Transcript q - Indico

Gaseous Particle Detectors
By Archana SHARMA
CERN Geneva Switzerland
UNAL
BOGOTA COLUMBIA
October 2012
1
Archana Sharma CERN, Geneva, Switzerland
• HIGH
• ENERGY
• PARTICLES
• ACCELERATORS
• INTERACTIONS
• DETECTORS
What is a TeV ?
Archana Sharma CERN, Geneva, Switzerland
7 TeV + 7 TeV
1 TeV = 1 Tera electron volt
= 1012 electron volt
Rate 40 MHz
The LHC will determine the Future course of High Energy Physics
3
Introduction
HEP experiments study the interactions of particles
by observing collisions of particles
Result: change in direction / energy / momentum of
original particles
And production of new particles
Archana Sharma CERN, Geneva, Switzerland
See http://cmsinfo.cern.ch/Welcome.html/
TOTAL
Gaseous Detectors
In CMS
~ 10,000 m2
Just in case you wonder why?
Known particles
Highly
Expected
Particles
Methodology
that disappeared
after the Big
Bang
E=mc2
Hypothetical
Or totally
unsuspected ?
6
SUSY
A Higgs Event in CMS
Methodology
2 muons
2 electrons
7
Archana Sharma CERN, Geneva, Switzerland
The ideal detector
With an “ideal” detector, we can reconstruct the
interaction, i.e. obtain all possible information on it. This
is then compared to theoretical predictions and
ultimately leads to a better understanding of the
interaction and properties of particles
For all particles produced, the “ideal detector” measures
energy, momentum, type by :
mass, charge, life time, spin, decays
Archana Sharma CERN, Geneva, Switzerland
Measure and derive
The mass, velocity, energy and charge (sign)
– from ‘tracking’ curvature in a magnetic field
Negative charge
Magnetic field, pointing
out of the plane
Positive charge
The lifetime t
from flight path before decay
t
Archana Sharma CERN, Geneva, Switzerland
Different type of particles to be detected
Charged particles
– e-, e+,
p (protons),
p, K (mesons),
m (muons)
Neutral particles
g (photons), n (neutrons), K0 (mesons),
n (neutrinos, very difficult)
Different particle types interact differently with matter
(detector) (for example, photons do not interact with a magnetic field)
Need different types of detectors to measure different types of particles
Archana Sharma CERN, Geneva, Switzerland
Principles of detection
Interaction of a particle with detector
Sensitive Material
Measureable Signal
Ionization
Excitation
e-
p
g
e-
p
p
p
Particle trajectory is changed due to
Bending in a magnetic field, energy loss
Scattering, change of direction, absorption
Archana Sharma CERN, Geneva, Switzerland
Ionization signals by using
Gaseous detectors:
MWPC and its derivatives
(Multi-Wire Proportional Chambers)
Drift Chambers (DCs)
TPC (Time Projection Chamber)
Archana Sharma CERN, Geneva, Switzerland
Charged Particle Radiation
Fast Electrons
b-particles emitted in nuclear decay
energetic electrons produced by any
process
Heavy Charged Particles
Energetic ions: alpha particles,
protons
Fission products
Products of nuclear reactions
14
Neutral Particles
Electromagnetic Radiation
X-rays emitted by re-arrangement of
electron shells in atoms
Gamma rays from transitions in the nucleus
Neutrons
Generated in various nuclear processes
Often subdivided into slow and fast
neutron sources
Electromagnetic Interaction of
Particles with Matter
Interaction with atomic
electrons. Particle
loses energy; atoms
are excited or ionized.
Interaction with atomic
nucleus. Particle undergoes
multiple scattering. Could
emit a bremsstrahlung
photon.
If particle’s velocity is greater
than the speed of light in the
medium -> Cherenkov
Radiation. When crossing the
boundary between media,
~1% probability of producing a
Transition Radiation X-ray.
Stopping Power
Linear stopping power (S) is the
differential energy loss of the
particle in the material divided by
the differential path length. Also
called the specific energy loss.
