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Gaseous Detectors
Introduction
• Gaseous detectors are mainly used to measure the
trajectory of a charged particle
• When a magnetic field is applied the momentum can be
determined by measuring the curvature of a particle track
• If the momentum is determined than the energy loss of the
particle can be related to its mass particle identification
EXAMPLE:
ZEUS
Drift Chamber
50 MeV
Inner radius
16 cm
Outer radius
85 cm
B-field = 1.5 Tesla
100 MeV
radius =
ρ=
1
ρ
0.3 B
p ⋅ sinθ
500 MeV
Typical example of modern gas-filled track detector. In the gas-filled volume, thin wires are
fixed, and put at specific potential. A fast charged particle creates electron-ion pairs, and the
electrons drift towards anode wire, causing a detectable (charge) signal. From the timing of
these signals, accurate data on the track position can be obtained.
EXAMPLE:
ATLAS
Drift tubes
Inner radius
500 GeV
5m
Middle radius 10 m
Outer radius
Sagitta = 5 mm
Sagitta
15 m
B-field = 0.6 Tesla
radius = R
Sagitta = L2/8R
Another example of high-precision tracking: the position of a track is measured by groups of
‘drift tubes’. The track curvature, and therefore the momentum of the particle, follows from
the sagitta.
First Gaseous Detectors
• Ionisation chamber
Gas filled space with anode and cathode. A current flows
after the passage or the absorption of a (possibly charged)
particle.
V
t
Discovery of
cosmic rays
V
The first gas-filled detectors were ionisation chambers. Here, a static potential between two
electrodes (for instance two plates, or a tube and a central wire) creates a static electric field.
The gas can be ionised by passing charged particles, absorption of X-rays, or neutrons. In this
ionisation process, electrons are separated from their atom or molecule, resulting in electronion pairs. Electrons drift towards the anode, and ions towards the cathode. This is an electric
current.
In 1912 it was noticed that if a tube + wire is charged and the battery is taken away, the
potential still dropped, even when external radiation sources were taken away. Victor Hess
concluded that the tube is still hit by ionisation radiation: cosmic rays.
Ionisation chambers are still used today as smoke detector. The air-filled gap between two
plates is crossed by alfa particles from a (low intensity!) source. These cause a measurable
current. Smoke particles absorb some alfa’s, and, in addition, reduce the current due to
recombination. As a result, the current decreases.
First Gaseous Detectors
• Geiger Counter: Hans Geiger, 1908, Manchester
[100 years ago!!]
wire
• Gas ionisation
• electron drift
• electron avalanche
• discharge propagation
• (self) quenching
Sequence of processes:
-
Ionisation of gas (for instance: dE/dX by passing charged particle)
Separation of electrons and ions
Drift of electrons along local electric field line towards anode (wire)
Electron avalanche onto the wire
Ions, created in avalanche, move backwards towards cathode tube
[Discharge quenching, propagation: not relevant for proportional tubes, but important
process in detectors in Geiger mode]
The layout of a spark chamber
Early Gaseous Detectors
• Spark chamber
Parallel conducting plates connected alternating to highvoltage supply (pulsed after external trigger) and ground.
Filled with noble gas. High voltage causes avalanche
formation leading to production of UV light seen as sparks
• Streamer chamber
Large gap spark chamber, with high voltage (typically
about 500 kV) applied during short time after external
trigger. Avalanches develop only in the vicinity of the
initial ionization, resulting in short segments,
"streamers",producing UV light.
Energy loss
• The energy loss depends on the particle and the medium it
traverses
• Fluctuations in the energy loss follow a Landau
distribution
• The average energy spent in the creation of an ion-electron
pair, W, is roughly constant for a given gas
dE W ⋅ N ionisation s
=
dx
pathlength
Energy loss
• Primary energy loss by ionisation of atoms leads to
creation of ion-electron pairs a gas volume.
• Two step process: freed electrons can cause secondary
ionisation (cluster, delta rays, number of electrons per
cluster)
Essential:
- number of clusters per mm tracklength
- number of electrons per cluster
specific for gas (and density ρ, thus T, P!)
