Why silicon detectors? Main characteristics of silicon detectors: Small band gap (E g = 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 X 0 for a typical 300  m thickness) E g =1.12 V

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 n+-type implant n-type bulk Charged particle -V +V electron hole P+-type implant Reverse bias E

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  m of Si = 84 keV  3.63 eV to generate a e-h pair in Si  ≈ electron- hole pairs  Q ≈ 4 fC small charge!!!!

Detecting photons 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 n+-type implant n-type bulk -V +V electron hole P+-type implant Reverse bias E photon photoelectron

Photon interactions Photoelectirc effect Compton scattering e + e _ production Mass attenuation coefficient (cm 2 /g) Silicon

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  10 8 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). N-type substrate P+ n+ Al P+ SiO 2 AC coupling to electronics SiO 2 Al DC coupling to electronics

DC vs. AC coupling 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. AlSiO 2 P+ n+ Al P+ SiO 2

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

Silicon microstrip detectors ParameterValue Depth300 μm Strip length10 mm Strip pitch100 μm Depletion voltage20-23 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 The signal from the detector is a small (amplitude ≈ few  A) and fast (τ ≈ ns) current pulse –Signal too small to be transmitted over long distances Need to amplify the signal Detector

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

The problem of noise A signal X fluctuates in time around its average value X 0 –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 x0x0  Noise can be due to External sources Can be screened Fluctuations in the electronic devices Cannot be eliminated, but it must be minimized

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 Detector AMPLIFIER SHAPER 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=t 0 : C 0 capacitor enabled  integrate current beween t 0 and t 1 –At t=t 1 : C 1 capacitor enabled  integrate current beween t 1 and t 2 Advantages: –No loss of information –Exact signal amplitude is read Disadvantages: –Huge amount of data –Transmission of analog data C0C0 C1C1 C2C2

Binary readout architecture 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…) V TH 6.4 mm

ADC readout architecture ADC = Analog to Digital Converter: –Signal above threshold  1 –Signal below threshold  0 Advantages: –Digitized information about amplitude –Robust Disadvantages: –Still large amount of data (especially in large systems) –Mix between digital and analog ADC 1 cm

Complete system Chip RX64 → counts incident photons on each strip of the detector 4 cm 6.4 mm 10 strip = 1 mm micro-bondings 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 Knowing from which strip the electric signal comes from,the position of the incoming X-ray phonton is reconstructed.