Energy loss through
ionization and atomic
excitation
Stopping Power of muons in
Copper
Particle Data Group
Bethe-Bloch Formula
m – electronic mass
v – velocity of the particle (v/c = b)
N – number density of atoms
I – ‘Effective’ atomic excitation energy – average value found empirically
Gas is represented as a dielectric medium through which the particle propagates
And probability of energy transfer is calculated at different energies – Allison Cobb
Particle Data Group
Bethe-Bloch Formula
Energy Loss Function
Rel
To mips
1.6
1.5
Fermi Plateau
1.4
Relativistic Rise
1.3
1.2
1.1
1
10
100
Minimum ionizing particles (mips)
19
Archana Sharma CERN, Geneva, Switzerland
Different Materials
20
Archana Sharma CERN, Geneva, Switzerland
Average Ionisation Energy
Few eV to few tens of eV
21
Energy-loss in Tracking Chambers
The Bethe Bloch Formula tool for Particle Identification
22
Archana Sharma CERN, Geneva, Switzerland
Straggling
Mean energy loss
Actual energy loss will scatter around the mean
value
Difficult to calculate
– parameterization exist in GEANT and some
standalone software libraries
– Form of distribution is important as energy loss
distribution is often used for calibrating the
detector
23
Archana Sharma CERN, Geneva, Switzerland
Straggling
Dec 2008
24
Alfons Weber
Archana Sharma CERN, Geneva, Switzerland
Electrons
Electrons are different light
– Bremsstrahlung
– Pair production
25
Archana Sharma CERN, Geneva, Switzerland
Multiple Scattering
Particles not only lose energy …
but also they also change direction
26
Archana Sharma CERN, Geneva, Switzerland
Range
Integrate the Bethe-Bloch
formula to obtain the range
Useful for low energy
hadrons and muons with
momenta below a few
hundred GeV
Radiative Effects important
at higher momenta.
Additional effects at lower
momenta.
Radiation Length
Mean distance over
which an electron
loses all but 1/e of its
energy through
bremsstralung
also
7/9 of the mean free
path for electronpositron pair
production by a high
energy photon
Energy Loss in Lead
Electron Critical Energy
Energy loss through
bremsstrahlung is
proportional to the
electron energy
Ionization loss is
proportional to the
logarithm of the electron
energy
Critical energy (Ec) is
the energy at which the
two loss rates are equal
Electron in Copper: Ec = 20 MeV
Muon in Copper: Ec = 400 GeV!
Photon Pair Production
Differential Crosssection
Total Cross-section
What is the
minimum energy
for pair production?
Probability that a photon
interaction will result in a pair
production
Electromagnetic cascades
Visualization of
cascades
developing in the
CMS
electromagnetic
and hadronic
calorimeters
Muon Energy Loss
For muons the critical
energy (above which
radiative processes
are more important
than ionization) is at
several hundred GeV.
Ionization
energy loss
Mean
range
Pair production,
bremsstrahlung
and photonuclear
Muon
Tomography
Luis
Alvarez
used the
attenuation
of muons
to look for
chambers
in the
Second
Giza
Pyramid
X-Ray Radiography
for airport security
Signals from Particles in a Gas Detector
Signals in particle detectors are mainly due to
ionisation
And excitation in a sensitive medium – gas
Also:
Direct light emission by particles travelling
faster than the speed of light in a medium
– Cherenkov radiation
Similar, but not identical
– Transition radiation
35
Archana Sharma CERN, Geneva, Switzerland
Cerenkov Radiation
Moving charge in dielectric medium
Wave front comes out at certain angle
slow
fast
1
cos c 
bn
36
Archana Sharma CERN, Geneva, Switzerland
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
Archana Sharma CERN, Geneva, Switzerland
Transition Radiation (2)
q1
medium
v3
Before the charge
crosses the surface,
apparent charge q1
with apparent
transverse vel v1
v1
vacuum
q3
q2
v2
After the charge
crosses the surface,
apparent charges q2
and q3 with
apparent transverse
vel v2 and v3
38
From Interactions to Detectors
Archana Sharma CERN, Geneva, Switzerland
Archana Sharma CERN, Geneva, Switzerland
Multiwire Proportional Chamber
Archana Sharma CERN, Geneva, Switzerland
Multiwire Proportional Chamber
and derivatives
Archana Sharma CERN, Geneva, Switzerland
Signal Creation
Charged particles traversing matter leaving excited atoms,
electron or holes and ions behind. These can be detected
using either excitation or ionization.
