PHYSICS 225, 2 ND 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 = I o exp(-µx), where µ = µ PE + µ PP + µ C & µ PE ~ Z 5, µ PP ~ Z 2, µ C ~ Z.
IONIZATION CHAMBERS Dosimeter, 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 METHODS Usually 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