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S. Sihotra
Deptt. Of Physics,
Panjab University, Chandigarh-160014
DETECTORS FOR NUCLEAR RADIATIONS
OVERVIEW OF LECTURE
Radiation Detectors – Principle & General Properties.
Gas Filled Detectors.
Scintillation Detectors.
Semiconductor Detectors.
Radiation
Ionizing Radiation
Protons
Heavy ions
Fission Fragments
Electrons
Photons
Neutrons
Non-Ionizing Radiation
Ultra Violet
Visible
Infra Red
Microwaves
Radiowaves
Radiation
Charged Particle
Protons
Heavy ions
Fission Fragments
Electrons
Neutral
EM Radiations
Neutrons
Why Study Radiation - Matter Interactions
• Universe is composed of radiation and matter.
• The study of interaction of radiation with matter is a strong probe
to understand evolution of universe.
Evolution of Universe
• The study of molecular, atomic, nuclear and nucleonic
structures is possible by probing with radiation of appropriate
size, charge and energy.
Microscopic Structures
• Laser and Plasma.
• Fabrication of microelectronic devices.
• Medical diagnostic and surgical tools.
• New Materials.
Technologies
Penetration Power of Radiations
Alpha / Heavy Ion
0-1b
-
42a ++
00
Beta
Gamma and X-rays
Neutron
Paper Plastic Lead Concrete
10n
RADIATION DETECTORS-GENERAL CHARACTERISTICS
PRINCIPLE OF RADIATION DETECTOR
Quantum of radiation, incident on a detector medium, deposits its energy in it.
Mechanism of energy deposition depends upon energy and nature of radiation.
The interaction time {~ns in gases and ~ps in solids} of radiation being very
small, whole of energy is deposited almost instantaneously in detector
medium.
These interactions produce a given amount of electric charge in the form of ion
pairs or electron-hole pairs in the active volume of the detector.
The electric charge is collected by imposition of external electric field.
The charge collection time depends upon nature of detector medium and
mobility of charge carriers.
The current flows for the time duration of charge collection resulting in output
current pulse.
Modes of Detector OperationAn output current pulse from the detector is measured with an
instrument whose response time defines the mode of operation of
detector.
Pulse mode
Useful for detailed radiation spectroscopy.
Detector & associated electronics records each quantum of
radiation that interacts in the detector.
Current mode
The measuring instrument has a fixed response time.
It records current averaged over many interactions.
Useful for radiation dosimetry.
Pulse mode is most frequently used.
Pulse Mode Operation
The pulse mode operation of a detector is possible only when
radiation influx is such that the time interval between two
successive interactions is much larger than the time required
for processing output current pulse.
The nature of signal pulse produced due to single quantum of
radiation depends upon the input characteristics of circuit
coupled to the detector (i.e. equivalent resistance and
capacitance of detector and circuit).
Detector RC
Circuit
V(t)
Decay time
Rise time
Time
If charge collection time >>> RC time constant, the current through
load resistance is instantaneous. This is used to derive the signal for
time of interaction of radiation.
If charge collection time <<< RC time constant, the output pulse is
used to derive the signal indicating radiation energy deposited in the
interaction.
Voltage /
curr
ent
Pulse Height Spectrum
Distribution of pulse heights.
Informs about flux incident on detector or emanated from the source.
It can be represented in two different forms.
dH
dN
H
Differential Pulse Height Spectrum
Max. No. of pulses
Max. Pulse Height
H
N
Integral Pulse Height Spectrum
Min. No. of Pulses
Total No. of pulses
Resolution of Detector
If mono-energetic source of radiation is counted with a detector, then one
observes distribution of pulse heights even though same energy is deposited
by each of the interacting quantum of radiation.
100% Energy
FWHMR
2
22
int
2
noiselstatistica
noiserandomrinsictotal
FWHM
FWHMFWHMFWHM
+
+
Energy
FWHM
Energy
Co
un
ts
• Operating Characteristics of detector &
their drift.
• Random Noise in detector and
associated electronics.
• Statistical fluctuations in number of
charge carriers produced.
