We report direct experimental measurements with picosecond time resolution of how high energy protons interact with water at extreme dose levels (kGy), delivered in a single pulse with the duration of less than 80 ps. The unique synchronisation possibilities of laser accelerated protons with an optical probe pulse were utilized to investigate the energy deposition of fast protons in water on a time scale down to only a few picoseconds. This was measured using absorbance changes in the water, induced by a population of solvated electrons created in the tracks of the high energy protons. Our results indicate that for sufficiently high doses delivered in short pulses, intertrack effects will affect the yield of solvated electrons. The experimental scheme allows for investigation of the ultrafast mechanisms occurring in proton water radiolysis, an area of physics especially important due to its relevance in biology and for proton therapy.

ion acceleration has made rapid progress in the past decade. 1,2 85, 751 (2013). 1. A. Macchi, M. Borghesi, and M. Passoni, Rev. Mod. Phys., 751 (2013). https://doi.org/10.1103/RevModPhys.85.751 75, 056401 (2012). 2. H. Daido, M. Nishiuchi, and A. S. Pirozhkov, Rep. Prog. Phys., 056401 (2012). https://doi.org/10.1088/0034-4885/75/5/056401 ion beam parameters increases, it becomes possible to find and develop applications utilizing their unique properties. Ion pulses from laser particle accelerators are created on the time scale of a picosecond, 3 2, 48 (2006). 3. J. Fuchs, P. Antici, E. d'Humieres, E. Lefebvre, M. Borghesi, E. Brambrink, C. A. Cecchetti, M. Kaluza, V. Malka, M. Manclossi, S. Meyroneinc, P. Mora, J. Schreiber, T. Toncian, H. Pepin, and P. Audebert, Nat. Phys., 48 (2006). https://doi.org/10.1038/nphys199 energy spectrum, close to their source they are significantly shorter 4 7, 10642 (2016). 4. B. Dromey, M. Coughlan, L. Senje, M. Taylor, S. Kuschel, B. Villagomez-Bernabe, R. Stefanuik, G. Nersisyan, L. Stella, J. Kohanoff, M. Borghesi, F. Currell, D. Riley, D. Jung, C.-G. Wahlström, C. Lewis, and M. Zepf, Nat. Commun., 10642 (2016). https://doi.org/10.1038/ncomms10642 ion beams from conventional accelerators. Furthermore, it is possible to synchronise multiple laser pulses, provided they come from the same source, with time jitter that is negligible on the picosecond time scale, and thus enabling the possibility to use one laser pulse to drive the acceleration process and one as a probe, in a pump/probe-configuration. 4 7, 10642 (2016). 4. B. Dromey, M. Coughlan, L. Senje, M. Taylor, S. Kuschel, B. Villagomez-Bernabe, R. Stefanuik, G. Nersisyan, L. Stella, J. Kohanoff, M. Borghesi, F. Currell, D. Riley, D. Jung, C.-G. Wahlström, C. Lewis, and M. Zepf, Nat. Commun., 10642 (2016). https://doi.org/10.1038/ncomms10642 conducted probing or imaging the actual acceleration process with shadowgraphy or using the accelerated protons to probe laser generated dense plasmas. 5 43, A267 (2001). 5. M. Borghesi, A. Schiavi, D. H. Campbell, M. G. Haines, O. Willi, A. J. MacKinnon, L. A. Gizzi, M. Galimberti, R. J. Clarke, and H. Ruhl, Plasma Phys. Controlled Fusion, A267 (2001). https://doi.org/10.1088/0741-3335/43/12A/320 protons interact with water with a time resolution of only a few picoseconds. The field of laser basedhas made rapid progress in the past decade.As control overbeam parameters increases, it becomes possible to find and develop applications utilizing their unique properties.pulses from laser particle accelerators are created on the time scale of a picosecond,and although they commonly have a broadspectrum, close to their source they are significantly shorterthan thebeams from conventionalFurthermore, it is possible to synchronise multiple laser pulses, provided they come from the same source, with time jitter that is negligible on the picosecond time scale, and thus enabling the possibility to use one laser pulse to drive theprocess and one as a probe, in a pump/probe-configuration.Until now, studies have beenprobing or imaging the actualprocess with shadowgraphy or using theto probe laser generated dense plasmas.In this paper, we present results where the short pulse duration, in combination with the exceptional synchronisation properties, is utilized to investigate how energeticwithwith a time resolution of only a few picoseconds.

