Generic Detector. Layers of Detector Systems around Collision Point

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1 Generic Detector Layers of Detector Systems around Collision Point

2 Tracking Detectors Observe particle trajectories in space with as little disturbance as possible 2 use a thin ( gm. cm ) detector Scintillators ( σ : cm) Scintillating fibres ( σ : 15µ ) ( σ : 15µ ) ( σ : 1µ ) Gas trackers Solid state trackers Gas Based Detectors Multiwire proportional chamber Drift Chamber Time projection chamber Gas microstrip GEM (gas electron multiplier)

3 Generic Detector small amplification?

4 Multiwire Proportional Chamber wire spacing = resolution cathode anode wires cathode Drift Chamber measure arrival time of charge = spatial resolution

5 Schematic of Wire Chamber Cell envelope to contain gas gas should not absorb electrons anode wire collects signal field shaping cathode wire mesh pc board Repeat n times

6 3 stages in signal generation 1) Ionization by track passing through cell 2) Ionization drifts in E field time 3) In high E field region near wire, primary ionization electrons gain enough energy to start ionizing the gas - Avalanche - More charges - Charge amplification - Noise free amplifier 7 ~1 microvolt signal if no amplification

7 Gas Amplification

8 Behaviour as Voltage Increased Collection Recombination dominated All charge collected Amplification by gas multiplication Still proportional particle ident Saturation Breakdown Geiger/Mueller Volts

9 Diffusion Ions & electrons diffuse in space E field determines average direction Collisions limit velocity Maximum average velocity =Drift velocity

10 Diffusion Ions and electrons diffuse under influence of electric field Maxwell velocity distribution 8kT v = π m v : 1 cms. v : 1 cms. e From Kinetic theory, after t, linear distribution due to diffusion 2 dn N x = exp dx 4π Dt 4 Dt I + number of particles Diffusion coefficient RMS Spread ( x) 2 σ = ( r) 6 σ = Dt Dt 2-d 3-d about 1mm after 1 sec in air

11 Mobility For a classical gas 2 q kt u µ = 3 π pσ m = E drift velocity electric field qm, ion charge and mass p gas pressure σ ion scattering cross section In argon µ = e 4 µ mns kv cm µ m ns µ.1 I + = kv cm Electrons collected quickly compared to +ve ions

12 Diffusion and Drift Chamber Accuracy 1 D= vλ 3 Diffusion coefficient from kinetic theory λ = 1 kt 2 σ p Mean free path D = π σ p ( kt ) 3 m In argon D : e 1µ 2 Diffusion gives limit on spatial accuracy drift chamber ns To reduce D Lower temperature Raise pressure (reduce mobility)

13 Working Gas Noble gases give multiplication at lowest electric field Polyatomic gases have nonionization energy loss mechanisms Choose cheap noble gas with low ionization potential Krypton X Xenon X Argon OK rare, expensive cheap welding etc Argon Cheap, safe, non-reactive remove electro-negative contaminants O, CO, H O Pure argon limited to gain 3 1 Many excited ions produced during avalanche ( 11.6 ) * + + Ar Ar + γ ev absorbed on cathode ( ) γ + cathode e photo emission returns to anode - breakdown Absorb - quenchers

14 Quenchers ( 11.6 ) * + + Ar Ar + γ ev Absorb γ + X X Poly-atomic gas e.g. Methane Typical gases * Rotational vibrational modes 8% Ar + 2% CH4 G : 9% Ar + 1% C H or add electronegative gas (a bit of poison) X + ( photo electron) X Typical 9% Ar 1% CO2 non-radiative + 7 G : 1 Polymerization Organic quenchers polymerize Deposits on cathodes high resistance ion buildup discharge sparks, broken wires Add non-polymerizing agent water methylal Magic Gas 75% Ar 24.5% 2 ( CH ).5% Freon trace methylal 1% HO CH CH 3 2 3

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16 Gas Admixtures R.S. Orr 29 TRIUMF Summer Institute

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19 Signal from Gas Counter V electrostatic energy of field W dw = 1 2 lcv 2 = lcv dv potential energy of q W = Qφ () r dφ() r dw = Q dr dr Anode charge q moved by dr dv length of counter Q dφ() r = dr lcv dr Cathode potential capacitance/unit length Electrons produced in avalanche close to anode wire Small dr small signal +ve ions drift across whole radius Large dr large signal V electron V ion dφ() r lcvdv = Q dr dr Q dφ() r dv = dr lcv dr ( r) a+ λ φ CV 2πε () r = ln a Q dφ Q a+ λ = dr ln lcv = dr 2πε l a ( r) b Q dφ Q b =+ dr ln lcv = dr 2πε l a + λ V electron a+ λ V ion a+ λ b = ln ln a a+ λ Typically 1% r a R.S. Orr 29 TRIUMF Summer Institute

20 Time Development of Signal Assume All signal comes from ions Start from a V t + Q µ CV Q t = ln 1+ t ln 1 2 = + 4πε πε a 4πε t ( ) ( r) t rt () rt () dv Q dφ V () t = dv = dr = dr dr lcv dr Q lcv = l = a CV 2πε a r a ( ) r Q r t n ln a 2πε l a dr dt r a ( ) r t + = µ E = rr d + µ CV = 2πε 2 = a + + µ CV 2πε t dt + µ CV πε 1 r t Typically get 5% of signal in 1 3 RC differentiation for fast signal T ~7ns R.S. Orr 29 TRIUMF Summer Institute

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