Excitation
Photons emitted by excited atoms can
be detected by photomultipliers or
semiconductor photon detectors
Ionization
If 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.
q
Total induced charge –q.
-q
Different charge positions
results in different charge
distributions but the total
charge stays –q.
q
-q
Signal Induction for Moving Charges
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.
q
q
-q
-q
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
Cathode
Charge is produced near the
track.
Electric Field
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.
Anode
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
Cathode
Parallel Plate
Ionization
Chamber
Anode
Anode wire
Circular
Cathode
Cylindrical
Ionization
Chamber
Charge Multiplication
At sufficiently high electric fields (50100kV/cm) electrons gain energy in excess of
the ionization energy, which leads to
secondary ionization, etc.
Townsend
Coefficient
Townsend Coefficient
Computed values of
the Townsend
coefficient as a
function of the
electric field for
different gases.
Avalanche
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.
Electrons: close
to the wire
Anode wire
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
LHC
Semi-proportional region: A=104-106
Saturation region: A > 108
(independent of the number of primary
electrons)
1970’s
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
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 tail
Total charge induced by the
electrons amount to only ~1-2
% of the total charge.
Properties of Gases
Properties of commonly used gases
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 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
Electrons moving in an electric field
may still attain a steady distribution if
the energy gain per mean free path
<< electron energy
Momentum 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)
Cross-section for electron collisions
in Argon
Poly-atomic gases
Poly-atomic molecular and
organic gases have other
modes of dissipating energy:
molecular vibrations and
rotations
Electron collision cross-sections for CO2
In CO2 vibrational collisions are
produced at smaller energies
(0.1 to 1 eV) than excitation or
ionization
Vibrational and rotational crosssections 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
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
vd
Lorentz Angle
B
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
Gases with low electron energies have
small Lorentz angle
F
Properties of Helium
Diffusion
Drift
HeliumEthane
Lorentz Angle for
Helium-Isobutane
Neon
Longitudinal
Diffusion
Constant for NeCO2 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
Argon
Possible gas for single photon
detectors
Lorentz Angle in
Ar/CO2
Xenon
In medical imaging, the gas choice
is determined by spatial resolution:
CO2 added to improve diffusion
Pure
Xenon
XenonCO2
DME
Transport Parameters for
Pure DME
Low diffusion
characteristic
s 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/C
O2
Ion Transport Properties
vi
Ion mobility 
E/p
Electric
field
Ion drift
velocity
pressure
Constant up to
rather high fields

Pollutants
Pollutants modify the transport
parameters and electron loss occurs
(capture by electro-negative
pollutants)
The static electric dipole moment of
water increases inelastic crosssection for low energy electrons thus
dramatically reducing the drift
velocity
Electron capture phenomenon has a
non negligible electron detachment
probability
Mean electron capture length
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.
Avalanches in a Geiger Discharge
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.
Cathode
Anode wire
Cathode
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 coordinates
Qi(X), Qi(Y): Charge on strips
Q(X), Q(Y): Total Charge
2D readout essential for
medical imaging applications.
Drift Chambers
D
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.
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.
Drift fields of 100-400 V/cm
Drift times 10 -100 μs
Distance up to 2.5 m
Gas volume
B
drift
Event display of a Au-Au collision
in STAR at RHIC. Typically ~200
tracks per events.
E
Wire chamber
Modern TPCs
STAR
TPC
ALICE
TPC
Gas for TPCs
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.
