General Characteristics of Gas Detectors

Signal Creation

Charged particles traversing matter leaving excited atoms, electron or holes and ions behind. These can be detected using either excitation or ionization.

ExcitationPhotons emitted by excited atoms can be detected by photomultipliers or semiconductor photon detectors

IonizationIf an electric field is applied in the detector volume, the movement of the electrons and ions induces a signal on metal electrodes. Signals are read out using appropriate readout electronics

Signal Induction

A point charge above a grounded metal plate induces a surface charge.

Total induced charge –q.

Different charge positions results in different charge distributions but the total charge stays –q.

-q

-q

q

q

Signal Induction for Moving Charges

-q

-q

q

q

Segment the grounded metal plate into grounded individual strips.The surface charge density from the moving charge does not change with respect to the infinite metal plate.

The charge on each strip depends on the charge position.

If the charge is moving, current flows between the strips and ground.

Signal Theorems

Placing charges on a metal electrodes results in certain potentials. A different set of charges results in a different set of potentials.

Reciprocity Theorem

Signal Theorems

Charge induced on an electrode is independent of the actual path.

Once all charges have arrived at the electrodes, the total induced charge in the electrodes is equal to the charge that has arrived at this electrode.

If one electrode encloses all the others, the sum of induced currents is always zero.

Charge Generation in a Gas

Amount of ionization produced in a gas is not very great.

A minimum ionizing particle (m.i.p.) typically produces 30 ion pairs per cm from primary ionization in commonly used gases (e.g. Argon)

The total ionization is ~100 ion pairs per cm including the secondary ionization caused by faster primary electrons.

Primary ionization

Secondary ionization

Charge Collection

Electric Field

Cathode

Anode

Charge is produced near the track.

Apply an electric field to move charge to electrodes.

Charge is accelerated by the field, but loses energy through collisions with gas molecules.Overall, steady drift velocity of electrons towards anode and positive ions towards the cathode.

Ion Mobility

Ions drift slowly because of their large mass and scattering cross-section. Similar spectrum to the Maxwell energy distribution of the gas molecules.Average drift velocity (W+) increases with the field strength (E) and decreases as the gas pressure, P, increases.

A pressure increase leads to a shorter mean free path (distance during which an ion is accelerated before losing its energy in a collision).The ion mobility, μ+, defined as μ+=W+(P/E), is constant for a given ion type in a given gas.

Electron Drift Velocity

The dependence of the electron drift velocity on the electric field varies with the type of gas used.

Electron Diffusion

Electron longitudinal (dashed) and tranverse (solid) diffusion.

Ionization Chamber Geometry

Anode

Cathode

Circular Cathode

Anode wire

Parallel Plate Ionization Chamber

Cylindrical Ionization Chamber

Charge Multiplication

At sufficiently high electric fields (100kV/cm) electrons gain energy in excess of the ionization energy, which leads to secondary ionization, etc.

Townsend Coefficient

Amplification/Gas Gain

Townsend Coefficient

Computed values of the Townsend coefficient as a function of the electric field for different gases.

Avalanche

Anode wire

Electrons: close to the wire

Positive Ions

Number of electrons and ions increases exponentially and quickly forms an avalanche.

Electrons move more rapidly than ions and development a tight bunch at the head of the avalanche.

Ions move significantly more slowly and have typically not moved from their original position when the electrons reach the anode.

Types of Avalanches

Proportional region: A=103-104

Semi-proportional region: A=104-106

Saturation region: A > 108

(independent of the number of primary electrons)

Streamer region: A > 107

Avalanche along the particle track

Limited Geiger region: Avalanche propagated by UV photons

Geiger region: A = 108

Avalanche along the entire wire

LHC

1970’s

1950’s

Proportional Counters

Space charge effects arise when the electron and ion density is so large that they modify significantly the electric field locally and reduce the ionization probability.

For low gains, this is unimportant and the size of the signal charge is proportional to the initial ionization. A detector operated in such a way is called a proportional counter.

Time Development of a Signal

Electron avalanche occurs very close to the wire, with first multiplication occuring ~2x the wire radius.

Electron move to the wire surface very quickly (<<1ns), but the ions drift to the tube wall more slowly (~100 μs).

Characterized by a fast electron spike and a long ion tailTotal charge induced by the electrons amount to only ~1-2 % of the total charge.

Properties of Gases

Properties of commonly used gases

2.2 Transport Parameters of Operational Gas Mixtures

Introduction

Particle physics experiments rely on the detection of charged and neutral particles by gaseous electronics

A suitable gas mixture within an electric field between electrodes detects charged particles

Ionizing radiation passing through liberates free charge as electrons and ions moving due to the electric field to the electrodes.