proton radiolysis in water has so far been limited in time resolution by available proton pulse durations and by electronic synchronisation between the proton pulse and a separate probe. This has made it impossible to make direct experimental studies of processes that are faster than a few hundred picoseconds. Another option is to use indirect scavenging techniques, where a chemical scavenger is employed to measure the yields of radiolytic species in the tracks of high energy protons. 6 109, 9393 (2005). 6. J. A. LaVerne, I. Stefanić, and S. M. Pimblott, J. Phys. Chem. A, 9393 (2005). https://doi.org/10.1021/jp0530303 7,8 47, 69 (2006). 7. S. Uehara and H. Nikjoo, J. Radiat. Res., 69 (2006). https://doi.org/10.1269/jrr.47.69 77, 1218 (2008). 8. G. Baldacchino, Radiat. Phys. Chem., 1218 (2008). https://doi.org/10.1016/j.radphyschem.2008.05.033 energy protons on water. The experimental research ofradiolysis inhas so far been limited in time resolution by availablepulse durations and by electronic synchronisation between thepulse and a separate probe. This has made it impossible to make direct experimental studies of processes that are faster than a few hundred picoseconds. Another option is to use indirect scavenging techniques, where a chemical scavenger is employed tothe yields of radiolytic species in the tracks of highBy varying the concentration of the scavenger, the yields can be accurately determined at different times using the rate constant for the reaction between the scavenger and the radicals. For picosecond time scales, however, this method becomes less reliable due to the very high concentration of scavengers needed. Traditionally, such experiments were performed in combination with Monte Carlo simulationsto study the early effects of the highon

energy protons propagate into water, the absorbance of the water changes for certain wavelengths. This process occurs when the protons deposit energy through ionising the water molecules and thereby releasing electrons. In the water, these secondary electrons will first become thermalized and then subsequently solvated/hydrated and, among other things, have the property of absorbing light in the visible and near-infrared region. 9 14, 22 (2012). 9. B. Abel, U. Buck, A. L. Sobolewski, and W. Domcke, Phys. Chem. Chem. Phys., 22 (2012). https://doi.org/10.1039/C1CP21803D solvated electrons has been known for decades, their exact properties and behaviour are not fully understood and are to some extent still debated. 10–12 329, 65 (2010). 10. R. E. Larsen, W. J. Glover, and B. J. Schwartz, Science, 65 (2010). https://doi.org/10.1126/science.1189588 331, 1387 (2011). 11. L. Turi and A. Madarasz, Science, 1387 (2011). https://doi.org/10.1126/science.1197559 331, 1387 (2011). 12. L. D. Jacobson and J. M. Herbert, Science, 1387 (2011). https://doi.org/10.1126/science.1198191 energy protons interact with water is highly relevant since as much as 66 % of radiation deposited into a cell is initially absorbed by water molecules. The radicals and solvated electrons formed in this interaction can then react with and damage the DNA in the cell. 13 112, 5578 (2012). 13. E. Alizadeh and L. Sanche, Chem. Rev., 5578 (2012). https://doi.org/10.1021/cr300063r As highpropagate intothe absorbance of thechanges for certain wavelengths. This process occurs when thethrough ionising themolecules and thereby releasingIn thethese secondarywill first become thermalized and then subsequently solvated/hydrated and, among other things, have the property of absorbing light in the visible and near-infrared region.Even though the existence ofhas been known for decades, their exact properties and behaviour are not fully understood and are to some extent still debated.From a biological perspective, the study of how highwithis highly relevant since as much as 66of radiationinto a cell is initially absorbed bymolecules. The radicals andformed in thiscan then react with and damage the DNA in the cell.

