Transcript Experimental technique in subatomic physics

Experimental technique in subatomic physics
Sources of particles and their acceleration
1. Particle sources
2. Particle motion through electric and magnetic fields
3. Accelerators
4. Systems of accelerators
Interaction of radiation with matter
5. Introduction – types of interactions
6. Passage of heavy charged particles through matter
7. Passage of light charged particles through matter
8. Passage of gamma rays through matter
Particle detectors
9. Introduction and their review
10. Particle and photon detector
11. Track detectors
12. Detector systems
13. Experiment control
Particle sources
Particles created by decay – are used for detector calibration but also for research
and applications (medical, material, …)
Secondary particles are created by reactions using accelerators – with high energy
Electron sources – 1) beta decay (continuous spectrum)
2) conversion electrons (discrete spectrum)
Examples of electron sources from beta decay:
Source
3H
32P
90Sr/90Y
99Tc
204Tl
Decay half-life
EMAX [MeV]
12.26 years
0.0186
14.28 days
1.710
27.7 years/64 hours
0.546/2.27
2.12∙105 years
0.292
3.81 years
0.766
Alpha sources – 1) alpha decay (discrete spectrum)
2) nuclear reaction (discrete spectrum)
Examples of alpha particle sources from decay:
Isotopes
T 1/2
Energy [MeV]
241Am
433 years
5.486 and 5.443
210Po
138 days
5.305
242Cm
163 days
6.113 and 6.070
Branching
85% and 12.8%
100%
74% and 26%
Charge of alpha particles is Z = 2 → high ionization losses and absorption during passage through
matter → alpha sources are given on underlay and conceal by extremely thin metal foil.
Gamma ray sources – 1) Gamma decay following beta decay (discrete spectrum)
2) Radiation produced during positron annihilation Eγ = 511 keV
3) Bremsstrahlung radiation
Examples of gamma ray sources:
Source
Decay type
Decay half-life
Energy [MeV]
22Na
β+, capture
2.603 let
0.511,1.275
54Mn
Electron capture
0.855 let
0.835
60Co
β-
5.27 let
1.173,1.333
133Ba
Electron capture
10.54 let
0.081,0.356
137Cs
β-
30.2 let
0.662
207Bi
Electron capture
31.8 let
0.57,1.06,1.77
Neutron sources – 1) Spontaneous nuclear fission
2) Induced nuclear fission, nuclear reactors
3) Nuclear reactions – connection of alpha decay and (α,n) reaction, of
gamma decay and reaction (γ,n)
4) Spallation reactions of relativistic protons with heavy nuclei
Example of neutron sources based on reactions:
Mostly alpha source and Be: Pu+Be, Am+Be
Source of nuclei (radioactive) – 1) nuclear fission – spontaneous and induced
2) nuclear reaction
3) spallation reaction
Ion source (for following acceleration)
Sources of antiparticles, strange baryons, mesons, mions, tauons … - exploitation of accelerators
and reactions of high energy particles with targets
Source of ultrarelativistic particles with minimal ionization (mions) – cosmic rays
Particle motion in electric and magnetic fields
Electric and magnetic field affect only motion of charged particles.
Homogenous electric field changes value of kinetic energy and momentum of charged particle.



d2 r
Fe  m 2  Q  E
dt
Component of velocity longitudinal with direction of electric field intensity is increasing.
Perpendicular component of velocity is not changed.
Potential difference V produces on distance d addition of EKIN:
ΔE KIN 
1
1
mv 2  mv 02  QEd  QV
2
2
Homogenous magnetic field changes only direction of motion (of momentum vector) of charged

particle. Lorentz force is: 
 
d2 r
Fm  m
 
v
If  B :
dt 2

 Q vB

motion on circle with radius r (centrifugal force is balanced by Lorentz force :
For angular velocity:
v2
mv
p
m
 QvB  r 

