phys586-lec20-ion3

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 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

 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 4

Geiger-Mueller Tube

 Gas    Multiple pulsing can be quenched by the addition of a small amount of chlorine (Cl 2 ) or bromine (Br 2 ) (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

 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

Geiger Tube

 How is 900V generated from 1.5V batteries?

 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



2 

GeV

  

 sin  2  2 0 .

 

  2 

   0 .

LB p T s

  cos  2    2 8  0 .

p T

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 

1  2

3   3  2   



p T



 3  2   0 .

2 8

Magnets

 Solenoid  Large homogeneous field  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

Magnets

Momentum Resolution

 ATLAS muon momentum resolution 19

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 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

 Typical parameters  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 one half or one-third Readout pitch stays the same 26

ATLAS Muon System

ATLAS Muon System - Barrel

ATLAS CSC’s

 Some numbers   16 four-layer CSC’s per side Both r (precision) and f measured for each layer (transverse) position is  Each CSC has 4 x 192 precision strips   Each CSC has 4 x 48 transverse strips 32,000 channels total 31

ATLAS CSC’s

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

 Some numbers   ~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

Micromegas

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

Micromegas

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 re establish E field after avalanche or streamer +++++++++++++++ _ _ _ _ _ _ _ _ _ _ _ Before

 0 .

1 cm 2 +++ +++++ _ _ _ _ _ _ _ 48 After

RPC’s

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

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 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 1 Gy = 100 rad MeV/kg 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?



XW D air D air

 2 .

58  10  4   

kg R

    33 .

97   

J C

   

 0 .

876  10  2   

Gy R

   54

Geiger Tube

Relations

 Absorbed dose and kerma



K col



 1 



is the radiative fraction

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