OPPORTUNITY TO JOIN IEEE AND NPSS

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1 OPPORTUNITY TO JOIN IEEE AND NPSS If you are NOT an IEEE Member, IEEE & NPSS offers you a FREE: Half-year membership in IEEE (value= ~$80)* Half-year membership in NPSS (value= ~$13)* Half-year subscription to web versions of the IEEE TNS and the TPS journals * Paid for by NPSS and by IEEE (membership expires in Jan 2008) Visit the IEEE booth in the Exhibit Hall Nuclear and Plasma Science Society

2 OPPORTUNITY TO JOIN IEEE AND NPSS If you ARE an IEEE Member (but not a member of NPSS), IEEE & NPSS offers you a FREE: Half-year membership in NPSS (value= ~$13)* Half-year subscription to web versions of the IEEE TNS and the TPS journals * Paid for by NPSS and by IEEE (membership expires in Jan 2008) Visit the IEEE booth in the Exhibit Hall Nuclear and Plasma Science Society

3 Scintillator Non-Proportionality William W. Moses, Woon-Seng Choong, Giulia Hull, Bryan Reutter, Steve Payne, Nerine Cherepy, and John Valentine Lawrence Berkeley National Laboratory and Lawrence Livermore National Laboratory June 4, 2007 This work supported by the National Nuclear Security Administration, Office of Defense Nuclear Nonproliferation, Office of Nonproliferation Research and Development (NA-22) of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.

4 Light Yield BGO NaI:Tl CsI:Tl LSO LaCl 3 :Ce LaBr 3 :Ce 8,200 photons/mev 38,000 photons/mev 60,000 photons/mev 28,000 photons/mev 50,000 photons/mev 63,000 photons/mev Fundamental Scintillator Constant

5 Non-Proportionality 1.05 Relative Light Yield Energy Deposit (kev) Light Yield Not Constant Depends on Particle Energy & Type

6 1950 s: Non-Proportionality First Studied Alkali Halides (NaI & CsI) Different Particle Types (γ, β, p, α, Light & Heavy Nuclei, Fission Fragments, ) ~4 Orders of Magnitude Energy Range Why Does Light Yield Depend on Particle Type & Energy?

7 Light Yield Correlated with Ionization Density Mechanism: Saturation of Luminescent Centers Scintillation Efficiency (dl/de) Ionization Density (de/dx) Work Stopped in Late 1960 s Figure from R. B. Murray & A. Meyer, Phys. Rev. 122, pp , 1961

8 1991: LSO Discovered BGO 662 kev LSO 662 kev Counts per Bin % fwhm Counts per Bin % fwhm Pulse Height Bin Pulse Height Bin ~4x More Light Than BGO, But Same Resolution Why Isn t Resolution Dominated by Counting Statistics?

9 1995: Non-Proportionality Resurrected to Explain Poor Energy Resolution Interest Steady Growing (for γ excitation )

10 How Does Non-Proportionality Affect Energy Resolution? The Onion Model

11 Layer 1: Photon Response

12 Initial Interaction: Compton vs. Photoelectric Scintillator 1.05 Incident Gamma Photoelectric Compton Photoelectric Relative Light Yield Energy Deposit (kev) Non-Proportionality + Multiple Energy Deposit Degraded Energy Resolution

13 Energy Resolution for Small LSO Crystal Counts per Bin LSO 2 mm Cube 662 kev 10.7% fwhm Counts per Bin LSO 1 cm Cube 662 kev 9.4% fwhm Pulse Height Bin Pulse Height Bin Large Difference in Photoelectric Fraction No Difference in Energy Resolution There Must Be Something More

14 Photoelectric Interactions W K L M Valence γ photoelectron Usually Occur with Inner Shell Electrons Inner Shell Hole Filled via Cascade

15 Simplified Cascade Diagram for NaI Different Photoelectron Energies 1 4 kev Auger Electrons 83% K-Shell Interactions ~30 kev Fluorescent X-Rays Many Energetic (>1 kev) Particles Created Fluorescent X-Rays & Auger Electrons Figure from B.D. Rooney & J.D. Valentine, IEEE Trans. Nucl. Sci. 44, pp , 1997

16 Cascade After Photoelectric Interaction 1.05 Photoelectron Initial Gamma Relative Light Yield Fluorescent X-Ray Auger Electrons Energy Deposit (kev) Non-Proportionality + Multiple Energy Deposit Degraded Energy Resolution

17 Monochromatic Gamma Scintillator Photodetector Photon Response Relative Light Yield Energy (kev) Structure in Photon Response Curve Includes Many Confounding Effects Figure from M. Moszyński, et al., Nucl. Instr. Meth. A-484, pp , 2002

18 Layer 2: Electron Response

19 Electron Response Monochromatic Electron Scintillator Photodetector Surface Effects Sample Charging

20 How Is Electron Response Measured? 662 kev Gamma Compton Scatters in in Scintillator Energy of Scattered Gamma Measured in in HPGe Plot Light Output vs. Electron Energy (E γ γ E HPGe ) Figure from J.D. Valentine, et al., Nucl. Instr. Meth. A-486, pp. 452, 2002

21 Second Generation Apparatus: SLYNCI (Scintillator Light Yield Nonproportionality Characterization Instrument) Collimated 1 mci Source See Poster PMo06 (W.-S. Choong) Scintillator on Hybrid Photodiode 30% HPGe Detector, 10 cm away from Scintillator Measures Electron Response in <1 Day

