Transcript Lecture 3 - Rad Detection and Measur 1 - gnssn

Radiation Detection & Measurements - 1
IAEA
International Atomic Energy Agency
Day 3 – Lecture 3
Objective
To learn about different types of
radiation detectors used in radiation
protection
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Contents
• Detector Material
• Detector Principles
• Detector Types
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Detectors
• The detector is a fundamental base in all practice
with ionizing radiation
• Knowledge of the instruments potential as well as
their limitation is essential for proper interpretation
of the measurements
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Detector Material
• Any material that exhibits measurable radiation related
changes can be used as detector for ionizing radiation.
•
•
•
•
Change of colors
Chemical changes
Emission of visible light
Electric charge
• Active detectors: immediate measurement of the change.
• Passive detectors: processing before reading
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Detector Material
• Any material that exhibits measurable radiation related
changes can be used as detector for ionizing radiation.
•
•
•
•
Change of colors
Chemical changes
Emission of visible light
Electric charge
• Active detectors: immediate measurement of the change.
• Passive detectors: processing before reading
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Detector Principles
• Gas filled detectors
• ionisation chambers
• proportional counters
• Geiger Müller (GM) tubes
• Scintillation detectors
• solid
• liquid
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• Other detectors
• Semi conductor
detectors
• Film
• Thermoluminescense
detectors (TLD)
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Detector Types
1) Counters
Gas filled detectors
Scintillation detectors
2) Spectrometers
Scintillation detectors
Solid state detectors
3) Dosimeters
Gas filled detectors
Solid state detectors
Scintillation detectors
Thermoluminiscent detectors
Film
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Detector Types
Effect
Type of Instrument
Detector
Electrical
1. Ionizing Chamber
1. Gas
2. Proportional Counter
2. Gas
3. GM Tube
3. Gas
4. Solid State Detector
4. Semiconductor
1. Film
1. Photographic
Emulsion
2. Chemical Dosimeter
2. Solid or Liquid
Light
1. Scintillation counter
1. Crystal or Liquid
Thermo-
1. Crystal
luminescense
1. Thermo - luminescense
dosimeter
Heat
1. Calorimeter
1. Solid or Liquid
Chemical
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Gas Filled Radiation Detectors
These detectors consist of:
•
a gas filled tube
•
a positive electrode (anode) and negative electrode
(cathode)
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Regions Of Operation For Gas-filled
Detectors
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Ionization Chamber
 Simplest of all gas filled radiation detectors
 An electric field (104 V/m) is used to collect all the ionizations
produced by the incident radiation in the gas volume
 In most ionization chambers, the gas between the electrodes
is air.
 The chamber may or may not be sealed from the atmosphere.
 Many different designs for the electrodes in an ionization
chamber, but usually they consist of a wire inside of a
cylinder, or a pair of concentric cylinders.
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Ionization Chamber
Negative ion
Positive ion
Electrometer
+
1234
HV
The response is proportional to
ionization rate (activity, exposure rate)
General Properties Of Ionisation Chambers
 High accuracy
 Stable
 Relatively low sensitivity
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Examples Of Ion Chamber
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Applications of Ion Chambers
 Current Mode
 Radiation Survey
 Radiation Source
Calibrator
 Radioactive Gases
Measurement
 Pulse Mode
 Counting
 Alpha
Spectroscopy
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General Properties of Ionisation
Chambers
•High accuracy
•Stable
•Relatively low
sensitivity
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Problems With Ion-chambers
•
A basic problem with ionization chambers is that they are
quite inefficient as detectors for x and gamma-rays.
•
Only a very small percentage (less than 1percent) of X- or
gamma rays passing through the chamber actually interact
with and cause ionization of air molecules.
• for x and gamma- rays, their response changes with photon
energy because photon absorption in the gas volume
• detection efficiency
and relative penetration of photons
through the chamber walls
both are energy-dependent
processes
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Proportional Counter
 Proportional counter are operated at an electric field strength
106 V/m for Gases at STP causing Avalanches
 Applications
 Low Energy X-Radiations
 Neutron Detection
 Spectroscopy
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Gas Multiplication and Avalanche in
Proportional Detector
The avalanche will stop after the electric field reduced to a
threshold caused by the space charge of accumulated positive ions
in the gas.
anode
V
 (r) 
r ln( b / a )
an electron
cathode
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Properties of Proportional Counter
 Can be applied to situations in which the number of ion pairs
generated by the radiation is too small to permit satisfactory
operation in pulse-type ion chambers.
