Experimental technique in subatomic physics

Sources of particles and their acceleration

1. Particle sources 2. Particle motion through electric and magnetic fields3. Accelerators4. Systems of accelerators

Interaction of radiation with matter

5. Introduction – types of interactions6. Passage of heavy charged particles through matter7. Passage of light charged particles through matter8. Passage of gamma rays through matter

Particle detectors

9. Introduction and their review10. Particle and photon detector11. Track detectors12. Detector systems13. 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 Decay half-life EMAX [MeV]

3H 12.26 years 0.0186 32P 14.28 days 1.710 90Sr/90Y 27.7 years/64 hours 0.546/2.27 99Tc 2.12∙105 years 0.292 204Tl 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] Branching

241Am 433 years 5.486 and 5.443 85% and 12.8% 210Po 138 days 5.305 100% 242Cm 163 days 6.113 and 6.070 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 0.511,1.27554Mn Electron capture 0.855 let 0.83560Co β- 5.27 let 1.173,1.333133Ba Electron capture 10.54 let 0.081,0.356137Cs β- 30.2 let 0.662207Bi 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.

EQdt

rdmF

2

2

e

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: QVQEdmv2

1mv

2

1E 2

02

KIN Δ

Homogenous magnetic field changes only direction of motion (of momentum vector) of charged particle. Lorentz force is: BvQ

dt

rdmF

2

2

m

If : Bv

QB

p

QB

mvrQvB

r

vm

2

motion on circle with radius r (centrifugal force is balanced by Lorentz force :

For angular velocity:m

QBQvBmv

r

vm

2

Mass is dependent on velocity for relativistic case: 2

2

0

cv

1

mm

General direction of velocity against direction of B → velocity decomposition:

andcosvv sinvv||

Projection of motion in the plane perpendicular on intensity of magnetic field– circle with radius:

cosQB

pcos

QB

mvr

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 – magnetic2) For detector systems – determination of charge,

momentum, mass of particle

Superconducting magnet of HADES spectrometerconstructed 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

Accelerator of Van de Graaff type (25URC Pelletron at Oak Ridge –USA)

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.

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.

Linear accelerator at CERN

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.

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.

Particlesource

cylindrical electrodes

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:

1

v

r

v

r22T

rr

Then it is valid:m

QB

We substitute and obtain: 222

KIN Br2m

QE

m

QB

r

v

It is valid for maximal energy: 22MAX

2MAXKIN BR

2m

QE

Protons with energy up to E ~ 15 MeV can be accelerated, ions with condition:

pm

e

m

Q

Microtron – accelerates electrons → early relativistic change of mass. We substitute:

20KIN

20KIN

20

2

cmE1

1

cm

QB

Ecm

QBc

m

QB

Orbital period:

20

KIN0

cm

E1

QB

m22T

Single acceleration supply energy m0c2 → phasing is conserved. Electron energies up to 20 MeV.

Principle of cyclotron. Historical WWW pages of the American Institute of Physics (AIP)

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 constantB) Proton synchrotron – velocity is changing in wide range → frequency of synchrotron is changing. Orbital radius is:

constB

v

cm

E1

Q

mr

20

KIN0

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 HERA DESY (Hamburg) 82 GeV LEP CERN (Schwitzerland) 92 GeV closed

Tunnel of accelerator TEVATRON 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 boundary2) Radial focusation – supports return of particles on stable path r0 with induction B(r0). Suitable course of magnetic induction B(r):

n

00 r

rrBrB

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.

Antiproton storage ring at FERMILAB

Sizes of large accelerators are given by available magnetic field intensity B ~ 2T for normal magnets a B ~ 9T for superconductive magnets.

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 interaction2) Hadrons – strong interaction3) 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: Indx

dEES ion

KINKIN

where nion is number of created pairs ion and electron and is mean energy needed for such pair creation ( this energy for heavy nuclei ~ 10∙Z [eV]).

I

Its nature is electromagnetic interaction. Formula for ionization losses was derived by H. Bethe a F. Bloch (Bethe-Bloch formula):

22

22e

22e

22

0

KINKIN I

c2mlnn

cm

ZeQ

4

1

dx

dE)S(E

where me is electron rest mass, β = v/c, γ = [1- β2]-1/2, and n is number of atoms in volume unit n = ρA0/A (ρ – density, A0 – Avogardo constant and A – atomic mass)

In the case v << c relativistic corrections can be neglected:

I

c2mlnn

cm

ZeQ

4

1

dx

dE)S(E

22e

22e

22

0

KINKIN

In this case: 2

220

2KIN

KIN p

m

v

1

dx

dE)S(E

where m0 is particle rest mass. Small velocities (γ =1) → for the same momenta S(EKIN) = f(m02).

1) Ionization quickly decreases with increasing velocity2) Minimum is in the range, where EKIN ≈ m0c2, γβ ≈ 3, β ≈ 0.97c3) Ionization increasing with further energy rising is more gradual

We can calculate range R of particles in matter using knowledge of ionization losses:

T

0 KIN

KINKIN

R

0

0

T KIN ES

dEdE

dE

dxdxR

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.

