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W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Interaction of Radiation with Matter Gamma Rays 1.

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Presentation on theme: "W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Interaction of Radiation with Matter Gamma Rays 1."— Presentation transcript:

1 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Interaction of Radiation with Matter Gamma Rays 1

2 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas  -Induced Processes  -rays (photons) come from electromagnetic transitions between different energy states of a system  important structural information Detection of secondary particles from: 1.Photo-electric absorption 2.Compton scattering 3.Pair production 4.  -induced reactions 1. Photo-electric absorption (Photo-effect) ħħ photon is completely absorbed by e -, which is kicked out of atom ħħ Electronic vacancies are filled by low-energy “Auger” transitions of electrons from higher orbits 2

3 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas 1. Photo-Absorption Coefficient Absorption coefficient   (1/cm) “Mass absorption” is measured per density    (cm 2 /g)  “Cross section” is measured per atom    (cm 2 /atom) Absorption Coefficient  (cm 2 /g) Pt Wave Length (Å) Absorption of light is quantal resonance phenomenon: Strongest when photon energy coincides with transition energy (at K,L,… “edges”) Probabilities for independent processes are additive:  PE =  PE (K)+  PE (L)+… 3

4 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas 2. Photon Scattering (Compton Effect)   ’ 4 e-e-

5 Compton Electron Spectrum Actually, not photons are detected, but recoil-electrons Recoil-e - spectrum true finite resolution Compton Edge Attention: not real !

6 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas 3. Pair Creation by High-Energy  -rays {e +, e -,e - } triplet and one doublet in hydrogen bubble chamber Magnetic field provides momentum/charge analysis Event A):  -ray (photon) hits atomic electron and produces {e -,e + } pair Event B): one photon converts into a {e -,e + } pair, energy  mass In each case, the photon leaves no detectable trace in the bubble chamber, before a first interaction with a charged particle, electron or nucleus (proton). Magnetic field e-e- e-e- e-e- e+e+ e+e+  -rays A B 6

7 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Dipping into the Fermi Sea: Pair Production Dirac theory of electrons and holes: World of normal particles has positive energies, E ≥ +mc 2 > 0 Fermi Sea is normally filled with particles of negative energy, E ≤-mc 2 < 0 Electromagnetic interactions can lift a particle from the Fermi Sea across the energy gap  E=2 m e c 2 into the normal world  particle-antiparticle pair Holes in Fermi Sea: Antiparticles Minimum energy needed for pair production (for electron/positron) Energy 0 -m e c 2 +m e c 2 normally filled Fermi Sea normally empty e - hole e - particle EE -[m e c 2 +E kin ] +[m e c 2 +E kin ] 7 Particle/anti-particle recombination releases this energy in 2 photons

8 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas The Nucleus as Collision Partner in Pair Production Increase with E  because interaction sufficient at larger distance from nucleus (less energy/momentum mismatch for e ± ) Eventual saturation because of screening of charge at larger distances Excess momentum requires presence of nucleus as additional charged body. e+e+ e-e- recoil nucleus  Pb 1barn = cm 2 8

9 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Efficiencies of  -Induced Electronic Processes Different processes are dominant at different  energies: Photo absorption at low E  Pair production at high E  Compton scattering at intermediate E . Z dependence important: Ge(Z=32) has higher efficiency for all processes than Si(Z=14). Take high-Z for large photo-absorption coefficient Response of detector depends on E  detector material detector shape 9

10 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas High-energy  -ray leading to e + /e - pair production, with e - stopped in the detector. e + stopped in the detector  annihilates with an e -  2 photons of E  = 511 keV each. a) Both photons are absorbed  full energy E  detected  event in FE peak. b) One photon escapes  event in SE peak FE-511 keV) c) Both photons escape  event in DE peak FE MeV) Relative probabilities depend on detector size! Photon-Escape Geometries for Real Detectors     keV    keV e+e+ e-e- Ge crystal 10

11 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Shapes of Low-Energy  Spectra Photons/  -rays are measured only via their interactions with charged particles, mainly with the electrons of the detector material. The energies of these e - are measured by the detector. The energy E  of an incoming photon can be completely converted into charged particles which are all absorbed by the detector,  measured energy spectrum shows only the full-energy peak (FE, red) Example: photo effect with absorption of struck e - Incoming photon may only scatter off an atomic e - and then leave detector  Compton-e - energy spectrum (CE, dark blue) Incoming  -ray may come from back-scattering off materials outside the detector  backscatter bump (BSc) measured energy measured intensity 11

