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Geiger-Mueller Tube
пѓ�Introduced in 1928 by Geiger and
Mueller but still find application today
пЃ®
Used in experiments that identified the He
nucleus as being the same as the alpha
particle
1
Geiger-Mueller Tube
пѓ� Operation
пЃ®
Increasing the high voltage in a proportional tube
will increase the gain
пЃ· The avalanches increase not only the number of
electrons and ions but also the number of excited gas
molecules
пЃ®
пЃ®
пЃ®
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
2
Geiger-Mueller Tube
3
Geiger-Mueller Tube
пѓ�Gas
пЃ®
пЃ®
пЃ®
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
4
Geiger-Mueller Tube
пѓ�Gas
пЃ®
пЃ®
пЃ®
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
5
Geiger-Mueller Tube
пѓ� Use
пЃ®
Geiger tubes are often used as survey
meters to detect or monitor radiation
пЃ· They are rarely used as dosimeters but there are
some applications
пЃ®
пЃ®
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
пЃ· For other gamma ray energies one must account
for differences in efficiency
6
Geiger-Mueller Tube
7
Geiger Tube
пѓ�How is 900V generated from 1.5V
batteries?
пЃ®
Diodes are nonlinear circuit elements that
only conduct current in one direction
8
Geiger Tube
пѓ� Voltage doubler
9
Geiger Tube
пѓ� On one half-cycle, D1 conducts and charges C1
to V
пѓ� On the other half-cycle D2 conducts and
charges C2 to 2V
пѓ� A long string of half-wave doublers is known as
a Cockcroft-Walton multiplier
10
Geiger Tube
пѓ� This can be extended to an n multiplier
11
Proportional Counters
пѓ� Many different types of gas detectors have
evolved from the proportional counter
12
Proportional Counters
пѓ� Most of these variants were developed to
improve position resolution, rate capability,
and/or cost
пЃ®
пЃ®
пЃ®
пЃ®
пЃ®
MWPC (multi-wire proportional tube)
CSC (cathode strip chamber)
Drift chamber (e.g. MDT)
Micromegas (micromesh gaseous detector)
RPC (resistive plate chamber)
пѓ� Nearly every application has made some
attempt to transfer to medical applications
13
Momentum Measurement
пѓ� Let v, p be perpendicular to B
qvB пЂЅ
пЃІ
p T пЃ›GeV
L
2пЃІ
пЃ± пЂЅ
2
mv
пЂЅ sin
пЃќ пЂЅ 0 . 3 B пЃІ пЃ›T
пЃ±
2
п‚»
пѓ—mпЃќ
пЃ±
2
0 . 3 LB
pT
пЃ± пѓ¶
пЃ±
0 .3 L B
пѓ¦
s пЂЅ пЃІ пѓ§ 1 пЂ­ cos пѓ· п‚» пЃІ
пЂЅ
2пѓё
8
8 pT
пѓЁ
2
2
14
Momentum Resolution
пѓ� The sagitta s can be determined by at least 3
position measurements
пЃ®
This is where the position resolution of the
proportional chambers comes in
s пЂЅ x2 пЂ­
pT
пЂ©
2
3
пЃі пЂЁs пЂ© пЂЅ
пЃі пЂЁ pT
x1 пЂ« x 3
2
пЂЅ
пЃі пЂЁx пЂ©
пЃі пЂЁs пЂ©
s
3
пЂЅ
пЃі пЂЁ x пЂ©8 p
2
2
0 . 3 BL
15
Magnets
пѓ� Solenoid
пЃ®
пЃ®
пЃ®
Large homogeneous
field
Weak return field in
return yoke
Dead material in
beam
пѓ� Toroid
пЃ®
пЃ®
пЃ®
пЃ®
Field always
perpendicular to p
(ideal)
Large volume
Non-uniform field
Complex
16
Magnets
пѓ� ATLAS
пѓ� CMS
17
Magnets
18
Momentum Resolution
пѓ� ATLAS muon momentum resolution
19
Multiwire Proportional Chambers (MWPC’s)
пѓ� Nobel prize to Charpak in 1992
пЃ®
пЃ®
Simple idea to extend the proportional tube
Effectively spawned the era of precision high energy
physics experiments
20
MWPC’s
пѓ� You might expect that because of the large C
between the wires, a signal induced on one
wire would be propagated to its neighbors
пѓ� Charpak observed that a positive signal would
be induced on all surrounding electrodes
including the neighbor wires (from the positive
ions moving away)
21
MWPC’s
пѓ�Typical parameters
пЃ®
пЃ®
пЃ®
пЃ®
пЃ®
пЃ®
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
22
Cathode Strip Chambers (CSC)
пѓ� The negative charge induced on the anode
induces positive charge on the cathodes
пЃ®
пЃ®
пЃ®
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 (~100Ојm vs 600Ојm for 2mm/в€љ12)
23
Cathode Signal
пѓ� Consider the geometry
пѓ� The cathode charge distribution is given by
пЃ®
Where О» = x/d and Ki are geometry dependent
