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PHYSICS 225,
2ND YEAR LAB
NUCLEAR
RADIATION DETECTORS
G.F. West
Thurs, Jan. 19
INTRODUCTION, -1
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“Radiation” here refers to ionizing
radiation such as α, β, γ nuclear
emanations, not low energy
electromagnetic (photonic) radiation.
Typically arising from spontaneous or
stimulated nuclear decay, e.g., neutron, γ
or X-ray irradiation of atoms.
Kinetic energy (non rest mass
component) >> 10 eV , typically > 1 keV.
But not HEP energies > 100MeV.
INTRODUCTION, - 2
EM SPECTRUM
INTRO - 3
EM spectrum
with
photon energies
INTRODUCTION, - 4
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X and γ rays are pure EM radiation of
sufficiently high energy that they exhibit
particle-like behaviour.
α, (He nucleii), β, (electrons), β+,
(positrons) radiation are massive
particles. Obviously, they behave
differently, but they may often be detected
by similar methods.
Other emissions in this energy range,
(e.g., neutrons) need separate discussion.
WHAT IS A PARTICLE DETECTOR ?
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An apparatus to detect a radiation flux, usually as a
stream of separate events;
i.e., by counting the individual particles as they pass
through a defined aperture.
Thus, the particle must interact with the detector and
deposit some, or all, of its energy into it.
The detector can therefore be thought of as a target
body, having a cross-section (a probability) for
interaction with the radiation.
Some radiation may go through the detector without
significant interaction, some may interact and be
absorbed or altered and thereby detected.
PARTICLE DETECTORS , continued
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Possible functions:
Simple detection (counting),
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Energy measurement (spectroscopy),
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Path tracking.
Basic types:
Ionization chamber
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Scintillation detector
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Solid state electronic detector
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Track imager
INTERACTION PHYSICS
Effect of an incoming γ ray
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Photoelectric Effect (PE) - knocks out an
electron (and may continue on to another
event).
Pair Production (PP) - converts to electronpositron pair.
Compton scattering (C) - elastic collisions
with free electrons (partial energy absorption
in each collision).
I = Io exp(-µx), where µ = µPE + µPP + µC
& µPE ~ Z5, µPP ~ Z2,
µC ~ Z .
IONIZATION CHAMBERS
Dosimeter, proportional counter, geiger counter
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Chamber filled with gas or insulating liquid.
Some of the radiation produces ion-electron
pairs in the medium. Most passes through
unaffected.
A voltage gradient is established in the gas,
usually by applying a few hundred volts
between a central wire and an outer
cylindrical conductor. These electrodes
collect any charges produced in the
medium.
IONIZATION CHAMBER
Voltage dependence
DOSIMETER
USES OF IONIZATION CHAMBERS
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Dosimetry (safety and radiation therapy)
Proportional and geiger counters for α, β
counting, where sample can be in the
chamber, or outside next to an ultra thin
window.
Particle tracking chambers.
SCINTILLATION DETECTORS
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Much larger capture cross section due to
use of solid target volume.
Particle-target interaction produces ions
and ions give off optical flashes when the
ions return to ground state.
Captured optical radiation is observed
with photomultiplier tube or photo diode
layer.
Classic scintillator is NaI crystal doped
with thallium impurity. Many others.
PHOTO-MULTIPLIER (PMT)
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Need for a forepump.
NaI SCINTILLATOR
ABSORPTION IN DETECTOR
SOLID STATE DETECTORS
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Use semiconductor materials, and
construction techniques.
Faster and much more precise energy
analysis.
Low capture cross-section.
Most need liquid nitrogen cooling.
SOLID STATE DETECTORS, - 3
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Note logarithmic count scales on both graphs
SOLID STATE DETECTORS - 2
TRACKING METHODS
Usually used with magnetic field for path analysis
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Wilson cloud chamber (historical)
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Bubble chambers
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Wire ion chambers
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Spark chambers
TRACKING METHODS
Bubble chamber
TRACKING METHODS
Wire chambers (spark, or ionization)
DOSIMETRY
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Quantities and Units
Quantity
Activity of
a source
Absorbed
dose
Equivalent
dose
Dimension
Disintegrations
/ second
Deposited
energy / kg
Equivalent
gamma dose
Unit (old
metric)
Curie
Rad
Rem
= 3.7e10 Bq
= 0.01 Gy
= 0.01 Sv
Unit
Becquerel
(Bq) 1
gray (Gy)
Joule / kg
Sievert
(Sv) eqJ/kg
SIU