How do we see particles? Detectors and data acquisition for physics experiments

How do we see How do we see particles?particles?

Detectors and data Detectors and data acquisition for acquisition for

physics experimentsphysics experimentsAlessandro ScordoAlessandro ScordoInternational International Masterclass 2013Masterclass 201304/02/2013 LNF, 04/02/2013 LNF, FrascatiFrascati

Telescopes

Human eyes

Microscope

Accelerators

Detectors

But where But where does does

it all start it all start from?from?

Electronic properties of materialsElectronic properties of materials

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

Electronic properties of materialsElectronic properties of materials

Everyone wants to be noble !!!

Water is a good example….

Electronic properties of materialsElectronic properties of materials

Atomic levels Molecular bands

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 reachedEquilibrium !!! … ?

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

Breakdown voltageVbr

Junctions are the basic devices for all semiconductor detectors!

V=RxI

What and how What and how we measure?we measure?

Energy Time Rate

What and how we measure?Momentum (or energy???)

Position Tracks Mass (or energy???)Multiplicity

Measuring energy: Measuring energy: the Bethe Bloch formulathe Bethe Bloch formula

Particles through matterParticles through matter

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.)

A clever example: A clever example: Silicon Drift DetectorsSilicon Drift Detectors

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.

Measuring position: strip detectors

Measuring rate Measuring rate

Particle identification Particle identification via Time of Flight (TOF)via Time of Flight (TOF)

TOF can be used for measurements of mass, energy, momentum (velocity) of a particle (particle identification)

e-

-

-

Particle identification Particle identification via Drift Chamber (DC)via Drift Chamber (DC)

We can identify particles, measuring charge, mass, momentum; we can reconstruct vertices and parent particles

Measuring multiplicityMeasuring multiplicity

Signal coming out from the detecor is then:

QDC spectrum is then composed by several peaks with fixed distance

Measuring multiplicityMeasuring multiplicity

Quantization in your pocket:

e- charge estimation

areadtiQ

t

Qi

iRV

t

t

tot

1

0

Ohm law

Current definition

Charge definition

b (time)

h (Volt Ω)

2

hbQ

C

sV

R

tVQ

nst

mVV

tot12

92

1051052

1025102

2

)525(

)520(

CAA

QQ

AAQQ

preamp

tote

preampetot

195

12

det

det

103,1105105,7

105

Is the result ok?errors…..

st

VV

tVVtARA

Qpreamp

e

9

3

2222

det

105

105

2

1

CQ

CQ

e

e

19

19

103,1

104,0

30 % error due to the big error estimation on measured values of t and V

We got a signal...We got a signal...

and now what?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 : 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

New physicists?

Questions?