# Interaction of Particles with Matter

## Presentation on theme: "Interaction of Particles with Matter"— Presentation transcript:

Interaction of Particles with Matter
Alfons Weber CCLRC & University of Oxford Graduate Lecture 2004

Nov 2004 Table of Contents Bethe-Bloch Formula Energy loss of heavy particles by Ionisation Multiple Scattering Change of particle direction in Matter Cerenkov Radiation Light emitted by particles travelling in dielectric materials Transition radiation Light emitted on traversing matter boundary

Nov 2004 Bethe-Bloch Formula Describes how heavy particles (m>>me) loose energy when travelling through material Exact theoretical treatment difficult Atomic excitations Screening Bulk effects Simplified derivation ala MPhys course Phenomenological description

Nov 2004 Bethe-Bloch (1) Consider particle of charge ze, passing a stationary charge Ze Assume Target is non-relativistic Target does not move Calculate Energy transferred to target (separate) ze b y r θ x Ze

Bethe-Bloch (2) Force on projectile
Nov 2004 Bethe-Bloch (2) Force on projectile Change of momentum of target/projectile Energy transferred

Bethe-Bloch (3) Consider α-particle scattering off Atom
Nov 2004 Bethe-Bloch (3) Consider α-particle scattering off Atom Mass of nucleus: M=A*mp Mass of electron: M=me But energy transfer is Energy transfer to single electron is

Bethe-Bloch (4) Energy transfer is determined by impact parameter b
Nov 2004 Bethe-Bloch (4) Energy transfer is determined by impact parameter b Integration over all impact parameters b db ze

Bethe-Bloch (5) Calculate average energy loss
Nov 2004 Bethe-Bloch (5) Calculate average energy loss There must be limit for Emin and Emax All the physics and material dependence is in the calculation of this quantities

Bethe-Bloch (6) Simple approximations for
Nov 2004 Bethe-Bloch (6) Simple approximations for From relativistic kinematics Inelastic collision Results in the following expression

Bethe-Bloch (7) This was just a simplified derivation
Nov 2004 Bethe-Bloch (7) This was just a simplified derivation Incomplete Just to get an idea how it is done The (approximated) true answer is with ε screening correction of inner electrons δ density correction, because of polarisation in medium

Nov 2004 Energy Loss Function

Average Ionisation Energy
Nov 2004 Average Ionisation Energy

Density Correction Density Correction does depend on material with
Nov 2004 Density Correction Density Correction does depend on material with x = log10(p/M) C, δ0, x0 material dependant constants

Different Materials (1)
Nov 2004 Different Materials (1)

Different Materials (2)
Nov 2004 Different Materials (2)

Particle Range/Stopping Power
Nov 2004 Particle Range/Stopping Power

Application in Particle ID
Nov 2004 Application in Particle ID Energy loss as measured in tracking chamber Who is Who!

Straggling (1) So far we have only discussed the mean energy loss
Nov 2004 Straggling (1) So far we have only discussed the mean energy loss Actual energy loss will scatter around the mean value Difficult to calculate parameterization exist in GEANT and some standalone software libraries From of distribution is important as energy loss distribution is often used for calibrating the detector

Straggling (2) Simple parameterisation Landau function
Nov 2004 Straggling (2) Simple parameterisation Landau function Better to use Vavilov distribution

Nov 2004 Straggling (3)

δ-Ray δ-Rays Energy loss distribution is not Gaussian around mean.
Nov 2004 δ-Rays Energy loss distribution is not Gaussian around mean. In rare cases a lot of energy is transferred to a single electron If one excludes δ-rays, the average energy loss changes Equivalent of changing Emax δ-Ray

Nov 2004 Restricted dE/dx Some detector only measure energy loss up to a certain upper limit Ecut Truncated mean measurement δ-rays leaving the detector

Electrons Electrons are different light Bremsstrahlung
Nov 2004 Electrons Electrons are different light Bremsstrahlung Pair production

Nov 2004 Multiple Scattering Particles don’t only loose energy … … they also change direction

Nov 2004 MS Theory Average scattering angle is roughly Gaussian for small deflection angles With Angular distributions are given by

Nov 2004 Correlations Multiple scattering and dE/dx are normally treated to be independent from each Not true large scatter  large energy transfer small scatter  small energy transfer Detailed calculation is difficult but possible Wade Allison & John Cobb are the experts

Correlations (W. Allison)
Nov 2004 Correlations (W. Allison) nuclear small angle scattering (suppressed by screening) nuclear backward scattering in CM (suppressed by nuclear form factor) electrons at high Q2 whole atoms at low Q2 (dipole region) Log cross section (30 decades) log kL 2 18 17 7 log kT Log pL or energy transfer (16 decades) electrons backwards in CM Log pT transfer (10 decades) Example: Calculated cross section for 500MeV/c  in Argon gas. Note that this is a Log-log-log plot - the cross section varies over 20 and more decades!

Signals from Particles in Matter
Nov 2004 Signals from Particles in Matter Signals in particle detectors are mainly due to ionisation Gas chambers Silicon detectors Scintillators Direct light emission by particles travelling faster than the speed of light in a medium Cherenkov radiation Similar, but not identical Transition radiation

Nov 2004 Cherenkov Radiation (1) Moving charge in matter slow at rest fast

Nov 2004 Cherenkov Radiation (2) Wave front comes out at certain angle That’s the trivial result!

Nov 2004 Cherenkov Radiation (3) How many Cherenkov photons are detected?

Different Cherenkov Detectors
Nov 2004 Different Cherenkov Detectors Threshold Detectors Yes/No on whether the speed is β>1/n Differential Detectors βmax > β > βmin Ring-Imaging Detectors Measure β

Differential Detectors
Nov 2004 Differential Detectors Will reflect light onto PMT for certain angles only  β Selecton

Ring Imaging Detectors (1)
Nov 2004 Ring Imaging Detectors (1)

Ring Imaging Detectors (2)
Nov 2004 Ring Imaging Detectors (2)

Ring Imaging Detectors (3)
Nov 2004 Ring Imaging Detectors (3) More clever geometries are possible Two radiators  One photon detector

Nov 2004 Transition Radiation Transition radiation is produced when a relativistic particle traverses an inhomogeneous medium Boundary between different materials with different n. Strange effect What is generating the radiation? Accelerated charges

Nov 2004 Transition Radiation (2) Initially observer sees nothing Later he seems to see two charges moving apart  electrical dipole Accelerated charge is creating radiation

Nov 2004 Transition Radiation (3) Consider relativistic particle traversing a boundary from material (1) to material (2) Total energy radiated Can be used to measure γ