Particle Physics — Understanding the Universe’s ‘LEGO Bricks’. — Professor Lee Thompson — 25th September 2017.

One of the reas­ons for research­ing the make-up of atoms is that look­ing closely at the smal­lest things can tell us much about the largest things, such as suns, stars and galaxies.

Only 5 or 6 types of ‘bricks’ can make lots of dif­fer­ent things.

A clearer under­stand­ing of the atom was made by Thomson in 1897 when he dis­covered that cath­ode rays are elec­trons, and that elec­trons exist at the ‘edge’ of atoms and are not tightly bound to the atom.

In 1909 Rutherford, Geiger and Marsden, made an appar­atus that would fit on a desk top.

It was a cyl­in­der, sur­roun­ded by pho­to­graphic film, with a small gold foil taget in the middle.  They dir­ec­ted a beam of alpha particles at the thin sheet of gold and the photo film would detect the alpha particles.  As it was thought that the interior of atoms was mainly empty space they expec­ted that the beam would pass through the foil without devi­ation.  However some of the particles were scattered and some even appeared to bounce back.

In 1911 Rutherford con­cluded that there was a very tiny, very dense centre to the atom.  He called this the nucleus.

Nowadays we want to look inside the nuc­leus and to do this we need a lot more energy so we use particle accelerators.

The largest accel­er­ator is at CERN on the Swiss/French border.

This is housed in a large cir­cu­lar tunnel that is 27 km in cir­cum­fer­ence and is 150 metres under­ground.  In it particles are accel­er­ated to near light speeds and are smashed into each other to find out what comes off so soph­ist­ic­ated particle detect­ors are required.

Four dif­fer­ent types of detector are used made up of gas, liquid and plastics.

The ATLAS detector is enorm­ous and weighs over 70,000 tons and is 44 metres wide.

Phenomenal com­put­ing power is needed to ana­lyse the particles given off after collision.

At school I was taught that atoms were made up of elec­trons orbit­ing the nuc­leus, which con­tained pro­tons and neut­rons.  If elec­trons were stripped of the atoms they were called beta particles.  A helium nuc­leus, con­sist­ing of 2 pro­tons and 2 neut­rons, was an alpha particle.

Now we have:

  • Electrons
  • Muons — heav­ier electrons
  • Tau — even heav­ier electrons

Inside Protons and Neutrons we have Quarks, iden­ti­fied as Up Quarks and Down Quarks. Quarks come in pairs or threes — Protons have 1 Up Quark and 1 Down Quark, Neutrons have 1 Up Quark and 2 Down Quarks.

Other particles, called nutri­nos, have been detec­ted and  there are sev­eral sources of these particles — the Sun, nuc­lear power sta­tions and CERN.  Earth is con­tinu­ally bom­barded by showers of nutri­nos, but they pass through us and pen­et­rate the Earth without appar­ently caus­ing us any harm.

The Japanese are research­ing nutri­nos.  They aim a beam of nutri­nos at a research centre 293 km away, which has a detector con­sist­ing of  a huge spher­ical tank con­tain­ing 50,00 tons of water.  The tank roof is lined with a huge number of light detect­ors cost­ing mil­lions of dol­lars.  If a nutrino hits and reacts with atom in the water a tiny flash of light is emit­ted and detec­ted.  This is a very rare event.

The point of all this research at CERN and Japan is that it has brought many advances to the man in the street.

The World Wide Web was devised at CERN to allow inter­na­tional com­mu­nic­a­tion for phys­i­cists. High per­form­ance com­put­ing hand­ling large data volumes. Medical phys­ics has developed dia­gnostic tools for NMR and PET treat­ments and Hadron ther­apy and the pro­duc­tion of isotopes.

Cosmic rays inter­act in the atmo­sphere to gen­er­ate muons, which can be used to detect illegal imports of terrorist-owned nuc­lear mater­i­als in con­tain­ers at our ports. Also muons  tomo­graphy can be used to exam­ine the interi­ors of pyr­am­ids and magma cham­bers and volcanoes.

It was an inter­est­ing talk, con­tain­ing masses of mater­ial, that Prof. Thompson made as straight­for­ward as he could for the non-physicists!