+ All Categories
Home > Documents > Gaseous Tracking Detectors

Gaseous Tracking Detectors

Date post: 15-Jan-2016
Category:
Upload: amaris
View: 44 times
Download: 0 times
Share this document with a friend
Description:
Gaseous Tracking Detectors. TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: A A A A A A A A A. P 2. Overview. Straw trackers MWPCs TPCs Micropattern gas detectors Momentum measurement Thanks to Christian Joram and others from whom I borrowed slides. - PowerPoint PPT Presentation
63
Gaseous Tracking Detectors
Transcript
Page 1: Gaseous Tracking Detectors

Gaseous Tracking Detectors

Page 2: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 2

Overview

Straw trackers MWPCs TPCs Micropattern gas detectors Momentum measurementThanks to Christian Joram and others

from whom I borrowed slides

Page 3: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 3

Gaseous tracking detectors

Provides economic tracking over large areas Measures primary ionization of charged tracks in the gas Works by having avalances of secondary ionization

initiated when the primary ionization hits a small-area

anode. This provides built-in amplification.

Page 4: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 4

Signal formation

Page 5: Gaseous Tracking Detectors

P 5

The ATLASTransition Radiation Tracker Installation Design details Simulation and performance Calibration and Alignment Particle ID

An example gaseous tracker

Page 6: Gaseous Tracking Detectors

Peter Hansen, Lecture on gaseous tracking detectors

P 6

Transition Radiation Tracker

Length: Total 6802 cm

Barrel 148 cm

End-cap 257 cm

Outer diameter 206 cm

Inner diameter 96-128 cm

# straws: Total 372 832

Barrel 52 544

End-cap 319 488

# electronic channels 424 576

Weight ~1500 kg

Barrel

End-caps

Gas 70%Xe+27%CO2+3%O2

• Xe for good TR absorption

• CO2 > 6% for maximum operation stability

Gas gain 2.5104

Straw diameter - 4 mm

Wire diameter - 30 μm

Polypropylene foil/fibre radiators

Page 7: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 7

protons

protonsA

tlas

CMS

ATLAS

Page 8: Gaseous Tracking Detectors

Peter Hansen, Lecture on geseous tracking detectors

P 8

TRT performance

Bd0J/ψ Ks

0

6 keV

0.3 keVMIP threshold – precise tracking/drift time determination

TR threshold – electron/pion separation

90% electron efficiency10-2 pion rejection

~1 TR hit

~7 TR hits

High-γ charged particles (e.g. electrons) emit transition radiation (X-rays) when they traverse the radiators, detected in the straw tubes as larger energy deposition (8-10 KeV)

20 GeV beam

Page 9: Gaseous Tracking Detectors

Peter Hansen, Lecture on gaseous tracking detectors

P 9

The TRT Straw

Page 10: Gaseous Tracking Detectors

P 10

TRT commisioning 2008-2009 The TRT barrel and end-caps

were installed in their final position inside the cryostat in 2008 with all services.

One “splash-event” in 2008 was very useful for timing all the channels relative to each other.

Commissioning in 2009 with cosmic rays. The tracking resolution was found to be 160 microns. The High Threshold probability was found as expected.

Page 11: Gaseous Tracking Detectors

The first collisions Dec2009

Peter Hansen, Lecture on gaseous tracking detectors

Page 12: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 12

The TRT gas

70% Xe (high amplification A=25000, absorbs X-rays) 27% CO2 (quenches ultraviolet, does not polymerize)

3% O2 (intercepts unwanted electrons)

Gas Z A ½ Emin Wi (dE/ dx)min np

(g/ cm3) (eV) (eV) (MeV/ gcm¡ 2) ions/ cmXe 54 131.3 5:49 10¡ 3 8.4 22 1.23 44

CO2 22 44 1:86 10¡ 3 5.2 33 1.62 34

Gas Z A ½ Emin Wi (dE/ dx)min np

(g/ cm3) (eV) (eV) (MeV/ gcm¡ 2) ions/ cmXe 54 131.3 5:49 10¡ 3 8.4 22 1.23 44

CO2 22 44 1:86 10¡ 3 5.2 33 1.62 34

Gas Z A ½ Emin Wi (dE/ dx)min np

(g/ cm3) (eV) (eV) (MeV/gcm¡ 2) ions/ cmXe 54 131.3 5:49 10¡ 3 8.4 22 1.23 44

CO2 22 44 1:86 10¡ 3 5.2 33 1.62 34

Gas Z A Emin Wi dE/dx Np

Xe 54 131.3 5.49 10-3 g/cm3

8.4 eV 22 eV 1.23 MeV/(g/cm2)

44 ion/cm

Co2 22 44 1.86 5.2 33 1.62 34

½

Page 13: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 13

Drift of electrons in a B field

New gas stabilizes

drift velocity in B field.

