Detectors Flashcards
Particles’ passage through matter
What is the fundamental principle of particle detection?
Particles must interact with the detector material and lose energy in a measurable way. The detection is based on observing this energy loss, enabling measurement of properties like momentum, energy, and type (mass, charge, spin, parity, couplings, lifetime, etc.).
Particles’ passage through matter
Which particles are considered stable in particle physics? How to detect unstable particles?
Stable particles: electron (e), proton (p [uud]), photon (γ), and neutrinos (ν). Other particles decay (after s = γvτ distance) and can only be observed through their decay products.
Particles’ passage through matter
How do* short-lived* and long-lived particles differ in terms of detectability?
Relativistic particles with lifetimes ≳ 10^(–10) s (e.g., μ, n, π±, K±) travel measurable distances (few meters) in detectors and can be directly observed. Shorter-lived particles decay too quickly and are detected via their decay products.
Particles’ passage through matter
What types of interactions are used to detect different particles?
Charged particles: ionisation, bremsstrahlung, Cherenkov radiation, and transition radiation
Photons: photoelectric effect, Compton scattering, and pair production
Hadrons: nuclear interactions
Neutrinos: (only) weak interaction
Particles’ passage through matter
What is the Bethe-Bloch formula and what does it describe? What corrections are applied?
The Bethe-Bloch formula describes the energy loss per unit path length of heavy charged particles through a medium due to ionisation.
- valid for 0,1 < βγ < 1000 and mid-Z materials
- maximal energy transfer (W(max)) depends on particle mass and velocity
Corrections:
- high energy corrections: density corrections due to the density effect (density dependent polarization od medium)
- low energy corrections: shell corrections due to the breakdown of the stationary electron assumption (speed of the incident particle is close to the orbit speed of the electron)
heavy charged = M»_space; m(e)
Particles’ passage through matter
What is a Minimum Ionising Particle (MIP)?
A particle that loses the least energy per unit length while traversing a medium. This occurs around βγ ≈ 3–4, with energy loss ~1–2 MeV/(g·cm²).
- e.g.: pions, muons
Particles’ passage through matter
How does the energy loss of a charged particle change with velocity?
At low velocities: energy loss decreases sharply (~1/β²)
At high velocities (βγ > 4, v ≈ c): relativistic rise, energy loss increases logarithmically (~ln(βγ)).
- saturation at large βγ due to density effect
Particles’ passage through matter
How is dE/dx used for particle identification?
Energy loss per distance (dE/dx) is velocity-dependent and helps distinguish particles of different masses at the same momentum. This is useful in tracking detectors like ALICE TPC.
- momentum can be measured by the track radius in the magnetic field
- particles w/ diff. mass get separated on the plot (dE/dx histogram): typically π/K/p separation
Particles’ passage through matter
What technique improves accuracy in dE/dx measurements?
The ‘truncated mean’ method excludes outliers caused by δ-electrons (tail in the Landau distribution that thin absorbers follow) to reduce the effect of fluctuations and improve resolution.
- it excludes the highest measured energy loss values from the average
The distribution is of the energy loss of single collisions.
Particles’ passage through matter
What is the Bragg peak and why is it significant?
The Bragg peak is the point near the end of a particle’s range where it deposits maximum energy. This is exploited in medical applications like proton therapy.
The range is the mean penetration length that is not a sharp value but fluctuates statistically.
Particles’ passage through matter
Why must Bethe-Bloch be modified for electrons?
Electrons are identical in mass to atomic electrons, making interactions indistinguishable and requiring a more complex treatment than for heavy particles. They are also highly relativistic.
Low energy positrons need different treatment in the calculation, as they are not identical (i.e. they are distinguishable).
Particles’ passage through matter
What is bremsstrahlung and when is it important?
Bremsstrahlung is radiation emitted by accelerating charges, especially significant for electrons due to their small mass.
- arises if particles are accelerated in the Coulomb field of a nulceus
- dE/dx ~1/m²
- critical energy: where in a given material the bremsstrahlung starts to dominate over the ionization (where ionisation energy loss = radiation energy loss)
- at high energies, it always dominates over ionisation
- E = E0 exp(-x/X0) for electrons where X0 is the radiation length
Particles’ passage through matter
How do muons interact with matter?
Muons mainly lose energy via ionisation. They can travel long distances through dense materials, making them easily detectable as they penetrate deeper than most particles.
- in particle accelerators, muons pass through the full detector leaving a long ionisation trail behind
- below 100 GeV, ionisation energy loss dominates
Particles’ passage through matter
What is Cherenkov radiation and when does it occur?
It occurs when a charged particle moves faster than the phase speed of light in a medium. It produces a coherent conical light (shock) wave (while conserving energy and momentum) detected via photodetectors.
- coherent emission because it is in phase with the particle velocity
- threshold: β > 1/n
- Cherenkov angle: cos(θ(c)) = 1/(n·β)
Particles’ passage through matter
How can Cherenkov detectors be used for particle ID?
By measuring the angle or presence of Cherenkov radiation, one can determine the velocity of the particle and, when combined with momentum data, infer its mass.
Particles’ passage through matter
What are RICH and DIRC detectors?
RICH (Ring Imaging Cherenkov): detects Cherenkov photons forming rings for full 4π coverage
- all photons emitted at same Cherenkov angle
- from the radius of the ring, the velocity can be determined
DIRC (Detection of Internally Reflected Cherenkov light): preserves angle info via light-guiding elements using total reflection to guide photons
Particles’ passage through matter
What is transition radiation and when is it emitted?
Transition radiation occurs when a charged particle crosses between inhomogenous materials with different dielectric properties, emitting X-ray photons, especially for highly relativistic particles (γ > 1000).
- the moving charged particle can be considered as an electric dipole with its mirror charge
- energy loss proportional to γ
- increase in energy loss primarily due to increase of photon energy
- low atomic number radiator needed to minimize photon absorption, otherwise TR photons won’t leave the material
Particles’ passage through matter
How is transition radiation used for particle identification?
TR is used for electron-pion separation by detecting emitted X-ray photons (so, energy loss), typically 5–15 keV, in a high-Z gas detector (e.g., Xe) with a low-Z radiator (e.g., Mylar).
