physics 225, 2 nd year lab
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PHYSICS 225, 2ND YEAR LAB
NUCLEAR
RADIATION DETECTORS
G.F. West
Thurs, Jan. 19
INTRODUCTION, -1 “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
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 ?
1. An apparatus to detect a radiation flux, usually as a stream of separate events;
2. i.e., by counting the individual particles as they pass through a defined aperture.
3. Thus, the particle must interact with the detector and deposit some, or all, of its energy into it.
4. The detector can therefore be thought of as a target body, having a cross-section (a probability) for interaction with the radiation.
5. Some radiation may go through the detector without significant interaction, some may interact and be absorbed or altered and thereby detected.
PARTICLE DETECTORS , continued
Possible functions:- Simple detection (counting), Energy measurement (spectroscopy), Path tracking.
Basic types:- Ionization chamber Scintillation detector Solid state electronic detector Track imager
INTERACTION PHYSICS
Effect of an incoming γ ray Photoelectric Effect (PE) - knocks out an
electron (and may continue on to another event).
Pair Production (PP) - converts to electron-positron 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 CHAMBERSDosimeter, proportional counter, geiger counter
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
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
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)
Need for a forepump.
NaI SCINTILLATOR
ABSORPTION IN DETECTOR
SOLID STATE DETECTORS
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
Note logarithmic count scales on both graphs
SOLID STATE DETECTORS - 2
TRACKING METHODSUsually used with magnetic field for path analysis
Wilson cloud chamber (historical)
Bubble chambers
Wire ion chambers
Spark chambers
TRACKING METHODS
Bubble chamber
TRACKING METHODS
Wire chambers (spark, or ionization)
DOSIMETRY 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= 3.7e10 Bq
Rad= 0.01 Gy
Rem= 0.01 Sv
Unit
SIU
Becquerel
(Bq) 1
gray (Gy)
Joule / kg
Sievert (Sv) eqJ/kg
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