The study of the drift and amplification of electrons in a uniform (or non-uniform field) has been an intensive area of research over the past century.

Requirements for Gas Mixtures

Fast: an event must be unambiguously identified with its bunch crossing

Leads to compromise between high drift velocity and large primary ionization statistics

Drift velocity saturated or have small variations with electric and magnetic fields

Well quenched with no secondary effects like photon feedback and field emission: stable gain well separated form electronics noise

Fast ion mobility to inhibit space charge effects

Electron-Ion Pair Production in a Gas

An ionizing particles passing through a gas produces free electrons and ions in amounts that depend on the atomic number, density and ionization potential of the gas and energy and charge of the incident particleNp: number of primary electron pair per cm.Nt: total number of electron ion pairs (from further ionization)

Electron Transport Properties

With no electric field, free electrons in a gas move randomly, colliding with gas molecules with a Maxwell energy distribution (average thermal energy 3/2 kT), with velocity v

When an electric field is applied, they drift in the field direction with a mean velocity vd

Energy distribution is Maxwellian with no field, but becomes complicated with an electric field

vd

Noble Gases

Cross-section for electron collisions in Argon

Electrons moving in an electric field may still attain a steady distribution if the energy gain per mean free path << electron energyMomentum transfer per collision is not constant.

Electrons near Ramsauer minimum have long mean free paths and therefore gain more energy before experiencing a collision.Drift velocity depends on pressure, temperature and the presence of pollutants (e.g. water or oxygen)

Electron collision cross-sections for CO2

Poly-atomic gases

Poly-atomic molecular and organic gases have other modes of dissipating energy: molecular vibrations and rotations

In CO2 vibrational collisions are produced at smaller energies (0.1 to 1 eV) than excitation or ionization

Vibrational and rotational cross-sections results in large mean fractional energy loss and low mean electron energy

Mean or ‘characteristic electron energy’ represents the average ‘temperature’ of drifting electrons

Electron Diffusion

vd

Electrons also disperse symmetrically while drifting in the electric field: volume diffusion transverse and longitudinally to the direction of motion

In cold gases, e.g. CO2, diffusion is small and the drift velocity low and unsaturated: non-linear space-time relation

Warm gases, e.g Ar, have higher diffusion. Mixing with polyatomic/organic gases with vibrational thresholds between 0.1 and 0.5 eV reduces diffusion

Lorentz Angle

B

F

Due to the deflection effect due to a B field perpendicular to the E field, the electron moves in a helical trajectory with lowered drift velocity and transverse dispersion

The Lorentz angle is the angle the drifting electrons make with the electric field

Large at small electric field but smaller for large electric fields

Linear with increasing magnetic field

θ

Gases with low electron energies have small Lorentz angle

Properties of Helium

Lorentz Angle for Helium-Isobutane

Drif

tD

iffus

ion

Helium-Ethane

Neon

Longitudinal Diffusion Constant for Ne-CO2 mixtures

Diffusion in Argon

Transverse Diffusion in Ar-DME mixture

No B field With B field

Transverse Diffusion in Ar-CH4

Argon

Drift Velocity for Pure ArgonLorentz Angle in Ar/CO2

Possible gas for single photon detectors

Xenon

In medical imaging, the gas choice is determined by spatial resolution: CO2 added to improve diffusion

Pure Xenon

Xenon-CO2

DME

Transport Parameters for Pure DME

Low diffusion characteristics and small Lorentz angles used to obtain high accuracy

Lorentz angle in DME-based mixtures

Introduced as a better photon quencher than isobutane.

Absoption edge of 195nm: stable operation with convenient gas multiplication factors

High gains and rates without sparking.

Townsend Coefficient

Mean number of ionizing collisions per unit drift length

Helium-Ethane

DME/CO2

Ion Transport Properties

Constant up to rather high fields

Ion mobility v iE / p

Electric field pressure

Ion drift velocity

Pollutants

Pollutants modify the transport parameters and electron loss occurs (capture by electro-negative pollutants)

The static electric dipole moment of water increases inelastic cross-section for low energy electrons thus dramatically reducing the drift velocityElectron capture phenomenon has a non negligible electron detachment probability

Mean electron capture length

Lecture 2.3: Wire-based Detectors

Geiger-Muller Counter

Tube filled with a low pressure inert gas and an organic vapor or halogen and contains electrodes between which there is a voltage of several hundred volts but no current. Anode is a wire passing through it. Cathode is the walls.Ionising radiation passing through the tube ionizes the gas. The free electrons are accelerated by the field. The avalanche begins as these in turn ionise more.

Anode wire

UV photonCathode

Cathode

Avalanches in a Geiger Discharge

1928

Multiwire Proportional Chamber

Invented by Georges Charpak in 1968. Nobel Prize in 1992.