12 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas measured energy (MeV) measured intensity Shapes of High-Energy  Spectra High-E  can lead to e + /e - pair production, e - : stopped in the detector e + : annihilates with another e - producing 2  -rays, each with E  = 511 keV. One of them can escape detector  single escape peak (SE) at FE-511 keV Both of them can escape detector  double escape peak (DE) at FE MeV The energy spectra of high-energy  -rays have all of the features of low-energy  -ray spectra e + /e - annihilation in vicinity of detector or housing  511keV  -rays FE 12 QuizzQuizz: Interpret  spectrum

13 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Reducing Background with Anti-Compton “Shields” High-energy  -rays produce e + /e - pairs in the primary Ge detector. All e + and e - are stopped in the Ge detector. e + finds an e - and annihilates with it, producing 2 back-to-back 511-keV photons. Escaping 511-keV photons are detected by surrounding annular scintillation detector. Escape events are “tagged” and can be rejected.    keV Ge crystal  e+e+ e-e-    keV  BGO Scintillator Photo Multiplier 13

14 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Compton Suppression Technique The figure shows a comparison between the “raw” 60 Co  - ray spectrum (in pink) and one (blue) where Compton contributions have been removed. The disturbing Compton background has been reduced by app. a factor 8 by eliminating all events, where a photon has been detected by the BGO scintillation shield counter in coincidence with a  -ray in the corresponding inner Ge detector. 14

15 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas  -Ray/Photon Detectors There is a variety of detectors for nuclear radiation, including  -rays. A special presentation is dedicated to the main detector principles. The next image shows a section of one of several modern 4  detector arrays, the “Gamma-Sphere.” The sphere surrounds the reaction chamber on all sides and leaves only small holes for the beam and target mechanisms. Each element of the array consists of two different detector types, a high-resolution Ge-solid-state detector encapsulated in a low-resolution BGO scintillation counter detecting Compton-scattered photons escaping from the Ge detectors 15

16 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Modern  Detectors: “Gammasphere” Compton Suppression 16

17 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas  -Induced Nuclear Reactions Real photons or “virtual” elm field quanta of high energies can excite (“heat”) nucleus with (one)  -ray to boiling point or beyond.  Nuclear spectroscopy via emitted characteristic radiation. Nucleus thermalizes energy, then “evaporates” particles and  -rays.  - induced reactions: ( ,  ’ ), ( , n), ( , p), ( ,  ), ( , f) Nucleus can emit directly a high- energy secondary particle or, usually sequentially, several low-energy particles or  -rays. n  nucleus   secondary radiation p incoming  -induced nuclear reactions are most important for high energies, E   (5 - 8)MeV 17

18 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas endendendend 18

19 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Quiz Try to identify the various features of the  spectrum shown next (well, it is really the spectrum of electrons hit or created by the incoming or secondary photons), as measured with a highly efficient detector and a radio-active A Z source in a Pb housing. The  spectrum is the result of a decay in cascade of the radio-active daughter isotope A (Z-1) with the photons  1 and  2 emitted (practically) together Start looking for the full-energy peaks for  1,  2,…; then identify Compton edges, single- and double- escape peaks, followed by other spectral features to be expected. The individual answers are given in sequence on the following slides. 19

20 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Spectrum of  Rays from Nuclear Decay 20

21 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Spectrum of  Rays from Nuclear Decay 11 21

22 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Spectrum of  Rays from Nuclear Decay 22 11 22

23 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Spectrum of  Rays from Nuclear Decay 22 11 CE  2 23

24 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Spectrum of  Rays from Nuclear Decay 22 11 SE  2 CE  2 24

25 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Spectrum of  Rays from Nuclear Decay 22 11 SE  2 DE  2 CE  2 25

26 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Spectrum of  Rays from Nuclear Decay 22 11 SE  2 DE  keV CE  2 26

27 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Spectrum of  Rays from Nuclear Decay 22 11 BSc SE  2 DE  keV CE  2 27

28 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Spectrum of  Rays from Nuclear Decay 22 11 BSc SE  2 DE  keV CE  2 Pb X-rays 28

29 W. Udo Schröder, 2009 Rad. Int. with Matter: Gammas Spectrum of  Rays from Nuclear Decay 1+21+2 22 11 BSc SE  2 DE  keV CE  2 Pb X-rays 29 Return


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