It occurs:

Passage of heavy charged particles through matter

1) ionization and excitation of atoms in matter – Bethe-Bloch formula ( even electrons capable of further ionization are created – δ electrons)2) elastic scattering – described by Rutheford equation:

QZe

bvm4

2cotg

20

Very small angles dominate:bmv4

QZe

2tan

2 20

222

0

22

bmv4

QZe

2min

2max

min

max

2

20

b

b2

bb22

0

2

b

b

b

b22

0

2

b

b

b

b

2

2

bb2

b

bln

mv

QZe

b

blnmv2

QZe

bdb2

dbb

1

mv4

QZe2

bdb2

bdb2b

max

min

max

min

max

min

max

min

max

min

max

min

Hence:

Number of scatters is done by number of atoms Na per volume unit, by layer thickness x and by cross section σ: 2

min2maxa

b

b

aaroz bbxNbdb2xNxNNmax

min

Mean quadratic deflection of multiple scattering is: 2roz

2 N

After substitution and modification:min

max22

222a

20min

max

2

20

a2

b

bln

vp

eZxQN

2

1

b

bln

mv

QZexN

2

1

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 distinguishedc) annihilation for positrons for EKIN < 100 MeV → S(EKIN)heavy ~ 1000∙S(EKIN)light for relativistic – difference smaller than 10 %

m

1

r

QZe

4

1

m

Fa

20

C

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:

and then: 2

2

radKIN m

Z~ES

a) Radiation losses are the largest for light particlesb) Radiation losses increase with Z of matter → large for heavy nuclei (big charge)

Critical EKIN → ionization losses equal to radiation losses

Radiation length X0 → EKIN = EKIN0/e by radiation

0

KIN

rad

KINradKIN X

E

dx

dE)S(E

0X

x

KIN0KIN0

KINKIN eEEX

dxEdE

and then

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

Quantum relativistic calculation: S(EKIN)rad ~ Z2EKIN

Radiation losses start with energy mec2 and higher critical EKIN increase linearly with EKIN

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:

nv

c

vt

tnc

cos n

1cos

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:

dsinc

Q

8

1d

n

11

Q

8

1dN 2

2

2

20

222

2

20

c

From this we obtain: 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

22

2

20

max n

11

Q

8

1N

cup 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).

Particle

Wavefront

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

x0eII(x)

where μ is absorption coefficient. Then we obtain classical formula:

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 27

5

h

Z~

for Eγ > mec2

h

Z~

5

γ e-

From momentum conservation law:

coshhcospcpcoscos

c

h0

c

h

sinhsinpcsinpsin

c

h0

We square and sum equations:

2222 hcoshh2hcp

From energy conservation law: EKIN = hν - hν’

Together it is valid: E2 = (m0c2 + EKIN)2 = m02c4 + p2c2

And then: p2c2 = EKIN2 + 2m0c2EKIN

We substitute:

hhcm2hhh2hcp 20

2222

and modify:

cos1cm

h1

hh

20

Compton scattering – photon scattering on electrons: Photon energy E = hν and momentum p = E/c = hν/c

e-

γ

e-

γ

reflected electron

targetelectron

scatte

red photon

incident photon

cos1hcm

cos1hE

20

2

KIN

energy of reflected electron:

minimal energy of scattered photon:2

0cmh

21

hh

photons are scattered to all angels, electrons only forward

For hν > m0c2 the cross section per atom is:

h

Z~

This process dominates in the 0.1 – 10 MeV energy range

Dependency of scattered photon energy Eγ on scattering angle ΘExample of cross section

dependency on photon energy

Photon energy

Cro

ss s

ecti

on

AngleS

catt

ered

ph

oton

en

ergy

e+

e-

γPair 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

2Z~

This process starts to predominate for Eγ≥ 10 MeV, for Eγ ≥ 100 MeV increasing of σ stopped.

Dependency of cross section on photon energy

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:

e+ + e- → γ + γ

photons have each energy 511 MeV (electron rest 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

Photon energy

Cro

ss s

ecti

on

Photon energy

Cro

ss s

ecti

on

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 energies4) 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:

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 voltageO

utp

ut

sign

al

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).

Schema of Geiger-Müller counters and its usage in dosimetry devices

Counter

Integrator

Amplifier

G.-M. tube

impulses

amplifiedimpulses

cathode

anode

Rays

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 schemeCombination 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.

Wilson chamber on PS (CERN – 1961)

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

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 systemsCommon 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 detectorMDC – drift chambersMAGNET – superconducting magnetTOF – time of flight wall from plastic scintillation detectorsSHOWER – shower detection – three chambers and between first and second is lead converter

Construction of HADES:

backside view - shower detectorsTOF wall and two segments of shower detectors

Insertion of Cherenkov detector and drift chambers

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.