detectors for nuclear radiationsscintillation counters the scintillation detectors convert the...

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

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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

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r Regio

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Regio

n o

f R

eco

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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

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on

Predominant interaction for low energy -rays.

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Compton Scattering'h

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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

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Compton Edge

For a given photon energy, the

Compton edge is given as:

MeV

cm

cm

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hE

cmh

cm

h

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o

c

256.0

2

1

2

1

2

1

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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

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-

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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.