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1
Geiger-Mueller Tube
Introduced in 1928 by Geiger and Mueller but still find application today Used in experiments that identified
the He nucleus as being the same as the alpha particle
2
Geiger-Mueller TubeOperation
Increasing the high voltage in a proportional tube will increase the gain The avalanches increase not only the number of
electrons and ions but also the number of excited gas molecules
These (large number of) photons can initiate secondary avalanches some distance away from the initial avalanche by photoelectric absorption in the gas or cathode
Eventually these secondary avalanches envelop the entire length of the anode wire
Space charge buildup from the slow moving ions reduce the effective electric field around the anode and eventually terminate the chain reaction
4
Geiger-Mueller Tube
Gas The main component is often argon or
neon However when the large number of
these noble ions arrive at the cathode and are neutralized, the released energy can cause additional free electrons to be liberated from the cathode
This gives rise to multiple pulsing (avalanches) in the G-M tube
5
Geiger-Mueller TubeGas
Multiple pulsing can be quenched by the addition of a small amount of chlorine (Cl2) or bromine (Br2) (the quench gas)
As we mentioned earlier, collisions between ions and different species of gas molecules tend to transfer the charge to the one with the lowest ionization potential
When the halogen ions are neutralized at the cathode, disassociation can occur rather than extraction of a free electron
6
Geiger-Mueller TubeUse
Geiger tubes are often used as survey meters to detect or monitor radiation They are rarely used as dosimeters but
there are some applications Survey meters generally have units of
CPM or mR/hr but beware/check the calibration information
If calibrated, the survey meter is calibrated to some fixed gamma ray energy For other gamma ray energies one must
account for differences in efficiency
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Geiger Tube
How is 900V generated from 1.5V batteries? Diodes are nonlinear circuit elements
that only conduct current in one direction
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Geiger Tube
On one half-cycle, D1 conducts and charges C1 to V
On the other half-cycle D2 conducts and charges C2 to 2V
A long string of half-wave doublers is known as a Cockcroft-Walton multiplier
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Proportional CountersMany different types of gas detectors
have evolved from the proportional counter
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Proportional CountersMost of these variants were developed
to improve position resolution, rate capability, and/or cost MWPC (multi-wire proportional tube) CSC (cathode strip chamber) Drift chamber (e.g. MDT) Micromegas (micromesh gaseous detector) RPC (resistive plate chamber)
Nearly every application has made some attempt to transfer to medical applications
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Momentum MeasurementLet v, p be perpendicular to
B
T
T
T
p
BLs
p
LB
L
mTBGeVp
mvqvB
8
3.0
82cos1
3.0
22sin
2
3.0
22
2
15
Momentum Resolution The sagitta s can be determined by at
least 3 position measurements This is where the position resolution of the
proportional chambers comes in
2
312
3.0
823
2
3
2
BL
px
s
s
p
p
xs
xxxs
T
T
16
MagnetsSolenoid
Large homogeneous field
Weak return field in return yoke
Dead material in beam
Toroid Field always
perpendicular to p (ideal)
Large volume Non-uniform field Complex
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Multiwire Proportional Chambers (MWPC’s)
Nobel prize to Charpak in 1992 Simple idea to extend the proportional tube Effectively spawned the era of precision high
energy physics experiments
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MWPC’sYou might expect that because of the
large C between the wires, a signal induced on one wire would be propagated to its neighbors
Charpak observed that a positive signal would be induced on all surrounding electrodes including the neighbor wires (from the positive ions moving away)
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MWPC’s
Typical parameters Anode spacing – 1-2 mm Anode – cathode spacing – 8 mm Anode diameter – 25 m Anode material – gold plated tungsten Cathode material – Aluminized mylar
or Cu-Be wire Typical gain - 105
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Cathode Strip Chambers (CSC)
