Resume: Interazione dei Fotoni con la materia … Effetto principale...
Transcript of Resume: Interazione dei Fotoni con la materia … Effetto principale...
Resume:Interazione dei Fotoni con la materia … Effetto principale e dominante (“per noi”): Compton Scattering
σ ÷ Z4-5/E 7/2σ ÷ Z/E
σ ÷ Z2 (alte en)σ ÷ Z2 lnE (basse en)
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REMEMBER ! In detector we detectElectrons ! NOT Photons !
Compton Scattering of Photon with energy E
Scattered Photon Energy E’: E’photon =
The energy difference between photon after and before scatteringis transfered to the electron, which has then kinetic energy T:
Two extrem cases:FORWARD-Scattering angle θ≈0° E’≈E and T≈0 (gamma perde pocco)BACK-Scattering θ≈180° (massima perdita del gamma)
E’=0.5*mec2=256keV (energia del gamma rimanentedopo compton) andTelectron=Egamma-256keV (maximum possible electron energyin one single interaction = ‘Compton edge’)
questo e l’energia RILEVATA, RILASCIATA nel rivelatoreTo detect whole energy we need to summ with another (type) interaction !
Scattered electron Energy T:
• General properties of radiation detectorsa) gas detectorsb) scintillatorc) semiconductord) neutron[e) momentum measurement]
-risoluzione energetica-risposta temporale veloce-tempo di recupero piccolo-linearita in energia-alta efficienza di rivelazione-basso costo-basso invecchiamento-possibilita di distinguere particelle-…
Cosa si chiede ad un buon rivelatore ?
gas filled detectorsGas chamber:Two electrodes “parallel” with oposite charge.
They operate by utilizing the ionisation produced by radiation as it passes through a gas.Such counter consists of two electrodes to which a certain electical potential is applied.Ionizing radiation dissipate part or all of its energy by generatingElectron-ion pairs. They move under the influence of the appliedElectric field. Their motion induced a current on the electrodes,Which can be measured.
For most gasses the average energy
required to produce an electron-ion pair is about 30 eV.
gas filled detectorsFor most gasses th average energy required to
produce an electron-ion pair is about 30 eV,This number takes into account all collissions, including those that lead to excitation.
If a 3-MeV alpha or beta particle deposits all of its energy, it will produce:3*10^6/30 = 10^5 electron-ion pairs.
A typical gas counter has a capacitance of about 50pF.The charge will be collected in a time around 1μsec.If all charge of 3 MeV alpha is collected we have:V=Q/C=10^5*1.6*10^-19 [C/el] / (50*10^-12[Farad])=0.5mVI=Q/t=1.6*10^-8 A
gas detectors – How they are Built:Ionisation chamber:Two electrodes “parallel” with oposite charge.
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Relationship between voltage applied to a gas-filled counterand the charge collected
Relationship between voltage applied to a gas-filled counterand the charge collected
REGION A:Voltage very low,Field not strong,electrons and ions move very slowHigh recombination rate.As Voltage increasesThe recomb-rate decreasesUntil it becomes ZERO.V=Va applied Voltage
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Relationship between voltage applied to a gas-filled counterand the charge collected
REGION BThe charge collectiod staysConstant despite aChange in the voltage,Because the recombinationrate is zero and No new chargeIs produced:IONIZATION REGION applied Voltage
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Relationship between voltage applied to a gas-filled counterand the charge collected
REGION CIncrease of collected chargeBecause electrons produceSecondary ionization.
charge multiplication.Field is very strong thatin a certain fraction of theCounter Volume electronsFrom primary ioiz.Accumulate enough energy Between collisions to produce additionalIonization.
Gas multiplication factor=The ratio of total ioniz. Produced divided primary ioniz.Independent of the primary ionization: PROPORTIONAL to prim ioniz:PROPORTIONAL COUNTER REGION
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Relationship between voltage applied to a gas-filled counterand the charge collected
REGION DField is so strong, that a single electron –ionpair generates an avalancheof electron-ion pairs.Signal with shape and heightIndependent ofPrimary ionization:GEIGER-Mueller-REGION applied Voltage
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Relationship between voltage applied to a gas-filled counterand the charge collected
REGION EIf voltage is higher thanVe, a single ionizingEvent initiates a continuousDischarge.NO counter anymore.Normally the counter willBe DESTROYED !
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Again: Riassunto in un grafico:
The region of operation depends on:
1. Form of chamber (cylindrical, flat, spherical,…)2. Size of wires (in cylindrical counters)3. Gas type (mixture)4. Gas Pressure5. Level of high voltage
Normally counters are manufactured to work in one region ONLY
Next Slides: Some Data
Two possible types:•current produced by motion of induced electron ion pairsis measured directly current (or integrating) chamber
•Charge is transformed into a Pulse pulse chamber
InPlanar GeometryElectrical field is constant
InCylindrical GeometryElectrical field is NOT constant:
With E=-gradV and V(r) from laplace equation we have
Pulse shape of a proportional Counter
The electrons move very fast to anode-wire:Produce secondary ionisation (in small volume around the wire)are collected before ions have moved appreciably.BUT their contribution es negligible, because they crossonly very small fraction of potential difference.Pulse is almost entirely due to motion of ions towards the catode !
Cylindrical Geometry
Potential Energy: U = ½*lenght*C*V2
Voltage pulse in proportional chamberIs a function of time(Kowalski)
PROPORTIONAL CHAMBER
Final PulseShape Planar Geometry
Teoretical
Practical
Practical(differentialcircuit)
Gas multeplication factor in proportional chamberElectromagnetic field inside cylindrical chamber:
The gas molteplicationfactor M, which is equalto the total number of free electronsproduced in the counter when ONE ion pairis produced by incident radiation.
K and V are constants of the GAS
FRISCH GRID
Zone with HV to accelerate
There are various types of particle counting instruments filled with gas:
i.Ionisation chamber counters (no secondary ions are produced)
ii.Proportional counters (secondary ions are produced but the number is proportional with initial energy of the radiation)
iii.Geiger-Müller counters (secondary ions are produced in large numbers and the number of ions is no longer proportional with radiation energy)
The main difference between these 3 types of counters simply depends on the voltage used for charging the condenser.
The performace dependsstrongly on the applied Voltage andthe gas pressureand type of gas!
Relationship between voltage applied to a gas-filled counterand the charge collected