Silicon Detectors and DAQ Silicon Detectors and DAQ principles for a physics principles for a physics

experimentexperiment

Masterclass 2011, 7-11 February 2011

Alessandro Scordo

Telescopes

Human eyes

Microscope

Accelerators

Detectors

But where does

it all start from?

Electronic properties of materials

Atoms are made of proton, neutrons (nucleus) and electrons

Valence and conduction electrons are responsible for the principal characteristics of different atoms

Electronic properties of materials

Everyone wants to be noble !!!

Water is a good example….

Electronic properties of materials

Atomic levels Molecular bands

Semiconductors

-For low temperature, valence band is full and conduction band is empty

-At higher temperatures, thermal excitations promote some electrons from valence to conduction band

Definition of semiconductor

If some electron is promoted in the conduction band, what may occur?

1) Drift: an external field can move these electrons

2) Multiplication; if the field is strong enough

3) Recombination: if nothing happens, electrons fall back to valence band

What happens then?

How can we describe the situation?

Physicians must be smart and clever….

holes !!!

h+

h+

h+

h+

....and do a smart use of drugs!!!

n doping p dopingWhy ?

p-n Junctions

Fermi level definition

Electrons and holes diffusion

Non equilibrium situation

Donors and acceptors ions field plays against diffusion and equilibrium is reached

Equilibrium !!! … ?

p-n Junctions

Equilibrium is reached when the two Fermi levels are at the same energy

A sort of slope is then created, hard to climb up and easy to roll down!

Equilibrium does not mean immobility!!!

p-n Junctions

Equilibrium means that electrons and holes diffusions are compensating each other!

A depletion region is formed where no charge carriers are present, and if some charged particles are created there they can be detected

The new slide shape of the bands can be used to improve this detection!

Direct polarization of the junction reduce (or reverse) the potential barrier, electrons are provided from the external voltage and current flows in the device

Reverse polarizaton increase the potential barrier and in principle no current flows!If the reverse bias is big enough, multiplication may occur and inverse breakdown let current flow in the device

p-n Junctions

p-n Junctions

Breakdown voltageVbr

Junctions are the basic devices for all semiconductor detectors!

Particles through matter

How can we detect them?

Energy loss; particle interacting with matter

Bethe - Bloch

Particles’ measurements

A particle passes through a silicon thickness, generating e-h pairs

e- and h+ are collected by anode and cathode (be aware of recombination…)

An electric field causes electron flow through the device and created charge can be collected (by capacitor for ex.)

SDD, a clever anti-recombination device

An electric field leads electrons, generated by particle flow (x-Rays or ionizing) to a small collector anode. At the same time holes are immediately removed from electron’s path by cathode strips.

Position measurements: strips !

We got the charge...

and now what?

Analog – Digital conversion

Digital signal; signal is a function of discrete numbers, F(N)

Analog signal; signal is a function of continuous numbers, usually time, F(t)

The world is analogic but Pc and analysis software can only work with digital informations…..

Analog signal have to be converted to digital signals!

Analog – Digital conversion

Sampling Quantization

Analog – Digital conversion

channels

Analog – Digital conversion

In this world…..

….this is poker !!!

Analog – Digital conversion

Converting analog signals into digital signals, some information may be lost … but are they really necessary?

From analog signals to files and histograms:

Data AQuisition methods

DAQ

What are we interested in ? Which information can we get?

Charge Timing Rates

DAQ : Discriminators

DAQ : QDC (charge to digital converter)

QDC values(integer numbers)

Histograms

DAQ : TDC (time to digital converter)

DAQ : Scaler

4 events in 10 seconds Rate = 0,4 Hz

A real example!

MPPC (Multi Pixel Photon Counters) detectors

Each pixel acts like a p-n junction

Breakdown current is used

Output signals are summed

MPPC (Multi Pixel Photon Counters) detectors

MPPCSignal coming out from the detecor is then:

QDC spectrum is then composed by several pixes with fixed distance

Dark Count0 ph

1 ph 2 ph 3 ph

0 ph

1 ph

2 ph

3 ph

Thermal generated faked signals

New physicists?

Questions?