Computed with MAGBOLTZ
S. Biagi, NIM A42(1999) 234
Cherenkov Radiation
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
Cerenkov Radiation in the
core of a nuclear reactor
RICH Detectors
ALICE HMPID
LHCb RICH Detector
Can be used for particle identification together with tracking detectors
COMPASS RICH
Event Display
Array of 8 MWPCs with
CsI photocathodes
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
Limitations of MWPCs
Rate Limitations
1.1
MWPC-Rate1 0120 0
1
Relative Gain
Wire spacing limits position
accuracy and two track
resolution to ~1mm
Electrostatic instability limits
the stable wire lengths
Widths of induced charges
define the pad response
function
Accumulation of positive ions
restrict the rate capabilities
0.9
0.8
0.7
0.6
rate (Hz/mm
2
)
0.5
10
3
10
4
10
5
10
6
10
92
7
Multi-Step Chamber
Chamber operation is more
stable and provides higher
gain.
High Gain of Multi-Step Chamber
10
7
MSC-Trend
Nov26,2 000
10
6
Gain
Divides the gain of the
MWPC into two parts
First allow electrons
produced by ionizing particles
to ‘pre-amplify’
Then proceed to the anode
for further amplification.
V =3 kV
10
V = 2.5 kV
2
5
2
V = 2 kV
2
10
10
10
4
3
2
0
0.5
1
1.5
2
2.5
3
3.5
4
V (kV)
1
93
Micro Strip Generation
Micro-Strip Gas Chamber
(MSGC) invented by Oed in 1988.
Field Configuration in an MSGC
A pattern of thin anodes and
cathode strips on a insulating
substrate with a pitch of a few
hundred μm.
Electric field from a drift
electrode above and
appropriate potentials applied.
94
Micro Strip Generation
Removes positive ions from the
vicinity of avalanches
High rate capabiity two orders
of magnitude higher than
MPWCs (106/mm2s)
~30μm position resolution
Double track resolution of
400 μm
Good energy resolution
Applications in X-ray
spectrometry and digital
95
radiography
Damage in MSGCs
Damage in a MSGC
Difficulties began when exposed to
highly ionizing particles (charge 3x mip)
Streamer to gliding discharge transition
damaged strips
Small anode-cathode distance in MSGC.
High electric field at stream tip and along
the surface. Streamer is followed by a
voltage and ionization dependent
discharge. Culprits are charging of
surface defects, long-lived excited states
and overlapping avalanches.
Field Along the Surface of MSGC
1.4 10 8
-1
)
Electric Field (V cm
Investigation showed that the streamer
mode is stable in a MWPC because the
electric field in the propagation direction
is weak
1.2 10 8
1 108
8 107
6 107
4 107
2 107
0
0
96
20
40
60
80
x (µm)
100
New Micropattern Era
Microneedle Concept
(1976)
No observable gas gain due
to fine needles (<<1μm)
and small amplification
region
Microdot Chamber
Schematic
Ultimate gaseous pixel device
with anode dotes surrounded
by cathode rings. Very high
gains (~106). Does not
discharge up to very high
97
gains.
Micro-Megas
Very asymmetric parallel plate chamber. Uses
the semi-saturation of the Townsend coefficient
at high fields (100kV/cm) in several gas
mixtures, to ensure stability in operation with
mips.
Excellent energy
resolution
Electrons drifting
from the sensitive
volume into the
amplication
volume with an
avalanche in the
thin multiplying
gap.
98
Micro-Megas
Energy Resolution of
a Micro-Megas
Detector
Large area (40 x 40 cm2)
Micro-Megas detector
installed in the
COMPASS experiment at
CERN.
99
Compteur a Trous (CAT)
A narrow hole micro-machined in an insulator
metallized on the surface as the cathode. Anode is
the metal at the bottom of the hole.
Electric
Field
Energy
Resolution
100
Compteur a Trous (CAT)
μCAT
Removing the insulator leaves
the cathode as a micro-mesh
placed with a thin gap above the
readout electrode (μCAT). Gains
of several 104.
VIP
An ingenious scheme of readout
from virtual pixels made by
current sharing (20 times finer
resolution compared to the
reasout cell) giving 400 times101
more virtual pixels.
Gas Electron Multiplier (GEM)
Chemical Etching
Process
Manufactured using standard
printed circuit wet etching
techniques.
Comprise a thin (~50μm) Kapton
foil, double-sided clad with copper
and holes are perforated through.
Two surfaces are maintained at a
potential gradient; providing field
for electron amplification and an
avalanche of electrons.