The creation of electron-ion pairs is a two-step process. Per unit length, there is a specific
probability that an electron is emitted from an atom or molecule. This electron may carry
sufficient energy in order to release a 2nd or even 3rd electron. The energy may be so large
that the electron creates a track next to the track of the original passing charged particle. Such
an event is called a δ–ray.
We now call a primary interaction a cluster: it contains at least one electron. A δ–ray is a
cluster with many electrons along a deviating track.
Z
A
δ
E,W
atomic number
atomic mass
specific density
ionisation potentials
dE/dX energy loss
np
number of clusters/cm
nT
total number of electrons/cm
Parameters of gases which are often applied in gaseous detectors.
Primary Ionization: HEED simulation
10. 5 clusters/mm at 7GeV
λ 95µ
µm
nav 2.4 electrons/cluster
long tail !
Values of dE/dX (left) expressed in clusters/mm, and the distribution of the number of single
electrons within a cluster (right).
Drift of charge
•
The electron mobility is much
larger than the ion mobility, for
example for pure Argon at 1
kV/cm it is
– Electrons:
r
v drift = µE
Electrons
0.4 10-3 cm2V-1m-1s-1 =
400 cm2V-1s-1 for electrons
– Argons ions in Argon gas
1.5 cm2V-1m-1s-1 for Ar ions
•
The mobility depends on the
field and (gas) pressure
Fernow, page 212-213
The drift speed of electrons and ions follows the mobility relation vdrift = µE, where µ is a
constant depending on the gas (mixture) and pressure (note that the mobility constants for
electrons and ions are very different constants). For ions, this relation holds up to high values
of E, but not for electrons. The collisions between electrons and atoms or molecules become
inelastic, reducing the average speed of the electrons. The electron drift speed as a function of
the electric field for a given gas mix can be obtained from simulations or from measurements
done with calibration chambers.
The drift velocity is an average of the erratic velocity v of the electrons or ions.
If a magnetic field is applied, a Lorentz force is introduced in the form of the ‘cyclotron
frequency’ ω:
Drift time
• General motion of charge in an EM-field
r
r
r r
dv
= eE + ev × B
m
dt
• But there is friction due to the gas
r
r r r r
dv
= eE + ev × B + Q( t )
m
dt
r
• Solve for stationary operation or dv = 0
dt
r
v r eB
and Q(t) = mA(t)
• Define µ = r ω =
m
E
The average of mA(t) is obtained by considering that this force is equal to
<dp/dt>, which can be written as: Σ pi / tN = N mvD / (N <∆t>) = mvD / τ.
The average force is balancing the force due to electric and magnetic fields,
so mA(t) has to be replaced by -mvD / τ in the time averaged equation:
eE + ev D × B − mv D / τ = 0
This can be written as:
eτ
v D = (E + v D × B)
m
The quantity eτ/m is equal to the mobility for P=P0 for the case that B = 0,
as in that case: µ = vD/E =eEτ/(Em)=eτ/m, with τ the τ for the situation with
drift field. From the equation it is seen that the definition of mobility can
be generalized to make the mobility the proportionality constant between
the drift velocity and the sizes of the electric and magnetic forces divided
the electron charge. The equation is a vector equation in vD with as solution:
vD =
µ
[E + µE × B +µ 2 (E • B)B]
1+ µ 2B2
For B perpendicular to E we have:
µE
µ 2EB
v D,// =
v
=
D, ⊥
1+ µ 2B2
1+ µ 2B2
Therefore: v D, ⊥ / v D, / / = µB , i.e. drifting is still in a straight line, but under an
angle of arctan (µB) with the electric field direction in the plane perpendicular
to B. The angle is called the Lorentz angle and is independent of the electric
field strength.
With ω = eB/m (cyclotron frequency) is µΒ equal to ωτ.
