Detectors for Particle Physicshep.fi.infn.it/ciulli/Site/Tecniche_files/lesson3.pdfDetectors for...

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1 Detectors for Particle Physics

Transcript of Detectors for Particle Physicshep.fi.infn.it/ciulli/Site/Tecniche_files/lesson3.pdfDetectors for...

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Detectors for Particle Physics

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Charged particles loose energy in matter

• Discrete collisions with the atomic electrons of the absorber material.

Collisions with nuclei not important (me<<mN).

• If are big enough ionization.

e-

θ

Instead of ionizing an atom, under certain conditions the photon can also escape from the medium. Emission of Cherenkov and Transition radiation (See later).

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Average differential energy loss

Bethe - Bloch formula

only valid for “heavy” particles (m≥mµ). dE/dx depends only on β, independent of m !

First approximation: medium simply characterized by Z/A ~ electron density

Z/A~0.5

Z/A = 1

“relativistic rise”

“kinematical term”βγ ≈ 3-4 minimum ionizing particles, MIPs

“Fermi plateau”

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OK but then?

• Particles going through a material leave free ions and electrons on their path...

But how can we see them?

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C. T. R. Wilson, 1912, Cloud chamber

First trackingdetector

Nice, but requires to scan a lot of pictures...... is there a faster method?

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Scintillation

Energy deposition by ionizing particle → production of scintillation light (luminescense) → electric signal from photodetector

Two material types: inorganic and organic scintillators

high light output lower light outputbut slow but fast

(100-1000 nsec) (1-10 nsec)

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Inorganic crystalline scintillators (NaI, CsI, BaF2...)

often ≥ 2 time constants: • fast recombination (ns-µs) from activation centre• delayed recombination due to trapping (≈ 100 ms)

Due to the high density and high Z inorganic scintillator are well suited for detection of charged particles, but also of γ.

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Liquid noble gases (LAr, LXe, LKr)

also here one finds 2 time constants: few ns and 100-1000 ns, but same wavelength.

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Organic scintillators

Fast energy transfer via non-radiative dipole-dipole interactions → shift emission to longer wavelengths → longer absorption length and efficient read-out device

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Scintillating fiber tracking

High geometrical flexibility Fine granularity Low mass Fast response (ns) (if fast read out) → first level trigger

Charged particle passing through a stack of scintillating fibers(diam. 1mm)

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Gas detector

The Geiger counter, later further developed and then calledGeiger-Müller counter

First electrical signal from a particle was however obtained in a different way

pulse

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Gas detectors

Fast charged particles ionize the atoms of a gasAssume detector, 1 cm thick, filled with Ar gas:

– (Noise of amplifier ≈1000 e- )

Gas amplification Consider cylindrical field geometry (simplest case):

1 cm

~ 100 e-ion pair

Electrons drift towards the anode wire

Close to the anode wire the field is sufficiently high (some kV/cm), so that e- gain enough energy for further ionization → exponential increase of number of e--ion pairs.

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Signal formation

Avalanche formation within a few wire radii and within t < 1 ns!

Signal induction both on anode and cathode due to moving charges (both electrons and ions).

Electrons collected by anode wire, i.e. dr is small (few µm). Electrons contribute only very little to detected signal (few %).

Ions have to drift back to cathode, i.e. dr is big. Signal duration limited by total ion drift time !

in gas STP for E=1kV/cmvdrift (ion) ~ 1 cm/µsvdrift (e) ~ 10 cm/µsvsound ~ 0.033 cm/µs

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Multi wire proportional chamber

Normally digital readout:spatial resolution limited to

( d=1mm, σx=300 µm )

Typical parameters: L=5mm, d=1mm,awire=20µm.

Measures only one coordinate

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Catode Strip Chambers

Crossed wire planes. Ghost hits. Restricted to low multiplicities. Also stereo planes (crossing under small angle).

1 wire plane + 2 segmented cathode planes

Analog readout of cathode planes. → σ ≈ 100 µm

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Drift chambers

Measure arrival time of electrons at sense wire relative to a time t0.

x

Typical electron drift velocity: 5 cm/µs

The spatial resolution is not limited by the cell size → less wires, less electronics, less support structure than in MWPC.

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Time Projection Chamber

full 3-D track reconstruction

• x-y from wires and segmented cathode of MWPC• z from drift time• in addition dE/dx informations

• Requires precise knowledge of vD → LASER calibration + p,T corrections

Space charge problem from positive ions, drifting back to midwall → gating Gate open Gate closed

ΔVg = 150 V

ALEPH TPC

Ø 3.6M, L=4.4 m

σRφ = 173 µmσz = 740 µm(isolated leptons)

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Momentum measurement

Sagitta from 3 measurements with error σ(x):

for N equidistant measurements, one obtains (R.L. Gluckstern, NIM 24 (1963) 381)

(for N ≥ ≈10)

From measured particle trajectory in magnetic field

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Multiple ScatteringThick material layer → the particle will undergo multiple scattering.

Contribution of multiple scattering to momentum error

X0 is radiation length of the medium (discuss later)

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Silicon detectorsMost detectors make use of reverse biased p-n junctions

thin depletion zone

no free charge carriersin depletion zone

diffusion of e- into p-zone, h+ into n-zone → potential difference stopping diffusion

(A. Peisert, Instrumentation In High Energy Physics, World Scientific)

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Silicon MicrostripsSpatial information by segmenting the p doped layer → single sided microstrip detector.

300µm

SiO2 passivation

readout capacitances

ca. 50-150 µm

defines end of depletion zone + good ohmic contact

Intrinsic resolution 10-40 µm

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Silicon caracteristics E(e--hole pair) = 3.6 eV, (≈ 30 eV for gas detectors).

High specific density (2.33 g/cm3) → ΔE/track length for

M.I.P.’s.: 390 eV/µm ≈ 108 e-h/ µm (average)

High mobility: µe =1450 cm2/Vs, µh = 450 cm2/Vs

Detector production by microelectronic techniques → small

dimensions → fast charge collection (<10 ns).

Rigidity of silicon allows thin self supporting structures. Typical

thickness 300 µm → ≈ 3.2 ⋅104 e-h (average)

But: No charge multiplication mechanism

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Silicon pixel detectors– Segment silicon to diode matrix– Requires sophisticated readout architecture

Low noise and occupancyHuge number of channel ⇒ very expensive!

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Why do we need so precise dets?

• There are particles with very small but not negligible lifetime– B mesons have τ ~ 1.5 psec and mass m ~ 5 GeV– For an energy of 20 GeV, γ ~ 4 – B will travel on average cγτ ~ 2 mm before decaying– We can not put detectors so close to the interaction point,

but we can extrapolate back our tracks from the nearest dets (typically at a radius of 5-10 cm)

• Do you think we can see it?

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A very nice one...

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Primary Vertex

Primary Vertex