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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i(t)

Time, ttc

ct

dttiQ 0 )(


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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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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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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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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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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.

Lec1 General Properties of Det

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