-and it does not eat glass

Page 14: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 14

GEANT4 Simulation

Primary clusters formed according to PAI model

Custom TR physics process

Fiber radiator

Photon x-sect

Page 15: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 15

Digitization simulation

Digitization includes Diffusion and capture Avalance formation Electronics shaping Noise Reflections from ends Propagation along wire TOF and T0 fluctuation Threshold fluctuations

Page 16: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 16

Custom ASIC readout

Page 17: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 17

The electric field

According to Gauss, the capacity per unit length, C, and the anode voltage, V0, determines the electric field:

Integrating from the straw wall to the wire radius and equating

The result to V0, gives

E(r) =CV0

2πε 0r

E(r) =V0

log(b a)

1

r=

0.2V0

r

C =2πε 0

log(b a)= 0.114 pf /cm

Page 18: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 18

Avalance development

The movement of a charge, Q, in a system with capacitance per unit length, C, by a distance dr gives a voltage signal v

Almost all avalance electrons are created in the last mean free path

v =Q / l

CV0

dV

drdr

v − = −Q / l

CV0

dV

drdr = −

Q / l

2πε 0

loga + λ

aa

a +λ

Page 19: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 19

Shaping

The positive ions drift to the cathode gives rise to:

This contribution is about 50 times larger than that of the electrons. But it is a slow signal.

By terminating the wire in a resistance, the signal is differentiated with a time constant, RC.

For the TRT, the rise-time is 8ns and the duration only 20ns.

v + = −Q / l

CV0

dV

drdr = −

Q / l

2πε 0

logb

a + λa +λ

b

Page 20: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 20

Pulse shaping

Problem:

Long tail (much longer than

the 25ns bunch spacing)

from the positive ions

moving outwards

Solution:

The ASDBLR front-end chip

restores the baseline within ~20ns

Page 21: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 21

A simple calculation of A

Balancing concerns, the optimum gas amplification is 25000 In any cascade process, we have

Leading to a total amplification of

At ionization energy, W=22eV, Xe presents a cross-section of

2 10-16 cm2 and the electron has a mean free path of

dN

dr(r) = α (r)N(r)

A = exp | α (r)dr |rthresh

ranode

λ =1

NπrXe2 = 3 • 10−5cm

Page 22: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 22

A simple calculation of A

The distance from the wire, where the avalance starts, is given by:

The Townsend coefficient is assumed to be proportional to the kinetic energy of the electrons

Assuming =log 2 / λ at threshold, we have

eE(rstart )λ = W ⇒ rstart =0.2V0

22V3 • 10−5cm

= 0.06μV0

22V

(r) = kNε (r) = kNeE(r)λ

(r) =0.0063V0

r

Page 23: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 23

A simple calculation of A

Finally we get

This leads for the TRT to the target amplification of 25000 at a voltage of 1513 Volts, probably by luck this is close to the true value 1530 Volts.€

A = exp(0.0063V0 ∗log(0.06

V0

2215

))

Page 24: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 24

Drift Chambers

Page 25: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 25

The driftvelocity - naively

Assuming the electron is brought to halt at each collision and that the mean free path is independent of energy, we have at 1mm from a TRT wire:

The correct answer is

5cm /μs

Page 26: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 26

Complications

The difference between prediction and fact is due to: Dependence on electron energy of cross-section

(mainly the Ramsauer minimum around 1eV) The quencher gas. The magnetic field bending the drift-trajectories up/down Diffusion

Page 27: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 27

modifications from quencher gas

Page 28: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 28

Diffusion (no field case)

The mean velocity of a particle in an ideal gas is given by Maxwell:

According to kinetic theory, a collection of particles localized at x=0 at t=0, will later have a distribution:

Where D is the diffusion coefficient

v =8kT

πm

dN

N=

1

4πDtexp(−

x 2

4Dt)dx

Page 29: Gaseous Tracking Detectors

Peter Hansen, Lecture on gaseous tracking detectors

P 29

The diffusion coefficient

According to statistical mechanics:

where the mean free path for an ideal gas is:

By substitition:

D =1

3vλ

λ =1

2

kT

σP

D =2

3 π

1

σP

(kT)3

m

Page 30: Gaseous Tracking Detectors

Peter Hansen, Lecture on gaseous tracking detectors

P 30

Diffusion in an electric field

A classical argument by Einstein gives for an ideal gas in thermal equilibrium with the drifting ions:

In practice, we parametrize the spread of the coordinate in

the drift direction as:

Where the characteristic energy

can be calculated for known cross-sections and energy-losses

of the electron-gas collisions.

D

μ=

kT

e

σx = 2Dt =2ε k x

eE

εk =eED(E)

v(E)

Page 31: Gaseous Tracking Detectors

Peter Hansen, Lecture on gaseous tracking detectors

P 31

The TRT resolution

The gasses in the TRT have a characteristic energy of about 2 eV. Thus we have for the coordinate perpendicular to the wires a spread of 0.114mm:

For an average of 10 primary ion pairs, the distance of the closest electron to the wire has a spread of about 0.012mm

The drift-time binning in 3.125ns contributes 0.043mm Noise and gain variations gives 0.035mm Uncertainties in wire position and time=0 gives 0.036mm All together this gives a coordinate resolution of about 0.132mm, in

excellent agreement with detailled calculations – and with data.

σx =2 × 2eV × 0.1cm

1530V /0.2eV /cm

Page 32: Gaseous Tracking Detectors

Peter Hansen lecture on gaseous tracking detectors

P 32

Drift time simulation

The leading edge of signal gives the drift time of the ionization electrons and hereby the distance from the charged particle to the wire

The simulation includes diffusion, Lorentz-forces, signal propagation and shaping, channel-to-channel fluctuations in threshold and noise amplitudes (deduced from the observed noise levels), the time structure of noise – and more €

r = vDt

Page 33: Gaseous Tracking Detectors

Peter Hansen lecture on gaseous trackling detectors

P 33

CTB data and simulation

Perfect agreement! But only if using an average threshold of 161eV -

where previous it was 300eV. The explanation is probably new noise and

threshold fluctuations in MC – but there is no profound understanding.

Residuals

fromThomas

Kittelmann thesis

Sigma=0.132mm

100 GeV

pions

Page 34: Gaseous Tracking Detectors

Peter Hansen, lecture on gaseous tracking detectors

P 34

CTB data and simulation

Also the Time Over Threshold is reasonably well simulated

And the hit efficiency is predicted to 95% in agreement with data

Page 35: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 35

Tracking performance in ATLAS

Page 36: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 36

Calibration

Calibration is concerned with T0, the R(t-T0) relation, the high threshold probability and noise removal.

The ”V-plot” of time versus track impact position is used

Page 37: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 37

Calibration

The tip of the V yields T0 (and, if a single wire is plotted,

also the wire position). The peak position in each 3ns bin of t-T0 yields R(t-T0) (Note that the average position is not good because of tails

at long arrival times for tracks passing close to the wire)

Page 38: Gaseous Tracking Detectors

Peter Hansen, Rome seminar, 04-May-07

P 38 Electron Identification in test beam

Performace of combined pion rejection at 90% electron efficiency

Universality of the HT probability

Page 39: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 39

Multiwire proportional counters

In 1968 it was shown by Charpak that an array of many closely spaced anode-wires in the same chamber could each act as an independent proportional counter.

This provided an affordable way of measuring particles over large areas, and the technique was quickly adopted in high energy physics.

Later it has found applications in all kinds of imaging of X-rays or particles from radioactive decay.

Page 40: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 40

Multiwire proportional counters

Page 41: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 41

Second coordinate –some ideas

Page 42: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 42

The TPC

Page 43: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 43

The TPC end-plate

Page 44: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 44

The TPC field cage

Experience shows that the greatest challenge in a TPC is to maintain a constant axial electric field.

This field is made by electrodes on the inner and outer cylindersof the Field Cage, connected to a resistor chain.

