Ionisation of H 2 O by Proton and Atomic Hydrogen Impact at velocities ~ the Bragg peak Sam Eden Institut de Physique Nucléaire de Lyon

The IPM Group Particle - Matter Interactions The IPM Group Particle - Matter Interactions Group members: Bernadette Farizon Michel Farizon Sam Eden Bruno Coupier Jean Tabet Helium impact upon ionic hydrogen clusters H 3 + (H 2 ) n Proton collisions with gas phase molecules (H 2 O, He, uracil, DNA bases) Atomic hydrogen collisions with gas phase molecules (H 2 O, He) Proton impact upon biomolecule - water clusters

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 Faria Ginette Jalbert Departamento de Física Nuclear, Instituto de Física, Universidade Federal do Rio de Janiero, Brazil

Introduction Proton impact upon gas phase H 2 O Neutral hydrogen atom impact upon gas phase H 2 O An experiment to observe collisions between protons and biomolecule – water clusters

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 H 2 O to interpret the proton – biomolecular cluster collision results

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 The energy regime (20–150 keV)

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

The experimental system Cross sections which are differential in terms of direct ionisation direct ionisation electron capture electron capture Proton impact upon gas phase H 2 O

The experimental system

Each product ion can be associated with a single incident proton The charge state of the detected projectile determines the ionisation process: Event by event analysis H + → direct ionisation (emission of at least one e - ) H→ single electron capture H - → double electron capture Large samples (typically > 10 events) Large samples (typically > 10 4 events) → precise cross sections for all but the rarest events

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 Experimental (continued…) Absolute cross sections → Calibration using previous cross sections for cation production and electron emission by H 2 O upon H + impact M.E. Rudd et al., Phys. Rev. A 31, 492 (1985)

The experimental system Cross sections Comparisons with proton impact ionisation Hydrogen impact upon gas phase H 2 O

The experimental system H Neutralising gas (Ar)

target ionisation σ 00 → 01 : H + (H 2 O + )* + e - projectile + target ionisation or electron loss + target ionisation σ 00 → 11 : H + + (H 2 O + )* + 2e - electron loss with target excitation σ 00 → 10 : H + + (H 2 O)* + e - σ i i → f f initial charge of projectile (0) initial charge of target (0) initial charge of target (0) final charge of projectile (0 or +1) final charge of target (0 or +1) Event by event analysis The first neutral hydrogen impact experiment to separates these processes

σ i f initial charge of projectile (0) final charge of projectile (0 or +1) σ+σ+σ+σ+ σ-σ-σ-σ- target cation production 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) Previously measured

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

Current work: An experiment to observe collisions between protons and biomolecular clusters Mixed clusters composed of one DNA base (or uracil) and n H 2 O molecules Fragmentation mechanisms and reactions within a cluster will be studied as a function of n Comparisons with gas phase results (H 2 O, uracil, and the DNA bases - currently being measured) Key information to quantify the direct and indirect effects of ionising radiation

1. Condensation under vacuum of vaporised biomolecules and water in the presence of electrons (≤ 100 eV) 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 4. Ionic cluster beam crossed with a monochromatic beam of protons ( keV) 5. Event by event coincidence analysis of projectiles and target products post-collision → analogous to present experiment The experimental system

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