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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. Geiger-Mueller Tube. Operation Increasing the high voltage in a proportional tube will increase the gain

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Geiger-Mueller Tube


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geiger mueller tube
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
geiger mueller tube2
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
geiger mueller tube4
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
geiger mueller tube5
Geiger-Mueller Tube
  • Gas
    • 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
geiger mueller tube6
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
geiger tube
Geiger Tube
  • How is 900V generated from 1.5V batteries?
    • Diodes are nonlinear circuit elements that only conduct current in one direction
geiger tube9
Geiger Tube
  • Voltage doubler
geiger tube10
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
geiger tube11
Geiger Tube
  • This can be extended to an n multiplier
proportional counters
Proportional Counters
  • Many different types of gas detectors have evolved from the proportional counter
proportional counters13
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
momentum measurement
Momentum Measurement
  • Let v, p be perpendicular to B
momentum resolution
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
magnets
Solenoid

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

Magnets
momentum resolution19
Momentum Resolution
  • ATLAS muon momentum resolution
multiwire proportional chambers mwpc s
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
mwpc s
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)
mwpc s22
MWPC’s
  • Typical parameters
    • 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
cathode strip chambers csc
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)
cathode signal
Cathode Signal
  • Consider the geometry
  • The cathode charge distribution is given by
    • Where λ = x/d and Ki are geometry dependent constants
cathode signal25
Cathode Signal
  • The shape is quasi-Lorentzian with a FWHM ~ 1.5 d, where d is the anode-cathode spacing
cathode signal26
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
atlas csc s31
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
drift chambers
Drift Chambers
  • Another variation on the MWPC is the drift chamber
drift chambers36
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
drift chambers37
Drift Chambers
  • Planar chambers
drift chambers38
Drift Chambers
  • CDF central tracker
atlas mdt s42
ATLAS MDT’s
  • Some numbers
    • ~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
micromegas
Micromegas
  • Principle of operation
    • Bulk micromegas use photolithographic techniques to produce narrow anodes and precise micromesh – anode spacing
resistive plate chambers rpc s
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
rpc s

+++++++++++++++

_ _ _ _ _ _ _ _ _ _ _

Before

+++ +++++

_ _ _ _ _ _ _

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

After

atlas rpc s

Bakelite Plates

X readout strips

Foam

Gas

HV

Grounded planes

Y readout strips

PET spacers

Graphite electrodes

ATLAS RPC’s

2mm gas gap

8.9kV operating voltage

atlas rpc s51
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
radiation units
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
radiation units53
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 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
geiger tube54
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?
    • The absorbed dose in air is
relations
Relations
  • Absorbed 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