When this happens... We see this... 12/6/2018 Steve Wotton

Particle trails Particles are too small to see with the naked eye but can leave visible traces. They also leave traces which are not directly visible but with a little assistance... 12/6/2018 Steve Wotton

Scintillation Particle Scintillator Passage of charged particles causes excitation of molecules. Molecules emit photons during de-excitation. Some of the photons cause lower wavelength photons to be emitted by fluorescence. These photons propagate to the end of the scintillator material and can be detected using light sensitive detectors. Can use scintillation for triggering (Did a particle go through my detector?), for tracking (Where did it go?) and for timing (How fast was the particle travelling? In which direction?). Use thin, light materials to avoid disturbing the particle too much e.g. plastic scintillator (polyvinyltoluene). Also use scintillation for calorimetry (energy measurement). Use heavy material to completely stop the incident particle so that all of its energy is deposited e.g. Lead-glass. Particle Light detector Scintillator 12/6/2018 Steve Wotton

Detecting light -2kV 0V Photon Electrons Dynode Photocathode A photon ejects an electron at the photocathode (25% of the time) by the photoelectric effect. An electric field accelerates the electron to the first dynode where it now has enough energy to eject a few more electrons. The electrons are accelerated to the next dynode where each electron again ejects a few more. After several stages (~12) there are enough electrons (>100000) that the total charge can be measured using sensitive electronics. Photomultiplier tube is sensitive to single photons and is fast. Photocathode Photon -2kV 0V Electrons Dynode 12/6/2018 Steve Wotton

Detecting charge Detecting charge Semiconductors Wire chambers A diode is a semiconductor p-n junction. Allows current to pass easily in one direction but not the reverse. A reverse biased diode contains a region depleted of charge carriers. A charged particle liberates electron-hole pairs in the depletion region which drift in the electric field towards implanted strips where the charge is detected using charge sensitive amplifiers. Very small feature size allows precise determination of particle position. Detecting charge Wire chambers Charged particles leave a trail of ionised gas atoms in a wire chamber. An electric field causes charge to drift towards sense wires at constant speed. Arrival time depends on the distance from wire. Charge flows along sense wire to the ends where it is detected using charge sensitive amplifiers. The amount of charge depends on particle type so wire chambers are used to track particles and identify the particle type. Sense wires p-type n-type Ionisation cloud Signal charge Charged particle Charged particle 12/6/2018 Steve Wotton

12/6/2018 Steve Wotton