1. 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)

2. 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

3. 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

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
≈ electron- hole pairs  Q ≈ 4 fC small charge!!!!

5. 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

6. Photon interactions Photoelectirc effect Compton scattering e+e _
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. 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

9. 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

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

11. 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

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

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

15. 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

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. Shaper SHAPER Detector AMPLIFIER Current-Voltage conversion
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. 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. 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…)

21. 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

22. 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.