r
QB QB
2
v
QB
m  mv   QvB   
r
m
Mass is dependent on velocity for relativistic case:
m
m0
1
v2
c2
General direction of velocity against direction of B → velocity decomposition:
and
v||  v  sin 
v  v  cos 
Projection of motion in the plane perpendicular on intensity of magnetic field– circle with radius:
r
mv
p
cos  
cos 
QB
QB
Constant velocity of motion in the direction of B. Resulting motion on helix with axis in the direction
of B.
If intensity of electric and magnetic fields are mutually perpendicular and at the same time they are
perpendicular on the direction of charged particle velocity, we can create situation, when electric and
Lorentz force cancel together. It is valid for magnitude of forces:
Fe = Fm
We substitute:
QE = QvB
→ v = E/B
Device using this phenomena is named velocity filter.
The usage of magnetic and electric field:
1) For accelerators – for acceleration (mainly electric)
for guiding and focussation of beam – magnetic
2) For detector systems – determination of charge,
momentum, mass of particle
Superconducting magnet of HADES spectrometer
constructed at GSI Darmstadt. Produced magnetic
field serve for determination of momentum of
electrons and positrons from dileptone pairs.
Accelerators
An accelerator consists of an ion source and an acceleration system alone.
Ion source – produces electrons or ions, it gets atoms of electrons or put on electrons.
Acceleration system - accelerates obtained charged ions or electrons
Subdivision based on determination: 1) Electron accelerators
2) Proton and light ion accelerators
3) Heavy ion accelerators
Subdivision based on path form: 1) Linear
2) Circular (cyclic) – accelerated particles are kept on circular
path by magnetic field
Acceleration by passage through potential difference
Linear accelerators:
1) Electrostatic – consist of high voltage source and acceleration
tube.
Voltage source:
A) Cockroft-Walton generator – voltage sources are connected
with set of cylindrical electrodes accelerating tube →
acceleration only in the gap between electrodes. Maximal
energy ~ 4 MeV.
Accelerator of Van de Graaff type (25URC Pelletron at Oak Ridge –USA)
B) Van de Graaff generator – accumulation of charge by isolated belt on high voltage electrode
connected with acceleration tube. Maximal energy up to 10 MeV. Tandem accelerator up to 20
– 30 MeV. Special proton tandem even up to 60 MeV.
2) Highfrequency – it consists of acceleration tube with set of cylindrical electrodes connected
to source of HF voltage.
Particle
source
cylindrical electrodes
Constant frequency → passage through gap with suitable voltage.
Velocity increases → increasing of electrode length.
The biggest linear accelerator (3 km) is Linac at SLAC (USA) –
it accelerate electrons on 50 GeV energies.
Linear accelerator at CERN
Highfrequency accelerator with carrier wave: acceleration tube –
waveguide conduct electromagnetic wave abducting particle.
Used for electron acceleration. Maximal energy 1 GeV.
Circulator accelerator:
1) Betatron – inductive accelerator of electrons. Electrons on the path with constant radius
are accelerated by force of electromagnetic induction.
Construction: nucleus, coil of electromagnet on it, inside acceleration tube.
The biggest betatron – electron energies ~ 340 MeV, commonly – up to 50 MeV.
Often as sources bremsstrahlung radiation for technical and medical purposes.
2) Cyclotron – time constant magnetic field hold particles on circular orbit. HF field accelerates
particle during passing through gap between D-shaped electrodes. Passing through gap 2× during
one cycle, during passing through opposite part of gap – opposite polarity of electric field.
Frequency of electric field switching is constant, cycle is: T  2  2  r  r  1

vr
vr

Then it is valid:   QB
m
v QB
Q2 2 2

 E KIN 
r B
r
m
2m
We substitute and obtain:
It is valid for maximal energy:
E
MAX
KIN
Q2 2

R MAX B2
2m
Protons with energy up to E ~ 15 MeV can be accelerated, ions
Q
e
with condition:

m
mp
Principle of cyclotron. Historical
WWW pages of the American
Institute of Physics (AIP)
Microtron – accelerates electrons → early relativistic change of mass. We substitute:

Orbital period:
T
2


QB
QBc 2
QB
1


2
2
m
m 0 c  E KIN m 0 c 1  E KIN m 0 c 2
2  m0  E KIN 
1 

QB  m0c 2 
Single acceleration supply energy m0c2 → phasing is conserved. Electron energies up to 20 MeV.
Synchrocyclotron (phasotron) – classical cyclotron during start of acceleration. Latter relativistic
increasing of accelerated particle mass → decreasing (supermodulation) of HF generator
frequency. Limitation given by magnet size. One from largest is at JINR Dubna –
E= 680 MeV for protons. Magnet has mass 7 000 tun and volume of vacuum space is 35 m3.
3) Synchrotron – intensity of magnetic field is changing. Orbital radius stays constant.
A) Electron synchrotron – for electrons v  c → frequency of synchrotron stays constant
B) Proton synchrotron – velocity is changing in wide range → frequency of synchrotron is
changing. Orbital radius is:
m0  E KIN  v
r
1 
  const
Q  m0c 2  B
Work in strobe like mode. The biggest proton synchrotron with weak focusation –
synchrophasotron at JINR Dubna (protons up to 10 GeV) – beam diameter a few cm.
Synchrotrons with strong focusation – beam diameter a few mm.
Acceleration tube
Quadrupol magnet
For synchrotron, acceleration tubes and focusing magnets alternate:
Schema of synchrotron with
strong focusation at CERN
Synchrotron – the biggest accelerators, diameters up to tenths km.
The biggest accelerators (strong focusation) are now:
Proton:
TEVATRON
FERMILAB (USA)
1000 GeV
HERA
DESY( Hamburg)
820 GeV
SPS
CERN (Schwitzerland)
450 GeV
LHC
CERN (Schwitzerland) 7 000 GeV
(in the construction)
Electron:
SLC
SLAC (USA)
50 GeV
Tunnel of accelerator TEVATRON
HERA
DESY (Hamburg)
82 GeV
LEP
CERN (Schwitzerland)
92 GeV closed at FERMILAB (Batavia, Ilinois,USA)
Focusation – keeps of losses of beam particles during acceleration. Focusation acts in two
directions:
1) Axial focusation – it is holding particle in the plane – achieved by form of magnetic field –
it is weaker on the boundary
2) Radial focusation – supports return of particles on stable path r0 with induction B(r0). Suitable
course of magnetic induction B(r):
r
Br   Br0  
 r0 
n
where n is field index and for radial focusation 0 < n < 1. This is weak focusation.
Phase stability – synchronization of particle motion with frequency of accelerating voltage is very
important. Such setting – behavior of HF field gives to particle right energy to be near to ideal
phasing:
1) Particle comes in the right time t0 → field is E0
2) Particle comes early t < t0 → field is E < E0 → decreasing of particle
3) Particle comes later t > t0 → field is E > E0 → increasing of particle
Strong focusation – strong forces are necessary. Accelerator is split to even number of sectors.
Magnets excite together with homogeneous field also inhomogeneous field with large field index n ~
300. Field indexes and gradients are in turn positive and negative → alternately radial focusation
and axial defocusation and vice-versa.
Stochastic cooling – information about
particle position is sending directly through
center of circle to the other side and before
accelerated particle coming HF is ready to
correct their transverse position, do not escape
from the beam.
Sizes of large accelerators are given by available
magnetic field intensity B ~ 2T for normal
magnets a B ~ 9T for superconductive magnets.
Antiproton storage ring at FERMILAB
Systems of accelerators
Achieving of still higher energies → construction of systems of accelerators and storage rings
System of accelerators at CERN (Switzerland)
View on placement of accelerator complex at CERN
Colliding beams – maximal value of available energy is in the centre of mass. For beam with energy
450 GeV:
1) fixed target – 29 GeV
2) colliding beams 900 GeV
Secondary beams – meson factories, interaction of primary particles on target. Secondary particles
are focused, formed and eventually further accelerated
Radioactive beams – production of radioactive nuclei and their follow-up acceleration
Luminosity: characterizes beam intensity of accelerator. Units [cm-2s-1]. Maximal present
values ~ 1033 cm-2s-1.
Introduction – types of interactions
Charged or neutral particle passage through matter → interaction of particle and matter.
1) Charged – electromagnetic interaction
2) Hadrons – strong interaction
3) Neutrina – only weak interaction
A) Charged particles – electric charge is interacting with atoms of matter → escape of electrons
from atomic shell → ionization losses → deceleration.
B) Gamma rays – without charge. They interact with electrons or Coulomb field of nucleus by
three processes (photoeffect, Compton scattering, pair production)
C) Neutrons – during reactions with nuclei (strong interaction) further particles (also charged) are
emmited
D) Neutrina – only weak interaction → only very small cross sections of interaction with matter.
These interactions, which convert kinetic particle energy to electrons created by ionization, make
possible detection of these particles.
Passage of charged particles through matter:
Quantity, which describes ionization properties of given material, is ionization losses (stopping
power) S(EKIN) = -dEKIN/dx, defined as amount of kinetic energy loosed by particle per unit of
path through matter:
dE KIN
SE KIN   
dx
 n ion I
where nion is number of created pairs ion and electron and I is mean energy needed for such
pair creation ( this energy for heavy nuclei ~ 10∙Z [eV]).
Its nature is electromagnetic interaction. Formula for ionization losses was derived by H. Bethe a F.
Bloch (Bethe-Bloch formula):