102
Gas Electron Multiplier (GEM)
Electric Field
When coupled with a drift
electrode above and a
readout electrode below, it
acts as a micropattern
detector.
Amplification and
detection are decoupled
 readout is at zero
potential. This permits
transfer to a second
amplification device and
can be coupled to
another GEM.
Avalanche
across a GEM
103
Other Micropattern Detectors
Gain with a
Micro-Wire
Many other detectors
following the GEM
concept
Micro-Wire (μDOT in 3D)
Micro-Pin Array (MIPA)
Micro-Tube
Micro Well
Micro Trench
Micro Groove
MIPA Array
104
MicroTube Detector
Microtube
Combination of laser micromachining and nickel electroplating
~150μm diameter cathode
Anode tube machined through
the well and plated alongside.
Electric field that increases
rapidly at the anode, but no
insulating material between
cathode and anode.
Allow for higher gas gains, better
stability (fewer discharges) and a
reduction of charging effects.
Similar performance to μDOT and
μCAT
Field across a
Microtube
105
Other Micropattern Detectors
Assembled GEM+MSGC
Studies have shown that
discharges in the presence of
highly ionizing particles appear in
all micropattern detectors at gains
of a few thousand
Vertex Reconstruction
Can obtain higher gains with poorly
quenched gases (lower operating
voltage and higher diffusion)
lowers charge density
Lowers photon feedback
probability
Safe operation of a combination of
an MSGC and a GEM has been
demonstrated up to gains of
~10000s
106
Larger GEMs
Triple GEMs operate
even more stably in
poor hadronic beam
environments
Discharge Probability for single,
double and triple GEMs
Larger GEMs are
segmented to reduce
capacity and limit the
energy in the
discharge
107
MSGCs for X-ray Imaging
Images of a snail shell taken with an
MSGC operating with Xe-Ch4 at 4 bar
Conventional film radiography
has excellent spatial resolution
but limited dynamic range
Conventially storage and
display media are the same.
The film image can saturate
and the display contrast is
fixed at the time of exposure.
A digital system has infinite
dynamic range and the display
contrast can be varied at will.
108
TPC Readout
It boasts a fast electron signal,
minimal magnetic distorting
effects and suppression of ion
feedback.
Fractional ion feedback in the TPC
drift volume
0.25
pos Ion DGem P
16.9.99
Fractional Ion Feedback
For the TESLA experiment at
the ILC, a double or triple GeM
is under consideration.
0.2
0.15
0.1
Special hexagonal pads are
being developed to provide 50
x 60 μm resolution
0.05
Measurement
S. Bachmann NIM 99
simulation A. Sharma unpublished 99
0
Drift Field (V/cm)
-0.05
0
500
1000
1500
2000
109
MICROMEGAS Xrays
MICROMEGAS
detectors have been
developed for X-ray
imaging.
Vertebra scanned with
a MICROMEGAS
Operate with pure
Xenon at atmospheric
pressure
110
Protein Crystallography SAXS
X-ray diffraction patterns of Cytochrome C
with different levels of contamination
Rapid analysis of single crystal
structures with X-ray diffraction
studies using MSGCs
Crystal structures of organic
molecules can be determined in
minutes using position and time
information
Fast time resolved
measurements off a time
variation of the SAXS pattern of
a protein sample in 10 ms.
X-ray diffraction insensities
111
Protein Crystallography SAXS
Diffraction pattern of a lipid
membrane made with the VIP
detector.
Complex algorithms made
for the cell border and
superimposing several shots
allow a high degree of detail
to be obtained
112
Digital Mammography
Benefits of the early detection of cancer
are obvious. Small tumours usually
detected in routine radiographic scanning
of the body
Current equipment limited by contrast
difference between malignant and benign
tissues
Combination of an x-ray converter, a
MSGC and visible photocathode shows
great promise.
Single photon detection with a CsI
photocathode coupled to 3/4 GEMs in
tandem and very large gains obtained in
Ar
113
Cherenkov Ring Imaging
Very high gains
observed with
cascade of four
GEMs and using
pure ethane as
the operating gas
114
Scintillation Light Imaging
A novel application
was developed by
integrating a MSGC
in a gas proportional
scintillation counter.