Efficiency of a Detector
o In general the particle detectors record all the quanta incident over them.
o In case of photon counters, all the quanta do not deposit their complete
energy and hence are not recorded. The concept of efficiency becomes
important for such detectors.
o The efficiency is defined in two different ways: absolute and intrinsic
int
int
4
det.
.
.
.
abs
abs
ectoronincidentquantaofNo
recordedpulsesofNo
sourcefromemittedquantaofNo
recordedpulsesofNo
Efficiency of detector depends upon:
Nature of detector medium
Dimensions of detector.
Source to detector distance.
Nature of radiation.
Dead Time of a Detector
In most of the detectors, there is a minimum time interval
between two successive interactions so that they can be
recorded as two separate pulses. This interval of time is called
dead time and detector remains insensitive to incident quanta
of radiation during this interval.
The dead time becomes severe when the incident radiation
flux is very high. Appropriate corrections are applied to
account for counting losses due to dead time of detector.
The dead time is attributed not only to the detector but the
associated pulse processing electronics also contributes to it.
GAS FILLED DETECTORS
Gas Filled Counters
Ammeter
Cathode
High Voltage
Gas at low pressure
The Gas-filled counters contain gas at low pressure filled in a
chamber with two electrodes inserted in it. These electrodes are
maintained at some potential difference through an external
source.
Anode
Regions of Gas Detector Operation
Pu
lse
Am
pli
tud
e
Applied Voltage
Ioniz
ation C
ham
ber
regio
n
Pro
port
ional re
gio
n
Lim
ited P
roport
ional re
gio
n
Geig
er
Mulle
r Regio
n
Regio
n o
f R
eco
mbin
ation
When a quantum of radiation enters the chamber, it causes
ionization of gas molecules or atoms. The ion pairs, so formed, move
towards their respective electrodes under the influence of the
imposed electric field.
REGION OF RECOMBINATION: When the applied electric field is low,
the ions suffer recombination while moving towards their respective
electrodes.
IONIZATION CHAMBER REGION: As the electric field is increased,
the recombination of ions gets suppressed and all the ions produced
in the interaction get collected at two electrodes. The pulse
amplitude gives the energy of radiation. The gas detectors operated
in this region is called ionization chamber.
PROPORTIONAL REGION: As the electric field is further increased,
the ions acquire sufficient kinetic energy to cause secondary
ionizations and pulse amplitude gets amplified but is proportional
to the number of primary ionizations caused. The gas counter
operated in this voltage region is called proportional counter.
REGION OF LIMITED PROPORTIONALITY: As the electric field is
further increased, the positive ions, moving towards cathode,
create space charge sheath around the anode which modifies the
shape of the electric field. The pulse amplitude is non-linearly
dependent on field and defines the region of limited
proportionality.
GEIGER-MULLER REGION: When the electric field is very high,
then avalanche production takes place till the space charge effect
of positive ions lowers the electric field strength below threshold
to cause any further ionization. In this region any interacting
radiation quantum produces output pulse of same saturated
amplitude irrespective of its energy. This is called Geiger-Muller
region.
• Low electric field.
• Ion pairs move slowly towards respective electrodes.
• Ions suffer recombination frequently.
• Current and voltage pulses of small amplitude.
Recombination Region
• Electric field increases to reduce recombination of ions to insignificance.
• Output pulse amplitude is proportional to radiation energy.
• Useful for radiation energy measurements.Ionization Chamber
• As electric field is increased , primary ion pairs cause secondary ionization.
• Output pulse amplitude gets amplified by a constant factor.
• Useful for radiation spectroscopic measurements. Proportional Counter
Limited Proportionality
GM Counter
• Further increase in electric field favors multiple secondary ionizations.
• The electrons get collected rapidly but positive ions, moving slowly, form acylindrical sheath around anode.
• This disturbs electric field and leads to limited proportionality betweenenergy and output pulse amplitude.
• Electric field is increased to a value where primary ion pairs cause further multiple secondary ionizations leading to avalanche production.
• Output pulse amplitude gets saturated for every quantum of radiation.
• Useful for radiation detection only.