MWPC

The particle ionizes the gas producing electrons and free ions.

The liberated electrons move rapidly move towards the anode wire and the ions towards the cathode plans

More electrons are liberated which in turn ionise the gas. An avalanche of charges is produced giving rise to and electric pulse on the anode wire.

MWPC

Grid of parallel thin anode wires between two cathode planes.

Electrons drift to the anodes and are amplified in avalanche.

Drift of ions produced in the avalanche induces a negative charge on the wire and positive charges on surrounding electrodes.

Positive induced charge on adjacent wires overcomes the negative charge due to the large capacitance between the wires

Two-Dimensional Readout

An MWPC with the cathode strips perpendicular to the wires. Charge profile recorded on both anodes and cathodes. Centre of gravity provides X and Y projections:

Xi;Yi: Strip coordinatesQi(X), Qi(Y): Charge on stripsQ(X), Q(Y): Total Charge

2D readout essential for medical imaging applications.

Applications of MWPCs

Applications include crystal diffraction, beta chromatography and dual energy angiography

Low dose X-ray digital radiography scanner based on the MWPC

Film of congenital hip dislocation in a 7-year old boy.

Satisfactory visualization of femoral architecture and bone structure

Applications in Medical Imaging

Regional uptake of deoxyglucose in a dog’s heart

M.G. Trivelli et al,Cardiovasclar Res.

26(1992) 330

Activity in a vasopressine-labelled rat’s brain (from CERN-Geneva hospital).

E. Tribollet et al, Proc. Natl. Acad. Sci. USA 88(1991) 1466

Drift Chambers

An alternating sequence of wires with different potentials, there is an electric field between the ‘sense’ and ‘field’ wires.The electrons move to the sense wires and produces an avalanche which induces a signal read out by the electronics.The time between the passage of the particle and the arrival of the electrons is measured measure of the particles position. Can increase the wire distance to save electronics channels.

D

Typical Geometries of Drift Chambers

W. Klempt. Detection of Particles with Wire Chambers, Bari ‘04

Straws

If a single wire breaks in an MWPC the entire detectors is impacted. A solution is to replace the volume, with arrays of individual single-wire counters, known as straws. Typically a wire is strung between two supports within a thin straw (either metallic or with the internal surface coated with a metal)

Portion of the ATLAS TRT End Cap

MDTs

The ATLAS barrel muon spectrometers uses Monitored Drift Tubes. These reconstruct tracks to 100 μm accuracy.

ATLAS MDTs

MDTs can also be used for making music!

MDT pipe organ made by Henk Tieke from NIKHEF, Amsterdam.

Time Projection Chamber (TPC)

A TPC is a gas-filled cylindrical chambers (with parallel E and B field) with MWPCs as endplates.

Event display of a Au-Au collision in STAR at RHIC. Typically ~200 tracks per events.

B

E

Gas volume

drift

Wire chamber

Drift fields of 100-400 V/cmDrift times 10 -100 μsDistance up to 2.5 m

Modern TPCs

STAR TPC ALICE TPC

Gas for TPCs

Computed with MAGBOLTZS. Biagi, NIM A42(1999) 234

A common gas filling used is 90% Argon, 10% CH4, but this has saturated drift velocity at low fields and transverse diffusion is reduced with a magnetic field.

Best choice is CF4 because it has low diffusion even without a magnetic field. Requires high drift fields.

Cherenkov Radiation

Cerenkov Radiation in the core of a nuclear reactor

Photons are emitted by a charged particle moving faster than the speed of light in a medium at an angle which depends on the particle’s velocity: β=1/n cos(θ)

θ

These are reflected on a spherical mirror. The radius of the ring is R = rθ/2

RICH Detectors

LHCb RICH Detector

ALICE HMPID

Can be used for particle identification together with tracking detectors

COMPASS RICH

Event Display

Array of 8 MWPCs with CsI photocathodes

Time Resolution

Time Resolution of Wire Chambers

It takes the electrons some time to move from their creation point to the wire. Fluctuations in this primary deposit and diffusion times to ~5ns

If one uses a parallel plate geometry with a high field, the avalanche starts immediately so that time resolutions down to 50 ps can be achieved. These detectors can be used for triggering.

Resistive Plate Chambers

Place resistive plates (Bakelite or window glass) in front of the metal electrodes

Sparks cannot develop because the resistivity and capacitance will allows only a very localized discharge.

Large area detectors can be made

Rate limit of kHz/cm2

CMS RPCs

ALICE TOF Detector

Large Time-Of-Flight (TOF) system with 50 ps time resolution made from window glass and fishing line (high precision spacers)

Before RPCs were available, very expensive photomultipliers were used with scintillators