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Gas Detectors [The Oldest Detectors]

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Gas Detectors [The Oldest Detectors] Gas Detectors [the oldest detectors] Basic principle: - gas ionization from radiation interaction; - electric signal originated by ion-electron pairs collected through an electric field. § Ionization chambers § Proportional counters § Geiger Muller counters Same basic principle, different intensity of applied electric field Ionization Process basic signal Pair formation: - direct interaction - secondary process (very energetic electrons can ionize …) # of pairs (e-,ion) depends on particle energy w ∼ 30 eV average energy per pair production: weak dependence on gas and particle type Example: Ep ∼ 1 MeV # pairs ∼ 30000 < 1 ! ! Thermal motion produces diffused charge transport from high energy regions Ions/electrons suffer of multiple collisions with gas molecules and lose energy ⇒ thermal equilibrium with gas, ricombination, … loss of signal v v = a τ = F/m τ v diffusion coefficient v v Electric Field is needed to efficiently collect the charges from interaction place Net effect: superposition of thermal velocity (random) and drift velocity (ordered) Drift velocity v def v E v ∝ µ drift p Typical values: vdrift = 1 m/s Transit time (1 cm distance) = 1 ms !!! [reduced mass compared to ions] Gas admixture 10%Ar+90% methane vdrift v- v+ 90%Ar+10% methane Region of operation of gas detectors 1 Nr. Collected Ions vs. Voltage Loss of original Signal 2 5 Signal proportional to primary ionization # 3 collected 4 ions 3 Signal proportional to primary+secondary ionization 4 1 2 5 Same Signal independent of energy Region of operation depends on ⇒ Ionization Chamber: the simplest gas detector Measured current is true indication of rate of formation of charges E = constant 10-100 V E ∼ 1/r Choice of gas: - air (γ detection) - Ar (more dense) to increase ionzation probability (p ∼ 1 atm) General constituents Mode of operation: § Container filled with gas - current mode [average rate info] § Two isolated electrodes - pulse mode [info on individual event] § High Voltage (low density and Z !!!) Signal type and size: α particle Eα = 3 MeV 5 w = 30 eV ⇒ n =Eα/w = 10 e-ions SMALL signals, amplification is needed ! Force acting to move charges to electrodes Energy absorbed by e- and e+ v v v τ = RC >>tc signal rise time depends on Time dependent signals observed in the detector charge collection time tc v v v v- - Δ v ( The pulse is chopped at a convenient time by a shaping circuit Ionization Chamber pulse mode operation with Frisch grid The fine mashing grid removes the pulse-amplitude dependence on position of interaction: • The grid is maintained to intermediate potential and is transparent to electrons • Incident radiation directed only into active volume Chathode-Grid in PRISMA Shield Ionization Chamber Frisch d = 1.6 cm @LNL d = 16 cm NO resistor ⇒ NO measurable signal The signal derives from electrons drift only ⇒ NO dependence on slow ion motion !!! Application 1 IC for Z identification of reaction products Bhete-Bloch dE mZ 2 × E ∝ × E = mZ 2 dx E Application 2 Ionization chamber 1 µCu (∼ 0.3 µg) of 241Am τ = 432 y - α passing through ionization air-chamber produce costant current - smoke absorbes α’s ⇒ reduced ionization, lower signal, allarm .. - α’a have low penetrability: they are stopped by the plastic of detector NEUTRON detectors Application 3 Detection of secondary events produced in - nuclear reactions: (n.p), (n,α), (n,γ), (n,fission) - nuclear scattering from light charged particles, than detected For slow and thermal n: ∼ 3000 b (n,p) and (n,α) mostly … ∼1/v Best reaction: 10B + n → 7Li* + α γ = 0.48 MeV • Natural abundance of 10B is high: 20% ⇒ material enriched with 10B increase the efficiency • Exotermic reaction: it occurs also for very low n energy For En = 0.025 eV, σ = 3770 b !! ⇒ ideal for n detection ⇒ useful for measuring the flux of thermal n Ionization chamber/proportional counters filled with BF3 gas [typical sized tubes: e.g., 2 - 5 cm diameter] Paraffine as moderator Ideal response Only determination of interaction, NOT energy Real Wall effect Lost Response prevents Lost 7Li Lost Lost 7Li precise energy α α Wall effect measurements Due to partial energy deposition of lost ion Proportional