proton pulse radiolysis experiment we present here, the protons were accelerated from thin aluminium foils in the target normal sheath acceleration (TNSA) regime. 14,15 7, 2076 (2000). 14. S. P. Hatchett, C. G. Brown, T. E. Cowan, E. A. Henry, J. S. Johnson, M. H. Key, J. A. Koch, A. B. Langdon, B. F. Lasinski, R. W. Lee, A. J. Mackinnon, D. M. Pennington, M. D. Perry, T. W. Phillips, M. Roth, T. C. Sangster, M. S. Singh, R. A. Snavely, M. A. Stoyer, S. C. Wilks, and K. Yasuike, Phys. Plasmas, 2076 (2000). https://doi.org/10.1063/1.874030 8, 542 (2001). 15. S. C. Wilks, A. B. Langdon, T. E. Cowan, M. Roth, M. Singh, S. Hatchett, M. H. Key, D. Pennington, A. MacKinnon, and R. A. Snavely, Phys. Plasmas, 542 (2001). https://doi.org/10.1063/1.1333697 electrons through the plasma and sets up a quasi-static electric field on the back of the foil. Contaminations, such as hydrocarbons and water, which can be found here become ionised and preferentially the protons ( H + ) are accelerated in the electric field to MeV-energies. This gives a characteristically broad, exponentially decaying, and in our case, non-relativistic proton energy spectrum, meaning that the protons within a pulse travel with different velocities. This proton pulse is also divergent. Due to these factors, it is important to study the proton-water interaction as close to the proton source as possible, to fully take advantage of the short proton pulses and also to reach the highest dose levels. In thepulse radiolysis experiment we present here, thewerefrom thin aluminium foils in the target normal sheath(TNSA) regime.Here, the leading edge of an ultrashort high intensity laser pulse fully ionises the front surface of a target foil and turns it into a plasma. The ponderomotive force of the laser pulse pushesthrough the plasma and sets up a quasi-staticon the back of the foil. Contaminations, such as hydrocarbons andwhich can be found here become ionised and preferentially the) arein theto MeV-energies. This gives a characteristically broad, exponentially decaying, and in our case, non-relativisticspectrum, meaning that thewithin a pulse travel with different velocities. Thispulse is also divergent. Due to these factors, it is important to study the proton-wateras close to thesource as possible, to fully take advantage of the shortpulses and also to reach the highest dose levels.

mirror onto a 12 μm thick aluminium foil at 30° angle of incidence. Protons were accelerated to energies up to ∼10 MeV, which was measured with stacks of radiochromic film, 16 80, 033301 (2009). 16. F. Nürnberg, M. Schollmeier, E. Brambrink, A. Blazevic, D. C. Carroll, K. Flippo, D. C. Gautier, M. Geissel, K. Harres, B. M. Hegelich, O. Lundh, K. Markey, P. McKenna, D. Neely, J. Schreiber, and M. Roth, Rev. Sci. Instrum., 033301 (2009). https://doi.org/10.1063/1.3086424 1 water in this cell was probed, perpendicular to the proton propagation direction, with a 1 ns long chirped laser pulse, originating from the same source as the main pulse. The chirp in the probe pulse was introduced in a double pass grating stretcher and was tuned in a dedicated grating compressor. In the beam path of the probe pulse, two mirrors were placed on a translation stage so that the relative delay between the proton pulse and the probe pulse could be varied over a few ns. The interaction area was imaged by a lens with 8 times magnification onto the entrance slit of a 1 m imaging Czerny-Turner spectrometer with a 10 × 10 cm 2 , 1200 lines/mm grating. The width of the proton beam, which the optical probe beam propagates through, is on the order of one hundred micrometers, and the transmission is assumed to be constant over this distance. Different wavelengths passed through the interaction area at different times since the probe pulse was chirped. The wavelength bandwidth of the probe pulse is only a few nm, and its optical spectrum is fully enclosed in the much broader spectral region of absorption of the solvated electrons. The spectrometer then separated the frequencies spatially, thus creating a time-resolved image of the interaction. The images were captured at the exit of the spectrometer by a 16-bit CCD camera with 20482 pixels on a 27.6 × 27.6 mm 2 sensor, with one axis corresponding to the time domain and the other axis the propagation depth of the laser-accelerated protons into the water. Unlike some other similar techniques that probe radiolytic processes with a chirped optical probe, 17,18 96, 25 (2004). 