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!