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Positron scattering and annihilation in organic molecules

Positron scattering and annihilation in organic molecules. G. P. Karwasz , A. Karbowski, K. Fedus Institute of Physics, Nicolaus Copernicus University , Toruń Jan Franz Faculty of Applied Physics and Mathematics Gdansk University of Technology. Rationale.

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Positron scattering and annihilation in organic molecules

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  1. Positron scattering and annihilation in organic molecules G. P. Karwasz, A. Karbowski, K. Fedus Institute of Physics, Nicolaus Copernicus University, Toruń Jan Franz Faculty of Applied Physics and Mathematics Gdansk University of Technology

  2. Rationale • J-PET aims to reveal the biological type of the tissue at the annihilation site. • Interactions between electron (positron) and atom (molecule, tissue) is described by cross sections. • This holds for all processes: implantation of protons, DNA damage, positron annihilation etc. • Many different diagnostic methods are not only „morphological” but also chemically-sensitive; positron annihilation, in principle, also • Positron annihilation in living bodies happens in the liquid environment • But liquids are not much eagerly measured

  3. Modelling positron/ electron/ proton tracks in condensed matter or positron generaton

  4. 2. Partial cross sections: 1. Total cross section (& annihilation rate) Elastic scattering e++A →e++A rotational excitation e++ H2O (J=0) → e+H2O (J=1) vibrational excitation e++AB(v=0) → e++AB(v>0) Positron attachment e++A → Ae+ (resonant state?) Electronic excitation e++A →e++A* Positronium formation e++A → A+ + Ps Ionization e++A →A++e+ + e-

  5. Electron-scattering cross sections M-Y. Song, …Karwasz, JCP Reference data, 2017

  6. Positron annihilation in solids Karwasz et. al. J Alloys & Compounds, 2003

  7. He bubbles in Si - quantization of S values: types of defects Brusa et al. PRB 2000, JAP 2001

  8. Positron lifetime in Cz-silicon • Positron lifetime in silicon (not implanted, not deformed) is some 220-222 ps; 260 ps in V2 in SiO2 230 ps • 1 year of measurements, • ~80 samples, some annealed under pressure • A tiny difference: • „virgin” 222 ±1 ps • „as grown” 221 ±1 ps • „under pressure” 226 ps • (or τ2~500 ps 1-4%)

  9. Oxygen in Cz-grown silicon Annealing at 450ºC makes the oxygen migrate into small SiOx clusters so it disappears from the intestitial positions i.e. becomes electrically inactive

  10. Positron annihilation in liquids Only long lifetime component

  11. Positron annihilation in liquids I2 τ2 P. R. Gray, C. F. Cook, G. P., Sturm, J. Chem. Phys. 48 (1968) 1145

  12. Positron annihilation in liquids P. R. Gray, C. F. Cook, G. P., Sturm, J. Chem. Phys. 48 (1968) 3.

  13. Consolati: Benzene unusual annihilation τ1 =148 ps τ2 = 416 ps τ3 = 1.18 ns τ4 = 3.26 ns I3 = 5.3% I4 = 39%

  14. Consolati & Quasso: Magnetic quenching

  15. fit, not ab-initio

  16. phase transitions C30H62 τ3 rises by a factor of 2

  17. „Free” volumes in polymers These are not phase transitions!

  18. „Free” volumes in poly-carbonate

  19. San Diego: energy-resolved annihilation rate in gas phase Zeff=185,000 cyclohexane Zeff= 18,400 benzene

  20. Zeff vs. TCS: noble gases Ar Xe Karwasz et al., NIMB (2006) Surko, Buckmann, Gribakin J. Phys. B (2005)

  21. Feshbach vibrational resonances

  22. Feshbach resonances Karwasz, EPJD 35 (2005) 267

  23. Gribakin, Young, Surko, Rev. Mod. Phys. (2010)

  24. Total cross sections (experiment) FIRST ACCELERATOR REMODERATOR STAGE INJECTION OPTICS DEFLECTOR Brusa, Karwasz, Zecca 2000-2004 Weak magnetic field (9 G) Angular resolution: L=100 mm/ G. P. Karwasz, R.S. Brusa, M.Barozzi and A.Zecca, Nuclear Instr. and Methods in Physics B 171, 178 (2000)

  25. Total cross sections (experiment)

  26. Total cross sections (experiment)

  27. Theory: Modified effective range (2006) Analytical solution of Schroedinger equation with V polarization

  28. we have even more than one explanation But this is still only a model, not mechanism

  29. Total cross sections Franz: εbind from Gribakin

  30. Benzene: uncertainty analisis Fedus: present work

  31. Cyclohexane: TCS ab initio theory

  32. Positron Lifetime System • - plastic scintilators (St. Gobain BC418) and photomultipliers RCA, • - 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, • 250 k-1000 k data in one run • χ2 better than 1.01; some runs repeated A. Karbowski, J. D. Fidelus, G. Karwasz, Materials Science Forum, 666 (2011) 155

  33. Results for methanol τ3 of some 2.6 - 3.2 ps, and changes with oxygen presence

  34. 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

  35. Results for benzene Solid benzene oxygen lowers o-Ps lifetime by factor of 2

  36. Results for benzene With oxygen I3 slightly higher (?)

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

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

  39. 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)

  40. Consolati, Quasso (2014) &Tanzi Marlotti (2018): contact density

  41. PALS: resume’ (25ºC) Mogensen Present Pretty good agreement with Mogensen for τ3 and I3 τ1 and τ2 require further analysis (with no 3:1 fix)

  42. 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? )

  43. Thank you for your attention!

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