Silicon Detectors and DAQ
principles for a physics
experiment
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
What happens then?
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
How can we describe the situation?
Physicians must be smart and clever….
h+
h+
h+
h+
holes !!!
....and do a smart use of drugs!!!
n doping
p doping
Why ?
p-n Junctions
Non equilibrium situation
Electrons and holes
diffusion
Donors and acceptors
ions field plays
against diffusion and
equilibrium is reached
Fermi level definition
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
Breakdown voltage
Vbr
V=RxI
Junctions are the basic devices
for all semiconductor detectors!
Particles through matter
How can we detect them?
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 antirecombination device
An electric field leads electrons, generated by particle flow (xRays 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
MPPC
Signal coming out from the detecor is then:
QDC spectrum is then
composed by several
pixes with fixed
distance
Questions?
New physicists?
An experience in the lab:
e
charge estimation
V R  i
Q
i
t
Ohm law
Current definition
t1
Qtot   i  dt  area
t0
Charge definition
b (time)
h (Volt Ω)
bh
Q
2
V  (20  5)m V
t  (25  5)ns
V  t 2 102 V  25109 s 
12
Qtot 

 5 10 C
2 R
2  5 10
Qtot  Qe   Adet  Apreamp
Qtot
5 1012
19
Qe  


1
,
3

10
C
5
Adet  Apreamp 7,5 10  5 10
Is the result ok?
errors…..
 t 
V  2
2
Q    V    t
 2R 
 2R 
2
2
V  5 103V
t  5 109 s
Q  1,6 10 C
12
19
Qe   1,3  10 C
Huge errors due to the big
error estimation on measured
values of t and V
Can you do it better ???