Transcript Document
Principles of Spectrometry Photoelectric Absorption. (with atoms, why??) Spectrum if all energy is captured in detector. Allows identification of gamma energy. • Large Detector (in depth?!). • Photo Peak or Full Energy Peak…? • Doppler. • Cost. • Crystal growth. Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). Auger 1 Principles of Spectrometry • Spectroscopy, energy deposition. • Consider what might escape. • Size and material of detector. • Shield lining. Considerations for shields and detectors? Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 2 Principles of Spectrometry HW 15 HPGe Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 3 Principles of Spectrometry • “Probability” for photoelectric absorption: • Strong Z dependence. • Considerations for shields and detectors. Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 4 Principles of Spectrometry Efficiency considerations. Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 5 Principles of Spectrometry NaI or BGO Efficiency vs. Resolution HPGe Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 6 Principles of Spectrometry Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 7 Principles of Spectrometry Compton Scattering. h ' h h 1 cos 1 2 m0 c In the figure: • Photoelectric suppressed. • Single Compton (effect of crystal dimensions). • Below 1.022 MeV. h Ec h 1 2 m0 c 2 h 1 cos 2 m0 c Ee h h ' h h 1 cos 1 2 m0 c Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 8 Principles of Spectrometry Reproduce the graph. h ' Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). h h 1 cos 1 2 m0 c 9 Principles of Spectrometry • Compton scattering is predominant for energies typical of radioisotope sources. HW 16 Let the recoil electron be emitted at an angle , calculate cot() as a function of tan(/2). • “Probability” per atom increases linearly with Z (Why). • Electron density. • Effect of binding energy. • Effect of polarized photons. • Compton fraction. Klein-Nishina Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). formula h m0c 2 10 Principles of Spectrometry HW 17 What is Thomson scattering. Note tendency for forward scattering at high energies. Considerations for shields and detectors? Sources vs. accelerators??? Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 11 Principles of Spectrometry If you need to measure gamma ray yield from nuclear reactions (using particle accelerators), the photon energies could be as high as 10 MeV. What is the typical Ge detector to be used? Prepared with Monte Carlo simulations. Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 12 Principles of Spectrometry Pair Production. Ee Ee h 2m0c2 Single peak if electron and positron “kinetic” energies are captured by the detector. But annihilation! Rest mass of electron and positron. Gamma must have 1.022 MeV minimum energy for PP to occur. Two annihilation photons are then created when positron recombines with an electron. These photons may or may not be captured, causing single and double escape peaks in the spectrum (effect of crystal dimensions). Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 13 Principles of Spectrometry Low-energy gamma. High-energy gamma. Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 14 Principles of Spectrometry Probability of pair production. Dependence on Z? HW 18 Recoil nucleus, Why? Effect of screening? Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 15 Principles of Spectrometry Importance in Linacs? Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 16 Principles of Spectrometry Very large detector. All secondary radiation, including Compton scattered gamma rays, Bremsstrahlung, x-rays and annihilation photons, are captured in detector volume Large photofraction (fraction of full energy events) is desired in any detector Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 17 Principles of Spectrometry Very small detectors. All secondary radiation, including Compton scattered gamma rays and annihilation photons, are not captured in detector volume (escaping). Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 18 Principles of Spectrometry A typical detector. Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 19 Principles of Spectrometry Effect of surrounding material. Detectors are normally shielded to minimize as much as possible the counting of ambient background radiation. X-rays and lining. Radiation Detection and Measurement, JU, First Semester, 2010-2011 (Saed Dababneh). 20