Why silicon detectors?

1. Eg=1.12 V Why silicon detectors? • 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)

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

3. L K Minimum Ionizing Particle 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

4. 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!!!!

5. photon photoelectron E 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 -V P+-type implant Reverse bias n-type bulk electron hole +V n+-type implant

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

7. 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

8. DC coupling to electronics Al SiO2 N-type substrate AC coupling to electronics Al SiO2 P+ P+ n+ 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).

9. Al SiO2 Al SiO2 P+ P+ n+ 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.

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

11. Silicon microstrip detectors

12. 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

13. Detector How to treat the signal • 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

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

15. X s x0 t 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

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

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

18. 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

19. C0 C1 C2 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

20. VTH 6.4 mm 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…)

21. ADC 1cm 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

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