Inorganic Scintillators

1. Inorganic Scintillators • Inorganic scintillators are inorganic materials (usually crystals) that emit light in response to ionizing radiation • NaI is the protypical example • Scintillation mechanism is different than for organic scintillators • Inorganic scintillators have higher Z and higher density (4-8 g/cm3 versus ~1 g/cm3) than organic scintillators • Higher Z and density translates into higher photon conversion efficiency and stopping power • Uses include calorimetry in particle physics, gamma ray spectroscopy, and medical/biological imaging

2. Inorganic Scintillators • The physical processes leading to scintillation in inorganic materials are complex and are dependent on the specific scintillator • A good picture to start with is that there is a core valence band and a conduction band • General steps to scintillation are • Initial electron-hole production and secondary production from the initial excitation energy • Thermalization • Transport and localization of electrons, holes, and excitons • Excitation and de-excitation of the luminescent centers

3. Inorganic Scintillators • Mechanism of luminescence in some inorganic scintillators

4. Inorganic Scintillators • Excitation/ionization • Causes the creation of (hot) electrons in the conduction band and (deep) holes in the inner core band • Relaxation • On a very short time scale (~1 fs), a large number of secondary electronic excitations occur • Radiative decay (secondary x-rays), Auger electrons, and inelastic electron-electron scattering

5. Inorganic Scintillators • Thermalization • Electrons and holes thermalize by making intraband transitions and by phonon production • Electrons end up at the bottom of the conduction band and holes end up at the top of the valence band • Occurs on ~ 1 ps time scale

6. Inorganic Scintillators • Transport/localization • Occurs on ~1-500 ns time scale • Electrons and/or holes migrate through the material and become trapped by impurities or activator ions, sometimes repeatedly, sometimes sequentially • Coulomb attraction can cause electrons and holes to form excitons or self-trapped excitons (STE) • Holes can become self-trapped (between two anions – called VK centers)

7. Inorganic Scintillators • Luminescence • During the localization stage, luminescent centers can be excited by various mechanisms and subsequently de-excite by the emission of scintillation light • Transport and forbidden transitions can be relatively slow • Two broad types of materials • Intrinsic or self-activated • Luminescence is produced by part of the crystal structure itself • External or activated • Must add some impurity to create energy levels between valence and conduction band

8. Inorganic Scintillators • Some examples

9. Inorganic Scintillators

10. Inorganic Scintillators • Scintillation efficiency for 1 MeV photons

11. Inorganic Scintillators • NaI(Tl) • High light output and widely used in gamma spectroscopy • BaF2 • Fastest known inorganic scintillator • BGO (Bi4Ge3O12) • Used in x-ray tomography and PET • LSO (Lu2O3-SiO2(Ce)) • Used in PET • PbWO4 • High density and radiation hard but low light yield • Used in CMS EM calorimeter

12. NaI(Tl) • Discovered by Hofstadter in 1948 but still a standard in gamma ray spectroscopy today • Activator is thallium (Tl) at 10-3 mole fraction • + Excellent light yield • + Relatively small non-linearity in energy response • - Hydroscopic (must be sealed) • - Damage from mechanical or thermal shock • - Slow (t ~ 230 ns (90%) and 0.15 s (10%))

13. NaI(Tl) • Tl+ is a well-known luminescent center because of its 5d106s2 configuration • Also the hole mobility is very small which increases the rise time of the luminescence • The excited states are P states which means the luminescence is spin-forbidden i.e. slow decay • Because fluorescence occurs through the activator sites in the forbidden band, NaI will be transparent to scintillation light • Very efficient transfer to activator sites results in high light output • 38k photons per MeV of energy deposited

14. NaI(Tl) • The band structure for NaI (Tl) looks something like

15. NaI(Tl) • In NaI, here are some of the trapping mechansims • And here are SOME of the recombination mechanisms

16. NaI(Tl) • Light output is well-matched to a PMT

17. CsI(Tl) • CsI(Tl) needles used in digital radiography

18. BaF2 • An example of intrinsic emission • The very fast transitions in BaF2 and CsF are due to an intermediate transition between the valence and core bands • Actually there are two components of light: one with t ~ 0.6 ns and one with t ~ 630 ns

19. BGO • Bismuth germanate (Bi4Ge3O12) • + High density (7.13g/cm3) and high Z (83) result in high probability for photoelectric absorption • + Rugged and not hydroscopic • + No afterglow (phosphorescence) • - t ~ 300 ns (90%) and 60 ns (10%) • - Lower light yield (about 10-20% of NaI) • Finds widespread application in PET and CT scanners

20. BGO • Another example of intrinsic emission • In this case the luminescence center is one of the constituents of the crystal • Ionization of Bi results in a hole in the 6s2 level and an excited electron in the 6s6p level of Bi3+ • Can also be interpreted as trapped exciton deexcitation • BGO emission is well-matched to sensitivity of photodiodes

21. Inorganic Scintillators

22. LSO • Lutetium Oxyorthosilicate (Lu2O3-SiO2(Ce)) • +Good light output (~75% of NaI) • +Relatively fast (t~47ns) • +High density (7.4g/cm3) • +Easily grown • -Contains 176Lu which is radioactive! • -Nonlinear response somewhat degrades energy resolution • Finds application in PET scanners

23. Inorganic Scintillators • Light output is strongly dependant on temperature

24. Inorganic Scintillators

25. Inorganic Scintillators • Properties

26. Inorganic Scintillators • Properties

27. Scintillator Comparison

28. Inorganic Scintillators • Inorganic scintillators have found wide application in HEP as calorimeters as they provide excellent energy resolution • Crystal Ball – NaI • L3, CLEO, KTeV, BaBar, BELLE – CsI • CMS, ALICE - PWO • R&D on inorganic scintillators has been spurred in part by HEP (but medical imaging is the primary driver)

29. Inorganic Scintillators • Still an active area of R&D

30. Gamma Camera • These images are made using gamma cameras • We will cover the details of these (and similar detectors) in upcoming lectures

31. Gamma Camera • A schematic of a standard gamma camera

32. CMS EM Calorimeter

33. CMS EM Calorimeter

34. CMS EM Calorimeter

35. CMS EM Calorimeter • 80,000 PWO crystals

36. CMS EM Calorimeter

37. Standard Model Massive Higgs Boson • Summary Higgs Mechanism Local Gauge Invariance Massive Gauge Bosons

38. Higgs Decay

39. Higgs Decay

40. CMS EM Calorimeter

41. CMS EM Calorimeter

42. CMS EM Calorimeter • PWO is relatively radiation hard for HEP

43. SPECT • Single Photon Emission Computed Tomography • Diagnostic technique in nuclear medicine utilizing many of the concepts we have covered to date • SPECT differs from PET in that only one photon is detected • Topographic techniques are used to locate the source of emission

44. Gamma Camera • These images are made using gamma cameras • We will cover the details of these (and similar detectors) in upcoming lectures

45. Gamma Camera • A schematic of a standard gamma camera

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