dE KIN
1 Q 2 Ze 2   2m e  2 c 2 2 
2
S(E KIN )  
dx

n ln 
40 m e  2 c 2  
I
    


β2]-1/2,
where me is electron rest mass, β = v/c, γ = [1and n is number of atoms in volume
unit n = ρA0/A (ρ – density, A0 – Avogardo constant and A – atomic mass)
dE KIN
1 Q 2 Ze 2
In the case v << c relativistic corrections can be neglected: S(E KIN )  

dx
40 m e  2 c 2
2 2
dE
1 m
S(E KIN )   KIN  2  0 2
In this case:
dx
v
p
  2m e  2 c 2 

n ln 
I

 
where m0 is particle rest mass. Small velocities (γ =1) → for the same momenta S(EKIN) = f(m02).
1) Ionization quickly decreases with increasing velocity
2) Minimum is in the range, where EKIN ≈ m0c2, γβ ≈ 3, β ≈ 0.97c
3) Ionization increasing with further energy rising is more gradual
R
0
T
dE KIN
dx
We can calculate range R of particles in matter
R   dx  
dE KIN  
dE KIN
SE KIN 
using knowledge of ionization losses:
0
T
0
For low energies and for the same EKIN of two particles → strong dependency on R is visible. It
decreases for high energies. The largest part of energy is released on the end of path (v<<c). Bragg
curve.
Passage of heavy charged particles through matter
It occurs: 1) ionization and excitation of atoms in matter – Bethe-Bloch formula ( even electrons
capable of further ionization are created – δ electrons)
2
2) elastic scattering – described by Rutheford equation: cotg    40 m v b
2
Very small angles dominate:
b max
Hence:
2 
  b2bdb

QZe
 
 tan   
2
2
 2  40 mv b
2 QZe 
2
2
b min
b max


4 0 mv
 2bdb
2
b min

2 2
1
b b db
min
b max
 
2
 QZe 2
b max



 b 
2 0 mv
2 2
ln b
b max
b min
2 b max
b min
 bdb
QZe

QZe2

4 0 mv 2 b 2
2
2
 QZe 
b max


ln
0 mv 2 
b min


2 b 2max  b 2min


b min
Number of scatters is done by number of atoms Na per volume unit, by layer thickness x and
b
by cross section σ:
max

