Physics of Particle Detection

(Everything from geiger counters to the ALEPH detector.)

Charged particles passing through matter

• lose energy
• scatter

primarily due to 

 (a) collisions with atomic electrons

• dominates energy loss, except for electrons

 (b) scattering from atomic nuclei

• dominates scattering

These interactions occur frequently

Other processes

 (c) bremsstrahlung

• for electrons and high energy muons (>0.1TeV)

 (d) coherent em radiation

• Cerenkov radiation (e.m. shock wave) for v > c/n
• Transition radiation (g >> 1) from layered materials
• Channelling radiation from particles channelled by crystals

 (e) inelastic nuclear reactions

• for hadrons always, and for all particles at high enough energies

Ionization Energy Loss

For a heavy (M>m_e), fast (b> a) charged (Q=ze) particle, the ionization energy loss per unit distance travelled in a medium is:

n	density of electrons in medium
I	effective ionization potential of atoms in medium (~10 eV)
d	"density effect" due to screening of charged particle's electromagnetic field by polarization of atoms in medium d asymptotes to [2 ln(g) + constant] for g>>1
C/Z	"shell corrections" important when b ~ batomic electrons ~ a
n	higher order QED corrections important when b ~ a

Effective ionization potential (I) / atomic number (Z)

Figure 22.4 of Review of Particle Properties , Phys. Rev. D 54, No. 1. (see also the Mass Spectrometer experiment in the 3/4^th year labs.)

Bremsstrahlung

All charged particles scatter from the atomic nuclei and electrons when passing through a medium. When a charged particle is accelerated it radiates, so scattering of charged particles produces radiation. The radiation produced by charge particles passing through a medium is know as "bremsstrahlung".

The mean bremstrahlung energy loss of a charged particle (mass M, charge ze) is

where the radiation length, X₀, for the medium (atom density n_a and atomic number Z) is approximately given by

Bremstrahlung dominates the energy loss of electrons above the critical energy, E_c; ionization dominates at lower energies.

Energy loss of electrons

The critical energy (in lead) of the electron is about 7 MeV; the critical energy of the next lightest particle (the muon) is about half a TeV.