22 Compton Interactions W K L M Valence γ Compton electron Usually Occur with Outer Shell Electrons All Energy Transferred to e (No Cascade)

23 Electron Response vs. Photon Response Photon Response Electron Response Relative Light Yield Rooney and Valentine NaI:Tl (Sample 2) NaI:Tl (Sample 1) Energy (kev) Electron Response Has Less Structure Photon Response Can Be Qualitatively Predicted from Electron Response & Cascade Figure on left from M. Moszyński, et al., Nucl. Instr. Meth. A-484, pp , 2002 Data on right from B.D. Rooney & J.D. Valentine, IEEE Trans. Nucl. Sci. 44, pp , 1997

24 Do Primary Compton & Core Holes / Cascade Completely Explain Resolution Degradation? Energy Resolution (fwhm) 70% 60% 50% 40% 30% 20% 10% 0% Counting Stats Electron Excited See Poster PMo31 (B. Reutter) Gamma Excited Excitation Energy (kev) No!!! There Must Be Something More

25 Electron Energy Deposit Still Non-Uniform! Landau Fluctuations Delta Ray γ e + e in Bubble Chamber

26 Layer 3: Ionization Density

27 Yield Depends on Electron Ionization Density Relative Light Yield Light Yield Ionization Density Energy (kev) Ionization Density (de/dx) Relative Light Yield Ionization Density (de/dx) Non-Proportionality + Non-Uniform Energy Deposit Degraded Energy Resolution

28 Compute Fluctuations in Light Output Along the Electron Track Bethe-Block Equation Gives ionization density (de/dx) as function of E Landau Equation Gives variation in ionization density (de/dx) Measured Electron Response Gives scintillation efficiency as function of E Preliminary work done by Steve Payne Compute Variance in in Light Produced at at Each Point Integrate Variance Along Track To Get Total Variance Gives Reasonable Prediction of Intrinsic Broadening

29 Layer 4: Exciton Interactions

30 Understand Shape of Electron Response Exciton Exciton Annihilation: Birk s Equation? 1.05 η ANNIH = 1 / [1 + a B (de/dx)] Relative Light Yield Exciton Formation: Diffusion Length? Hecht Equation? η EXC = 1 b e/h exp [- b EXC (de/dx)] Energy Deposit (kev) Could Fundamental Cause of Non-Proportional Electron Response Be Exciton Exciton Annihilation?

31 How Can We Test This? Change Temperature Change exciton mobility Measure Decay Times Observe exciton kinetics Change Dopant Concentration Examine luminescent center saturation Add Defects Examine competition from non-radiative centers Many Variables to Study Many Studies Necessary

32 Conclusion: A Few Layers Peeled, but Plenty of Onion Left!

33 kev kev Top View of Apparatus kev kev kev kev 1 40 kev kev kev kev 125 cps System Event Rate Expected

34 Understand Shape of Electron Response Exciton Formation: Hecht equation η EXC = 1 b e/h exp [- b EXC (de/dx)] Exciton Exciton Annihilation: Birk s equation η ANNIH = 1 / [1 + a B (de/dx)] Electron Response η EXC η ANNIH LaCl 3 LaBr 3 Material Fundamental Cause of Fundamental a B b e/h b EXC Cause R FWHM (%) of (cm/mev) (cm/mev) predicted R FWHM (%) measured Non-Proportional Electron Response Non-Proportional NaI(Tl) Electron Response 5.5 LaBr 3 (Ce) ~1 May Be Exciton Exciton Annihilation LaCl 3 (Ce) May Be Exciton Exciton Annihilation

35 Light Output per kev of Electron Energy for Several Scintillators From W. Mengesha, T. Taulbee, B. Rooney and J. Valentine, Light yield nonproportionality of CsI(Tl), CsI(Na), and YAP, IEEE Trans Nucl Sci 45, pp , Relative Light Output NaI:Tl CsI:Tl CsI:Na Relative Light Output CaF2:Eu LSO YAP BGO GSO BaF2 LaCl Electron Energy (kev) Electron Energy (kev) Ideal Scintillator Would Be Horizontal Line

36 Relate Linearity to Energy Resolution Linearity Energy Resolution % Relative Light Output LaCl3 NaI Ideal Energy Resolution (% fwhm) 25% 20% 15% 10% 5% LaCl3 NaI Ideal Electron Energy (kev) 0% Gamma Energy (kev) Study With Monte Carlo Simulation

37 What Can Be Improved? Conventional PMT (non-linearity issues) Simple Coincidence Hybrid Photodiode (much more linear) More Sophisticated ~10% HPGe Detector >1 Ci Source Multiple Channels ~1 m Away From Sample ~2 m Away From Sample Higher Rate Calibration Events Multiple Detectors Smaller Source, Closer More to Efficient Sample Scintillator Decay Time Detectors Closer to Sample Lots of Opportunities for Improvement Expect Measurement Time <1 Day

38 How Do We Measure Electron Response? Ge1 Ge5 Ge3 Ge2 Ge4 Light Yield Signal Detected by the Photomultiplier Tube Electron Energy = 662 (Energy of the Scattered Photon Detected by the HPGe)

39 Analysis Method