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

3

Geiger-Mueller Tube

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

7

Geiger-Mueller Tube

8

Geiger Tube

How is 900V generated from 1.5V batteries? Diodes are nonlinear circuit elements

that only conduct current in one direction

9

Geiger TubeVoltage doubler

10

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

11

Geiger TubeThis can be extended to an n multiplier

12

Proportional CountersMany different types of gas detectors

have evolved from the proportional counter

13

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

14

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

17

MagnetsATLAS CMS

18

Magnets

19

Momentum ResolutionATLAS muon momentum resolution

20

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

21

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)

22

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

23

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)

24

Cathode SignalConsider the geometry

The cathode charge distribution is given by

Where λ = x/d and Ki are geometry dependent constants

25

Cathode SignalThe shape is

quasi-Lorentzian with a FWHM ~ 1.5 d, where d is the anode-cathode spacing

26

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

27

ATLAS Muon System

28

ATLAS Muon System - Barrel

29

ATLAS CSC’s

30

ATLAS CSC’s

31

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

32

ATLAS CSC’s

33

ATLAS CSC’s

34

ATLAS CSC’s

35

Drift ChambersAnother variation on the MWPC is the

drift chamber

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

37

Drift ChambersPlanar chambers

38

Drift ChambersCDF central

tracker

39

ATLAS MDT’s

40

ATLAS MDT’s

41

ATLAS MDT’s

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

43

Micromegas Detector

44

Micromegas Principle of operation

Bulk micromegas use photolithographic techniques to produce narrow anodes and precise micromesh – anode spacing

45

Micromegas

46

Micromegas

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

49

RPC’s

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

55

Geiger Tube

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