Exploring Scintillators: A Glowing Study of Particle Detection

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

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 particlesusing intense high-energy heavy-ion beam “beam” is injected every 3 seconds from a synchrotron Neon-20 beam from accelerator GSO = Gd2SiO3

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 matterparticle 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 re=classical radius of electron me=mass of electron Na=Avogadro’s number c=speed of light z=charge of incident particle β=v/c of incident particle γ=(1-β2)-1/2 Wmax=max. energy transfer in one collision 0.154 MeV cm2/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 identificationfor z=1 particles, the measurement of energy loss and momentum gives their mass Energy loss Momentum z=1

13. Interaction of photons with matter • photoelectric (Eg < a few MeV) • a photon is absorbed by an atom, and an electron is ejected. • Ee = Eγ - BE(binding energy : ~ a few eV) • cross section ~ f(Eγ-7/2, Z5) • 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 (Eg > a few MeV) • e-e+ creation (kinematically forbidden in free space) • Coulomb field of nucleus or electron • cross section ~ f(Z2) (nucleus) • ~ f(Z) (electron) e- γ e- γ e- γ 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/cm2)) into a plastic scintillator (1 g/cm3, scintillation yield: 1 photon/100 eV). • 2 MeV / 100 eV = ~2 x 104 photons • Assumption of PMT • collection eff. * Quantum eff ~ 0.1*0.25 = 2.5%: gain ~ 106 • photo-electron: 2x104 x 0.025 x 106 = ~ 5x108 = 8x10-11C = Q • time duration ~ 50 ns • I = dQ/dt = 8x10-11 / 5x10-8 = 1.6x10-3A • 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 ~106 amplification) • high gain (>106-7) (even single photon counting) • variety of choices (size, gain, sensitivity) • cost (~104-5 JYen)

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

21. pulse height (ch) 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

22. Various inorganic crystals

23. Electric signals from some inorganic scintillators

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

25. Light Output (L) L/MZ2 Responses of NaI(Tl) to various heavy-ion beams

26. photo-peak Compton edge at θ=180º Ｅγ,out Ｅγ,in θ NaI(Tl) electron Pulse Height Organic scintillator • low Z materials • γ (~1MeV) interacts mostly via Compton scattering • pulse height is determined by Compton-scattered electron energy

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 g rays ( 511 keV) with back-to-back radiation. • It is easy to define the location of positron emitter (e.g. 11C, 18F), thanks to the g 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 g g 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