Transcript Ionization

Ionization
Measuring Ions
I
E
+
A
-
V
• A beam of charged particles
will ionize gas.
– Particle energy E
– Chamber area A
• An applied field will cause ions
and electrons to separate and
move to charged plates.
– Applied voltage V
– Measured current I
Saturation
• Ion – electron pairs created will
recombine to form neutral atoms.
– High field needed to collect
all pairs
– V > V0
I
I0
• Uniform particle beam creates
constant current.
– Saturation current I0
V0
Ion
Saturation
recombination
V
Saturation Current
• A uniform beam is defined by
fluence rate and energy.
– Intensity is the product
• Energy per area per time:
 
E

• The number of ion pairs N is:
• The number of ions depends on
the gas.
– Ionization energy W
• The saturation current is
proportional to intensity
N  E /W
• The saturation current is:
A
I 0  Ne
 AE eA
e

I0 


W
W
Ionization Energy
• W values measure the average
energy expended per ion pair.
– Electrons uniform with
energy
– Protons above 10 keV
similar to electrons
• W for heavy ions increases at
low energy.
– Excitation instead of
ionization
Gas
He
H2
O2
CO2
CH4
C2H4
Air
Wa
43
36
33
36
29
28
36
Wb (eV/ion pair)
42
36
31
33
27
26
34
Electrometer
Typical Problem
• A good electrometer can
measure a current of 10-16 A.
• What is the corresponding rate
of energy absorption in a
parallel-plate ionization
chamber with W = 30 eV/ip?
Answer
• The energy rate is related to the
intensity.
 A  I 0W
E  
e
– (10-16 C/s)(30 eV)/(1.6 x 10-19
C) = 1.88 x 104 eV/s
– Equivalent to one 18.8 keV
particle per second
Smoke Detector
• Many household smoke detectors are ionization chambers.
– Electric field from a battery
– 241Am alpha source (.5 mg)
• Smoke interrupts saturation current through recombination.
howthingswork.com
Liquid Argon
• Liquid noble gases can be used
in ionization chambers.
– Liquid argon, krypton,
xenon
• An applied field of 1.1 MV/m
used to suppress scintillation in
liquid Ar.
• Focus on an example from the
Dzero electromagnetic
calorimeter.
• Liquid argon parameters
– Density 1.41 g/cm3
– Boiling point 87 K
– W value 23.6 ev/ion pair
Uranium Cell
4.0 2.3 4.3
mm mm mm
depleted
uranium
readout
pad
• Uranium plates are alternated
with readout pads.
– Separated by liquid argon
liquid Ar
gaps
• Readout pads are 5-layer
printed circuit boards.
– Outer readout pads
– Inner layer readout wires
– Ground planes to reduce
crosstalk
– Resistive coat at 2.5 kV
Shower Production
• Uranium acts as an absorber.
– Density 19.05 g/cm3
– Interaction primarily in
uranium
– 4 cm for electromagnetic
incident
particle
depleted
uranium
• Shower particles ionize liquid
argon in the gaps.
– Measured on circuit board
pads
Sampling Calorimeter
• The energy loss in the uranium is much greater than in the argon.
• Ionization is a sample of the particles in the shower.
– Readout signal is proportional to a sample of the shower energy.
• A sampling calorimeter loses some resolution due to statistics.
– Gains in flexibility to construct optimal layer thicknesses
High Voltage Response
Pulse Mode
• Ionization signals are read out
as individual pulses.
– Cell is a capacitor CD
– Total charge dQ
proportional to ionization
CF
iin
+
CD
• Charge sensitive preamp
integrates current pulse to get
charge.
– Measure as voltage change
dQ  CF - dvout 
dvout  -
i dt
dQ
 - in
CF
CF
vout
Pedestal
• Integrating the charge
means selecting a sample
time and initial voltage.
– 2.4 ms
– Subtract the baseline
voltage
• The distribution with no
input signal is the pedestal.
– Asymmetric due to
uranium noise
Cell Noise
• The pedestal is not constant.
– Variation of the pedestal contributes to statistical error.
– Depends on cell capacitance
– High voltage on picks up uranium noise
Resolution
• Total energy for a particle is
due to a sum of N channels.
N
E   Ei
i 1
• Resolution varies with total
energy
• Signal variance S2 depends on a
number of sources of error.
– Statistical channel error s
– Channel crosstalk error c
sE
E

1
E
S 2  Ns 2 + N ( N - 1)c 2
Electromagnetic Calibration
• Electrons of known energy are used to calibrate the cells.
– Initial digital counts Aj
– Cell calibration bj
– Tower calibration a
– Offset for other material d
E  a  b j Aj + d
• Compare beam momentum to measured energy for various energies.
Beam Response
Measured Resolution
• Energy resolution is measured as a
fraction s/E.
– From mean and standard
deviation fit to Gaussian
• Resolution is fit to a quadratic as a
function of momentum
– Channel-to-channel variation C
– Statistical sampling S
– Energy-independent noise N
2
2
S
N
s 
2
+ 2
  C +
p
p
E
2
• Fit results:
– C = 0.003 ± 0.003
– S = 0.157 ± 0.006
– N = 0.29 ± 0.03
• Quoted resolution:
s
E

15.7%
E

GeV