The expressions for B perpendicular to E then become:
1
1
-> the drift velocity in the field
v D,// = µE
= v D,//,no B
2 2
1+ ω τ
1+ ω 2 τ 2
direction decreases for increasing B
ωτ
ωτ
v D, ⊥ = µE
= v D,//,no B
v D, ⊥ / v D, / / = ωτ
1+ ω 2 τ 2
1+ ω 2 τ 2
v
µE
v D = v 2D, ⊥ + v 2D, / / =
= D,//,no2B 2 -> the total drift velocity decreases
2 2
for increasing B
1+ ω τ
1+ ω τ
Operation Mode
•
Ionisation chambers are used
for dosimetry: only the
integrated signal over time is
relevant (charge collection
similar to solid state detectors,
see next week)
•
Most particle detectors are in
the proportional mode where
the total number of secondary
electrons is proportional to the
number of primary electrons
•
Geiger counter discharges at
every single ‘event’
Fernow, page 206
The development of avalanches
With a wire potential below a certain value, no avalanche will occur, and all primary charge is
simply collected onto the wire (gas gain = 1). With increased (positive) wire potential,
avalanches develop and a larger charge signal is collected on the wire. Up to gas gains of 100
k, the charge signal is proportional to the primary ionisation. The detectors is in the
proportional mode.
Avalanche close to wire
F.Sauli, CERN-77-09
Elementary gaseous detecor:
The Proportional tube
The charge signal from the wire
is proportional to the primary
ionistation charge
Time development
The charge signal development, as it appears on the wire of a proportional counter, is mainly
determined by the displacement of the ions, created in the avalanche, moving from the wire
surface towards the cathode tube. This causes the long ‘ion tail’ in the signal, which can be
overcome by means of a differentiating filter.
Multi Wire Proportional Chamber
•
•
•
•
The signal on the wire provides
the coordinate in 1-dimension
The resolution is determined by
the wire pitch P. No drift time:
σ = P /√12
Typical dimension
– Thickness 10 mm
– Pitch 2 mm
– Surface 1000 x 1000 mm
Mind the electrostatic force
between the two wires
The Multi Wire Proportional Chamber (Wire Chamber). A particle crossing the detector
plane vertically will, in general, result in one wire to give a (negative) charge signal. Tilted
tracks crossing more than one ‘cell’, associated with a wire, may ‘fire’ more (adjacent) wires.
Drift Chamber
• After the MWPC introduction of the drift time
measurement
• To improve the electric field configuration field wires are
used
The drift chamber. The moment of passage of a particle is known from another detector (i.e.
a scintillator). The primary electrons drift towards the anode wire in a homogeneous electric
field. Arrived at the anode wire an avalanche occurs and a charge signal appears. The time
period between the scintillater signal and the signal from the wire and is called the drift time.
This drift time can be converted to a drift distance if the drift velocity is (empirically) known.
In order to provide a homogeneous drift field, more wires are applied, put a suitable potential,
possibly by means of a resistor chain.
Drift velocity as a function of the electric field for several gas mixing rations Ar/Ethane
In the right-hand figure above, a multi-sampling drift chambers is shown. The position of a
track, crossing a drift cell, can be accurately reconstructed.
-
Lorentz Angle
+
B is perp. to image
Due to the Lorentz force, the direction of drifting electrons is affected by the presence of a
magnetic field (this field is required for the measurement of the momentum of charged
particles).
In planar drift chambers (depicted above), the Lorentz force can be compensated by a tilt of
the electric field. The potential of the field wires is carefully set.
Cylindrical Wire chamber
• In a Drift Chamber the drift is orthogonal to the wires and
also to the magnetic field. Lorentz angle is important
B
E in plane
electron
With field wires individual driftcells
can be created
Cylindrical Drift Chamber
C.Joram. CERN Academic Training 1997-1998
Drift chambers have been made for many applications, in a large variety of geometries.
An important application has been the vertex detector (see Semiconductor Detectors). These
detectors are placed as close as possible to the interaction point of collider experiments. These
detectors must measure tracks with the best possible spatial resolution, and must be as light as
possible in order not to disturb tracks (multiple scattering).