Scintillation images of alpha
tracks in Ar-CF4
A reflective CsI
photocathode was
deposited on the
microstrip plate surface of
the MSGC that serves as
the VUV photosensor for
the scintillation light from
xenon GPSC
115
X-ray imaging: Radiology and Diagnostics
13 kV X-ray absorption
radiography of a fish bone
taken at 2 atm using a
GEM + MSGC
combination.
3 mm x 10 mm 50 kV xray image of a digit of a
mouse.
Radiography
of a small bat
using GEM
and 50µm x
50 µm 2dreadout
116
Imaging of Polarized X-rays
Measurements of
X-ray polarization
are used to
investigate
pulsars,
synchrotron
nebulae, etc
Some X-ray
polarimters have
been developed
using GCPs and
GEMs
Photoelectrons from
GCP Polarimeter
Emission direction
of the primary
electron depends
on the incident Xray polarization, it
can be measured
117
GEM for Plasma Diagnostics
Imaging the dynamics of
fusion plasmas has been
attempted at the Frascati
Tokamak Upgrade to
exploit the sensitivity of
the GEM to soft X-rays.
Time resolved plasma
diagnostics are made with a
GEM and individual pixel
readout.
Counts integrated in 50 μs for
four adjacent pixels at the
Frascati Tokamak
Reconstruction
of
photoelectrons
with a GEM+
micropixel
readout
118
Aging Effect in Wire Chambers
The degradation of operating conditions of wire
chambers under sustained irradiation are the main
limitation to the use of gas detector in high-energy
physics.
‘Classical aging effects’ are deposits formed on
electrode surfaces by chemical reactions in avalanche
plasma near the anode.
During gas avalanches many molecules break up and
form free radicals (unionized atomic or molecular
particles with one or more unsatisfied valence bonds).
Free-radical polymerization is regarded at the
dominating mechanism of wire chamber aging
Aging Effects (cont)
Free radicals either recombine to form the original molecules
or cross-linked molecular structures of increasing weight
Leads to the formation of deposits (conducting or insulating)
on electrode surfaces.
Decrease of the gas gain (due to modification
of electric field)
Excessive currents
Sparking and self-sustained discharges
Radiation-induced degradation depends on
the nature and purity of the gas mixture
different additives and trace contaminants
materials in contact with the gas
materials of the electrodes
electric field configuration
Aging Effects in Wire Chambers
Premature aging in Ar/CH4
Free radicals are hydrogen deficient and are
therefore able to make bonds with hydrocarbon
molecules. Therefore CH4 polymerizes in the
avalanche plasma, which causes premature aging.
Aging rate of Ar/hydrocarbon gases can be reduced
by adding oxygen-containing molecules, which
allows large systems to operate at low intensity with
only a small performance loss.
Not trustworthy for long-term, high-rate experiments.
Silicon Deposits
Si-deposits on anode wires
Silicon has been detected in the
analysis of many wire deposits,
although the source has not been
clearly identified in all cases.
Si-compounds can be found in many
components including lubricants,
adhesives, rubber, silicon-based
grease, oils, O-rings, fine dust, gas
impurities, diffusion pumps, molecular
sieves, and many more.
Most dangerous are Si-lubricant traces
used for the production of gas system
components. Cleaned by flushing the
system with DME.
The Malter Effect
Microscopic insulating layer deposited on a cathode from
quencher dissociation products and/or pollutant molecules.
Some metal oxide coatings, absorbed layers or even the
cathode material itself may not be initially conducting
enough inhibit neutralization of positive ions from the
avalanche. These ions generate a strong electric field
across the dielectric film and cause electron field-emission
at the cathode.
Positive feedback between electron emission at the cathode
and anode amplification leads to the appearance of dark
current, increased rate of noise pulses and finally
exponential current growth (classical Malter breakdown)
Adding water prevents Malter discharges, because water
increases the conductivity of partially damaged electrodes
Malter Effect
Malter Effect first imaged in the
CRID RICH detector.
Wire deposits in CH4+TMAE after a
charge dose of 6x10-3 C/cm
Aging from α-particles
Energy loss per α track can be
103 times larger than primary
ionization with X-rays or MIPS.
