Positronium formation in atoms and molecules – from experiment to modelling

1. 2nd Jagiellonian Symposium on Fundamental and Applied Subatomic Physics Cracow, 06 June 2017 Positronium formation in atoms and molecules – from experiment to modelling Grzegorz Karwasz, Kamil Fedus, Andrzej Karbowski, Jan Franz* Institute of Physics Nicolaus Copernicus University Toruń, Poland *Gdańsk Technical University, Institute of Physics e-mail: kamil@fizyka.umk.pl

2. Outline • Why we need cross sections? • Positron scattering in gases by beam methods • Total (& elastic cross sections) • Positron annihilation in (degassed) liquids • ? Conclusions

3. Rationale (0): TOF - PET diagnostic tool in biological tissues • The concept of time-of-flight means simply that for each annihilation event, we note the precise time that each of the coincident photons is detected and calculate the difference. • Applying the energy thresholds for measured photons as in traditional lifetime measurements should, in principle, to allow for 3D monitoring of positronium lifetimes. • 3D monitoring of oxygen level should be possible. J – PET in Cracow plastic scintillators allow to improve time resolution

4. Protonoterapia W przypadku zastosowania cząstek, jonizacja polega na tym, że przechodząc przez tkankę, przykładowo protony, wybijają elektrony krążące wokół jąder atomowych. Robią to tym skuteczniej, im wolniej się poruszają - w efekcie dawka jonizacji posiada ostre maksimum na końcu zasięgu protonu. Ilustruje to pik Bragga, którego położenie zależy od energii wiązki, przykładowo: 3 cm dla protonów o energii E=60 MeV i aż 30 cm dla protonów o E=230 MeV. Dzięki temu, regulując ich energię, określamy obszar, w którym najwięcej komórek będzie zniszczonych - „celujemy” w obszar nowotworu. (C) G. Kontrym-Sznajd

5. What cross-sections do we need?

6. Modeling of positron/ electron/ proton tracks in condensed matter or positron generaton

7. Rationale (1): unexpected results for positron scattering in gas phase

8. Trento low-energy gas-phase positron beam experiment G. P. Karwasz, R.S. Brusa, M.Barozzi and A.Zecca, Nuclear Instr. and Methods in Physics B 171, 178 (2000)

9. Rationale (1): positron scattering in gases

10. Rationale (2): liquids vs. gases • Positron annihilation is the fundamental process in PET, and also in radiation therapies. • 60 and over yrs of positron annihilation studies (in condensed) matter match very poorly with experiments in gas phase (*1977) • Till recently (2004) positron scattering at low energies did not match with theories, particularly at thermal energies (reasons will be shown here) • Studies of annihilation in liquid phase are usually limited to the long lifetime component, that is used to be attributed to o-positronium formation. The lifetime of o-Ps in vacuum is 143 ns and it annihilates in three γ-quanta. • Note that studies of o-Ps are to be considered methodologically „pure” only in large really free volumes, like in some double-matrix glasses, like so-called vycor (lifetimes up to 50 ns, or so). • In liquids (or polymers) measured lifetimes are of few (2-3) ns. Several processed are called in help, like o-Ps→p-Ps transition and/or electron capture from the medium. • Numerous models allow to relate the lifetime to the volume (Eldrup, Consolati, Goworek). We like the most the model of quantum well by Radek Zalewski.

11. Positronium formation in gas-phase organic molecules

12. Positron cross-sections for simple targets EPs = I– 6.8eV

13. Cross-sections for positroniumformation in noble gases

14. Cross-sections for positronium formation in large organic systems Where is the positronium ? EPs=(9.24-6.8)eV ? EPs=(8.95-6.8)eV ? G.P. Karwasz, et al.., Acta Phys. Pol. 107 (2005) 666.