N roz  N a x  N a x  2 bdb  N a x b 2max  b 2min

b min
 2  N roz 2
Mean quadratic deflection of multiple scattering is:
2
 QZe 
b max
1
1 N a xQ 2 Z 2 e 2 b max
2
 ln

ln
After substitution and modification:   N a x 
2 
2
2 2
2

mv
b
2

p
v
b min
min
0
 0

Path of particle is therefore crinkle, beam diverges. Heavy particles have small crinkle - range is
very well defined.
Passage of light charged particle through matter
Passage of electrons and positrons through matter:
1) Ionization and excitation of atoms – Bethe-Bloch formula has within parenthesis different form
than for ionization losses for heavy particles:
a) electron can transfer during collision large part of energy
b) exchange effects – impinging and impacted electrons can not be distinguished
c) annihilation for positrons
for EKIN < 100 MeV → S(EKIN)heavy ~ 1000∙S(EKIN)light
for relativistic – difference smaller than 10 %
2) Bremsstrahlung – if motion of charged particle is not uniform rectilinear → emission of
electromagnetic radiation → particles loose energy – radiation losses. In classical
approximation losses are proportional to acceleration S(EKIN)rad ~ a2. In the case of Coulomb
interaction:
FC
1
QZe 1
a 

m
4 0 r 2 m
and then:
SE KIN rad
Z2
~ 2
m
a) Radiation losses are the largest for light particles
b) Radiation losses increase with Z of matter → large for heavy nuclei (big charge)
Critical EKIN → ionization losses equal to radiation losses
Quantum relativistic calculation: S(EKIN)rad ~ Z2EKIN
Radiation losses start with energy mec2 and higher critical EKIN increase linearly with EKIN
Radiation length X0 → EKIN = EKIN0/e by radiation
and then
E
 dE 
S(E KIN ) rad    KIN   KIN
dx  rad
X0

x

dx
X0
dE KIN  E KIN
 E KIN  E KIN0e
X0
Dependency of EKIN on thickness absorbing material → exponential law
Electrons are strongly scattered because of small mass , big radiation losses → well defined range
does not exist
Very high energy → radiation losses → creation of high energy photons
high energy photons → creation of electron and positron pairs
Creation of electromagnetic shower
Cherenkov radiation – particle velocity in material v > c’ = c/n
(n – index of refraction) → irradiation of Cherenkov light:
c
t
c
n
cos  

vt nv
cos  
Particle
1
n
Wavefront
From this equation we derive :
Threshold velocity exists βmin = 1/n. For βmin emission is directed in the direction of particle motion.
For lower velocity emission does not arrive.
For ultrarelativistic particles cosΘmax = 1/n.
For water: n = 1.33 → βmin = 0.75, for electron EKIN = 0.26 MeV
cosΘmax = 0.75 → Θmax= 41.5o
Number of photons N(ν) in the
interval from ν up to ν+dν:
From this we obtain:
N d 
1

2
80
Q2 
1 
1 Q2


1

d


 2 sin 2 d
2 
2 2 
2
c  n  
80 c
1) Spectrum is same for particles with the same charge Q.
2) N(ν) is changing with β from Nmin(ν) = 0 for βmin = 1/n
Q2 
1 
N max   

