Ionization

Measuring Ions

• 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

I

V

A

E

Saturation

• Ion – electron pairs created will recombine to form neutral atoms.

– High field needed to collect all pairs

– V > V0

• Uniform particle beam creates constant current.

– Saturation current I0

I0

V0

I

VIon recombination

Saturation

Saturation Current

• A uniform beam is defined by fluence rate and energy.

– Intensity is the product

• The number of ions depends on the gas.

– Ionization energy W

• The saturation current is proportional to intensity

• Energy per area per time:

• The number of ion pairs N is:

• The saturation current is:

E

WEN /

ANeI 0

W

eA

W

AEeI0

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 W W (eV/ion pair)

He 43 42

H2 36 36

O2 33 31

CO2 36 33

CH4 29 27

C2H4 28 26

Air 36 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.

– (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

e

WIAE 0

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

• Uranium plates are alternated with readout pads.

– Separated by liquid argon

• 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

4.0 mm

2.3 mm

4.3 mm

depleted uranium

liquid Ar gaps

readout pad

Shower Production

• Uranium acts as an absorber.

– Density 19.05 g/cm3

– Interaction primarily in uranium

– 4 cm for electromagnetic

• Shower particles ionize liquid argon in the gaps.

– Measured on circuit board pads

depleted uranium

incident particle

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

• Charge sensitive preamp integrates current pulse to get charge.

– Measure as voltage change

outF dvCdQ

F

in

Fout C

dti

C

dQdv

iin vout

CD

CF

Pedestal

• Integrating the charge means selecting a sample time and initial voltage.

– 2.4 s

– 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.

• Resolution varies with total energy

• Signal variance S2 depends on a number of sources of error.

– Statistical channel error – Channel crosstalk error c

222 )1( cNNNS

N

iiEE

1

EEE 1

Electromagnetic Calibration

• Electrons of known energy are used to calibrate the cells.

– Initial digital counts Aj

– Cell calibration j

– Tower calibration – Offset for other material

• Compare beam momentum to measured energy for various energies.

jj AE

Beam Response

Measured Resolution

• Energy resolution is measured as a fraction /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

• Fit results:

– C = 0.003 ± 0.003

– S = 0.157 ± 0.006

– N = 0.29 ± 0.03

• Quoted resolution:

2

222

2

p

N

p

SC

E

GeV%7.15

EE