Download Radiation Detectors

Radiation Sensors
Zachariadou K. | TEI of Piraeus
Radiation Sensors
Part-V
Semiconductor Sensors
Part-V
Semiconductor Sensors
The course is largely based on :
 G. F. Knoll, “Radiation detection and measurement” ; 3rd ed., New
York, Wiley, 2000
 Gordon Gilmore & John D. Hemingway, “ Practical Gamma-Ray
Spectrometry”; Willey , 21008
Types of detectors
Gas detectors
Gas-filled detectors consist of a volume of gas between two electrodes
Scintillators
the interaction of ionizing radiation produces UV and/or visible light
Solid state detectors
crystals of silicon, germanium, or other materials to which trace amounts
of impurity atoms have been added so that they act as diodes
Other , Cerenkov etc…
Semiconductor Detectors
Semiconductor Detectors
They work on same principle as gas-filled detectors:
Gas-filled detectors: production of ion pair
Semiconductors: production of electron-hole pairs
Advantages
Drawbacks
 Only 3eV are required for ionization compared
to about 30eV required to create an ion pair in
typical gas filled detectors
 Limitation to small sizes
Good stability
 Radiation-induced damage
 Thin entrance windows
 Need for cooling (thermal
noise)
Simplicity in operation
High Energy resolution
Compact size
 Fast timing characteristics
 Variable effective thickness to match the
requirements of the application
Band structure-Carriers in an
Electric Field
Insulators: Eg>5eV
Conduction Band
-
-
+
+
-
+
Valence Band
Semiconductors: Eg~1eV
Eo + Eg
Air: >35eV
EF
Scintillators~15eV
Eo
Thermal excitation: A valence
electron gains sufficient thermal
energy to be elevated to the
conduction band
0
Probability per unit
time for thermal
excitation:
P(T )
 Eg

2 KT
 CT 3 2 e 



Depends on the ratio of Eg
over the absolute
temperature
Materials with large Eg have low
thermal excitation probability
Migration in an electric field
 At low to moderate
values of the electric field
intensity:
The drift velocity is
proportional to the electric
field
v h  h  E
v e  e  E
In Gases:
mobility of free electrons >>
mobility of positive ion
In Semiconductors:
mobility of electrons ~ mobility
of holes
7
 At higher electric field:
the drift velocity rises slowly
with the field and reaches a
saturation velocity:
Saturation velocity : v e ~ 10 cm s
Time to collect the carriers over
typical dimensions (0.1cm) :
t  10 ns
Semiconductors are among the fastestresponding radiation detectors
Semiconductors
basics
Ionization energy
Unbiased p-n junction
If it functions as a detector
This detector would very poor performance
n
e-
 Charge carriers migrate
across the junction
p
+
+
o
-
 Conduction electrons in the pside will combine with holes
vice versa
-
 Accumulated space charges create an electric field
that opposites the conduction carriers migration
Depletion region
The space charges do not contribute to conductivity.
The depletion region has very high resistivity electron-holes pairs
created by the passage of radiation will be swept out 
their motion is an electrical signal.
p
n
e-
+
-
+
-
o
eND
 (x)  
eNA
d 2 (x)
ρ(x)
dt
2
a x0
0 xb

 (x)
ε
The thickness of the
depletion region is small
E(x)
d
E
dx
Contact potential :~1V
V(x)
V
The electric field is not enough to
make the charge carriers to move
fast incomplete charge collection
-a
0
b
biased p-n junction
p
n
+
-
e-
o
+
 Forward bias
The contact potential is
reduced by bias V
Large currents are conducted
p
n
 Reverse bias
+
-
e-
o
+
-
The contact potential is
increased by bias V
The minority carriers are
attracted across the junction.
The reverse current is very low
+
e-
-
+
eND
eNA
o
 (x)  
-
d 2 (x)
ρ(x)
dt
2

 (x)
ε
a x0
0 xb
d (-a) d (b)

0
dt
dt
 eN D
x  a   a  x  0

d  
E

dx  eN A
x  b  0  x  b

 
E(x)
Reverse bias V
 (a)  V  (b)  0
V(x)
V
-a
0
b
 eND
2



x

a
V  a  x  0
 2
 ( x)  
 eNA x  b 2 0  x  b
 2
e-
For x=0:
+
-
+
-
V
o
eND 2 eNA 2
a 
b
2
2
d ab b 
N D a  N Ab
if
2V
d
eN
N D  N A
Resistivity :

where μ is the mobility of the
majority carrier
 eND
2
 2 x  a   V  a  x  0
 ( x)  
 eNA x  b 2 0  x  b
 2
1
 
N= dopant concentration
2V
d
μρ
High resistivity(ρ)  large depletion
region (d) (detecting region)
e-
+
-
+
-
o
d
Higher reverse bias
The capacitance
per unit area
decreases
Small capacitance means less
electronic noise resulting to better
energy resolution
2V
μρ
Thicker depletion region
C
ε
Ne