Diffusion
Diffusion. Primary electrons on a track left by a passing particle drift towards the anode wire.
Allthough the primary electrons have a common drift velocity, they will spread due to the
stochastic drift process. This diffusion is proportional to the square root of the drift time, and
thus the drift distance. The diffusion has a Gaussian character and the spatial spread can be
described by σdiff. Per electron the diffusion can be approached as σdiff = σd sqrt(D), where D
equals the drift length (often in cm), and σd a constant (gas property) in µm/sqrt(cm).
The values for longitudinal and transversal diffusion are different, in general.
Gas Gain
• The increase in the number of electron after travelling a
distance y+dy
dN
= αN
dy
α = Townsend first multipication coefficient
• The electron multiplication or gain is
M(x) = e αx
• The maximum gain is up to 107
Fernow, page 214
Performance (=precision) of Drift Chambers
‘Spatial resolution’
‘ time resolution’
- timing measurement: slewing; avalanche fluctuations, noise
- fluctuation in # primary electrons
- electron diffusion during drift
- variation in drift velocity
- E-field variation
- Temperature and pressure variations
wire
el.
el.
δt
Track
Sci
el.
Beam
Cross
t0
Time measurement:
- time interval (tstop – tstart)
All (cable) delays to be measured?
The calibration of drift chambers. First the time measurements must be corrected for the
delays due to signal propagation over wires and cables, and for the propagation of the particle
itself. The calibration system is usually integrated in the design of the electronic readout
system.
Drift Time Spectrum
Irradiate chamber homogeneously (with cosmics)
time spectrum
dift distance
t0
tmax
D-T relation
Dmax R-T relation
t0
tmax
drift time
The drift velocity is the main parameter to be known when operating drift chambers. In a
planar drift chamber with a constant drift velocity, and thus a linear relation between drift
time and drift distance (linear r-t relation), the drift time spectrum has a flat distribution if we
take care for a homogeneous irradiation (by particle tracks, or by a radioactive source). In an
experiment one can often use the data itself for obtaining these spectra.
Round ATLAS drift tube
dia. 30 mm
Above the drift time spectrum of a drift tube is shown. Since the electric field is proportional
with 1/R, the drift velocity is by no means constant within the tube. The empirical r-t relation
can be obtained by integration of the drift time spectrum.
Y :: 1/ Vdrift
Vdrift = dR/dt
D-t relation is integral
of drift time spectrum
The r-t relation of a drift chamber with hexagonal cells, obtained from the drift time spectrum.
Note the significant tail, associated with very long drift times. This is due to areas in the drift
cell with a very low electric field.
Time Projection Chamber (TPC): 2D/3D Drift Chamber
The Ultimate Wire (drift) Chamber
track of
charged
particle
E-field
(and B-field)
Wire plane
Wire Plane
+
Readout Pads
Pad plane
Time Projection Chamber
• Large drift volume of several meters to measure accurate
drift time.
• Electron drifts parallel to B-field
• Avalanche induces signal on sense wires & readout pads
Electron drift
Avalanche
Readout pads
Sense wires
The principle of the Time Projection Chamber.
If the magnetic field is parallel to the electric field, the diffusion is reduced, much to the
benefit of the precision of track position measurements.
With a magnetic field in the direction of the drift
field the transverse diffusion will be reduced:
ρ sin θ/2
under influence of the magnetic field
the distance transverse to the direction
of the fields between successive collisions
φ
is equal to 2ρ sin(θ/2), with θ=vT∆t/ρ.
ρ
Projected on one of the transverse directions
the result is: 2ρ sin (vT∆t/(2ρ) ) cos φ.
