Inorganic Scintillators

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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/cm 3 versus ~1 g/cm 3 ) 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 1

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 2

3 Inorganic Scintillators Mechanism of luminescence in some inorganic scintillators 3

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 4

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 5

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 V K centers) 6

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 7

8 Inorganic Scintillators Some examples 8

9 Inorganic Scintillators n Light output L = photons E γ E β S Q β is the electron - hole conversion efficiency S is the transfer efficiency Q is the luminescent quantum yield = g 9

10 Inorganic Scintillators Scintillation efficiency for 1 MeV photons 10

11 NaI(Tl) Inorganic Scintillators High light output and widely used in gamma spectroscopy BaF 2 Fastest known inorganic scintillator BGO (Bi 4 Ge 3 O 12 ) Used in x-ray tomography and PET LSO (Lu 2 O 3 -SiO 2 (Ce)) Used in PET PbWO 4 High density and radiation hard but low light yield Used in CMS EM calorimeter 11

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(τ ~ 230 ns (90%) and 0.15 s (10%)) 12

13 NaI(Tl) Tl + is a well-known luminescent center because of its 5d 10 6s 2 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 13

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

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

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

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

18 An example of intrinsic emission BaF 2 The very fast transitions in BaF 2 and CsF are due to an intermediate transition between the valence and core bands Actually there are two components of light: one with τ ~ 0.6 ns and one with τ ~ 630 ns 18

19 BGO Bismuth germanate (Bi 4 Ge 3 O 12 ) + High density (7.13g/cm 3 ) and high Z (83) result in high probability for photoelectric absorption + Rugged and not hydroscopic + No afterglow (phosphorescence) - τ ~ 300 ns (90%) and 60 ns (10%) - Lower light yield (about 10-20% of NaI) Finds widespread application in PET and CT scanners 19

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 6s 2 level and an excited electron in the 6s6p level of Bi 3+ These self-trapped excitons give photons on recombination BGO emission is well-matched to sensitivity of photodiodes 20

21 Inorganic Scintillators 21

22 LSO Lutetium Oxyorthosilicate (Lu 2 O 3 - SiO 2 (Ce)) +Good light output (~75% of NaI) +Relatively fast (τ~47ns) +High density (7.4g/cm 3 ) +Easily grown -Contains 176 Lu which is radioactive! -Nonlinear response somewhat degrades energy resolution Finds application in PET scanners 22

23 Inorganic Scintillators Light output is strongly dependant on temperature 23

24 Inorganic Scintillators 24

25 Properties Inorganic Scintillators 25

26 Properties Inorganic Scintillators 26

27 Scintillator Comparison 27

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 28

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

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

31 Gamma Camera A schematic of a standard gamma camera 31

32 CMS EM Calorimeter 32

33 CMS EM Calorimeter 33

34 CMS EM Calorimeter 34

35 CMS EM Calorimeter 80,000 PWO crystals 35

36 CMS EM Calorimeter 36

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

38 Higgs Decay 38

39 Higgs Decay 39

40 CMS EM Calorimeter 40

41 CMS EM Calorimeter 41

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