### Announcement:

Redo problem 3 of homework 5 for extra credit. (Pair creation problem.)

# Particle Detection and Measurement

There are literally hundreds of different particle detectors out there, far too many for me to discuss in this course. Luckily, they are based on a small number of possible interactions of particles with bulk matter, and a small number of quantities that can be measured. I will focus on these interactions and how they are used to measure quantities about particles.

## Particle Measurements

Assume a particle is passing through your experimental apparatus (detector). What information can you measure about that particle? Position, direction, charge, momentum, energy, mass, point of origin, point of decay, spin orientation.

## Interactions of Particles with Bulk Matter

We detect particles using sensors made from bulk matter: gases, liquids, and solids. Let's consider the general types of processes that can lead to "signals" in bulk matter, and how the travel of a particle is effected. I'll limit the discussion to relativistic particles, since this is generally the case in particle physics.

ProcessDescription

ionization

passing particle ionizes an atom, leaving behind an electron and positive ion.

scintillation

passing particle excites (or ionizes) an atom which emits light (visible or u.v.) as it returns to an unexcited state

cascade of secondary particles created when particles that interact electromagnetically and strongly pass through high A/Z material.

emission of light (visible and u.v.) when the speed of a particle, βc, exceeds the speed (phase velocity) of light in a medium, c/n where n is the index of refraction (analogy of sonic boom for a vehicle exceeding the speed of sound)

emission of light (visible and u.v.) when a particle passes from one medium to another of differing EM properties

In the list of interactions I neglect several novel ones, for simplicity. These include phonon excitations in a crystal at low temperatures, and radio frequency Cerenkov radiation.