The negative charge induced on the anode induces positive charge on the cathodes This provides a second detectable signal If the surface charge density is sampled by
separate cathode electrodes then the location of the avalanche can be determined
If the cathode pulse heights are well measured the position resolution can be precisely determined (~100μm vs 600μm for 2mm/√12)
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Cathode SignalConsider the geometry
The cathode charge distribution is given by
Where λ = x/d and Ki are geometry dependent constants
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Cathode SignalThe shape is
quasi-Lorentzian with a FWHM ~ 1.5 d, where d is the anode-cathode spacing
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Cathode Signal In order to reduce
the number of readout channels one can use capacitive coupling between strips Strip pitch is
one-half or one-third
Readout pitch stays the same
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ATLAS CSC’sSome numbers
16 four-layer CSC’s per side Both r (precision) and (transverse) position
is measured for each layer Each CSC has 4 x 192 precision strips Each CSC has 4 x 48 transverse strips 32,000 channels total
36
Drift ChambersAdvantages
Better position resolution Smaller number of channels
Disadvantages More difficult to construct Need time measurement
The position resolution of drift chambers is limited by diffusion, primary ionization statistics, path fluctuations, and electronics
Many different geometries are possible
42
ATLAS MDT’s
Some numbers ~1200 drift chambers with ~400000
drift tubes Covers ~5500 m2
Optical monitoring of relative chamber positions to ~ 30m
Ar:CO2 (93:7) pressurized to 3 bar Track position resolution ~ 40m
44
Micromegas Principle of operation
Bulk micromegas use photolithographic techniques to produce narrow anodes and precise micromesh – anode spacing
47
Resistive Plate Chambers (RPC’s)
Principle of operation Very high electric field (few kV/mm) induces
avalanches or streamers in the gap High resistivity material localizes the
avalanche Signal is induced on the readout electrodes
48
RPC’s Avalanche mode
Like a proportional chamber
Streamer mode Small “spark”
Excellent time resolution
1-2 ns In both cases charge
must recover to re-establish E field after avalanche or streamer
+++++++++++++++_ _ _ _ _ _ _ _ _ _ _
Before
2cm 1.0r
+++ +++++_ _ _ _ _ _ _
After
50
ATLAS RPC’s
Bakelite Plates Foam
PET spacers Graphite electrodes
X readout stripsHV
Y readout strips
Grounded planes
Gas
2mm gas gap8.9kV operating voltage
51
ATLAS RPC’s
A few notes on linseed oil The linseed oil lowers the current
draw through the gas and the singles rate by a factor of 5-10 It makes a smooth inner surface which gives a
uniform electric field It absorbs UV photons produced in the avalanche
Babar RPC’s had problems associated with linseed oil
52
Radiation UnitsExposure
Defined for x-ray and gamma rays < 3 MeV Measures the amount of ionization (charge
Q) in a volume of air at STP with mass m X == Q/m
Basically a measure of the photon fluence (= N/A) integrated over time
Assumes that the small test volume is embedded in a sufficiently large volume of irradiation that the number of secondary electrons entering the volume equals the number leave (CPE)
Units are C/kg or R (roentgen) 1 R (roentgen) == 2.58 x 10-4 C/kg Somewhat historical unit (R) now but sometimes
still found on radiation monitoring instruments X-ray machine might be given as 5mR/mAs at 70
kVp at 100 cm
53
Radiation UnitsAbsorbed dose
Energy imparted by ionizing radiation in a volume element of material divided by the mass of the volume
D=E/m Related to biological effects in matter Units are grays (Gy) or rads (R)
1 Gy = 1 J / kg = 6.24 x 1012 MeV/kg 1 Gy = 100 rad
1 Gy is a relatively large dose Radiotherapy doses > 1 Gy Diagnostic radiology doses < 0.001 Gy Typical background radiation ~ 0.004 Gy
54
Geiger TubeNotes
Survey meters generally have units of CPM or mR/hr
Generally the Geiger tube is not used to determine the absorbed dose
The G-M tube scale is in mR/hr – what is the absorbed dose?
The absorbed dose in air is
R
GyXD
C
J
R
kgCD
XWD
air
air
air
2
4
10876.0
97.33/
1058.2
56
RelationsAbsorbed dose and kerma
In theory, one can thus use exposure X to determine the absorbed dose Assumes CPE Limited to photon energies below 3 MeV
CPE assumesrelation above The
ionconsideratunder material the
as wellasenergy kineticelectron on the depends
fraction radiative theis
1
g
g
gKKD col