Ionisation of H 2 O by Proton and Atomic Hydrogen Impact at velocities ~ the Bragg peak

1. Ionisation of H2O by Proton and Atomic Hydrogen Impact at velocities ~ the Bragg peak Sam Eden Institut de Physique Nucléaire de Lyon s.eden@ipnl.in2p3.fr

2. The IPM GroupParticle - Matter Interactions Group members: • Bernadette Farizon • Michel Farizon • Sam Eden • Bruno Coupier • Jean Tabet http://lyoinfo.in2p3.fr/ipm Helium impact upon ionic hydrogen clusters H3+(H2)n Proton collisions with gas phase molecules (H2O, He, uracil, DNA bases) Atomic hydrogen collisions with gas phase molecules (H2O, He) Proton impact upon biomolecule - water clusters

3. Collaborations • Tilmann Märk Paul Scheier Institut für Ionenphysik, Innsbruck, Austria • Saïd Ouaskit Faculté Ben M’sik, Casablanca, Morocco • Marie-Christine Bacchus LASIM, UCBL, Lyon, France • Alain Bordenave-Montesquieu Patrick Moretto-Capelle IRSAMC, Toulouse, France • Nelson Velho de Castro FariaGinette JalbertDepartamento de Física Nuclear, Instituto de Física, Universidade Federal do Rio de Janiero, Brazil

4. Introduction • Proton impact upon gas phase H2O • Neutral hydrogen atom impact upon gas phase H2O • An experiment to observe collisions between protons and biomolecule – water clusters

5. Why study water? • Incident rays can damage living tissue through direct (or primary) particle-biomolecule interactions… … And through interactions with secondary species • These secondary fragments and electrons create the track patterns along the path of the energetic particle • The water content of a living cell is ~ 70% by weight • We will need absolute cross sections for gas phase H2O to interpret the proton – biomolecular cluster collision results

6. The energy regime(20–150 keV) • At these energies, KE transfer is mainly to bound electrons in the absorbing medium (excitation, ionisation) • Coincides with the Bragg peak (~ 100 keV / amu) • Proton therapy: incident protons of velocity ~ the Bragg peak dramatically degrade the reparability of DNA

7. Charge transfer events Changes in the charge state of the projectile are understood to play a critical role in the occurrence of the Bragg peak in irradiated media seeM. Biaggi et al., Nucl. Instr. Methods Phys. Res. B 159, 89 (1999)

8. Proton impact upon gas phase H2O • The experimental system • Cross sections which are differential in terms of • direct ionisation • electron capture

9. The experimental system

10. Event by event analysis Each product ion can be associated with a single incident proton The charge state of the detected projectile determines the ionisation process: • H+→ direct ionisation (emission of at least one e-) • H → single electron capture • H-→ double electron capture Large samples (typically > 104 events) → precise cross sections for all but the rarest events

11. Experimental (continued…) The system can be configured to detect either positive or negative ions Processes in which two or more product ions are formed in a single collision event can be identified Absolute cross sections → Calibration using previous cross sections for cation production and electron emission by H2O upon H+ impact M.E. Rudd et al., Phys. Rev. A 31, 492 (1985)

12. Hydrogen impact upon gas phase H2O The experimental system Cross sections Comparisons with proton impact ionisation

13. The experimental system Neutralising gas (Ar) H

14. Event by event analysis σi i → f f initial charge of projectile (0) final charge of target (0 or +1) initial charge of target (0) final charge of projectile (0 or +1) • target ionisationσ00 → 01: H + (H2O+)* + e- • projectile + target ionisationorelectron loss + target ionisationσ00 → 11: H+ + (H2O+)* + 2e- • electron loss with target excitationσ00 → 10: H+ + (H2O)* + e- The first neutral hydrogen impact experiment to separates these processes

15. Previously measured σi f initial charge of projectile (0) final charge of projectile (0 or +1) σ+ σ- target cation production total electron (or anion) production R. Dagnac et al., J. Phys. B 3, 1239 (1970) L.H. Toburen et al., Phys. Rev. 171, 114 (1968) M.A. Bolorizadeh and M.E. Rudd, Phys. Rev. A 33, 893 (1986)

16. Absolute cross sectionsEach H impact measurement is accompanied by an H+ result with the same target conditions → calibration carried out by comparison with previous H+ impact cross sections M.E. Rudd et al., Phys. Rev. A 31, 492 (1985) U. Werner et al., Phys. Rev. Lett. 74, 1962 (1995)

17. Current work:An experiment to observe collisions between protons and biomolecular clusters • Mixed clusters composed of one DNA base (or uracil) and n H2O molecules • Fragmentation mechanisms and reactions within a cluster will be studied as a function of n • Comparisons with gas phase results (H2O, uracil, and the DNA bases - currently being measured) • Key information to quantify the direct and indirect effects of ionising radiation

18. The experimental system 1. Condensation under vacuum of vaporised biomolecules and water in the presence of electrons (≤ 100 eV) 4. Ionic cluster beam crossed with a monochromatic beam of protons (20 - 150 keV) 5. Event by event coincidence analysis of projectiles and target products post-collision → analogous to present experiment 2. Acceleration of ions and ionic clusters (up to 30 keV) 3. Energy and mass selection → monochromatic beam of ionic clusters comprising one biomolecule and n water molecules

19. Acknowledgements • Our collaborators • The technical staff at IPNL And the financial support of • The French research council CNRS • The French and Austrian Governments through the PICS and Amadee programmes • The French Ministère de la Recherche