Post on 08-Nov-2014
description
Proportional Counters
Amplifying Field
• Gas counters at the ionization plateau collect virtually all ion pairs produced.
• At higher field the electrons gain energy to ionize other atoms.
– More electrons than initial count of ion pairs
– Gas amplification
I
V
E
Avalanche
• Many electrons reach the anode for each initial pair.
– Typically 104 electrons
– An “avalanche”
Proportional Region
• Ionization chambers at increased voltage move from an ionization plateau to the proportional region.
– Counters operating in this region are proportional counters.
Cylindrical Chamber
• Cylindrical geometry is common for proportional counters.
– Grounded outer cathode
– High voltage anode
• The avalanche is limited to a region near the wire.
I
V
)/ln( abr
VE
Single Track
• A single track in a chamber creates many avalanches.
– All contribute to one pulse
• Timing is based on first avalanche arrival.
– Usually nearest point in the field.
• Accurate time-to distance conversion requires uniform field.
Multiwire Proportional Chamber
• An array of proportional readout wires can be placed in an array.
– Invented in 1968 by Georges Charpak
– Used in many discoveries
– Received the 1992 Nobel Prize
• Provides excellent position resolution for charged particle tracks.
• For 90% Ar, 10% CO2
– A = 14 / (cm-Torr)
– B = 180 V/(cm-Torr)
Gas Gain
• Gain in the proportional region is exponential with the wire radius a.
• The Townsend coefficient depends on the field E.
– Adjusted by pressure P
aKeG
EBPAeP
/
Parallel Cathode Chamber
• A parallel plate chamber may have a single anode wire at center.
• The cathodes are at high positive voltage Vp compared to the case.
– 2-3 kV
• The anode is at a higher voltage Vw compared to the case and wire.
– 4-5 kVcathode pads anode wire
D0 central muon drift cell
grounded shell
Equipotentials
Gain Comparison
• The gas gain can be measured by comparing pulse height to voltage difference.
– Field approximated by cylindrical formula.
– Expect 204 V for factor of e
– 250 V yields factor of e
pw VVV 0
)/)((
1
0
0
EBPaV
dV
Drift Velocity
• The important function of a proportional wire chamber is to measure the distance.
– Particle to wire
– Need drift velocity
• The drift velocity also is a factor of the voltage difference.
Drift Linearity
• Conversion of time to distance is easiest with strong linearity.
• Particles are measured externally and compared to test cell.
• At right, noise dominates over non-linearity.
Drift Residuals
• With multiple drift cells resolution can be determined through residuals.
– Three displacements
– Ideal residual equals 0
= 0.31 mm
Cathode Pads
• Measurement along the length of the wire gives a third dimension.
– Timing on wire gives 9 cm to 28 cm resolution
• Dividing the charge on the pads acts like a vernier to subdivide the longitudinal coordinate.
– Repeat pattern longer than wire resolution.
Charge Ratios
• The signal is not as linear in this coordinate.
Pad Residuals
• Resolution is improved by staggering the phase of the pad pattern.
• Residuals can be applied compared to get resolution.
– External wire chambers used for figure at right
= 2.7 mm
Gas Fill
• The avalanche relies in electrons moving toward the anode.
• Electronegative gases like O2 pick up electrons.
– O2 drifts toward anode
– No avalanche
• Preferred gases are noble gases and hydrocarbons
– Hydrocarbons are flammable
– Noble gases excite and emit photons
• Gas mixes can quench photons and extra electrons but remain non-flammable
Oxygen Contamination
• Oxygen is an electronegative impurity.
– Reduced gain
– Increases with impurity
– Equivalent to 110 V drop at wire
• Gain also decreases with distance.
– Greater attachment of ions
Water Contamination
• Water added to the gas causes non-linearities to the drift times.
– Electronegative impurity
– 3000 ppm at left
– Different than O2
• Effect of water is dependent on the field strength.