This ages shows up as hair-like
deposits within the irradiated
area.
Aging rate in Ar/CF4/CH4
~100x higher in a 100 MeV αbeam than with a Fe55 sources
with similar current densities.
High Voltage
Triple GEM detectors are more reliable
and radiation hard than double GEMs,
with the same total gain. This gain is due
to the reduction in the electric strength
across a single GEM.
GEMs age less than other wire-detectors
because multiplication and readout are
separate (gas amplification only within
GEM holes) and the rate of impurity
polymerization caused by the lower
electric field.
Triple GEM Detector of the
COMPASS experiment
Damage to MSGCs
Discharges
measured in the
CMD MSGC
prototypes
Strip Damage due
to discharges and
sparks
Triple GEM Ageing test
Gain of 2x104
Total integrated charge of 13 C/cm2 is expected in 10 years of
operation in LHCb
50 MHz/cm2 X-rays, in 10 days a total charge of 20 C/cm2 was
integrated
Less than 5% change in the
chamber behavior
CMS high-η - maximum integrated charge
LHCb
129
Conclusion and Outlook
Multiwire chambers have matured since their
introduction over the last few decades, with
several applications in particle physics and
diagnostics of various kinds.
The last decade has seen several novel
developments in Micropattern Gaseous Detectors.
Understanding of the discharge mechanisms in
these devices has also improved allowing
amelioration of their design.
 Progress in manufacture of customized readout
boards has evolved revolutionizing the potential
applications of these detectors in radiology,
diagnostics, astrophysics and other fields.
130
[1] Phil. Mag. Xiii(1896)392; Conduction of electricity through gases (Ist ed, 1903); Proc. Of Royal Society, A81(1908)141
[2] G. Charpak et al Nucl. Instr. And Meth. 62(1968)262-268
[3] A. Breskin et al, Nucl. Instr. And Meth 161(1979)19; F. Sauli Physica Scripta 23(1981)526; G. Charpak and F. Sauli Ann. Rev. 34(1984)285, See also
C. Grupen, Particle Detectors, Cambridge Press, 1996
[4] E. Babichev et al (1992) Nucl Instrum Methods Phys Res A323: 49, and references therein, S. Baru et al, Novosibirsk Preprint 35(1989) 98-39
[5] G. Kalifa et al, Pediatric. Radiology 28(1998)557
[6] E. Babichev et al in the Proceedings of Frontier Detectors for Frontier Physics Elba, Italy May 2000 & A. Porosev, in Proceeding of Frontier Detector
for Frontier Physics, Elba May 2000.
[7] Martinez-Davalos et al, Evaluation of a new low-dose digital x-ray system. Phys Med Biol 38(1993)141
[8] CMS Technical Design Report. CERN/LHCC 98-6
[9] A. Oed, Nucl. Instr. And Meth. A263(1988)35
[10] F. Sauli and A. Sharma, Annual Rev. of Nucl. Sci. 49(1999)41
[11] R. Bouclier et al Nuclear Physics B 61B (1998)315
[12] B. Boimska et al Nuclear Physics B 61B (1998) 498
[13] B. Schmidt , Nucl. Instr. And Meth. A419(1998)230
[14] V. Peskov et al, IEEE Transactions,
Nucl. Sci. NS 45(1998)244
[15] A. Bressan et al Nucl. Instr. And Meth. A 424(1998)321
[16] P. Fonte et al Nucl. Instr. And Meth. A 419(1998) 405
[17] J. Kadyk et al Nuclear Physics B 61B (1998) 258
[18] S.F. Biagi et al, Nucl. Instrum. Methods A371(1995)12
[19] A. Bressan, A. Buzulutskov, L. Ropelewski, F.Sauli and L. Shekhtman, Nucl. Instr. And Meth. A 432(1999)119-124
[20] Y. Giomataris et al Nucl. Instr. And Meth. A 376(1996)29
[21] A. Sharma, ICFA Bulletin Fall 1999 http://www.slac.stanford.edu/pubs/icfa
[22] F. Kuune in Proceeding of Frontier Detector for Frontier Physics, Elba May 2000
[23] F. Bartol et al., J. Phys. III France 6 (1996)337, G. Chaplier et al Nucl. Instr. And Meth A426(1999)
[24] A. Sarvestani et al Nucl. Instr. And Meth. A 419(1998)444
131
[25] N. Pavel, Siegen, Priv. Com. Oct 2000
[26] F. Sauli Nucl. Instr. And Meth. A 386(1997)531
[27] B. Adeva et al., Nucl. Instr. And Meth A 419(1998)405
[28] P. Rehak et al Proceedings IEEE Nuclear Science Symposium 1999, Seattle USA
[29] W. K. Pitts et al Nucl. Instr. And Meth A438(1999)277
[30] W. K. Pitts See also Proceedings, IEEE Nuclear Science Symposium 1999, Seattle USA
[31] R. Bellazzini et al, Nucl. Instr. And Meth. A 423(1998)125
[32] R. Bellazzini et al, Nucl. Instr. And Meth. A 424(1998)444
[33] C. Richter, PhD. Thesis October 2000 submitted to University of Heidelberg, Germany.