15. vo rcycl= mv┴/ eB v┴= vosin θ θ vo=√(2E/m) R Benzene - angular resolution correction! Ps signal? Karwasz et al., NIM B, 266/3 (2008) 471

16. Quantum phenomenaat low-energy interaction (below Ps) Resonant annihilation for large organic molecules C. Surko et al. (San Diego University)

17. (bound state) Surko et al.. (virtual state) Theoretical analysis of benzene cross sections Bayesian analysis and DFT calculations of elastic scattering cross-sections

18. Positronium formation in liquid-phase organic molecules

19. The results of analyses of lifetime spectra for different liquids τ2= 3.2 – 4.6 ns I2= 30-48% P. R. Gray, C. F. Cook, G. P., Sturm, J. Chem. Phys. 48 (1968) 3.

20. The results of analyses of lifetime spectra for different liquids P. R. Gray, C. F. Cook, G. P., Sturm, J. Chem. Phys. 48 (1968) 3.

21. The results of analyses of lifetime spectra for benzene (C6H6) 3= 3.1 – 3.3 ns air lowers 3 to 2.4 - 2.6 ns O. E. Mogensen, Positron Annihilation in Chemistry, Springer-Verlag, Berlin 1995. P. R. Gray, C. F. Cook, G. P., Sturm, J. Chem. Phys. 48 (1968) 1145. G. Consolati, D. Gerola, F. Quasso, J. Phys.: Condens. Master 3 (1991) 7739. A. Bisi, G. Consolati, N, Gambara and L. Zappa, Nuovo Cimento D 13 (1991) 393. J. A. Merrigan, J. H. Green, S. J. Tao: In Physical Methods of Chemistry, ed. by A.Weissberger (Wiley, New York 1972) Vol. 1, Pt. IID. K. P. Singh, R. M. Singru, C. N. Rao, J. Phys. Atom. Molec. Phys. 4 (1971) 261. C. R. Hatcher, T. W. Falconer, W. E. Millett, J. Chem. Phys. 32 (1960) 28. J. Lee, and G. J. Celiitans, J. Chem. Phys., 42 (1965) 437 S. Berko, A. J. Zuchelli, Phys. Rev. 102 (1955) 724. G. R. A de Blonde, PhD thesis: Positron annihilation in condensed hydrocarbons, The University of Manitoba (1972). B. J. Brown, Aust. J. Chem. 27 (1974) 1125.

22. The results of analyses of lifetime spectra for cyclohexane (C6H12) J. Lee, and G. J. Celiitans, J. Chem. Phys., 42 (1965) 437 P. R. Gray, C. F. Cook, G. P., Sturm, J. Chem. Phys. 48 (1968) 1145. G. R. A de Blonde, PhD thesis: Positron annihilation in condensed hydrocarbons, The University of Manitoba (1972). O. E. Mogensen, Positron Annihilation in Chemistry, Springer-Verlag, Berlin 1995. B. Zgardzińska, T. Goworek, Chemical Physics, 421 (2013) 10. What we do not like in reported data is that they do not refer to other than τ3 lifetimes…

23. The results of analyses of lifetime spectra for methanol (CH3OH) J. Lee, and G. J. Celiitans, J. Chem. Phys., 42, 437 (1965). P. R. Gray, C. F. Cook, G. P., Sturm, J. Chem. Phys. 48 (1968) 1145. B. J. Brown, Aust. J. Chem. 27 (1974) 1125. O. E. Mogensen, Positron Annihilation in Chemistry, Springer-Verlag, Berlin 1995.

