Testing and Evaluation of Scintillators

Transcription

1 Institut für Physik, Martin-Luther-Universität Halle-Wittenberg February 18th, 2009

2 Preface: Moved to IZM (With a Little Help from my Friends)

3 Preface: Proud Father

4 Preface: Table of Contents 1 Preface 2 Methods 3 Scintillators 4 Timing Resolutions 5 Conclusion

5 Positrons in 5 Minutes How to explain positrons in 5 minutes (or less). One talk to teach them all. 1 1 Sorry for that cheap Lord of the rings reference:-)

6 Antimater and Albert Einstein Dirac[1] found solutions with positive charge in his electron-theory (1928) Anderson[2] found that the particle was not the proton but the positron, anti-particle of the electron (in 1933) positrons annihilate with electrons, always one e + with one e thanks to Einstein[3] the annihilation results in γ-radiation where E γ = (m e + + m e )c 2 Figure: Einstein 1921

7 Conservation of the pulse Another physics law pi = 0 The pulse of a system has to be conserved. Effect on positron-electron-annihilation: Two γ-quants are emitted in opposite directions Both have energy of 511keV Figure: Feynman-diagram of electron-positron-annihilation

8 Conservation of the pulse Another physics law pi = 0 The pulse of a system has to be conserved. Effect on positron-electron-annihilation: Two γ-quants are emitted in opposite directions Both have energy of 511keV Positron is in ground-state But: Electron is in excited state γ-energy changes due to p of the electron Electron-energy depends on the state, core-electrons have higher energy than valence-electrons Figure: Feynman-diagram of electron-positron-annihilation (extended)

9 Statistics (More or less:) Important Electron-positron-annihilation is a highly statistical process! Because the diffusion of the positron in the solid is a random-walk. Annihilation is influenced by: Electron density Electron energy Atomic structure Defects, voids, charge für 22-Na: Quant (1.27 MeV) Positroneneinfang durch Kristalldefekte Quant (511 kev) Figure: Positrons in the solid Positronen-Wellenfunktion wird im Defekt lokalisiert Annihilationsparameter ändern sich, wenn Positron im Defekt zerstrahlt Defekte können nachgewiesen werden (Identifizierung und Quantifizieru

10 Lifetime Lifetime of the positron is influenced mainly by two effects: 1 The lower the electron density the lower the chance to hit an electron the higher the lifetime. 2 One (or many) missing atoms/cores build a potential well the positron can t escape (because normal cores repulse the positron). positron lifetime increases but this only works in neutral or negativ traps Si; 90degree-setup; 1.5cm BaF Time in samples (1 sample = 250ps) Figure: Positron-Lifetime in Cz-Silicon

11 Doppler-broadening and Angular-correlation two techniques for one effect both measure the electron-energy from the energy-shift of the γ-quants Doppler-broadening: measures the energy-shift in γ-direction (usually p z ) can be measured with one detector, gives high background two detectors in coincidence give low background but longer measurement-time energy-range is 511keV ± 10 kev While the spectra can be calculated theoretically, most times results are compared to a defect-free spectrum Experimental Techniques Intensity [arbitrary units] GaAs Plastically deformed Reference A s ray energy [kev] Fig Doppler-broadening spectra of as-grown zinc-doped gallium positron trapping (reference) compared with plastically deformed Figure: 1997b). The line Doppler-Spectrum[4] shape parameters S and W are determined by the ind divided by the area below the whole curve. The curves are normalized The background correction is often performed as the subt line. More sophisticated treatments use a realistic backgroun eled by a non-linear function. This function takes into account at a given -ray energy is proportional to the sum of anni higher energies. Despite such a background reduction, the Do slightly unsymmetrical. The calculation of the W paramete quently carried out only in the high-energy wing of the D quality Doppler-broadening spectra are obtained by the coi preferably with a setup using two Ge detectors. Arnold In this Krille case, t A w