Photon-Matter Interaction
Photoelectric Absorption
Compton Scattering
Pair Production
Other Interactions
Rayleigh Scattering
Thomson Scattering
Delbruck Scattering
The interaction of photons with matter
leads to either partial or complete transfer
of energy in contrast to continuous slowing
of charged particles. These processes lead to
sudden changes in photon history.
Photoelectric Absorption
In this process, a photon is completely absorbed by
one of the bound electrons of the absorber atom,
thereby resulting in its ejection.
BKe EEE -
Photoelectron
X-ray photon
X/BLBKKX EEE -
-ray photon interacts with the absorber atomand it completely disappears.
Probability of photoelectric absorption
τ is high for high Z.
Ph
oto
ele
ctr
ic A
bso
rpti
on
Predominant interaction for low energy -rays.
2/7
5
E
kZ
Compton Scattering'h
Ee
h
-+
-
)cos1(1
)cos1(
2
2
cm
h
cm
h
hE
o
o
e
In Compton scattering,
the incident gamma-
photon is deflected
through an angle w.r.t its
original direction.
The photon transfers a
portion of its energy to
the electron (at rest)and
originates as scattered
photon.
The electron receives
the momentum and
energy imparted by
incident photon and
recoils.
)cos1(1
'
2
-+
cm
h
hh
o
Compton Edge
For a given photon energy, the
Compton edge is given as:
MeV
cm
cm
h
hE
cmh
cm
h
hEE
e
o
c
256.0
2
1
2
1
2
1
21
2
0
2
0
2
2
0
+
The Compton Scattering assumes that electron is free
or unbound.
If the binding energy of the scattering electron is
considered, then for low energy incident photon
energies, the shape of Compton continuum is affected.
The upper extreme of the continuum is rounded off
during its rise and fall has finite slope.
Energy
• It occurs for the photon having
energy greater than 1.022 MeV.
Interaction
• When passing through the absorber medium, the photon interacts with the field of
nucleus resulting in creation of electron-positron pair.
• The electron and positron share the kinetic energy
Products
• Electron and positron lose their kinetic energy in travelling few mms in medium.
• Positron combines with electron of medium causing emission of two 0.511 MeV
photons.
Signal
• The time required for the positron to slow down and annihilate is small.
• Consequently annihilation radiations appear more or less promptly.
-
2
022.1E
Pair Production
Taken from Glenn F. Knoll
Small size detectors
Taken from Glenn F. Knoll
Intermediate detector size
Escape Peaks
Gamma-ray Attenuation
When a collimated beam of mono-energetic
gamma- and X-rays passes through an absorber
medium of variable thickness, it suffers
exponential attenuation in its intensity. This
attenuation in intensity is expressed as
where µ is linear attenuation coefficient defined
as sum of all gamma- or X-ray interaction
probabilities per unit path length with the
absorber atoms.
The linear attenuation coefficient is dependent
on density of absorber which is replaced by mass
attenuation coefficient
x
oeII -
)()()( ppcspe ++
t
o
x
o eIeII '
-
-
SCINTILLATION DETECTORS
Scintillation Counters
The scintillation detectors convert the energy of incident quantum
of radiation into visible photons. Whenever a material is supplied
with energy, it results in excitation of electrons to higher excited
states. The subsequent de-excitation of these electrons can be:
Prompt leading to fluorescence (Fast component of output)
OR
Delayed resulting in phosphorescence and/or delayed
fluorescence
(Slow component of output).
Scintillation Materials
Scintillation detectors use different kind of materials like
Organic Scintillators
Liquid Organic Scintillators : Anthracene NE213, NE216.
Plastic Scintillators: Styrene, NE102, NE105, Pilot B, Pilot F
Loaded Liquid Scintillators: NE311 (B loaded), NE 313 (Gd loaded)
Inorganic Scintillators
NaI(Tl), CsI(Tl), BGO, BaF2, ZnS, CaF2
Scintillation detector Principles
Fluorescence is the prompt emission of visible radiation
from a substance following the excitation by some means.
Phosphorescence corresponds to the emission of longer
wavelength light than fluorescence, and with a characteristic
time that is generally much slower.
Delayed fluorescence results in the same emission
spectrum as prompt fluorescence but again is characterized
by a much longer emission time following excitation.