24. ORTEC Positron Lifetime System

25. ORTEC Positron Lifetime System

26. ORTEC Positron Lifetime System • - plastic scintilators (St. Gobain BC418) and photomultipliers RCA, • - data acquisition MAESTRO by ORTEC, • measured time resolution (in metals) FWHM = 180 ps • degassing system with rotary pump (10-3 Tr @ LN2 freezing) • temperature regulation (Peltier) 0 - 30º C • positron source 22Na (15 μCi) in kapton 7 μm thick foil, • 250k-1000 k data in one run • data analysis - LT 9 programme by J. Kansy, • 3:1 ratio between o-Ps and p-Ps fixed (the latter 124 ps) • χ2 better than 1.01; some runs repeated A. Karbowski, J. D. Fidelus, G. Karwasz, Materials Science Forum, 666 (2011) 155

27. Results for methanol • Quite in line with previous measurements: τ3 of some 2.6 - 3.2 ps • In other words: nihil novae sub Sole

28. Cancer and oxygen • The link between oxygen and cancer is clear. In fact, an underlying cause of cancer is usually low cellular oxygenation levels. • In 1931 Dr. Warburg won his first Nobel Prize for proving cancer is caused by a lack of oxygen respiration in cells. He stated in an article titled "The Prime Cause and Prevention of Cancer... the cause of cancer is no longer a mystery, we know it occurs whenever any cell is denied 60% of its oxygen requirements..." Source: http://www.cancerfightingstrategies.com/oxygen-and-cancer.html

29. Results for benzene Solid benzene so, it is oxygen in air which lowers o-Ps lifetime

30. Results for benzene Within error bar I3 is constant. This is non in-line with predictions, that oxygen makes quenching o-Ps→p-Ps Re-analysis to be done: 3:1 ratio is to be released…

31. Results for benzene: τ2 No clear conclusion: 380 ps is too close to the source component … However, the intensity is much more than the source itself, and it changes when benzene gets solid

32. The results of analyses of lifetime spectra for benzene (C6H6) Lee: no „I3” is given

33. Results for cyclohexane τ3 lifetime drops down by a factor of 2 in presence of oxygen! (and intensity slightly rises!)

34. Results for cyclohexane: τ2 τ2 for degassed cyclohexane not much different from that in benzene, but intensity I2 much higher (50% vs 40%)

35. The results of analyses of lifetime spectra for cyclohexane (C6H12) J. Lee, and G. J. Celiitans, J. Chem. Phys., 42 (1965) 437 P. R. Gray, C. F. Cook, G. P., Sturm, J. Chem. Phys. 48 (1968) 1145. G. R. A de Blonde, PhD thesis: Positron annihilation in condensed hydrocarbons, The University of Manitoba (1972). O. E. Mogensen, Positron Annihilation in Chemistry, Springer-Verlag, Berlin 1995. B. Zgardzińska, T. Goworek, Chemical Physics, 421 (2013) 10.

36. Results for methanol τ3 lifetime (again) drops down by a factor of 2 in presence of oxygen!

37. Results for methanol: τ2

38. Conclusions @ 25ºC: present vs Mogensen Mogensen Present Pretty good agreement with Mogensen for τ3 and I3 τ1 and τ2 require further analysis(with no 3:1 fix)

39. Conclusions • Oxygen reduces strongly the ortho-positronium lifetimes (not intensities) in all studied liquids • τ2 (in degased liquids): 400 ps (methanol) > 390 ps (benzene) > 380 ps (cyclohexane) • I2(in degassed): 70% methanol > 48% cyclohexane > 41% benzene • I3(in degassed): 44% benzene > 39% cyclohexane > 22% methanol • Does it reflect a higher (??) relative positronium formation cross section in gas-phase benzene? • First, we need calculations of the elastic part of total cross section • Little dependence on temperature in C6H6 and C6H12, stronger in CH3OH (? effect of rising pressure in the sample cell → higher concentration of O2 dissolved?/ polar character of CH3OH? )

40. Perspectives • Re-calculation of τ3 and I3 releasing 3:1 ratio • Futher ab-initio calculation: cyclohexane and methanol • More liquids to be measured • More precise measurements in gas phase @ 0.2-20 eV • Positron scattering in gas phase – the low-energy beam machine at testing with electrons @ UMK

41. Thank you for your attention!