Detecting Particles

Key concept

charged particles leave tracks

• When charged particle passes through a substance it causes ionisation

• Electrons are knocked out of the atoms

• Leaves a train of ions as the particle travels

• We can detect a particle if you make a trail of ions and then take a photo

How does a cloud chamber work?

• It contains supercooled vapour that is in a gas state at temperatures where it is usually condensed

• As a charged particle moves through it, it ionises the vapour causing it to condense

• The train of condensation is used to interpret the ionising property of the particle

• Heavy and short tracks = high ionisation (e.g alpha particles)

• Long and thin tracks = low ionisation (e.g. beta particles)

How do bubble chambers work?

• Contains hydrogen in a liquid state when it is at a temperature higher than its boiling point using high pressure

• If charged particles pass it causes the liquid to form bubbles of gas in its trail

• We can quick take a photo to see the trail before the bubbles get too big

What is the limitation of cloud and bubble chambers?

They only show charged particles

• Today they aren’t used anymore

• Now, scientists use detectors that give out electrical signals that are logged directly on the computer (easier) e.g drift chambers

How are magnetic fields used to detect particles?

When a charged particle is in a magnetic field, it causes it to move in a circular path

• It’s radius will decreases as it is losing energy

• The radius is proportional to the momentum r = p/BQ

• The direction of the spiral will be determined by the charge of the particle

• Scientists use this to identify ions

• If ions are all sent into the detector at the same speed, their spiral will be proportional to their masses

• As r = mv/BQ

How do we detect neutral particles?

• They don’t make tracks

• They only make tracks when they decay into 2 charged particles

• We can fire them from the interaction point in a detector and observe tracks shown

• This can be seen as a V shape where 2 oppositely charged particles have come from the decay of a neutral particle

What does the distance between the interaction point and the V depend on?

The half life of the neutral particle

• longer half lived particles travel further before decaying

What is the relativistic time dilation?

• When particles move close to the speed of light they experience this

• Time seems to run more slowly for the moving particle than for the observer

How can we recognise the tracks in bubble chambers and interpret them?

• Straight lines - lines of incoming beams (Can ignore them)

• A spiral coming from an straight line - shows that an electron has been knocked out of the hydrogen (ionised)

• The direction of these spirals show us the direction of negative particles (negative = clockwise)

• The point where several curved tracks are coming from is the point of interaction