Transcript phys586-lec20-ion3

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
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Geiger-Mueller Tube
 Operation
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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
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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
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Geiger-Mueller Tube
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Geiger-Mueller Tube
Gas
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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
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Geiger-Mueller Tube
 Gas
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Multiple pulsing can be quenched by the addition
of a small amount of ethyl alcohol, chlorine (Cl2),
or bromine (Br2) (the quench gas)
As mentioned in an earlier lecture, 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
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Geiger-Mueller Tube
 Use
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Geiger tubes are often used as survey
meters to detect or monitor radiation
 They are rarely used as dosimeters but there are
some applications
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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-Mueller Tube
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Geiger Tube
How is 900V generated from 2x1.5V
batteries?
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Diodes are nonlinear circuit elements that
only conduct current in one direction
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Geiger Tube
 Voltage doubler
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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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Geiger Tube
 This can be extended to an n multiplier
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Cockcroft-Walton Accelerator
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Proportional Counters
 Many different types of gas detectors have
evolved from the proportional counter
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Proportional Counters
 Most of these variants were developed to
improve position resolution, rate capability,
and/or cost
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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 Measurement
 Let v, p be perpendicular to B
qvB 
mv2

pT GeV   0.3B T  m
L
 
 sin 
2
2 2
0.3LB

pT

 2 0.3L2 B

s   1  cos   

2
8
8 pT

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Momentum Resolution
 The sagitta s can be determined by at least 3
position measurements
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This is where the position resolution of the
proportional chambers comes in
x1  x3
s  x2 
2
3
 s    x 
2
  pT   s 
pT

s

3
 x 8 p
2
2
0.3BL
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Magnets
 Solenoid
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Large homogeneous
field
Weak return field in
return yoke
Dead material in
beam
 Toroid
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Field always
perpendicular to p
(ideal)
Large volume
Non-uniform field
Complex
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Magnets
 ATLAS
 CMS
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Magnets
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Momentum Resolution
 ATLAS muon momentum resolution
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Multiwire Proportional Chambers (MWPC’s)
 Nobel prize to Charpak in 1992
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Simple idea to extend the proportional tube
Effectively spawned the era of precision high energy
physics experiments
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MWPC’s
 You 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
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Anode spacing – 1-2 mm
Anode – cathode spacing – 8 mm
Anode diameter – 25 mm
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
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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 Signal
 Consider the geometry
 The cathode charge distribution is given by
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Where λ = x/d and Ki are geometry dependent
constants
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Cathode Signal
 The shape is quasiLorentzian 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 onehalf or one-third
 Readout pitch
stays the same
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ATLAS Muon System
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ATLAS Muon System - Barrel
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ATLAS CSC’s
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ATLAS CSC’s
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ATLAS CSC’s
 Some numbers
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16 four-layer CSC’s per side
Both r (precision) and f (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
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ATLAS CSC’s
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ATLAS CSC’s
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ATLAS CSC’s
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Drift Chambers
 Another variation on the MWPC is the drift
chamber
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Drift Chambers
 Advantages
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Better position resolution
Smaller number of channels
 Disadvantages
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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
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Drift Chambers
 Planar chambers
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Drift Chambers
 CDF central tracker
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ATLAS MDT’s
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ATLAS MDT’s
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ATLAS MDT’s
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ATLAS MDT’s
Some numbers
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~1200 drift chambers with ~400000 drift
tubes
Covers ~5500 m2
Optical monitoring of relative chamber
positions to ~ 30mm
Ar:CO2 (93:7) pressurized to 3 bar
Track position resolution ~ 40mm
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Micromegas Detector
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Micromegas
 Principle of operation
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Bulk micromegas use photolithographic techniques
to produce narrow anodes and precise micromesh –
anode spacing
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Micromegas
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Micromegas
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Resistive Plate Chambers (RPC’s)
 Principle of operation
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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
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RPC’s
 Avalanche mode
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Like a proportional
chamber
 Streamer mode
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Small “spark”
 Excellent time
resolution
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1-2 ns
r  0.1 cm 2
 In both cases charge
must recover to reestablish E field after
avalanche or streamer
+++++++++++++++
___________
Before
+++
___
After
+++++
____
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RPC’s
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ATLAS RPC’s
HV
X readout
strips
Y readout
strips
Bakelite
Plates
Gas
Foam
PET spacers
2mm gas gap
8.9kV operating voltage
Grounded
planes
Graphite
electrodes
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ATLAS RPC’s
A few notes on linseed oil
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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
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Babar RPC’s had problems associated with
linseed oil
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Radiation Units
 Exposure
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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 (F = 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)
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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
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Radiation Units
 Absorbed dose
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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
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1 Gy is a relatively large dose
 Radiotherapy doses > 1 Gy
 Diagnostic radiology doses < 0.001 Gy
 Typical background radiation ~ 0.004 Gy
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Geiger Tube
 Notes
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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?
Dairabsorbed
 XW dose in air is
The
Dair
Dair
 C / kg 
J
 2.58 10 
  33.97 
 R 
C 
 2  Gy 
 X  0.876 10 

 R 
4
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Geiger Tube
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Relations
 Absorbed dose and kerma
D  K col  K 1  g 
g is the radiative fraction
g depends on the electron kinetic energy as well as
the material under considerat ion
The above relation assumes CPE
 In theory, one can thus use exposure X to
determine the absorbed dose
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Assumes CPE
Limited to photon energies below 3 MeV
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