1. Nishina School Sep/29-Oct/8/2009 Scintillator 渡邊康 Wata-nabe Yasushi

2. What is scintillation  A flash of light produced in a phosphor by absorption of an ionizing particle or photon (The American Heritage Dictionary). Excited atoms (or molecules) in the medium releases the energy due to de-excitation by photons. How atoms (or molecules) are excited?

3. Anyway… Let’s see scintillators!

4. Contents Introduction Interaction of particles with matter  interaction of charged particles in matter  interaction of photons with matter scintillators  inorganic crystals response to γ-ray response to charged particle  organic scintillators response to γ-ray response to charge particle Some applications

5. How do we investigate atomic nucleus? - We can never see nucleus by our eyes. Need probe to reach nucleus. Particles as the probe: typical example  Charged: proton, electron, ion, meson  Neutral massless: photon  Neutral massive: neutron We cannot see even particles directly They have kinetic energy: Energetic particles

6. How do we see energetic particles? Need converter the kinetic energy into measurable quantity How it works? When putting energetic particle into converter  The particle kicks electrons of the converter producing electron-hole (ionization) detect the electrons or holes with electronics  gas chamber (by Taketani, Yesterday)  semiconductor detector (by Nishimura, Tomorrow) exciting atom or molecule (excitation) produce photon due to de-excitation  detect the photons by electronics: Scintillator!! N.B Both cases are through “kicking electrons”. It means Electro- Magnetic interaction is needed. Q: How do we detect particle which does not have EM interaction. e.g. neutron

7. Scintillation excited with charged particles using intense high-energy heavy-ion beam “beam” is injected every 3 seconds from a synchrotron Neon-20 beam from accelerator GSO = Gd 2 SiO 3

8. The scintillator can be used as  presence of a charged particle  energy measurement  the measure of the time when it passes Types of materials that scintillates  non-organic crystals  organic plastics (+ organic liquids)

9. Interaction of particles with matter charged particles  losing energy due to ionization  losing energy due to photon emission bremsstrahlung (mainly electron, positron)  nuclear reaction photons  photoelectric  Compton  pair production other important electromagnetic processes  Cherenkov radiation, transition radiation  (multiple scattering)

10. Energy loss of charged particles in matter particle loses its energy by ionization and excitation Beth-Bloch formula ( a few % accuracy) Some useful formula and scales (exclude electron/positron) for estimation of the range of particles Minimum ionizing Same dE/dx for same charge r e =classical radius of electron m e =mass of electron N a =Avogadro’s number c=speed of light z=charge of incident particle  =v/c of incident particle  =(1-  2 ) -1/2 W max =max. energy transfer in one collision 0.154 MeV cm 2 /g Particle energy [MeV]

11. Energy loss measurement is good for particle identification one must determine their mass (m) and charge (z) what one can measure are...  Momentum with magnetic spectrometer  velocity (time-of-flight for a certain distance) with scintillator  energy loss with scintillator or semiconductor  total energy with scintillator or semiconductor

12. An example of particle identification for z=1 particles, the measurement of energy loss and momentum gives their mass particle mass (MeV/c 2 ) electro n e0.511 muonμ106 pionπ140 kaonK494 protonp938 duetrond1876 z=1 Energy loss Momentum

13. Interaction of photons with matter photoelectric (E  < a few MeV)  a photon is absorbed by an atom, and an electron is ejected. E e = E γ - BE(binding energy : ~ a few eV) cross section ~ f(E γ -7/2, Z 5 ) higher Z is favored for γ-ray detection Compton scattering  scattering of photons by an electron  cross section : Klein-Nishina formula  cross section ~ f(Z) : number of electrons Pair production (E  > a few MeV)  e - e + creation (kinematically forbidden in free space)  Coulomb field of nucleus or electron  cross section ~ f(Z 2 ) (nucleus)  ~ f(Z) (electron)  e-e-  e-e-  e-e- e+e+

14. Energy dependence of photon-Lead cross section

15. For high energy photons and electrons pair production + bremsstrahlung emitted from electron(positron)  electromagetic showers in matter a GEANT simulation for electromagnetic shower of 300 MeV in material (CsI)

16. Scintillators… Photomultiplier tube (PMT) Need to explain PMT before the detail of scintillators. PMT converts scintillation light (photon) to electric signal.