1



802 c 2  n 2 
1
up to
for β →1
N(ν) is independent on ν → dN(ν) ~ dν.
Spectrum is continuous
Usage: velocity determination, threshold detectors (separation of fast and slow particles).
Passage of gamma rays through material
Photons are neutral but they interact with material by electromagnetic interaction → they loose
energy.
Absorption of radiation at material → change of intensity I:
dI = I(x+dx) – I(x) = - μI(x)dx
where μ is absorption coefficient. Then we obtain classical formula:
I(x)  I 0 e   x
Three specific processes contribute to the absorption:
Photoeffect – whole energy of gamma photon is transferred to electron. Photon kinetic energy is split
to kinetic energy of electron EKINe and energy of its binding in atom (ionization potential) of i shell Ii:
EKINe = h ν - Ii
(Ii < 0)
Cross sections of this process:
for Eγ < mec2
for Eγ > mec2
~
Z5
h 
Z5
~
h
72
γ
e-
Compton scattering – photon scattering on electrons:
Photon energy E = hν and momentum p = E/c = hν/c
From momentum conservation law:
h
h 
0
cos   pcos  pc cos   h  h  cos 
c
c
h 
0
sin   p sin   pc sin   h  sin 
c
incident photon
We square and sum equations:
p 2 c 2  h   2h h  cos   h 
2
2
target
electron
reflected electron
From energy conservation law: EKIN = hν - hν’
E2 = (m0c2 + EKIN)2 = m02c4 + p2c2
Together it is valid:
And then:
p2c2 = EKIN2 + 2m0c2EKIN
We substitute:
p2c2  h   2h h   h   2m0c2 h  h 
2
and modify:
h  
2
h
h
1  cos  
1
m 0c 2
eγ
eγ
minimal energy of scattered photon:
energy of reflected electron:
h  
h
h
1 2
m 0c 2
2