constants
24
Cathode Signal
пѓ� The shape is quasiLorentzian with a
FWHM ~ 1.5 d,
where d is the
anode-cathode
spacing
25
Cathode Signal
пѓ� In order to reduce
the number of
readout channels
one can use
capacitive coupling
between strips
пЃ® Strip pitch is onehalf or one-third
пЃ® Readout pitch
stays the same
26
ATLAS Muon System
27
ATLAS Muon System - Barrel
28
ATLAS CSC’s
29
ATLAS CSC’s
30
ATLAS CSC’s
пѓ� Some numbers
пЃ®
пЃ®
16 four-layer CSC’s per side
Both r (precision) and f (transverse) position is
measured for each layer
пЃ· Each CSC has 4 x 192 precision strips
пЃ· Each CSC has 4 x 48 transverse strips
пЃ· 32,000 channels total
31
ATLAS CSC’s
32
ATLAS CSC’s
33
ATLAS CSC’s
34
Drift Chambers
пѓ� Another variation on the MWPC is the drift
chamber
35
Drift Chambers
пѓ� Advantages
пЃ®
пЃ®
Better position resolution
Smaller number of channels
пѓ� Disadvantages
пЃ®
пЃ®
More difficult to construct
Need time measurement
пѓ� The position resolution of drift chambers is
limited by diffusion, primary ionization
statistics, path fluctuations, and electronics
пѓ� Many different geometries are possible
36
Drift Chambers
пѓ� Planar chambers
37
Drift Chambers
пѓ� CDF central tracker
38
ATLAS MDT’s
39
ATLAS MDT’s
40
ATLAS MDT’s
41
ATLAS MDT’s
пѓ�Some numbers
пЃ®
пЃ®
пЃ®
пЃ®
пЃ®
~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
42
Micromegas Detector
43
Micromegas
пѓ� Principle of operation
пЃ®
Bulk micromegas use photolithographic techniques
to produce narrow anodes and precise micromesh –
anode spacing
44
Micromegas
45
Micromegas
46
Resistive Plate Chambers (RPC’s)
пѓ� Principle of operation
пЃ®
пЃ®
пЃ®
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
47
RPC’s
пѓ� Avalanche mode
пЃ®
Like a proportional
chamber
пѓ� Streamer mode
пЃ®
Small “spark”
пѓ� Excellent time
resolution
пЃ®
1-2 ns
r п‚» 0 . 1 cm
пѓ� In both cases charge
must recover to reestablish E field after
avalanche or streamer
+++++++++++++++
___________
Before
+++
___
After
2
+++++
____
48
RPC’s
49
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
50
ATLAS RPC’s
пѓ�A few notes on linseed oil
пЃ®
The linseed oil lowers the current draw
through the gas and the singles rate by a
factor of 5-10
пЃ· It makes a smooth inner surface which gives a uniform
electric field
пЃ· It absorbs UV photons produced in the avalanche
пЃ®
Babar RPC’s had problems associated with
linseed oil
51
Radiation Units
пѓ� Exposure
пЃ®
пЃ®
пЃ®
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
пЃ· Basically a measure of the photon fluence (F = N/A)
integrated over time
пЃ· 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)
пЃ®
Units are C/kg or R (roentgen)
пЃ· 1 R (roentgen) == 2.58 x 10-4 C/kg
пЃ· Somewhat historical unit (R) now but sometimes still
found on radiation monitoring instruments
пЃ· X-ray machine might be given as 5mR/mAs at 70 kVp at
100 cm
52
Radiation Units
пѓ� Absorbed dose
пЃ®
пЃ®
пЃ®
пЃ®
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)
пЃ· 1 Gy = 1 J / kg = 6.24 x 1012 MeV/kg
пЃ· 1 Gy = 100 rad
пЃ®
1 Gy is a relatively large dose
пЃ· Radiotherapy doses > 1 Gy
пЃ· Diagnostic radiology doses < 0.001 Gy
пЃ· Typical background radiation ~ 0.004 Gy
53
Geiger Tube
пѓ� Notes
пЃ®
пЃ®
пЃ®
пЃ®
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?
D air absorbed
пЂЅ XW dose in air is
The
D air пЂЅ 2 . 58 п‚ґ 10
пЂ­4
пѓ¦ C / kg пѓ¶
пѓ¦ J пѓ¶
пѓ§
пѓ· пѓ— 33 . 97 пѓ§ пѓ·
пѓЁ R пѓё
пѓЁC пѓё
D air пЂЅ X пѓ— 0 . 876 п‚ґ 10
пЂ­2
пѓ¦ Gy пѓ¶
пѓ§
пѓ·
пѓЁ R пѓё
54
Geiger Tube
55
Relations
пѓ� Absorbed dose and kerma
D пЂЅ K col пЂЅ K пЂЁ1 пЂ­ g пЂ©
g is the radiative
fraction
g depends on the electron
the material
kinetic
energy as well as
under considerat ion
The above relation
assumes
CPE
пѓ� In theory, one can thus use exposure X to
determine the absorbed dose
пЃ®
пЃ®
Assumes CPE
Limited to photon energies below 3 MeV
56
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