Counters Basic Principle: signal moltiplication to amplify charge produced by original (e-, ion) pair ⇒ Avalanche Formation: by increasing E electrons acquire kinetic energy to ionize … 6 Ethreshold ∼ 10 V/m, p ∼ 1 atm dn = α dx fractional increase of electrons n per unit path length dx Townsend coefficient n(x) = n(0)eαx α = 0 if E < Eth density of electrons increases exponentially in the α ∝ E if E > Eth direction of motion of the avalanche Townsend coefficients Ar-CO2 mixture Best Geometry: Cylindrical Polarity is very important: electrons must by attracted to the center a = anode radius b = cathod radius Electrons are more mobile than ions Example: ⇒ Avalanche looks like a liquid drop with electrons at the head V = 2000 V a = 0.008 cm ⇒ E(a) = 106 V/m b = 1 cm N.B. in planar geometry equivalent field requires V ∼ 105 Volt a = anode radius b = cathod radius Uniform multiplication is obtained only in a confined region d ∼ 5 a Pulse signal from a cylindrical proportional counter τ = RC The pulse is usually cut by an RC differentiating circuit MonteCarlo simulation Total Charge produced (E) Q = n0 e M Gas multiplication Factor M = 102 - 104 Multi Wire Proportional Counter Application [invented by G. Charpak, CERN 1968, Nobel Price in 1992] Detector sensitive to position of radiation interaction: - 2 planes of wires (anodes and coathods) - Δx ∼ 2-3 mm - size ∼ 1 m2 Wire read-out may be very complicated ⇒ read-out of more than one wire at just one end Electric field lines and constant potential energy surface Position reconstructed by charge division Read-Out methods 1. Separate wire read-out complicated and costly (too many wires) 1. Analog methods: - center of gravity - charge division Center of gravity Chatode in strips: More layers: x, y, diagonal … Signals from each strip is stored and center of gravity is calculated correction factor for noise effects Charge Division Charge collected at the two ends of the resistive anode is divided in proportion to the length of the wire from the point at which the charge is injected y Q = A L QA + QB - 3 electrode structure: central cathod 2 anodic wire planes (X and Y) - cathode: 3300 wires of 20µm 0.3 mm spacing negative high voltage: 500-600 V - X plane: 10 x 100 wires each 1mm spacing -- Y plane: common to all cathode, 13 cm 13 cm - 130 wires, 1 m long, 1mm steps 1 m - spatial resolution: ∆X ~ 1mm, ∆Y ~ 2mm MWPPAC: the Focal plane detector of the PRISMA spectrometer at LNL Laboratory (Padova) Focal Plane Detectors of PRISMA: in-beam tests (ε ~ 100%) IC MWPPAC 36 208 o 195 MeV S + Pb, Θlab = 80 dispersion in X Z=28 (DIPOLE) E (a.u.) (a.u.) E Δ Z=16 56 124 X position (channels) 240 MeV Fe+ Sn, Θlab = 70° Z=26 ∆t ~ 300 ps different shapes ∆X = 1 mm due to ∆Y = 2 mm PRISMA optics focusing (a.u.) E ∆E/E < 2% in Y Δ (QUADRUPOLE) ΔZ/Z ~ 60 Y position (channels) E (a.u.) Proportional Scintillator Counter Application Hybrid detector !!! Low field UV High field Relative gain per collected e- With noble gases 75-97 % of energy gained by charge electrons from electric field is converted into light light secondary Energy resolution for low radiation is light 2 times better X 10 than with ordinary proportional counters Geiger-Muller Counters for High Electric Fields: Secondary avalanches are produced by photons emitted by excited atoms M ∼ 1010 Entire volume gas is involved in the process ⇒ Loss of information on space ⇒ Loss of information on energy (NO spectroscopy) ⇒ identical output Noble gases To avoid formation of negative ions being based on multiplication process 400-1200 V depending on the gas portable instruments for radiation monitoring Dead Time The Geiger discharge stops when an high ions (+) concentration reduces the field E below the moltiplication threshold 50-100 µs Second full pulse appears when ALL ions (+) are collected to the cathode ⇒ G-M tubes are limited to application with low counting rates Method to improve effective counting range 1. Time-to-first-count-method: 1-2 ms The high voltage is switched between two values: - Normal operating voltage - lower value below avalanche threshold 2. Self-Quencing: Quenching gas is consumed addition of quench gas (5-10%) (molecule disassociate) with complex molecule absorbing UV photons (ethyl alcohol …) à Limit 109 counts .
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