lec 7 generl properties 2

7/28/2019 Lec 7 Generl Properties 2

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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)
mailto:[email protected]:[email protected]


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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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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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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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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

lec 7 generl properties 2

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