A –Level Physics: Nuclear Physics Particle Detectors

Objectives:

FLASHBACK FLASHBACK: What is meant by the young modulus of a material? How does this differ from the stiffness constant? (6 marks)

Explain the following terms: Starter Activity Explain the following terms: Ionisation Annihilation Creation Particle Decay

Types of Particle Detectors To be able to study particle interactions, scientists required a method to detect them. When the first research in this field was being undertaken, bubble and cloud chambers were the first devises used to do this. They both work with a common principle whereby charged particles that go through the chamber leave a path of detectable ionised particles. They only differ by their choice of ionisable particles. -400BC Cloud Chambers Cloud chambers utilise super saturated air to show the ionising particles (e.g. alpha particles) As the ionising radiation passes through the cold damp air, tiny water droplets appear around the ions. This leaves a photographable trail 5.1 x 10-10 N

Types of Particle Detectors Bubble Chambers -400BC Bubble chambers utilise superheated liquid hydrogen on the verge of boiling to visualise the particle trajectories They were the main tool to use in the 1950s and 60s and was kept in a strong magnetic field to provide the particles with a centripetal force thus causing a curved path of motion When ionised, hydrogen bubbles form and this can be photographed and tracked 5.1 x 10-10 N

Particle Detector Images The images gained of the particle interactions are somewhat similar but with the big distinction that the paths are mostly straight in cloud chambers and curved in bubble chambers. -400BC Cloud Chambers Why might the alpha particle (bottom path) have thick and short paths whereas the beta (electron, top paths) have thin, longer paths? Alpha- larger, more ionising, therefore lose momentum and kinetic energy quicker. Beta- smaller, less ionising, take longer to lose momentum and kinetic energy

Particle Detector Images This is photograph of the interaction path of many particle particles! There are a few simple points that if you remember, you’ll be able to identify different particles (or at least their properties!) The momentum of a particle is proportional to its radius (r=p/Be) If the field is INTO the screen/paper then negative particles curve clockwise and positive curve anticlockwise Neutral particles leave NO track and travel in straight lines. Any gaps in a path are likely to be neutral particles Bubble Chambers -400BC Alpha- larger, more ionising, therefore lose momentum and kinetic energy quicker. Beta- smaller, less ionising, take longer to lose momentum and kinetic energy

Particle Detector Images Bubble Chambers -400BC Note: The ‘X’ symbol shows that the magnetic field is going INTO the paper Q: Explain how to use the following diagram of the bubble chamber paths of an alpha particle, beta particle, beta positive particle, and a gamma ray to determine which path corresponds to which particle. 1- positive particle, equal in momentum (and likely, mass) to another particle (4), so must be the position (beta positive) 2- it’s a positive particle so must be the alpha 3- lots of momentum as limited ionisation so is gamma 4- beta particle (electron) as it is negatively charged and going clockwise