Transcript Proportional Counters

Proportional Counters
Amplifying Field
I
+
• 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
E
-
V
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
I
+
-
• Cylindrical geometry is
common for proportional
counters.
– Grounded outer cathode
– High voltage anode
V
E
r ln( b / a)
• The avalanche is limited to a
region near the wire.
V
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.
Gas Gain
• Gain in the proportional region
is exponential with the wire
radius a.
• The Townsend coefficient a
depends on the field E.
– Adjusted by pressure P
G  Keaa
a
P
 Ae - BP / E
• For 90% Ar, 10% CO2
– A = 14 / (cm-Torr)
– B = 180 V/(cm-Torr)
Parallel Cathode Chamber
• A parallel plate chamber may have
a single anode wire at center.
grounded shell
• 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 kV
cathode pads
anode wire
D0 central muon drift cell
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
V0  Vw - V p
dV0
1

V0
(aa )( BP / E )
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
s = 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
s = 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.