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Pakistan Institute of Engineering & Applied Sciences(PIEAS)
Dr. Nasir M Mirza
Deputy Chief Scientist,
Department of Physics & Applied mathematics,
PIEAS, P.O. Nilore, 45650, Islamabad.
Email: [email protected]
Ph: +92 51 9290273 (ext: 3059)
Lectures onRadiation Detection
Delivered in Professional Training Course onRadiation Safety & RWM at PNRA (April, 2008)
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General Properties of Radiation Detectors
Recommended Text Books
1. Glenn F Knoll sRadiation Detection & Measurement (recentedition).
Lecture 7:
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Simple Detector System-pulse counting
Detector Pre-Amp Amplifier
High Voltage
Power Supply
Single
Channel
Analyzer
Count Rate
Meter
Counter
Timer
Oscilloscope
Multi-
channelAnalyzer
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Detection System
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Simple Detector System-pulse counting
Detector- converts energy of radiation pulse (neutron, gamma, beta,
alpha) into charge to be collected:
Geiger-Mueller counters
Proportional counters
Scintillation counters
Semiconductor detectors
Pre-amplifier- couple detector output to analysis system
Detector Bias -voltage required for detector to convert energy tomeasurable charge
Amplifier- amplify collected charge (gain) and shape the signal SCA/Discriminator-convert shaped pulses into logic pulses
Scaleror Counter - count pulses
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Pulse Height Spectra
Detector is generally operated in pulse mode;
Each and individual pulse amplitude carries information
regarding charge, energy, etc.
Interaction of mono-energetic radiation in a detector produce
many pulses of different height (especially interactions)
Amplitude is different due to
Variation in energy
Fluctuations in detector response due to different microscopic
interactions
Pulses are distributed in different heights
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Pulse Height Spectrum
Differential spectrum: Area undercurve represents the number of pulses
with amplitude between H and H+dH
Integral of spectrum: Always a
decreasing function
Maximum pulse height observed
Frequentpulses
Total
pulsesCounting plateaus occur on
the integral curve in areas
with few counts
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Pulse Height Spectra (Contd.)
Number of pulses between H1 and H2 =
Total number of pulses in the distribution =
H5 = Maximum pulse height observed
no pulse beyond this point
H4 = Peak in spectrum
amplitude about which a large number of pulses may befound
H3 = Valley in the spectrum
amplitude about which a small number of pulses may befound.
dH
dH
dNH
H
2
1
dHdH
dN
0
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Pulse Height Spectra (Contd.)
For physical interpretation of differential pulse height spectrum always use area
under the curve between two given limits of pulse height as (dN/dH) has no physicalmeaning itself unless multiplied by dH.
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Pulse Height Spectra (Contd.)
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Pulse Height Spectra (Contd.)
I ntegral Pulse Height Distr ibution Ordinate will always be a monotonically decreasing function of H
Due to finite amplitude of pulsed, at H=0 the value of ordinate
will be No (total number of pulses observed)
H5 is the end point of the graph as no pulse larger than that
amplitude is produced
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Pulse Height Spectra (Contd.)
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Comparison of Differential & Integral Spectra
Both spectra convey exactly same information and one canbe derived from the other
Differential spectrum Integral spectrum
Maxima (peak) Maximum slope
Minima (valley) Minimum slope
Differential spectra show clear difference in pulse height
changeand so is extensively used
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Counting Curves and Plateaus
Obtained when detector operated in Pulse mode
Main purpose to draw such counting curves is to findthe point of maximum stability pulse height
Different ways for achieving the counting curves
Discriminator level
Amplification
Amplifier gain
Applied voltage to detector
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Counting Curve
Hd = discrimination level. Pulse height required to be
counted as a pulse
Pulse height can be changed
via amplifier gain
Varying the gain determineshow many pulses are counted
Greater gain=more pulses
counted
Total # of counts is fixed
In counting curve, plateau is
located at height range with
lowest total counts
For stability, operate on plateau
(small drifts in gain have little
influence on total counts)
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Energy Resolution
Measurement of energy distribution of the incident radiationknown as Radiation Spectroscopy
Response of two detectors to a monoenergetic source for equal
number of pulses (area under the curve is same in both cases)
Broad distribution bad resolution more fluctuations even
in the case of same energy deposition
Narrow distribution good resolution less fluctuations
(improved ability to resolve fine details in incident energy of
the radiation)
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Energy Resolution (Contd.)
Full Width at Half Maximum (FWHM) is defined as the widthof the distribution at a level just half of the maximum ordinate
of the peak
Then using FWHM, resolution R and %R are defined as
Smaller the R better is the resolution
oH
FWHMR 100%
oH
FWHMR
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Energy Resolution
Response to mono-energetic radiation source Perfect resolution-delta function
Poor resolution leads to a broader, shorter spectrum
Some detection needsidentification of
radiation energy that is
spectroscopy;
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Energy Resolution
RFWHM
H0
2.35
N
Sources of fluctuation:
Drift of detector Random noise in detector and instrumentation
Signal fluctuation (sets minimum theoretical resolution)-the
same energy deposition may create slightly different charge in
detector
R 2.35F
N
F=Fano factor
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Detection Efficiency
Absolute
Efficiency
abs no. pulses detectedno. radiation quanta emitted by sourc
Intrinsic
Efficiency
int no. pulses recorded
no. radiation quanta incident on detecto
Dependent on detector properties and details of counting geometry
abs int
4= solid angle
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Solid Angle
w
1 Unit Sphere =
4
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Dead Time
In nearly every detector system, there will be a minimum amountof time which must separate two events for both to be recorded
as separate pulses.
Minimum separation time is known as dead-time
In some cases set by detector propertiesIn some cases set by signal processing system properties
Dead time losses can be severe at high counting rates
A detection system will have a time period t, known as the dead
time, during which new incoming pulses cannot be registered
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Dead Time-models
Paralysable: Dead time is extended by the new incoming pulse
Non-paralysable: True events which occur during the dead period
are lost; Real systems operate in a mode between these two ideal
explanations
4 of 6 counted
3 of 6 countedDead time t
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Dead Time Nonparalysable
Definitions
n=true interaction ratem=recorded interaction rate
tsystem dead time
Fraction of time detector is dead = mt
Rate at which true events are lost = nmt= nm
n m1 mt
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Dead Time ParalysableDefinitions
n=true interaction rate
m=recorded interaction rate=rate of occurrences of time intervalsbetween true events which exceed t
tsystem dead time
The distribution of intervals between random events occurring at anaverage rate n is:
P1 t dt nent
dt
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Measuring Dead Time-two source method
Two source method-observe the counting rate from two sources
individually and in combination.n1, m1= source 1 counting rate
n2 , m2= source 2 counting rate
n12 , m12= source 1 plus source 2 counting rate
nb, mb = background counting rate
n12 nb n1 nb n2 nb
n12 nb n1 n2
Non-paralysable model:m12
1 m12t
mb
1 mbt
m1
1 m1t
m2
1 m2t
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Measuring Dead Time-two source method
Solving gives:
t X 1 1 Z
Y
X m1m2 mbm12
Y m1m2 m1 mb mbm12 m1 m2
Z Y m1 m2 m12 mb
X2
Dead time can be calculated from measured count rates
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Measuring Dead Time-decaying source method
Allows a comparison of model to data
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