PHYSICS 225, 2 ND YEAR LAB

1. PHYSICS 225, 2ND YEAR LAB NUCLEAR RADIATION DETECTORS G.F. West Thurs, Jan. 19

2. 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.

3. INTRODUCTION, - 2 EM SPECTRUM

4. INTRO - 3 EM spectrum with photon energies

5. 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.

6. WHAT IS A PARTICLE DETECTOR ? • 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.

7. 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

8. 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 .

9. 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.

10. IONIZATION CHAMBER Voltage dependence

11. DOSIMETER

12. USES OF IONIZATION CHAMBERS • Dosimetry (safety and radiation therapy) • Proportional and geiger counters forα, β counting, where sample can be in the chamber, or outside next to an ultra thin window. • Particle tracking chambers.

13. 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.

14. PHOTO-MULTIPLIER (PMT) • Need for a forepump.

15. NaI SCINTILLATOR

16. ABSORPTION IN DETECTOR

17. 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.

18. SOLID STATE DETECTORS, - 3 • Note logarithmic count scales on both graphs

19. SOLID STATE DETECTORS - 2

20. TRACKING METHODSUsually used with magnetic field for path analysis • Wilson cloud chamber (historical) • Bubble chambers • Wire ion chambers • Spark chambers

21. TRACKING METHODS Bubble chamber

22. TRACKING METHODS Wire chambers (spark, or ionization)

23. DOSIMETRY • Quantities and Units