The average of the square of the displacement
in one of the transverse directions becomes:
s
2
⊥,1D
1 direction
=
∫
2π
0
cos 2 φ dφ
∫
2π
0
dφ
∞
vTt
∫ (2ρsin 2ρ )
0
Averaging over all
angles -> π/(2π) =1/2
Without magnetic field:
D T.B =
Dno B
s
2
⊥,1D
=
2
θ/2
1 2τ 2 v 2T
e− t / τ
τ 2 v 2T
dt =
=
2 2
2
2 1+ τ v T /ρ 1+ ω 2 τ 2
τ
∫
2π
0
cos 2 φ dφ
∫
2π
0
dφ
∞
∫v
0
2 2
T
t
e−t / τ
dt = v 2T τ 2
τ
1+ ω 2 τ 2
Gas Mixtures: Ageing
• At the electrodes (anode or cathode) a deposit can develop.
This reduces the liftime of the detector. The deposit can be
formed by polymers. Addition of gas with a low ionisation
potential can be put to avoid polymerisation (like
propylalcohol (C3H7OH) or methylacohol(OCH3CH2)
• If instead an inorganic quenching gas is used
polymerisation can be avoided altogether (like CO2 which
is has the advantage of being un-inflammable but it is
‘toxic’)
The negative effect of the deposit (in terms of the variation of the signal amplitude) is about
proportional with the total collected charge per unit surface on the electrode (or, more
practically, the total collected charge per unit wire length).
Ageing is very important for chambers placed in a high-radiation environment.
× 100%
C.Joram. CERN Academic Training 1997-1998
Unsolved issue for gaseous detectors: ageing.
× 100%
C.Joram. CERN Academic Training 1997-1998
Micro Pattern Gas Chambers (MPGCs)
Get rid of wire: granularity, 2D versus 1D
- MSGC
- Micromegas
- GEM
- GridPix chambers
Micro Strip Gas Counter
Wire chambers:
MSGCs:
granularity ~ 1 mm
granularity 200 µm
No more wires!!
The Micro Strip Gas Counter. Here, the wires are replaced by strips printed on a glass
substrate. These ‘wires’ can be placed with a pitch of ~0. 1 mm.
Not often applied:
…sparks……!
The electric field of a MSGC. Note the local strong field at the glass’ surface near the anode
strip. This causes instabilities which are hard to prevent.
Micro Patterned Gaseous
Detectors (MPGDs
(MPGDs))
• High field created by Gas Gain Grids
GEM
• Most popular: GEM & Micromegas
Micromegas
improved granularity : wire chambers
react on COG of many electron
clouds/clusters
Micro Pattern Gas Detectors. Here, the wires with their strong E-field close to their surface,
ate eliminated and replaced by perforated foils.
The perforated Micromegas foil is placed parallel to the readout anode, at a distance of 50
µm. By applying some 400 V between the foil and the anode, electron avalanches, and thus
gas amplification occurs. A drift field above the foil takes care for the transport of primary
electrons into the holes of the foil.
Time Projection Chamber (TPC): 2D/3D Drift Chamber
The Ultimate Wire (drift) Chamber
track of
charged
particle
E-field
(and B-field)
Wire plane
Wire Plane
+
Readout Pads
Pad plane
The MPGD can be well applied as Time Projection Chamber.
55Fe
Cathode (drift) plane
Micromegas
Drift space: 15 mm
Baseplate
MediPix2 pixel sensor
Brass spacer block
Printed circuit board
Aluminum base plate
Very strong E-field above (CMOS) MediPix!
Layout of the GridPix detector, where the readout anode plane is replaced by a pixel chip.
Each hole in the Micromegas foil is associated with its own pixel.
He/Isobutane
80/20
Modified MediPix
GridPix:
the electronic bubble chamber
14 mm
δ-ray!
Efficiency for
detecting single
electrons:
< 95 %
A cosmic muon observed with the GridPix detector. Note the δ- ray.
Resistive Plate Chambers
cheap, fast signal, good timing
(but noisy)
+
- 10 kV
Principle of the Resistive Plate Chamber. It consists of two parallel conducting plates with
high-resistive layers at their inside face. The high-potential difference appears at the surface
of the high-resistive layers, and a strong electric avalanche field exists between the layers.
The primary ionistation due to a passing charged particle causes a detectable avalanche.