interactions of particles with matter - department of physics...nov 2020 a. weber 5 bethe-bloch (1)...
TRANSCRIPT
Interaction of Particles
with Matter
Alfons Weber
STFC/RAL & University of
Oxford
Graduate Lecture 2020
Nov 2020 A. Weber 2
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 2020 A. Weber 3
Nov 2020 A. Weber 4
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 2020 A. Weber 5
Bethe-Bloch (1)
Consider particle of charge ze, passing a
stationary charge Ze
Assume
Target is non-relativistic
Target does not move
Calculate
Momentum transfer
Energy transferred to target
ze
Ze
br
θx
y
Nov 2020 A. Weber 6
Bethe-Bloch (2)
2
0
1
2x
Zzep dtF
c b
Force on projectile
Change of momentum of target/projectile
Energy transferred to target
2 23
2 2
0 0
cos cos4 4
x
Zze ZzeF
r b
2 2 2 4
2 2 2
0
1
2 2 (2 ) ( )
p Z z eE
M M c b
Nov 2020 A. Weber 7
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
2 2 2 4 2
2 2 2
0
1
2 2 (2 ) ( )
p Z z e ZE
M M c b M
2 4
2 2 2 2
0
2 1( )
(4 )e
e
z eE b E
m c b
Nov 2020 A. Weber 8
Bethe-Bloch (4)
Energy transfer is determined by impact
parameter b
Integration over all impact parameters
bdb
ze
2 (number of electrons / unit area )
=2 A
dnb
db
Nb Z x
A
Nov 2020 A. Weber 9
Bethe-Bloch (5)
Calculate average energy loss
There must be limits
material dependence is in the calculation
of the limits
max
max
min
min
max
min
2 2
2
2 2
2
2
2
0
dd ( ) 2 ln
d
ln
with 24
bbe
e b
b
Ee
E
A
e
m cn ZzE b E b C x b
b A
m c ZzC x E
A
eC N
m c
Nov 2020 A. Weber 10
Bethe-Bloch (6)
Simple approximations for
From relativistic kinematics
Inelastic collision
Results in the following expression
min 0 average ionisation energyE I
2 2 2 22
2
0
22 lne em c m cE ZzC
x A I
2 2 22 2 2
max 2
22
1 2
ee
e e
m cE m c
m m
M M
Nov 2020 A. Weber 11
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 (polarisation in medium)
2 2 2 222max
2 2
0
21 ( )2 ln
2 2 2
e em c m c EE ZzC
x A I
Nov 2020 A. Weber 12
Energy Loss Function
/ stopping powerE
x
Nov 2020 A. Weber 13
Average Ionisation Energy
Nov 2020 A. Weber 14
Density Correction
Density Correction does depend on
material
with
x = log10(p/M)
C, δ0, x0 material dependant constants
Nov 2020 A. Weber 15
Different Materials (1)
Nov 2020 A. Weber 16
Different Materials (2)
Nov 2020 A. Weber 17
Particle Range/Stopping Power
Nov 2020 A. Weber 18
Energy-loss in Tracking Chamber
Nov 2020 A. Weber 19
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
Nov 2020 A. Weber 20
Straggling (2)
Simple parameterisation
Landau function
Better to use Vavilov distribution
2
2
1 1( ) exp ( )
22
with e
f e
E E
m c ZzC x
A
Nov 2020 A. Weber 21
Straggling (3)
Nov 2020 A. Weber 22
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 2020 A. Weber 23
Restricted dE/dx
Some detector only measure energy loss
up to a certain upper limit Ecut
Truncated mean measurement
δ-rays leaving the detector
2 2 2 22
2 2
0
2
max
212 ln
2
( ) 1
2 2
cut
e e cut
E E
cut
m c m c EE ZzC
x A I
E
E
Nov 2020 A. Weber 24
Electrons
Electrons are different light
Bremsstrahlung
Pair production
Nov 2020 A. Weber 26
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 2020 A. Weber 27
Multiple Scattering
Particles don’t only loose energy …
… they also change direction
Nov 2020 A. Weber 28
MS Theory
Average scattering angle is roughly
Gaussian for small deflection angles
With
Angular distributions are given by
0
0 0
0
13.6 MeV1 0.038ln
radiation length
x xz
cp X X
X
2
2 2
0 0
2
2
00
1exp
2 4
1exp
22
space
plane
plane
dN
d
dN
d
Nov 2020 A. Weber 29
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
Nov 2020 A. Weber 30
Correlations (W. Allison)
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!
