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7/28/2019 Lec1 General Properties of Det
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General Properties of Radiation Detectors
Recommended Text Books
1. Glenn F Knoll sRadiation Detection & Measurement (recentedition).
Lecture One:
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Simple Detector
It Must convert energy into charge via interaction of radiation with
detector
Heavy charged particle (fission fragment) or electrons (beta)-
Coulomb interactions create electron-ion (ionization) pairs if
sufficient energy is transferred to orbital electrons
Gamma rays-interact to transfer energy to electrons
Photoelectric effect
Compton Scattering
Pair Production
Neutrons -no charge. Must convert neutron energy to a
secondary particle that can create charge
Neutron capture and alpha emission.
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Simplified Detector Model
Consider a hypothetical detector subjected to some type of radiation;
To better understand the process, let us consider only a single particle or radiation
quantum interaction;
As stopping times are very small, so deposition of energy can be considered
instantaneous (varying from nano-seconds to mili-seconds)
Due to radiation interaction the output will be Q amount of charge produced within the
detector.
DetectorSource Shield
Radiation
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Simplified Detector Model (Contd.)
An electric field to collect the charge created is applied
Collection time depends on detector
Collection time shows the mobility and average distance covered (to be
collected at electrodes) of the charge carriers
Output of our detector current flowing for time equal to that of charge
collection
An obvious thing about the charge collected is
dttiQct
0
)(
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Simplified Detector Model (Contd.)
i(t)
Time, ttc
ct
dttiQ 0 )(
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Simplified Detector Model (Contd.)
If irradiation rate is high then overlapping of such pulses cantake place
In case of low irradiation rate, one pulse processed at a time
(simplified case easy to understand)
Magnitude and duration of each current pulse may vary
depending upon type of interaction
Time intervals between successive current pulses are also
randomly distributed
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Simplified Detector Model (Contd.)
i(t)
Time, t
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Simple Detector-radiation pulses
Current coming out of thedetector must equal the total
charge deposited by the
radiation
Pulses can vary depending on
the type and energy of the
radiation
Current pulse from detector
is connected to preamplifier
with characteristic resistance
and capacitance
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Modes of Detector Operation
Current Mode
Pulse Mode
Mean Square Voltage (MSV) Mode
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Current Mode
For a fixed response time T and sequence of events, timedependent current will be
Normally T is greater as compared to average time betweenindividual current pulses
Larger T to minimize statistical fluctuations in the signal but
slows the response
An average current is recorded which is given by
tdtIT
tI
t
Tt
1
qW
ErrQIo
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Current Mode (Contd.)
Where,
r event rate
Q charge produced for each event
E= Average energy deposited per event
W Average energy required to produce a unit charge
pair (i.e. electron ion pair)
q 1.6x10-19 C
qW
ErrQIo
An average current is recorded which is given by:
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Current Mode (Contd.)
Used for high count rate events
Largely applied in the field of radiation dosimetery
For steady state irradiation of detector, average current can
also be written as the I = Io +
i(t)
Here i(t) is a random time-dependent variable due to random
nature of radiation events interacting in detector
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Current Mode (Contd.)
Fractional standard deviation =
Fractional S.D in number of events (according to Poissons
Statistics) =
Comparing both fractional standard deviations gives
I
o
t
I
rT
rT
n
n
n
n
nI
tn
o
I
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Current Mode (Contd.)
Which simplifies to
(Eqn. A)
This result is useful in estimating the uncertainty associatedwith a given current mode measurement
oII rQ r
t QTrT rT
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Mean Square Voltage (MSV) Mode
MSV mode operations are based on fluctuations (SD) incurrent signal
The mode is applicable in the mixed mode environment of
radiation
In reactors where n and gamma-rays signals are discriminated
Average current is blocked whereas fluctuations in current are
taken into account
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MSV Mode (Contd.)
I(t)
Io
Time, t
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MSV Mode (Contd.)
The output is the square of standard deviation
Using Equation (A), we get
True application of MSV mode in mixed mode environment
Simple current mode will equally weight mixed radiation
MSV mode will square the charge produced by individual
radiation
MSV mode enhances relativly large amplitude and is mostly
used in reactor instrumentation
22 QT
rtI
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Pulse Mode
Used when we need to preserve the information on amplitude
and timing of individual event
Simplified circuit used for pulse mode operation is
DetectorC R V(t)
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Pulse Mode (Contd.)
In the figure
R input resistance of current
C equivalence capacitance (detector + measuring circuit)
V (t) time dependent voltage on which pulse mode operation is based
characterizes the frequency response of a first-order, linear time-invariant
system.
= R C. It is the time required to charge the capacitor, through theresistor, to 63.2 (~ 63) percent of full charge; or to discharge it to 36.8 (~37) percent of its initial voltage.
Two extreme cases are discussed RC > tc
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Pulse Mode (Contd.)
V(t)
Time, ttc
The Signal voltage V(t) for the case of small time constant load current.
Case 1: RC
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Pulse Mode (Contd.)
V(t)
Time, ttc
Case 2: RC >> tc
Vmax= Q/C
The Signal voltage V(t) for the case of large time constant load current.
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Small RC (RC
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Large RC (RC >> tc)
Charge is accumulated on capacitor and during charge
collection time very little current flows in the load resister
If there is sufficient time between pulses, capacitor will
discharge through resistance and voltage across load drops to
zero
Here the pulse formation time consists of two parts
Time for pulse to reach maximum
Time for signal voltage to restored to zero
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Large RC (RC >> tc) (Contd.)
The time to reach the maximum is characterized by thedetector (means charge collection time)
The trailing edge is circuit dependent as time to drop the signalvoltage to zero depends on time constant of circuit
Q = CVmax OR Vmax = Q/C
Q is total charge created within the detectorby one radiationinteraction
C is a fixed quantity so the amplitude of the signal is a
measure of charge generated within a detector by incidentradiation
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Large RC (RC >> tc) (Contd.)
So the output of pulse mode operation Consists of sequence of individual signal pulses i.e.
individual interaction of pulses
Pulse occurrence rate can be measured
Pulse amplitude is measured
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Advantages of Pulse Mode Operation
Pulse mode is the most common choice of radiation detector
operation because:
Its Sensitivity is much larger than Current or MSV mode as each
single quantum of energy is detected separately;
Pulse amplitude carries information of energy and charge; hencespectroscopy is possible;
However in MSV mode this amplitude is averaged over time so
information about energy is lost.
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Pulse Collection
RC=time constant for pre-amplifier
tc=charge collection time indetector
RCtc Circuit responds slow
compared to charge collectiontime. Little current flow in circuit
as charge collects. Amplitude of
signal pulse proportional to charge
created.