Geiger-Mueller Tube


Used in experiments that identified the He nucleus as being the same as the alpha particle
1

Geiger-Mueller Tube
 Operation

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
2

Geiger-Mueller Tube
3

Geiger-Mueller Tube



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




Multiple pulsing can be quenched by the addition of a small amount of chlorine (Cl or bromine (Br
2
) (the quench gas)
2
)
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
 Use

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
6

Geiger-Mueller Tube
7

Geiger Tube


Diodes are nonlinear circuit elements that only conduct current in one direction
8

Geiger Tube
 Voltage doubler
9

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
10

Geiger Tube
 This can be extended to an n multiplier
11

Proportional Counters
 Many different types of gas detectors have evolved from the proportional counter
12

Proportional Counters
 Most 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
13

Momentum Measurement
 Let v, p be perpendicular to B qvB p
T
 mv

GeV



2
L
 sin

2

2
0 .
3 B


T
 m



2
 
0 .
3 LB p
T s
  cos

2
 

2
8

0 .
3 L
2
B
8 p
T
14

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 s
 x
2
 x
1

2 x
3
 
3

2
   p
T
T

 s
 

3

2
 
0 .
3 BL
2
8 p
15

 Solenoid

Large homogeneous field

Magnets
Weak return field in return yoke
 Toroid

Field always perpendicular to p
(ideal)

Large volume

Dead material in beam

Non-uniform field

Complex
16

 ATLAS
Magnets
 CMS
17

18

Momentum Resolution
 ATLAS muon momentum resolution
19

 Nobel prize to Charpak in 1992

Simple idea to extend the proportional tube

Effectively spawned the era of precision high energy physics experiments
20

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)
21

MWPC’s


Anode spacing – 1-2 mm





Anode – cathode spacing – 8 mm
Anode diameter – 25 m m
Anode material – gold plated tungsten
Cathode material – Aluminized mylar or
Cu-Be wire
Typical gain - 10 5
22

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)
23

Cathode Signal
 Consider the geometry
 The cathode charge distribution is given by

Where λ = x/d and K i constants are geometry dependent
24

Cathode Signal
 The shape is quasi-
Lorentzian with a
FWHM ~ 1.5 d, where d is the anode-cathode spacing
25

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
26

ATLAS Muon System
27

ATLAS Muon System - Barrel
28

ATLAS CSC’s
29

ATLAS CSC’s
30

ATLAS CSC’s
 Some numbers


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
31

ATLAS CSC’s
32

ATLAS CSC’s
33

ATLAS CSC’s
34

Drift Chambers
 Another variation on the MWPC is the drift chamber
35

Drift Chambers
 Advantages

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
36

Drift Chambers
 Planar chambers
37

Drift Chambers
 CDF central tracker
38

ATLAS MDT’s
39

ATLAS MDT’s
40

ATLAS MDT’s
41

ATLAS MDT’s



~1200 drift chambers with ~400000 drift tubes
Covers ~5500 m 2



Optical monitoring of relative chamber positions to ~ 30 m m
Ar:CO
2
(93:7) pressurized to 3 bar
Track position resolution ~ 40 m m
42

Micromegas Detector
43

Micromegas
 Principle of operation

Bulk micromegas use photolithographic techniques to produce narrow anodes and precise micromesh – anode spacing
44

Micromegas
45

Micromegas
46

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
47

 Avalanche mode

Like a proportional chamber
 Streamer mode

Small “spark”
RPC’s
 Excellent time resolution

1-2 ns
 In both cases charge must recover to reestablish E field after avalanche or streamer
+++++++++++++++
_ _ _ _ _ _ _ _ _ _ _
Before r

0 .
1 cm
2
+++ +++++
_ _ _ _ _ _ _
48
After

RPC’s
49

ATLAS RPC’s
HV
X readout strips
Bakelite
Plates
Foam
Gas
Grounded planes
Y readout strips
PET spacers
2mm gas gap
8.9kV operating voltage
Graphite electrodes
50

ATLAS RPC’s


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
51

Radiation Units
 Exposure


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)
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
52

Radiation Units
 Absorbed 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 10 12 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
53

Geiger Tube
 Notes

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

XW
D air
D air

2 .
58

10

4



C / kg
R 



33 .
97



J
C 



X

0 .
876

10

2



Gy
R 


54

Geiger Tube
55

 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

Assumes CPE

Limited to photon energies below 3 MeV
56