d
2V
We use largest
possible voltage
up to fully deplete
the junction
Semiconductor Radiation
sensors
The module is reverse-biased-->a depletion region is set up with an electric-field that
sweeps charge-carriers to the electrodes.
 When a charged particle passes across the silicon strip electron-hole pairs are
created.
The electric field in the depletion region sweeps the new electron-hole pairs to the
electrodes where they are collected
 The time taken for collection decreases as the bias voltage is increased. In a silicon
detector 300 m thick, electrons are collected in about 10 ns and holes in about 25 ns.
Semiconductor Radiation
sensors
 Germanium  need for
cryogenics
Energy to create +- pair = 2.9 eV
Silicon can be used at room
temperature.
Energy needed to create +- pair= 3.6
eV
Less performance for energetic
radiation such as  rays (it s a light
material : atomic number 14)
 CdTe is the most often used
because it combines heavy
materials (atomic numbers 48 and
52) with relatively high bandgap
energies.
Why Ge over Si ?
ZGe > ZSi (32 vs 14)
 photo-electric effect x 60
 Compton scattering x 2
Semiconductor detectors
Operational characteristics
 Leakage current
 Noise and Energy Resolution
 Bias voltage
 Pulse rise time
 Radiation damage
 Channeling
 Entrance window
 Energy calibration
 Pulse height defect
Leakage current
Deteriorates
energy resolution
Bulk leakage current
Minority carrier current – Mostly small
thermal generation of electron-hole pairs
in the depletion region– Need for cooling
Silicon resistivity : 50,000Ωcm
For bulk 1cm2
R=5000Ω
Leakage current I=0.1A
If V=500V
The current is of the order of 10-6A by
a pulse of 105 radiation induced
carriers
Surface leakage current
The Leakage current must not
exceed 10-9A
Contamination of the surfaces
clean techniques
Detector noise
For silicon diode detectors 3 contributions to noise are most
significant:
 Fluctuations in the bulk generated
leakage current
Parallel noise
 Fluctuations in the surface leakage
current
 Poor electrical contacts
series noise
Detector Bias Voltage
Incomplete charge collection.
 Low Bias Voltage & electric field:
 Sufficiently high Bias Voltage for
complete charge collection  saturation
region
 Higher Bias Voltage  multiplication
region
The pulse height rises with
applied voltage
Corresponds to the region of
ion saturation in a gas-filled
ion detector
The electrons liberated by the
incident radiation gain enough
energy from the electric field to
create further electron-hole pairs .
Basis of the operation of silicon
avalanche detectors`
Pulse rise time
Semiconductors are among the fastest radiation detectors.
Pulse rise time of the order of 10ns or less
The rise time of the output pulse limited by the time required for complete
migration of the electrons-holes created by the incident radiation from
their point of formation to the opposite extremes of the depletion region
The time is minimized with
 High electric field
 Small depletion width
Dead layer
Energy loss before the particle reaches the active volume
of the detector
The dead layer =metalic electrode + thickness of silicon beneath
the electrode in which charge collection is inefficient.
The dead layer can be a function of the applied voltage
Radiation damage
The energy that goes to the creation of electron-hole
pairs leads to fully reversible processes
BUT the Non-ionizing energy transferred to
the atoms cause irreversible changes
Increase in leakage current
Loss in energy
resolution of the
detector
Channeling
In crystalline materials:
The rate of energy loss of a charged
particle may depend on the
orientation of its path with respect
to the crystal axes.
Particles traveling parallel to crystal
plane show lower energy loss
The energy deposition depends
on the crystal orientation
Channeled particles penetrate
farther in the crystal
To minimize the channeling,
detectors are fabricated from silicon
cut so that the (111) orientation is
perpendicular to the wafer surface
Energy calibration
The response of semiconductor diode radiation detectors when
applied on the measurement of fast electrons, protons, alphas: is
Linear
The energy calibration obtained for one particle type is very close to
that obtained using a different radiation type
Most common calibration source: 241-Am
Pulse Height defect
Response of semiconductor detectors to
very heavy ions (fission fragments)
Pulse height defect:
The pulse height observed is
substantially less than that
observed for a light ion of the same
energy
is the difference between
the true energy of the heavy ion and its apparent
energy (as determined from an energy calibration of
the detector obtained using alpha particles)
Applications of Semiconductor
sensors
Charged particle spectroscopy
Heavy ion and Fission Fragment
Particle identification (Energy loss)
(For particle identification through dE/dx we choose
detectors thin compared with the particle range)
X ray spectroscopy with silicon p-i-n diode
Germanium detectors for
Gamma-ray spectroscopy
For gamma ray detection large
depletion region is needed
2V
d
eN
Using Silicon or Germanium depletion
beyond 2-3 mm are difficult to achieve.
For depletion voltage
V<1000V
d~10mm
and N=1010 atoms/cm3
A. Reduce impurity
concentration to achieve large
depletion regions
B. Reduce impurity concentration by
Lithium ion drifting
High Purity Germanium (HPGe) or
intrinsic germanium : ultra pure
Germanium
Ge(Li) detectors
HPGe type is now in favor because they don’t
need permanent cooling as Ge(Li) while
detection efficiency and energy resolution are
essentially identical
Ge-detectors
CONFIGURATIONS
 larger depleted volume
 more efficient detection
Ultra-pure Ge (HPGe)
impurity concentration: ~109-1010 cm-3 !
(Ge concentration ~ 1022 cm-3)
-HV
p+
n
 Planar Configuration
n+
signal
 Coaxial Configuration
p+
n
n+
maximization of the sensitive volume
(diameter ~8 cm, length ~7-8 cm)
p+
n
n+
-HV
signal
Ge Energy resolution
Excellent energy resolution in gamma ray spectroscopy
The energy resolution: combination of 3 factors:
FWHM:
2
WT
2
 WD
2
 WX
Inherent statistical spread in
the number of charge carriers
WD2  (2.35) 2 FE
For F=0.08
E=1333MeV
ε=2.96eV
WD=1.32KeV
2
 WE
Contributions of
electronic noise
Variations in the charge collection
efficiency Most significant in detectors with
large volumes and low average electric field
F=Fano Factor value
ε=value necessary to create an electron-hole
pair
E=incident gamma-ray energy
Ge Energy resolution
Comparative pulse height spectra recorded by NaI (Tl) and a Ge(Li) detector
Silicon detectors and the CMS experimentmore
by Caio Laganá
(http://www.academia.edu/1680843/Silicon_detectors_and_the_CMS_experiment