Organic Scintillators
Singlet StatesTriplet States
Ab
so
rpti
on
Flu
ore
sce
nce
Ph
osp
ho
resce
nce
Delayed fluorescence
Inorganic Scintillators
fluo
resce
nce
Phosphorescence
Quenching
Conduction Band
Valence Band
Band G
ap
Scintillation Counter
Taken From internet
Features of Scintillation Detectors Energy Resolution: The Scintillation detectors result in production of large number
of photons per interaction of radiation quantum. As a result they have
commendable energy resolution.
High Efficiency: These detectors are available in sufficiently large size and hence
they are characterized by large efficiency.
Fast Detectors: The interaction time of radiation in a scintillation material is small
and photons are produced almost instantaneously. These detectors are suitable for
timing measurements.
Wide Usage: Due to commendable energy resolution and high efficiency, these
detectors find applications in charge particle as well as gamma-ray spectroscopy.
Afterglow: In many scintillators, the slow component due to phosphorescence or
delayed fluorescence results in afterglow and degrades the performance.
SEMICONDUCTOR DETECTORS
Another method is byusing a semiconductor thatworks like a solar cell. Lightincident on the cellproduces electron-holepairs which can becollected by applying avoltage to the cell.
Since the signal is small,amplifiers are used tomake the signal largeenough ( ~ 1 Volt) to becounted in a countingcircuit.
Semiconductor Detector
Semiconductor Detectors
OVER-DEPLETED PN DIODES
These are pn-diodes fabricated out of highly pure semiconductor materials such as
Germanium or Silicon. The diode is applied reverse bias sufficient to cause its
depletion region to span nearly whole of the volume. These are called over-depleted
diodes.
When a radiation quantum enters the depletion region, it creates electron hole
pairs. These charge carriers are swept by the high reverse bias voltage leading to a
current pulse.
HIGH RESOLUTION
The creation of an electron-hole pair in depletion region requires very small amount
of energy hence a very large number of them are created in each interaction. This
reduces the statistical fluctuations. Hence these detectors are characterized by small
FWHM or sharp peaks in differential pulse height spectrum.
HPGe Detector
0.15% FWHM
NaI Detector
4.7% FWHM
EFFICIENCY
These detectors are not available in as large volume as scintillators. Hence their
efficiency are relatively lower.
TIMING
The creation of electron hole pairs and subsequent current pulse is almost
instantaneous. As a result, these detectors are useful for timing measurements.
CRYOGENIC REQUIREMENTS
Since the applied reverse bias is of the order of few hundred to few thousands
volts, the steady state leakage current flows through detector’s active volume. As a
result, these detectors are required to be operated at liquid nitrogen temperature
(78K).
Preamplifier Requirement
The output signal from these detectors is very small and a preamplifier is coupled
very closely. It provides initial amplification and shaping of the pulse depending
upon type of measurement (energy or timing) to be made.
Radiation Damage Prone
The operation of semiconductor detectors is based upon the near perfection of
crystalline lattice which prevents trapping of charge carriers. Extensive use of such
detectors cause disruption of lattice due to radiation damage. The damage is severe
for passage of charge particle than gamma-rays or electrons.
Block Diagram for Energy Counters
Amplifier
DiscriminatorADC/MCA
Counter
Detector
Preamplifier
Method of Compton Suppression
HPGe
-Source
Anti Compton Shield
allowed
RejectReject
Taken from Glenn F. Knoll
Influence of surrounding materials on detector response.
FWHM ~ 2 keV at 1.2 MeV
P/T ~ 30% for single crystal improved to 60% by ACS
unsuppressed
suppressed
A single HPGe Detector
8-9-2011BHU-SS-2011
IUAC INGA set up
THANKS
HPGe Detector
0.15% FWHM
NaI Detector
4.7% FWHM
12/18/2018
HIGH RESOLUTION
The creation of an electron-hole pair
in depletion region requires very
small amount of energy hence a very
large number of them are created in
each interaction.
This reduces the statistical
fluctuations and these detectors are
characterized by small FWHM or
sharp peaks in differential pulse
height spectrum.