 A little higher sensitivity than the ionisation chamber
 Used for particles and low energy photons
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GM Counters
 When the electric field strength across a proportional counter
is increased (> 106 V/m), the device enters a GM region of
operation.
 GM counter is gas-ionization device in which, the ionization
effect creates a response which can be converted to an
electrical output.
 It is a gas-filled detector designed for maximum gas
amplification effect.
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GM Tube Structure
 The center wire (anode) is maintained at high positive voltage
relative to the outer cylindrical electrode (cathode).
 The outer electrode may be a metal cylinder or a metallic film
layer on the inside of a glass or plastic tube.
 Some GM counters have a thin radiation entrance window at
one end of the tube.
 The cylinder or tube is sealed and filled with a special gas
mixture, typically argon plus a quenching gas.
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Fill Gases
• Gases used in a Geiger tube must meet some of the same
requirements as for proportional counters.
• noble gases are widely used for the principal component of
the fill gas in G-M tubes, with helium and argon the most
popular choices.
• A second component is normally added to most Geiger gases
for purposes of quenching, the electron avalanches.
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Uses of GM Tubes
Simple, low cost, easy to operate
Pulse type counter that records
number of radiation events
All energy information is lost-no
ability to do spectroscopy
Dead time greatly exceeds any other
commonly used radiation detector
It has a high sensitivity but has a
lower accuracy.
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Types of Geiger-Mueller (GM) Tubes
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Scintillation Detectors
 Scintillation is a means of detecting the presence of
ionizing radiation
 Ionizing radiation interacts with a scintillator which
produces a pulse of light
 This light interacts with a photocathode which results in
the production of an electron
 The electron is multiplied in a photomultiplier tube that
has a series of focused dynodes with increasing potential
voltage which results in an electrical signal
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Scintillation Detectors
 The number of counts is dependent on the activity that is
present
 The energy of the electron, and consequently the
associated current is proportional to the incident energy
of the ionizing radiation
 By analyzing the energy and corresponding number of
counts, the nuclide and activity may be determined
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Scintillation Detectors
There are several types of Scintillator Detectors:
• scintillator NaI (sodium iodide): restricted to the detection
of the gamma;
• plastic scintillator: solution of fluorescent compounds
included in a transparent plastic material (gantry);
• scintillator ZnS (Zinc Sulfide): used for the detection of
alpha radiation
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Scintillation Detector (alpha)
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Alpha Scintillation Detector
The photomultiplier tube is located in the handle.
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Scintillation Detection (photon)
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Spectral Analysis
• Scintillation detectors, when used with a
multichannel analyzer (MCA) provide
information on the energy of a photon
that has interacted with the detector as
well as the activity present
• The spectra can be analyzed to
determine which isotopes are present
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Thermolumniscent Dosimeter (TLD)
Thermoluminescence Mechanism:
• Thermoluminescence is the emission of light from a crystal
on heating, after removal of excitation (i.e. ionizing
radiation).
• Radiation dose causes the electrons in the crystal to move
from low energy states to higher energy states.
• Some of these excited electrons are trapped in metastable
states
• These photons can be collected with a photomultiplier
tube.
• By proper calibration, the dose delivered to the crystal can
be measured.
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Simplified scheme of the TLD
process
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Thermoluminescence
TLD principle
thermoluminescent
material
photomultiplier
emitted light
heating filament
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TLD glow curves
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TLD
Advantages:
Disadvantages:
• Small size
• Time consuming
• High sensitivity
• No permanent record
• Integrating
• Tissue equivalent
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BF3 Neutron Detectors
BF3 Tube Construction
• Tube dimensions and geometry
 Large size tubes at higher pressure of fill gas
 Constructed of cylindrical geometry
• Cathode
• Al : low neutron absorption cross-section
• SS : preferred over Al because Al show alpha activity
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BF3 Neutron Detectors
Ageing effect
• Degradation in performance after operation of 1010 1011 registered counts
Detection Efficiency
• Efficiency decreases abruptly with increase of neutron
energies
• Dead spaces for charge collection reduce detection
efficiency
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Lithium Containing Slow Neutron Detectors
• Neutron induced reaction is detected by lithium based
scintillators
• LiI(Eu) scintillator function like NaI(Tl) detector
• Crystal size is greater than the range of reaction
products, pulse height response is free of wall effect
and a single is formed
• Scintillation efficiency is almost same for heavy charged
particles and secondary electrons
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The 3He Proportional Counter
Design of 3He Tube
• Diameter as large as possible
• Pressure of 3He is increased to reduce range of charged
particles
• Add a small amount of a heavier gas to increase stopping
power
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Solid State Detectors
• Solid State detectors are also called Semiconductor detectors
• In these radiation detector, a semiconductor material such as a
silicon (Si) or germanium (Ge) crystal constitutes the detecting
medium.
