their own detector

And place it on top of the roof of the school