Radiation Detectors

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Radiation Sensors
Zachariadou K. TEI of Piraeus
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Part-VSemiconductor Sensors
Radiation Sensors
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Part-VSemiconductor 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

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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

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Semiconductor Detectors
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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
• Good stability
• Thin entrance windows
• Simplicity in operation
• High Energy resolution
• Compact size
• Fast timing characteristics
• Variable effective thickness to match the
  requirements of the application

• Limitation to small sizes
• Radiation-induced damage
• Need for cooling (thermal noise)

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Band structure-Carriers in an Electric Field
Insulators Eggt5eV Semiconductors Eg1eV Air
gt35eV Scintillators15eV
Thermal excitation A valence electron gains
sufficient thermal energy to be elevated to the
conduction band
Depends on the ratio of Eg over the absolute
temperature
Probability per unit time for thermal excitation
Materials with large Eg have low thermal
excitation probability
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Migration in an electric field
In Gases mobility of free electrons gtgt mobility
of positive ion In Semiconductors mobility of
electrons mobility of holes

• At low to moderate values of the electric field
  intensity
• The drift velocity is proportional to the
  electric field

Saturation velocity

• At higher electric field
• the drift velocity rises slowly with the field
  and reaches a saturation velocity

Time to collect the carriers over typical
dimensions (0.1cm)
Semiconductors are among the fastest-responding
radiation detectors
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Semiconductors basics
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Ionization energy
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Unbiased p-n junction
If it functions as a detector This detector would
very poor performance

• Charge carriers migrate across the junction

• Conduction electrons in the p-side will combine
  with holes vice versa

• Accumulated space charges create an electric
  field that opposites the conduction carriers
  migration

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.
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?(x)
The thickness of the depletion region is small
E(x)
Contact potential 1V
V(x)
V
The electric field is not enough to make the
charge carriers to move fast? incomplete charge
collection
0
-a
b
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biased p-n junction
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• Forward bias

The contact potential is reduced by bias V
Large currents are conducted

• Reverse bias

The contact potential is increased by bias V
The minority carriers are attracted across the
junction. The reverse current is very low
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?(x)
E(x)
Reverse bias V
V(x)
V
-a
b
0
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For x0
N dopant concentration
if
Resistivity
where µ is the mobility of the majority carrier
High resistivity(?) ? large depletion region (d)
(detecting region)
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Higher reverse bias
Thicker depletion region
The capacitance per unit area decreases
We use largest possible voltage up to fully
deplete the junction
Small capacitance means less electronic noise
resulting to better energy resolution
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Semiconductor Radiation sensors

• The module is reverse-biased--gta 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.

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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 gt ZSi (32 vs 14) ?
photo-electric effect x 60 ? Compton scattering
x 2
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Semiconductor detectorsOperational
characteristics

• Leakage current
• Noise and Energy Resolution
• Bias voltage
• Pulse rise time
• Radiation damage
• Channeling
• Entrance window
• Energy calibration
• Pulse height defect

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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 50,000Ocm
R5000O
Leakage current I0.1A
For bulk 1cm2
If V500V
The current is of the order of 10-6A by a pulse
of 105 radiation induced carriers
The Leakage current must not exceed 10-9A

• Surface leakage current

Contamination of the surfaces? clean techniques
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Detector noise
For silicon diode detectors 3 contributions to
noise are most significant

• Fluctuations in the bulk generated leakage
  current
• Fluctuations in the surface leakage current

Parallel noise

• Poor electrical contacts

series noise
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Detector Bias Voltage
Incomplete charge collection. The pulse height
rises with applied voltage

• Low Bias Voltage electric field

Corresponds to the region of ion saturation in a
gas-filled ion detector

• Sufficiently high Bias Voltage for complete
  charge collection ? saturation region

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

• Higher Bias Voltage ? multiplication region

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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

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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
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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
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Channeling
The rate of energy loss of a charged particle may
depend on the orientation of its path with
respect to the crystal axes.
In crystalline materials
Particles traveling parallel to crystal plane
show lower energy loss
Channeled particles penetrate farther in the
crystal
The energy deposition depends on the crystal
orientation
To minimize the channeling, detectors are
fabricated from silicon cut so that the (111)
orientation is perpendicular to the wafer surface
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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
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Pulse Height defect
Response of semiconductor detectors to very heavy
ions (fission fragments)
The pulse height observed is substantially less
than that observed for a light ion of the same
energy
Pulse height defect 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)
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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

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Germanium detectors for Gamma-ray spectroscopy
For gamma ray detection large depletion region is
needed
Using Silicon or Germanium depletion beyond 2-3
mm are difficult to achieve.
For depletion voltage Vlt1000V and N1010
atoms/cm3
d10mm
High Purity Germanium (HPGe) or intrinsic
germanium ultra pure Germanium
A. Reduce impurity concentration to achieve large
depletion regions
Ge(Li) detectors
B. Reduce impurity concentration by Lithium ion
drifting
HPGe type is now in favor because they dont need
permanent cooling as Ge(Li) while detection
efficiency and energy resolution are essentially
identical
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Ge-detectors CONFIGURATIONS

• larger depleted volume
• more efficient detection

Ultra-pure Ge (HPGe) impurity concentration
109-1010 cm-3 ! (Ge concentration 1022 cm-3)

• Planar Configuration
• Coaxial Configuration? maximization of the
  sensitive volume
• (diameter 8 cm, length 7-8 cm)

n
p
n
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Ge Energy resolution

• Excellent energy resolution in gamma ray
  spectroscopy
• The energy resolution combination of 3 factors

• Contributions of electronic noise

FWHM

• Variations in the charge collection efficiency
  Most significant in detectors with large volumes
  and low average electric field

• Inherent statistical spread in the number of
  charge carriers

FFano Factor value evalue necessary to create
an electron-hole pair Eincident gamma-ray energy
For F0.08 E1333MeV e2.96eV
WD1.32KeV
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Ge Energy resolution
Comparative pulse height spectra recorded by NaI
(Tl) and a Ge(Li) detector
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Silicon detectors and the CMS experimentmore by
Caio Laganá (http//www.academia.edu/1680843/Silic
on_detectors_and_the_CMS_experiment