Why silicon detectors? Main characteristics of silicon detectors: Eg=1.12 V Main characteristics of silicon detectors: Small band gap (Eg = 1.12 V)  good resolution in the deposited energy 3.6 eV of deposited energy needed to create a pair of charges, vs. 30 eV in a gas detector Excellent mechanical properties Detector production by means of microelectronic techniques small dimensions spatial resolution of the order of 10 m speed of the order of 10 ns small amount of material (0.003 X0 for a typical 300 m thickness)

Detecting charged particles The impinging charged particles generate electron-hole pairs ionization Electron and holes drift to the electrodes under the effect of the electric field present in the detector volume. The electron-hole current in the detector induces a signal at the electrodes on the detector faces. Metal contact Charged particle -V P+-type implant Reverse bias n-type bulk E electron hole +V n+-type implant

Charged particle detection Energy loss mainly due to ionization Incident particle interacts with external electrons of Si atoms All charged particles ionize Amount of ionization depends on: particle velocity particle charge medium density K L Minimum Ionizing Particle

Charged particle signal evaluation dE/dx values from Bethe-Block formula are average values The ionization process is statistical  fluctuations Thick absorber: many collisions with atoms  Gaussian distribution Thin absorber: few collisions with atoms  Landau distribution Minimum Ionizing Particle dE/dx most probable value in 300 mm of Si = 84 keV 3.63 eV to generate a e-h pair in Si ≈ 25000 electron- hole pairs  Q ≈ 4 fC small charge!!!!

Detecting photons Reverse bias E Photons are not ionizing particles The impinging photons which interact in the detector volume create an electron (via Photoelectric, Compton or Pair Production) The electron ionizes the surrounding atoms generating electron-hole pairs Electron and holes drift to the electrodes under the effect of the electric field present in the detector volume. The electron-hole current in the detector induces a signal at the electrodes on the detector faces. Metal contact photon photoelectron -V P+-type implant Reverse bias n-type bulk E electron hole +V n+-type implant

Photon interactions Photoelectirc effect Compton scattering e+e _ Mass attenuation coefficient (cm2/g) Silicon Photoelectirc effect Compton scattering e+e _ production

Photon signal evaluation Not all photon interact and can be detected Typical X photons in mammography 20 keV (mammography) Photoelectric effect ≈ 30% of incident photons do photons do photoelectric effect in 300 mm of Si ≈ all photon energy converted in electron energy ≈ 5000 electron-hole pairs  Q ≈ 1 fC Small charges Just to compare… in a 1 cm x 1 cm x 300 mm pure Si volume at 25°C there are 4.5108 free e-h pairs due to thermal excitation  need for reverse biased junction  need for amplification

Silicon Microstrips detectors Micro-strip detector: silicon detector segmented in long, narrow elements. Each strip is an independent p-n reverse-biased junction Provides the measurement of one coordinate of the particle’s crossing point with high precision (down to 10 m). SiO2 Al DC coupling to electronics N-type substrate P+ n+ Al SiO2 AC coupling to electronics

DC vs. AC coupling DC coupling: AC coupling: Al SiO2 DC coupling: the readout electronics is connected directly to the strips Problem: the first stage of the preamplifier sinks the leakage current Preamplifier working condition affected by leakage current fluctuations Problems due to radiation damage which make the leakage current increase AC coupling: the readout goes through a decoupling capacitor The decoupling capacitance which must be much larger than the capacitance to the neighbours to ensure good signal collection (over 100 pF). The capacitor is integrated directly on the strips, using as plates the metal line and the implant and a thin SiO2 layer as dielectric. P+ n+ Al SiO2

Silicon microstrip detectors Strip p+ connected to ground, high (40-100 V) positive voltage on backplane n+

Silicon microstrip detectors Parameter Value Depth 300 μm Strip length 10 mm Strip pitch 100 μm Depletion voltage 20-23 V Leakage curr. (22º C) 50-60 pA

Pad and pixel detectors PAD detector: silicon detector segmented in both directions Matrix of small diodes  true 2 dimensional information Problem: difficult interconnection with electronics Solution: PIXEL detectors Readout electronics designed in form of a matrix each channel has exactly the same surface as a detector element Bump bonding: small ball of solder between detector and electronics Higher cost due to complex electronics and bump bonding

How to treat the signal Detector The signal from the detector is a small (amplitude ≈ few mA) and fast (τ ≈ 10-20 ns) current pulse Signal too small to be transmitted over long distances Need to amplify the signal

Amplifier Detector AMPLIFIER Signal increased by a factor of 10 But it’s noisy… Need to put a second stage to decrease noise

The problem of noise A signal X fluctuates in time around its average value X0 The distribution of the signal value follows a normal distribution The s of the gaussian is a measurement of the noise of the system t X x0 s

Low pass filtering Normal solution: put a filter (shaper) after the amplifier Filter = elaboration on the signal consisting in a selection on its frequencies

Shaper SHAPER Detector AMPLIFIER Current-Voltage conversion Changed signal shape (semi-gaussian pulse shaping) Current-Voltage conversion Different time scale

Readout architecture The signal after the shaper is a continuous function (analog) Infinite number of “points” Not good for computer storage and analysis Digital signals: Discrete number of signals in time domain (sampling) Select a finite number of “points” Store the value of the signals in this discrete set of “points” Discrete signal amplitude (digitization) Loss of information The extent of this loss depends on the number of bits used to represent the amplitude Technology: Discrete components vs. integrated (VLSI) circuits

Analog readout architecture Sampling: At t=t0: C0 capacitor enabled  integrate current beween t0 and t1 At t=t1: C1 capacitor enabled  integrate current beween t1 and t2 Advantages: No loss of information Exact signal amplitude is read Disadvantages: Huge amount of data Transmission of analog data

Binary readout architecture VTH 6.4 mm Discriminator: Signal above threshold 1 Signal below threshold  0 Advantages: Simple and fast Small amount of data (good for large detectors with many cahnnels) Disadvantages: Reduced information Threshold scans needed to access to analog quantities (gain, noise…)

ADC readout architecture ADC = Analog to Digital Converter: The larger the number of bins, the smaller the loss of information Advantages: Digitized information about amplitude Robust Disadvantages: Still large amount of data Mix between digital and analog 1 cm

Complete system micro-bondings 4 cm 6.4 mm 10 strip = 1 mm Silicon microstrip detector each strip is an independent detector which gives an electric signal when an X-ray photon crosses it and interacts with a silicon atom 4 cm Chip RX64 → counts incident photons on each strip of the detector 10 strip = 1 mm 6.4 mm micro-bondings Knowing from which strip the electric signal comes from,the position of the incoming X-ray phonton is reconstructed.