log kL
2
18
17
7
log kT
whole
atoms at
low Q2
(dipole
region)
electrons
at high
Q2
electrons
backwards in
CM
nuclear small angle
scattering (suppressed
by screening)
nuclear backward
scattering in CM
(suppressed by nuclear
form factor)
Log pL or
energy transfer
(16 decades)
Log pT transfer
(10 decades)
Log cross
section
(30
decades)
Nov 2020 A. Weber 31
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 2020 A. Weber 32
Moving charge in dielectric medium
Wave front comes out at certain angle
Cherenkov Radiation
1cos c
n
slow fast
Nov 2020 A. Weber 33
Cherenkov Radiation (2)
How many Cherenkov photons are
detected?2
2
2
2
2 2 2
0 2 2
( )sin ( )d
1( ) 1 d
11
with ( ) Efficiency to detect photons of energy
radiator length
electron radius
c
e e
e e
e
zN L E E E
r m c
zL E E
r m c n
LNn
E E
L
r
Nov 2020 A. Weber 34
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
Nov 2020 A. Weber 35
Threshold Counter
Particle travel through radiator
Cherenkov radiation
Nov 2020 A. Weber 36
Differential Detectors
Will reflect light onto PMT for certain
angles only β Selection
Nov 2020 A. Weber 37
Ring Imaging Detectors (1)
Nov 2020 A. Weber 38
Ring Imaging Detectors (2)
Nov 2020 A. Weber 39
Ring Imaging Detectors (3)
More clever geometries are possible
Two radiators One photon detector
Nov 2020 A. Weber 40
Transition Radiation
Transition radiation is produced, when a
relativistic particle traverses an
inhomogeneous medium
Boundary between different materials with
different diffractive index n.
Strange effect
What is generating the radiation?
Accelerated charges
Nov 2020 A. Weber 41
22 vq
vacuummedium
Before the charge crosses
the surface,
apparent charge q1 with
apparent transverse vel v1
After the charge crosses
the surface,
apparent charges q2 and q3
with apparent transverse
vel v2 and v3
11 vq
33 qv
Transition Radiation (2)
Nov 2020 A. Weber 42
Transition Radiation (3)
Consider relativistic particle traversing a
boundary from material (1) to material (2)
Total energy radiated
Can be used to measure γ
22 2
2
2 2 2 2 2 2 2
d 1 1
d d / 1/ 1/
plasma frequency
p
p
N z
Nov 2020 A. Weber 43
Transition Radiation Detector
Nov 2020 A. Weber 44
ATLAS TRTracker
ATLAS
ExperimentInner Detector:
pixel, silicon and straw tubes
Combination of Central Tracker and
TR for electron identification
Nov 2020 A. Weber 45
Atlas TRT (II)
Nov 2020 A. Weber 46
Atlas TRT (III)
TRT senses
ionisation
transition radiation
only electron produce
TR in radiator
e± / π separationElectrons with radiator
Electrons without radiator
Bod -> J/yKo
s
High threshold hits
Nov 2020 A. Weber 47
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 2020 A. Weber 48
Bibliography
This lecture https://www2.physics.ox.ac.uk/contacts/people/weber
PDG online: Experimental Methods https://pdg.lbl.gov/2020/reviews/contents_sports.html
Passage of particles through matter
Particle detectors …
References therein, especially Rossi
Lecture notes of Chris Booth, Sheffield http://cbooth.staff.shef.ac.uk/phy6040det/
Or just it!