12 Preface Methods Scintillators Timing order to Resolutions obtain the defect-specific plots, a normalization Conclusion for saturat trapping, i.e. for capture of all positrons in defects, is necessary. This by correlated positron lifetime measurements, which provide the fract trons trapped in the defect. Such contour plots can also be directly com theoretical calculations to improve the understanding of the electronic Doppler-broadening and Angular-correlation the defect. The first depth-resolved measurements of the two-dimensional angu tion of annihilation radiation were reported by Peng et al. (1996). The d ture of the SiO2/Si interface was studied by the combination of slo beam measurements and 2D-ACAR. The positron beam system used two techniques for one effect krypton moderator and an initial positron beam flux of about s 1 resolution was obtained by the variation of the incident positron en both measure the electron-energy from the energy-shift of the γ-quants Angular-correlation: measures the energy-shift perpendicular to the γ-direction (p x and p y ) has to be done in coincidence 1D- and 2D-measurements are possible typical range is ±10 20mrad, resolution is 0, 2mrad measurement for one spectrum is typically several days Gives good results on the electronic structure, but evaluating the spectra requires many theoretical calculations. Fig Combined perspective and contour plots of the measurement of two angular correlation of annihilation radiation in gallium arsenide exhibiting no Figure: ping in defects (Tanigawa ACAR-Spectrum[4] et al. 1995). The upper panel shows the contour m mentum distribution p in the (100) plane.

13 Overview 6 2 Experimental Techniques Birth -ray 1.27 MeV + e source t 1. Positron lifetime Sample Annihilation -rays MeV 2. Angular correlation p xyx, y xy x,, y mc 22 Na Diffusion L nm 100 µm 0 Thermalization 12 (10 s) 3. Doppler broadening MeV ± E, E = p c/2 z 22 Fig Scheme of different positron experiments. Positrons from an isotope source (

14 Digital Positron Lifetime: Setup Detektor Detektor Detektor In1 In2 Detektor Digitizer PC In1 In2 Digitizer PC

15 Preface Methods Scintillators Timing Resolutions Conclusion Digital Positron Lifetime: Digitizer Detektor Detektor In1 In2 Digitizer PC

16 Preface Methods Scintillators Timing Resolutions Conclusion Digital Positron Lifetime: Photomultiplier Detektor Detektor In1 In2 Digitizer PC

17 Digital Positron Lifetime: Scintillator Detektor Detektor In1 In2 Digitizer PC

18 Methods Time between two signals is needed Independent method: Correlation of channels not so exact Time of minimum: Hard to determine Constant threshold trigger: Very inaccurate because of variable pulse height Constant fraction: Best method so far

19 True Constant Fraction pcf Input Voltage [V] Pulse Time [samples]

20 True Constant Fraction pcf Input Voltage [V] Minimum -1.4 Pulse Time [samples]

21 True Constant Fraction pcf Input Voltage [V] Zeroline Minimum -1.4 Pulse Time [samples]

22 True Constant Fraction pcf Input Voltage [V] CF Zeroline Minimum -1.4 Pulse Time [samples]

23 Low-Pass Constant Fraction lp-pcf Input Voltage [V] Ch 1 - Raw Input Voltage [V] Ch 2 - Raw Time [samples] Time [samples] Input Voltage [V] Ch 1 - LowPass Input Voltage [V] Ch 2 - LowPass Time [samples] Time [samples] Butterworth-Filter (implementation taken from [5]) Followed by true constant fraction as before

24 Differentiated Constant Fraction dpcf Input Voltage [V] Ch 1 - Raw Input Voltage [V] Ch 2 - Raw Time [samples] Time [samples] Input Voltage [V] Ch 1 - Diff. Input Voltage [V] Ch 2 - Diff Time [samples] Time [samples] Noise disturbs the direct differentiation especially on small pulses Therefor...

25 Differentiated Constant Fraction dpcf Input Voltage [V] Ch 1 - Raw Input Voltage [V] Ch 2 - Raw Time [samples] Time [samples] Input Voltage [V] Ch 1 - LP + Diff. Input Voltage [V] Ch 2 - LP + Diff Time [samples] Time [samples]...signal is filtered by low-pass first Then true constant fraction is applied.

26 Scintillators Looking at different scintillation materials for pulse-shape, energy resolution and timing resolution.

27 BaF 2 Barium fluoride BaF2 0 Input Voltage [V] Time [samples] Fast and slow component Fastest risetime currently available (1.1ns)

28 BaF 2 Barium fluoride 1e+06 1e+05 Hamamatsu with BaF2 Ch 1 Ch 2 1e+04 Counts 1e+03 1e+02 1e+01 1e Input Voltage [-V] Fast and slow component Fastest risetime currently available (1.1ns)

29 LSO Lu 2SiO 5 - Lutetium oxyorthosilicate Input Voltage [V] LSO Time [samples] Better energy resolution than BaF 2 Has intrinsic decay of 1 67Lu

30 LSO Lu 2SiO 5 - Lutetium oxyorthosilicate 1e+07 1e+06 Hamamatsu mit LSO (-2.5kV) Channel 1 Channel 2 1e+05 Counts 1e+04 1e+03 1e+02 1e+01 1e Input Voltage [-V] Better energy resolution than BaF 2 Has intrinsic decay of 1 67Lu