Some useful elements are: A Gating Grid to avoid space-charges. Tight mechanical tolerances wrt the ideal cylindrical shape (while

keeping the material budget low). Severe cleaniness, (the tiniest piece of fiber in the cage may short-cut

two electrodes and distort the field.) As little as possible of insulator exposed to the drift-volume to avoid

build-up of charge on the insulator. Perfect matching of equipotential surfaces at the end plane is needed to

avoid transverse field components.

Page 45: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 45

Electron drift in E and B fields

The TPC is immersed in a magnetic field parallel to the electric field.

this Langevin equation becomes in the stationary state

Introducing the mobility

and the cyclotron frequency

we get

mdv

dt= eE + e(v × B ) + Q(t)

0 = eE + e(v D × B ) −m

τv D

μ =eτ

m

ω =eB

m

v D = μE +τv D × ω

Page 46: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 46

Electron drift in E and B fields

Solving for the drift velocity:

Since the coordinate resolution is high in the azimuthal direction, in order to measure the momentum well, a component of the velocity due to field distortions is very dangerous. But high ω helps!

At high ω, vD is suppressed by powers of ω

except for the effect of a B component. However, B is zero on the average according to Ampere.

v D =μE

1+ω 2τ 2 [ ˆ E +ωτ ˆ E × ˆ B +ω 2τ 2( ˆ E • ˆ B ) ˆ B ]

φ

Page 47: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 47

Electron diffusion in E and B fields

In the transverse projection an electron follows the arc of a circle with radius = vT /ω, where the mean squared velocity projected onto the transverse plane is:

After a time, t, the electron has reached a transverse distance of

so the spread after one collision is:

vT2 =

2λ2

3τ 2

2ρ sinvT t

δ 2 =1

2

dt

texp(−

t

τ)[2ρ sin

vT t

2ρ]2 =

1

2

τ 2vT2

1+ω 2τ 20

Page 48: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 48

Electron diffusion in E and B fields

After a longer time, the transverse spread is

Thus in large magnetic fields the transverse diffusion is reduced by a factor 1+ω22

e.g. for Ar/Ethane and B=1.5Tesla the reduction is a factor 50. This is what makes a TPC possible!

Thus you can get a precision of about

for about 30 points on each track over a 1m-2m radial distance from the collision point.

Note that this is without any significant multiple scattering and with a good resolution also in the longitudinal direction.

It is also relatively CHEAP, since it is mainly gas.

σT2 (t) =

t

2

τvT2

1+ω 2τ 2 = tD0

1+ω 2τ 2

200μ

Page 49: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 49

The ALEPH TPC

Page 50: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 50

TPC calibration

Page 51: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 51

Why use silicon instead of gas

Page 52: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 52

Micropattern gaseous detectors

Silicon detectors are still very expensive over large areas,

and they suffer from radiation damage. Although they give 10000 more charges, they have no

inbuilt amplification and are slow to read out. So if gaseous counters could be made small, fast and

precise, they would be quite attractive. The way to go is to employ the same precision methods for

fabricating micro-structures in silicon on the gas detector readouts.

Page 53: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 53

Micro gaseous detectors

Page 54: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 54 Gas Electron Multipliers (no spark)

Page 55: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 55

Two GEM’s are better than one

Page 56: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 56

Thin gap chambers

Page 57: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 57

Resistive plate chambers

Page 58: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 58

From coordinates to momenta

Given a number of ”track hit-coordinates” from the various detector elements, the parameters of the track is determined from a least squares fit.

In a solenoidal field geometry, the parameters are those of a helix:

The track position at closest approach (perigee) to the beamline (3 parameters)

The angle of the track to the beamline (polar angle) The signed curvature +-1/R

Page 59: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 59

Momentum measurement

Page 60: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 60

Momentum accuracy

Page 61: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 61

Multiple scattering

Page 62: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 62

Total momentum error

Page 63: Gaseous Tracking Detectors

Peter Hansen, Lecture on tracking detectors

P 63

Technology choices

Since the TPC is so superior why is it not used at the LHC?

This is because it is too slow. It is in fact used in ALICE who

do not depend on a very high intensity beam.

Why then are the micropattern gas detectors not used?

Well, they are for trigger chambers. But for the main

trackers the technology was deemed too risky at the time of

decision.


Recommended