17. Plastic scintillator for charged particle As an example to generate electric signal w/ PMT  Assumption of incident particle and scintillator Putting minimum ionizing particle (~2MeV/(g/cm 2 )) into a plastic scintillator (1 g/cm 3, scintillation yield: 1 photon/100 eV). 2 MeV / 100 eV = ~2 x 10 4 photons  Assumption of PMT collection eff. * Quantum eff ~ 0.1*0.25 = 2.5%: gain ~ 10 6 photo-electron: 2x10 4 x 0.025 x 10 6 = ~ 5x10 8 = 8x10 -11 C = Q  time duration ~ 50 ns I = dQ/dt = 8x10 -11 / 5x10 -8 = 1.6x10 -3 A electric signal (pulse height): V= IR= 1.6x10 -3 x 50(Ω ） = 80 mV

18. Scintillators inorganic scintillators: NaI(Tl), CsI(Tl), BaF2, BGO, GSO...  response is generally slower (~a few 100 ns)  some crystals are hygroscopic  high Z, high density : larger stopping power, photon detection  large light output organic scintillator (mostly plastic, but liquid is also used)  smaller Z  fast response (~ a few ns)  large light output  very flexible (thickness, size, shape etc.)

19. Training using detectors in this Nishina School NaI(Tl) scintillator (inorganic)  scintillation yields : ~ 1/25 eV: better energy resolution than plastic  decay time ~ 250 ns: slow  scintillation yield is still the largest among various scintillators plastic scintillator (organic)  scintillation yields : ~ 1/100 eV:  decay time ~ a few ns: fast: good for high intensity beam  cheap photomultiplier tube  scintillation-to-(electric signal) converter (with ~10 6 amplification)  high gain (>10 6-7 ) (even single photon counting)  variety of choices (size, gain, sensitivity)  cost (~10 4-5 JYen)

20. Inorganic scintillator (NaI(Tl)) widely used for (low energy) γ-ray measurement  high Z : enhancing photoelectric effect

21. γ-ray energy and pulse height Pulse height is proportional to the γ-ray energy  ex. radiation sources such as 137Cs : 0.662 MeV 60Co : 1.17, 1.33 MeV pulse height (ch)

22. Various inorganic crystals CrystalNaI(Tl)CsI(Tl)CsIBaF 2 BGOLSO(Ce)GSO(Ce) Density (g/cm 3 ) 3.674.51 4.897.137.406.71 Melting Point (ºC) 651621 1280105020501950 Radiation Length (cm) 2.591.85 2.061.121.141.37 Interaction Length (cm) 41.437.0 29.921.82122 Refractive Index 1.851.791.951.502.151.821.85 Hygroscopicity YesSlight No Luminescence (nm) 410560 420 310 300 220 480420440 Decay Time (ns) 2301300 35 6 630 0.9 3004060 Light Yield (%) 10045 5.6 2.3 21 2.7 97530

23. Electric signals from some inorganic scintillators

24. Responses of NaI(Tl) to various heavy-ion beams 7,8 Li, 11,12 Be, 14,15 B and 17,18 C using RIPS

25. Responses of NaI(Tl) to various heavy-ion beams L/MZ 2 Light Output (L)

26. Organic scintillator low Z materials  γ (~1MeV) interacts mostly via Compton scattering pulse height is determined by Compton-scattered electron energy Pulse Height Compton edge at θ=180º Eγ (MeV) Compton Edge (MeV) 137 Cs 0.6620.478 60 Co 1.17, 1.33 0.960,1. 12 NaI(Tl) photo-peak  Ｅ ,in electron Ｅ ,out

27. Another type of organic scintillator - liquid scintillator similar properties as plastic scintillator  ex. a special feature of the liquid scintillator fast and slow decay components of scintillation having different sensitivity to the energy loss densities analyzing the pulse shape -> particle identification  Good for neutron detection

28. Applications with using scintillators Positron Emission Tomography (PET)  Positron (e + ) annihilate with electron (e - ) and produce two  rays ( 511 keV) with back-to-back radiation.  It is easy to define the location of positron emitter (e.g. 11 C, 18 F), thanks to the  rays characteristic. How?. Wait an experiment in next week.  The location of cancer is easily defined (red arrow in top figure) if the positron emitter to accumulate there.   Scintillator

29. PET Cancer: continuing to grow  Cancer cell consumes much larger amounts of glucose than normal cell  Labeled glucose with positron emitter should accumulate at cancer cell. glucose Fludeoxyglucose (FDG)

30. Do we need the back-to-back radiation? - Can we define the source direction with a single photon?  Klein-Nishina formula Compton scattering shows it K-N differential cross section

31. Summary The scintillators are most versatile detector  Presence of a charged particle Even position resolution is adequate  Energy measurement charged particles, photon and neutron, too  Definition of timing Easy to use  Robust, no need to cool… If you need more resolution  Energy -> semiconductor (Ge, Si)  Position resolution -> gas chamber or Silicon