h  1  cos  
E KIN 
m 0 c 2  h 1  cos  
photons are scattered to all angels, electrons only forward
For hν > m0c2 the cross section per atom is:
~
Z
h
Photon energy
Example of cross section
dependency on photon
energy
Dependency of
scattered photon
energy Eγ on scattering
angle Θ
Scattered photon energy
Cross section
This process dominates in the 0.1 – 10 MeV energy range
Angle
Pair production – possible only for these conditions:
1) Energy hν > 2×me0c2 ~ 1.022 MeV.
2) Only in the matter – part of momentum is
transferred to nucleus
e+
γ
e-
 ~ Z2
After their creation, positrons loose energy by ionization and
bremsstrahlung radiation as electrons. After loose of EKIN, positron is
captured by electron – positronium creation (τ = 10-10s) → annihilation:
Cross section
This process starts to predominate for Eγ≥ 10 MeV, for Eγ ≥ 100 MeV increasing of σ stopped.
e+ + e- → γ + γ
Photon energy
Three mentioned processes give independent contributions to
photon absorption:
μ = μfe + μComp + μpar
For very high energies of photons or electrons:
γ → creation of e+e-→ bremsstrahlung γ → creation
of e+e-→ bremsstrahlung γ → …
electromagnetic shower is created
Dependency of cross section
on photon energy
Cross section
photons have each energy 511 MeV (electron rest energy)
Photon energy
Introduction – review of detectors
Experiments depend on detection and determination of particle characteristic. Detection is enabled
by particle interaction with matter. Part or whole kinetic energy is changed to other form. In
modern experiments mostly to electric voltage or current signal on the end.
Division of detectors into:
1) Counters – electric signal during particle passage (can depend on its energy, charge, …)
2) Track detectors – trace particle tracks
Quantities characterizing detector:
1) Sensitivity – capability to produce measurable signal for given particle type and energy.
It depends on: 1) cross sections of ionizing reactions, 2) detector mass, 3) detector noise and
4) its thickness and type of material surrounding sensitive volume of detector
2) Response – dependency between particle energy and detector output (total charge or amplitude of
current pulse).
3) Response function – spectrum of monoenergetic beam is observed by detector as complicated
function mostly near to Gauss function with tail to lower energies
4) Death time – time necessary for creation and processing of signal at detector.
5) Detection efficiency – ratio between number of particles detected and emitted by source –
absolute efficiency. It consists of intrinsic efficiency and geometrical efficiency (acceptance).
6) Energy resolution – the smallest distinguishable energy difference ΔE between two near energies.
Monoenergy beam → ideally δ-function – really peak with finishing width (mostly it has
Gaussian form. Resolution is mostly given in form of halfwidth – FWHM). Relative resolution
ΔE/E at [%] is used.
7) Time resolution – the smallest distinguishable difference of time – definition similar as for energy
8) Spallation resolution – the smallest distinguishable difference of tracks – definition similar as for
previous
Detectors of particles and photons
A) Gas filled (ionization) detectors:
measure ionization produced by passage of charged particle through matter. Electric field →
electron-ion pairs are not recombined → they drift to electrodes → number of pairs is proportional
to transferred energy → electric signal is proportional to transferred energy
Detector construction: 1) Chamber filled by easy ionizing material
2) Cathode and anode and HV between them
Dependency of current on voltage:
Output signal
I) range of Ohm´s law (recombination range) – ionization of gas, but ions mostly disappeared by
recombination
II) ionization range – all ions are collected on electrodes, only minimal recombination - ionization
chambers
III) proportional range – impact ionization starts act,
created ions are accelerated enough for further ionization
– proportional counters
IV) Geiger range – every primary ionization leads to big
current increase – Geiger-Müler counters
V) discharge range – discharge occurs
High voltage
1) Ionization chambers – they work with lower HV value → they do not amplification → small
output signal – they are better for fragments with larger charge. They are working also for high
radiation intensities.
2) Proportional counters – cylindrical cathode is around thin wire anode, factor of
amplification 105, signal is big enough also for particles with minimal ionization ( 1 – 10 mV).
3) Geiger-Müller counters – discharge occurs, necessity of its quenching, always high pulse
~1.6 V, factor of amplification 1010, lowly sensitive to voltage changes. Disadvantages: signal
does not depend on type and energy of particle, long time of regeneration ( ~ 1 ms).
G.-M. tube
anode
amplified
impulses
impulses
Counter
Rays
cathode
Amplifier
Integrator
Schema of Geiger-Müller counters and its usage in dosimetry devices
B) Solid state detectors:
Scintillation detectors: ionization excites atoms and molecules → during deexcitation light is
produced → light is changed to electric impulse by photomultipliers (amplification ~ 104 – 107). It
is needed ~ 10 times more energy per photon then for electron-ion pair.
Two types of scintillation materials:
1) Anorganic – BaF2, BGO, CsI, NaI conversion decay
constant (~ 10-6 s)
2) Organic – plastic scintillator – fast decay constant (~ 10-8 s)
Photomultiplier scheme
Combination of different scintilators – conversion decay constant is different for different
particles → pulse form analysis → particle identification.
Very good time resolution ~ 0.2 ns (for v = c spatial resolution 6 cm) → frequent usage for TOF
(time of flight) methods – start - start detector, beam detector or cyclotron frequency.
scintillation detectors for TOF wall of HADES spectrometer