• In the detecting medium electron-hole pairs are produced when a
particle of ionizing radiation pass through it
• As a result a pulse of current generated is measured
• Operation of HPGe detectors require Liquid Nitrogen
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Solid State Detectors
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Using Solid as Detection Medium
 In many radiation detection applications, the use of solid medium
is of great advantage
 For high energy electrons and gammas, solid state detectors are
much smaller than gas filled detectors
 Energy resolution can be improved by increasing number of
charge carriers – possible in semiconductors
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Semiconductor Detectors
• Desirable features of – (semiconductor diode detectors) or solid
state detectors
• Superior Energy Resolution
• Compact Size
• Fast Timing Characteristics
• Effective Thickness – Can be varied according to the
requirement
• Semiconductor Materials
• Silicon – Used for charged particle spectroscopy
• Germanium - Used for gamma ray spectroscopy
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Semiconductor Detectors
•
When a positive voltage is applied to the n-type material and
negative voltage to the p-type material, the electrons are pulled
further away from this region creating a much thicker depletion
region
•
The depletion region acts as the sensitive volume of the
detector
•
Ionizing radiation entering this region will create holes and
excess electrons which migrate and cause an electrical pulse
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Semiconductor Detectors
Reverse Bias
Anode (+)
++
++
++
++
++
++
++
------
Cathode (-)
Intrinsic/Depletion Region
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Semiconductor Detectors
Gamma rays transfer energy to
electrons
(principally
by
compton scattering) and these
electrons traverse the intrinsic (+)
(-)
region of the detector
e
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Film Badge Dosimeter
Open Window
0.8 mm Pb filter
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Cu filters (0.05,
0.3 and 1.2 mm)
Kodak Type 2
Radiographic Film
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Film Dosimeter
 Film dosimeters (commonly known as film badges) consist of
a piece of photographic film in a holder
 The holder is fitted with a range of filters which allows us to
distinguish between beta, x-ray, gamma and thermal neutron
radiations and also allows determination of the personal dose
equivalent for Hp(10), Hp(0.07) and Hp(3)
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Film Dosimeter
 By determining the degree of blackening (optical density) on
the developed film and comparing it with calibrated films that
have been exposed to known doses, it is possible to
ascertain both the total dose received by the wearer and also
the contribution to total dose by each type of radiation
 The various filters used in film badges to ascertain whole
body Hp(10), skin Hp(0.07) and eye Hp(3) doses are shown in
the following Figure and Table
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Film Dosimeter
Filter Material
Open Window
Plastic (50 mg cm-2)
Plastic (300 mg cm-2)
Dural (0.040”)
Sn + Pb (0.028” 0.012”)
Cd + Pb (0.028” 0.012”)
Lead (0.012”)
Indium (0.4 g)
Application
beta and very soft x-rays
 and x-ray dose and energy*
 and x-ray dose and energy*
 and x-ray dose and energy*
 and x-ray dose and energy*
slow neutrons**
edge shielding+
neutron accident monitoring
*quantitative determination of
** by gamma emitted after capture by cadmium
+to prevent overlap of film blackening due to angled incident radiation
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Film Badge Dosimeter
A
B
Film
Package
C
D
E
C
B
A
D
E
O
A - Plastic filter
B to E - Metallic filters
O - Open window
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Film Badge Dosimeter
• The density on the film
results from three basic
sources:
Al Filter
Black =
exposed
White = not
exposed
 Base+Fog
 Exposure
Pb Filter
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Where to Get More Information
 Cember, H., Johnson, T. E, Introduction to Health Physics,
4th Edition, McGraw-Hill, New York (2009)
 International Atomic Energy Agency, Postgraduate
Educational Course in Radiation Protection and the Safety of
Radiation Sources (PGEC), Training Course Series 18,
IAEA, Vienna (2002)
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