[34] F. Gomez Private Communication and Internal Notes. T. Nunez, and P. Vasquez Theses 1999, Santiago University, Spain.
[35] J. Benlloch et al, IEEE trans NUCl. Sci. NS-45(1998) 234, J. Benlloch et al NIMA 419 (1998) 410
[36] B. Ketzer, Contribution to this Conference
[37] M. Ziegler et al hep-ex/0007007/July 2000, LHC-B Internal Tracking Notes: 99-024, 2000-013, 2000-15, 2000-056, U. Straumann Proceedings, Imaging
2000, Stockholm.
[38] M. Dixit et al, IEEE Trans. 47(1998)809; D. G. Gobbi et al, Phys. Med. Biol. 44(1999)1317
[39] G. Charpak and M. Meynadier Priv. Com. Oct. 2000
[40] M. Dixit in Proceedings of Imaging 2000, Stockholm.
[41] A.Bressan et al Nucl. Instr. And Meth. A 425(1999)254, 262
[42] The future TESLA Linear Collider at DESY Hamburg, DESY 97/048
[43] A. Bressan et al Nucl. Instr. And Meth. A 423(1999)424
[44] A. Sharma, To appear in Nucl. Instr. and Meth. as Proceedings of Symposium on Applications of Particle Detectors in Medicine, Biology and
Astrophysics, 5-8 October 1999, Siegen, Germany, also printed as CERN-OPEN-99/373.
[45] S. Ochi et al, SPIE Proceedings 5(1998)324
[46] DIGITAL MAMMOGRAPHY Project MICADO within Weizman, Pisa, Brussels, and Agfa Gaevert.
[47] A. Breskin et al NIM A 442(2000)58
[48] V. Peskov et al NIM A433(1999)492,V. Peskov etal IEEE Trans. Nucl. Science NS-45(1999)244, and contribution to this conference.
[49] A. Sharma CERN Preprint 99/372, Submitted for publication to Nucl. Instr. & Meth. (1999), See also Proceedings of Imaging 2000, Stockholm, June 2000.
[50] C. Richter et al To Appear in the Proceedings of Frontier Detectors for Frontier Physics, Elba 2000.
[51] A.Buzulutskov et al. Nucl. Intr. And Meth A443(2000)164-180, A 433(1999)471, andContribution to Nuclear Science Symposium and Medical Imaging
conference, Oct 2000
[52] J. Va’vra Priv. Com. April 2000, and [49]
[53] J.F.C.A. Veloso et al Nucl. Instr. And Meth. A In Press
[54] J.F.C.A.Veloso et al Nuc. Instr. And Meth A In Press
[55] F. Fraga et al, Contribution to Nuclear Science Symposium and Medical Imaging conference, Oct 2000
[56] H. Sakurai et al. "A new type of proportional counter using a capillary plate" NIM A374(1996)341-344,
132
[57] T.Masuda et al. IEEE Trans NS 47, 2000.
[58] R. Bellazzini et al, Proceedings of Imaging 2000, Stockholm..