Everything you wanted to know about the TPC but were afraid to ask

85 Pages·2013·13.36 MB·English

Everything you
wanted to
know about
the TPC but
were afraid
to ask
Juniors Day 21.3.2013 Jens Wiechula - Universität Tübingen 1
Everything you
were afraid to
know about
the TPC and
never wanted
to ask
Juniors Day 21.3.2013 Jens Wiechula - Universität Tübingen 2
Outline
• Working principle of a Time Projection Chamber
• TPC basics
• Structure of the ALICE TPC + Auxiliary Systems
• Reconstruction & Calibration
• Performance
• Analysis
Juniors Day 21.3.2013 Jens Wiechula - Universität Tübingen 3
Structure of a TPC
Field cage
High voltage electrode (negative)
)
m
R
u
t
n
e
m R
)
o t
m f
e ri R
d
l r
c
i n e
t o
r r d
a t R
p c i
v
l
e e i
r ( d
u
s d R
a el Driftgas e
e
m fi g
a
(
d ic R t
l r l
e t o
Fi c V
e
c El R
i
t
e
n
g
R
a
M
R
Multiwire proportional chamber (0V)
Juniors Day 21.3.2013 Jens Wiechula - Universität Tübingen 4
Working principle of a TPC
• Two coordinates (x,y)
given by the projection
on the pad plane
• Third coordinate (z)
given by the drift time
and drift velocity (z =
v x t )
Drift Drift
• Anode: 1400 - 1650 V
• Cathode: 0 V
• Gating: -100 ± 90 V
open closed
Gas gain ≈ 2∙104
●
Juniors Day 21.3.2013 Jens Wiechula - Universität Tübingen 5
Why use a TPC
A TPC is the perfect detector for HI collisions …
• almost the whole volume is active
• minimal radiation length (field cage, gas)
• easy pattern recognition (continuous tracks)
• PID information from ionization measurements
Juniors Day 21.3.2013 Jens Wiechula - Universität Tübingen 6
TPC basics
• Energy loss of charged particles
• Ionisation
• Gas amplification
• Drift velocity
• Diffusion
Juniors Day 21.3.2013 Jens Wiechula - Universität Tübingen 7
The Bethe-Bloch-Formula
[ ]
〈 dE 〉 Z 1 2m c2β2 γ2 δ(β)
2 2 e 2
=−4π N ρm c z ln −β −
dx A e A β2 I 2
0
• dE/dx first falls ∝ 1/β2
(kinematic factor)
• a minimum is reached at βγ≈4
(Minimum Ionising Particle - MIP)
• then again rising due to the ln
γ2 term (relativistic rise:
contributions of more distant
particles due to the relativistic
expansion of the transverse E-
Field)
• at high γ the relativistic rise is
cancelled by the “density
effect” (fermi plateau:
polarisation of medium screens
more distant atoms; described by
the δ parameter)
Minimum Ionising Particle (MIP)
Juniors Day 21.3.2013 Jens Wiechula - Universität Tübingen 8
Ionisation
Distinguish between primary and
secondary ionisation:
Atoms become excited or suffer
primary ionisation , electrons
with energies above 100 eV can
make secondary ionisation.
dE
Δ x
Δ E dx
n =n +n = =
total primary secundary
W W
i i
n ≈3…4⋅n
total primary
W = mean energy loss per produced
i
ion pair (W > I ) ≈ 30 eV
i 0
∆E = total energy loss
Juniors Day 21.3.2013 Jens Wiechula - Universität Tübingen 9
Measurement of ionisation
Example: 1 cm gas counter, filled with Neon;
n ≈10 /(cm atm), n ≈30
prim total
1 cm
~ 30 e-Ion pairs
≈ 30 Electron-ion-pairs are hard to detect!
Amplifier noise is typically ≈1000 e- (ENC) !
(cid:222) Number of electrons has to be increased noiselessly!
(cid:222) Gas amplification
Juniors Day 21.3.2013 Jens Wiechula - Universität Tübingen 10