Electronic Excitation of C2F4

Report
Ionisation of H2O by Proton and
Atomic Hydrogen Impact at velocities
~ the Bragg peak
Sam Eden
Institut de Physique Nucléaire de Lyon
[email protected]
The IPM Group
Particle - Matter
Interactions
Group members:
Bernadette Farizon
Helium impact upon ionic hydrogen
clusters H3+(H2)n
Michel Farizon
Proton collisions with gas phase
molecules (H2O, He, uracil, DNA bases)
Sam Eden
Atomic hydrogen collisions with gas
phase molecules (H2O, He)
Proton impact upon biomolecule - water
clusters
Bruno Coupier
Jean Tabet
http://lyoinfo.in2p3.fr/ipm
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 H2O
Neutral hydrogen atom impact upon
gas phase H2O
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 H2O
to interpret the proton – biomolecular cluster collision
results
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
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
see M. Biaggi et al., Nucl. Instr. Methods Phys. Res. B 159, 89 (1999)
Proton impact upon
gas phase H2O
The experimental system
Cross sections which are differential in
terms of
direct ionisation
electron capture
The experimental system
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
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)
Hydrogen impact upon
gas phase H2O
The experimental system
Cross sections
Comparisons with proton
impact ionisation
The experimental system
Neutralising
gas (Ar)
H
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)
target ionisation
σ00 → 01
final charge of projectile (0 or +1)
: H + (H2O+)* + e-
projectile + target ionisation or electron loss + target ionisation
σ00 → 11
: H+ + (H2O+)* + 2e-
electron loss with target excitation
σ00 → 10
: H+ + (H2O)* + eThe first neutral hydrogen impact experiment
to separates these processes
Previously measured
σi f
initial charge of projectile (0)
σ+
final charge of projectile (0 or +1)
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)
Absolute cross sections
Each 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)
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
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)
The experimental
system
4. Ionic cluster
beam crossed with a
monochromatic
beam of protons
(20 - 150 keV)
3. Energy and mass selection →
monochromatic beam of ionic
clusters comprising one biomolecule
and n water molecules
5. Event
by event
coincidence
analysis of
projectiles
and target
products
post-collision
→ analogous
to present
experiment
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

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