Sunday, 10 January 2010

Bubble chamber

Introduction

Bubble chamber photographs were used to make many important discoveries and measurements in particle physics. They also provide a unique window on the properties of elementary particles recording in great detail the trajectories of charged particles and their decays and interactions. We have selected a small number of photographs taken at the 2 metre bubble chamber at CERN near Geneva. These are used, together with accompanying text and questions, to illustrate various physics concepts and to help you gain experience with simple calculations which are encountered in your Particle Physics A level topic.

The clarity of bubble chamber photographs has helped particle physicists to become very familiar with the signatures of many different types of particles including the electron, proton, pion, kaon and antimatter. Even neutral particles can sometimes be detected especially if they decay to charged particles within the bubble chamber. We hope that these specially selected photos will also help you in your understanding of this very exciting field. All the pictures were recorded in a bubble chamber filled with liquid hydrogen exposed to a "beam" of negative kaon particles (K-) each with the same momentum. Each picture shows a small region of the chamber typically 20 cm in length and the distance on the photographs roughly correspond to the actual distance in the chamber. The paths of all moving charged particles in the chamber are recorded on the photograph as a series of small dots (bubbles). The crosses are reference marks on the walls of the chamber to allow three dimensional reconstruction of a collision when pictures from several cameras are used. In this exercise we only study the photographs from a single camera.

The only target particles are the proton and electron of the hydrogen atom and we expect that most beam particles will pass through the mainly empty spaces of each atom without deviation. The beam kaons are always travelling away from the side of the photo where the picture number is written. The second number is the photograph number used for identification later. As they all have the same direction and momentum and are travelling in a uniform magnetic field, the curvature and direction of each beam particle should be the same. The beam particles are negatively charged and if you look along the track you will see that these curve to the right. The magnetic field strength is 1.78 Tesla and you should be able to work out its direction from the direction of curvature of the beam particles. In most photographs you will also see a few particles at a different angle to the majority of the beam particles. These are due to cosmic rays or particles produced in interactions outside the bubble chamber.

In order to help you identify the particle interactions and decays on the enclosed photographs we now summarise a few important properties of the bubble chamber tracks produced by different types of particles. The electron (and positron) have a much smaller mass than other particles. When they experience an electromagnetic force they are accelerated more than other particles and this leads to a loss in their energy because they radiate energy as photons. This loss of energy results in the characteristic spiral nature of an electron (or positron) track where the radius of the track gradually decreases.

When a beam particle collides with a proton then the kinetic energy of the beam can be converted into mass energy and extra particles can be produced. The number and type of particles produced must be consistent with the conservation laws of the strong force. These include momentum, energy, charge, strangeness, baryon number and lepton number conservation. As the beam (K-) particle has a strangeness of -1 and the proton has a strangeness of zero, the produced particles must also have a total strangeness of -1. The target proton is a baryon with baryon number =+1 and the beam particle (K-) is a meson, with no baryon number, so the produced particles must have a combined baryon number of +1.

It is often difficult to use the track properties to distinguish between different particles. However the large mass of the proton means it can sometimes be clearly identified by the density of bubbles (or darkness) of the track. The number of bubbles per centimetre is inversely proportional to the square of the particle velocity. Where two particles have similar momentum then the velocity of a particle will be inversely proportional to its mass. As the proton is the heaviest stable particle and is around seven times heavier than the pion its bubble density will be around fifty times larger than a pion. Consequently very dark tracks are often caused by protons.

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