31 LaBr 3 (Ce) Lanthanum bromide Input Voltage [V] LaBr3(Ce) Time [samples] Very good energy resolution (real photo-peaks) Very pricey, very hygroscopic

32 LaBr 3 (Ce) Lanthanum bromide 1e+06 1e keV 1.27MeV 1.27MeV + 511keV 1e+04 Counts 1e+03 1e+02 1e+01 Ch 1 Ch 2 1e Input Energy [-V] Very good energy resolution (real photo-peaks) Very pricey, very hygroscopic

33 ZnO Zinc oxide ZnO 0 Input Voltage [V] Time [samples] Up to now only used as powder for α-particles Current research project...

34 ZnO Zinc oxide Hamamatsu with ZnO Ch 1 Ch Counts Input Voltage [-V] Up to now only used as powder for α-particles Current research project...

35 Efficiency Efficiency by comparison and 1/r 2 -rule. Lower limit (x 0, r ): efficiency of a single scintillator point Upper limit (x, r 0): maximum digitizer transfer rate double-log plot fitted with arctan-function

36 Efficiency Counts per time [1/s] Relative Efficiency (1/r^2) LSO y0 = ZnO-Big y0 = ZnO-Small y0 = LaBr3(Ce) y0 = LaBr3(Ce) y0 = e-07 1e-06 1e Distance [1/cm^2] Relation: LSO 6 BaF 2 [6]

37 Timing Resolutions Lets take a look at the timing resolutions. Good energy resolution = good timing resolution? Best method for different pulse shapes?

38 LaBr 3 (Ce): Si Data File Variance Lt 1 [ns] Lt 2 [ns] I 2 [%] fwhm1 [ns] pcf-lt01-hl pcf-lt02-hl pcf-lt03-hl pcf-lt04-hl pcf-lt05-hl Coarse FWHM - LP-pCf Coarse FWHM - diff-pcf Determined FWHM [ns] (Filled: HL, Empty: LH) Order = 1 Order = 2 Order = 3 Order = (Filled: HL, Empty: LH) Order = 1 Order = 3 Order = Cutoff Frequency [sampling rate]

39 LaBr 3 (Ce): Si Data File Variance Lt 1 [ns] Lt 2 [ns] I 2 [%] fwhm1 [ns] pcf-lt01-hl pcf-lt02-hl pcf-lt03-hl pcf-lt04-hl pcf-lt05-hl lp pcf-lt06-hl lp pcf-lt07-hl lp pcf-lt09-hl dpcf-lt10-hl dpcf-lt11-hl

40 LSO: Si Timing Resolution [ns] pcf (0.568ns) Order 1 Order 2 Order 3 Order Cufoff Frequency [sampling rate] LSO with lp-pcf Method Variance Lt 1 [ns] fwhm 1 [ns] pcf (cf0.1) pcf (cf0.5) lp-pcf (cf0.5) dpcf (cf0.5)

41 BaF 2 : Si Method Variance Lt 1 [ns] Lt 2 [ns] I 2 [%] fwhm 1 [ns] pcf lp pcf dpcf dpcf dpcf (3) 0.32% dpcf % pcf % lp pcf % 0.252

42 One slide to show them all. 2 pcf lp-pcf dpcf LaBr 3 (Ce) 449ps 352ps 346ps LSO 275ps - 568ps 355ps - 462ps 298ps BaF 2 258ps 252ps 275ps 2 Sorry again.

43 Conclusions Aim from literatur: timing FWHM 150ps to 100ps. Feels like: Back to square one What is missing? Efficiency measurement with BaF 2 Lifetime measurements with ZnO Data is there, evaluation has to run. Measurements with plastic scintillators. Maybe its the tubes? More testing with XP20Z8 with second electronics. Gain experience with XP2020 Look at Hamamatsu R4700U

44 Conclusion: Thanks for your attention! Get the slides at P. A. M. Dirac. The quantum theory of the electron. Proc. R. Soc. London, Ser. A, 118: , C. D. Anderson. The positive electron. Phys. Rev., 43: , A. Einstein. Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig? Annalen der Physik, 323: , R. Krause-Rehberg and H. Leipner. Positrons in Solids. Springer-Verlag, S. D. Stearns. Digital Signal Analysis. Hayden Book Company Inc., A. Krille. Aufbau und optimierung eines digitalen positronen-lebendauer-spektrometers, 2008.