Detection of particles Flashcards
How are interacting particles detected?
Interacting particles are (relatively stable) particles have a large enough lifetime that can go through some substantial amount of our detector and create signals.
For example Photons, Protons, Neutrons, Muon, charged/neutral pions
What are the important properties in interacting particles?
- Charged vs. uncharged
- rest mass
- particles type /possible interactions
How do particles interact?
Charged particles: - Ionisation - Scattering - Energy loss by radiation, Bremsstrahlung (electron/positions) - Emitting -Cherenkov radiation *Neutral particles need to convert to charged particles to be detected Photons: - Pair production - Photoelectric effect - Compton scattering
How do you detect charged particles that interact with atomic electrons?
Interaction with the
atomic electrons. The
incoming particle loses energy in lab frame) and the atoms are excited or ionized.
The mean rate of energy loss by moderately relativistic charged heavy particles is described by the “Bethe
equation” which is valid in the region 0.1 < βγ < 1000 between this region the energy loss initially decreases with increasing energy and then rises logarithmically with energy.
How do you find the energy lose per centimetre ,=?
Inorder to get the energy loss per centimetre you will need to multiply with the density of the
material.
What does the graph of the Bethe formula tell us (the mean energy loss for charged particles in different media as a function of βγ?
Valid in the region 0.1 < βγ < 1000
Low velocity regime: Energy loss
decrease like β^-2 (precise dependence ~ β^-5/3)
MIP: at βγ =4-5
the energy loss falls as a function of βγ and
reaches a minimum which is quite important. the minimum ionisation
Relativistic rise : Energy loss increases like ln(βγ)2 (relativistic extension of electric field). The electric field from the primary charged particle flattens and so allows collisions with more distant atoms
This rise does not go to infinity, and this is what the term delta in our formula
does, because at some stage atomic electrons that are in a large distance from our travelling
particle are getting “screened”
What does the Bethe formula depend on?
- Material dependence
- Function of βγ but type of particle enters via its mass
We need to convert from βγ to momentum and vice versa
βγ = p/Mc
Calculating energy loss using the material dependence empirical
approximation?
minimum ionization energy loss for different materials _min =2.35 – 0.28ln(Z) came up with this ability line
where Z is the atomic number
What do detectors do?
Helping us reconstruct charged particle trajectories
are based on “seeing” the produced ionization. The detectors we will see depend on the particle interacting with their medium and producing some amount of ionization. Then we get signals and then we try to figure out the
properties of the particle
How do you detect charged particles that interact with the atomic nucleus?
Interaction with the atomic
nucleus. The particle is deflected causing multiple scattering of the particle in the material. During this scattering, a Bremsstrahlung photon can be emitted.
What is multiple scattering?
A charged particle traversing matter will be scattered by the electric field potentials of nuclei and
electrons.
Multiple-scattering processes are dominated by deflections in the electric field of nuclei.
Does not cause energy loss but alters the direction of the particle.
This leads to the particle being deflected by many small-angle scatters.
The result is that the particle will exit the material with a small angle θ_0 with respect its initial direction
The fact that it introduces “noise” is another reason we want “thin” detector when following a particle’s trajectory.
How can multiple scattering be described?
Considering many identical particles, one gets a distribution of their angles θ that follows a Gaussian distribution
A Gaussian distribution approximately describes the projected angular distribution.
θ_0=13.6MeV/βcpzsqrt(x/X_0)[1+ 0.038ln(xz^2/X_0β^2)
p,βc, and z are the momentum, velocity, and charge number of the incident particle,and x/X0 is the thickness of the scattering medium in radiation lengths.
Why do we want tracking detectors to be thin?
Multiple scattering is needs to accounted for the trajectories of charged particles (amount of material expressed in radiation lengths)
thin units of radiation length
we must minimize the use of high-Z material.
How are photons detected?
“indirectly” via interactions producing charged particles which are detected through their subsequent ionisation in our detectors
What happens to photons in pair production?
High energy photon interact with matter mainly via pair production , where they are “converted” to an
electron-positron pair.
Pair production can not happen in vacuum . We need material , recoil for energy and momentum conservation. P_γ=P_e+ + P_e- + P_R