(plastic material of Bicron company)
3) Semiconductor detectors – creation electron hole pair ~ 3 eV → big signal also for small
transferred energy. Output signal proportional to ionization losses → particle energy. Very
good energy resolution. The used materials – silicon and germanium.
Cooling by liquid nitrogen. Very good detectors for determination of energy of low energy
photons and electrons.
Recently as position sensitive detectors – thin silicon wafers ( ~ 200 – 300 μm). → SSD –
silicon strip detectors and SDD – silicon drift detectors.
EUROGAM II detector system
Position sensitive silicon drift detector
C) Cherenkov detectors:
They use Cherenkov phenomena for particle velocity determination, they work as threshold
detectors.
Schema of Cherenkov detector
Mirror of the Cherenkov detector
of HADES spectrometer
Reading electronics for photon
detectors detecting light rings of
the created Cherenkov radiation
D) Calorimeters – devices, which absorbs total particle energy and their output
is proportional to this energy. Based on shower creation (electromagnetic or
hadron). Nuclear interaction → smaller σ → hadron shower is longer →
hadron calorimeter is bigger than electromagnetic. Types of calorimeters: 1)
homogenous – whole volume is sensitive 2) consisted alternately of converter
(shower is developed by it – iron, lead) and of sensitive volume (for example
lead glass).
Calorimeter of NA49 experiment (CERN)
Track detectors
Ionization changes state of chamber content → visible tracks
Nuclear photoemulsion – higher content of bromide (up to 85%), thicker layers, bigger sensitivity.
Often for cosmic ray studies.
Cloud chamber - closed volume filled by gas and ingredient of saturated steam. Passage of
charged particle + supersaturated steam → condensation of vapor on ions → photography of
illuminated trace from droplets. Against of obtaining of saturated steam: expansion (Wilson) and
diffusion. Placement to magnetic field.
Bubble chambers – basin with liquid nearly below boiling point → charged particle + superheated
liquid → boil in ion neighboring → photography of illuminated bubbles. Simultaneously target and i
detector. Contents for example liquid hydrogen, deuterium, propane, xenon or Freon. Placement
to magnetic field. Position resolution ~ 200 μm.
Bubble chamber Gargamel (CERN) Reaction photography from v bubble chamber
Wilson chamber on PS
(CERN – 1961)
Spark chambers – registration of spark discharge created by ionization in the field produced by
HV of two electrodes. Contained of some thin conductive plates alternately grounded and on high
potential. Fill is inert gas.
Discharge (streamer) chambers – modification of spark chamber. Only two electrodes (spacing ~
50 cm). Very short HV pulses (~ 20 ns) on them. Spark discharge is quickly stopped – plasma
cloudlet is created → light point. This is photographed.
Pictures from streamer chamber: S+AU on SPS and
anti-p+Ne on LEAR (CERN)
Electronic registration of particle track:
Proportional chambers
Drift chambers – charged electrons and ions created by
ionization drift in strong electric field. Position is done
by electrode to which drift and by drift time (constant
velocity of drift is assumed). Path of ionizing particles
in space can be determined.
Reconstruction of Pb+Pb collision
Drift chamber of NA49 experiment (CERN)
Complicated detector systems
Common detection of big number of different particles and determination of their characteristics –
systems of big of detectors of different types.
Example of setup for highenergy experiments:
Beam detectors - start detectors – track detectors in target surroundings (SSD and SDD) – drift
chambers – superconducting magnet – drift chambers – shower detectors – TOF walls
from plastic scintillation detectors – calorimeters.
Setup of dilepton spectrometer HADES:
RICH – Cherenkov detector
MDC – drift chambers
MAGNET – superconducting magnet
TOF – time of flight wall from plastic scintillation detectors
SHOWER – shower detection – three chambers and between first
and second is lead converter
Construction of HADES:
backside view - shower detectors
Insertion of Cherenkov detector
and drift chambers
TOF wall and two segments of shower
detectors
Electronic control of experiment
Big number of detectors, big number of data → electronic acquisition and analysis of data → from
different signals (pulses) produced by detectors energy information, relative time differences must
be obtained → conclusion about event rejection or event taking.
Electronics for signal processing: mostly weak signal → preamplifiers and amplifiers. they can be
used also for pulse shaping. Dividing (splitter) to energy (analog form of pulse) and time (digital
form of pulse) lines.
Analog forma – pulse hold continuous information in form of continuous change of some of its
characteristic
Digital (logic) form – discrete values of some quantities hold transferred information
Conversion of analog signal to digital one and vice versa is made by appropriate converters
Fast signals – time of signal increasing in the range of few ns.
Slow signals – time of signal increasing in the range of hundreds and more ns.
Standardization of logical signals (NIM, ECL, …)
Discriminators – create signal only if voltage of input pulse will be
higher then given value.
Coincidence technique, amplitude discriminators:
Using coincidence units, and delay lines, logic signals are analyzed and
logical circuits make possible creation triggers (rules for event selection).
These blocks made possible also creation of right timing for starting of data
read out and .
Computer controlled electronics make possible data acquisition, on line monitoring and their
preliminary analysis. Detector control and operating and also of line data analysis are done by
computers.