17. I. A. Shkrob, D. A. Oulianov, R. A. Crowell, and S. Pommeret, J. Appl. Phys., 25 (2004). https://doi.org/10.1063/1.1711178 78, 1099 (2009). 18. V. De Waele, U. Schmidhammer, J. R. Marquès, H. Monard, J.-P. Larbre, N. Bourgeois, and M. Mostafavi, Radiat. Phys. Chem., 1099 (2009). https://doi.org/10.1016/j.radphyschem.2009.06.027 proton pulse. To determine the time resolution of the system, the spectrum of the chirped probe pulse was cut at a certain wavelength, thereby creating a step function in the wavelength domain. The 10 % −90 % rise time of the response in the spectrometer (giving the limit for the lowest resolvable time structures) was then corresponding to 12 ps ± 1.5 ps for a probe pulse duration of 1 ns. In a similar way, the spatial resolution was determined, by imaging a sharp edge placed in the position of the water cell to be 2.5 μm ± 0.5 μm. Our experimental scheme thus allows for imaging both the temporal and spatial evolution, along the proton propagation axis, of the optical absorbance as the proton pulse propagates through the water cell. The experiments were performed at the TARANIS laser at Queen's University, Belfast, UK. The laser delivered 10 J in 600 fs long pulses with a central wavelength of 1053 nm and a repetition rate of approximately 1 pulse per 10 min. The laser pulses were focused with an f/2 off-axis paraboliconto a 12m thick aluminium foil at 30° angle of incidence.weretoup to ∼10 MeV, which waswith stacks of radiochromic film,that change color when absorbing radiation. 1 cm behind the target a water-cell was placed (see Fig.). The optical absorbance of thein this cell was probed, perpendicular to thepropagation direction, with a 1 ns long chirped laser pulse, originating from the same source as the main pulse. The chirp in the probe pulse was introduced in a double passstretcher and was tuned in a dedicatedcompressor. In the beam path of the probe pulse, twowere placed on a translation stage so that the relative delay between thepulse and the probe pulse could be varied over a few ns. Thearea was imaged by a lens with 8 times magnification onto the entrance slit of a 1 m imaging Czerny-Turnerwith a, 1200 lines/mmThe width of thebeam, which the optical probe beam propagates through, is on the order of one hundred micrometers, and the transmission is assumed to be constant over this distance. Different wavelengths passed through thearea at different times since the probe pulse was chirped. The wavelength bandwidth of the probe pulse is only a few nm, and its optical spectrum is fully enclosed in the much broader spectral region of absorption of theThethen separated the frequencies spatially, thus creating a time-resolved image of theThe images were captured at the exit of theby a 16-bit CCD camera with 2048pixels on asensor, with one axis corresponding to the time domain and the other axis the propagation depth of the laser-acceleratedinto theUnlike some other similar techniques that probe radiolytic processes with a chirped optical probe,our scheme includes an intrinsic temporal synchronization between the chirped pulse and apulse. To determine the time resolution of the system, the spectrum of the chirped probe pulse was cut at a certain wavelength, thereby creating a step function in the wavelength domain. The 10−90rise time of the response in the(giving the limit for the lowest resolvable time structures) was then corresponding to 12 psps for a probe pulse duration of 1 ns. In a similar way, the spatial resolution was determined, by imaging a sharp edge placed in the position of thecell to be 2.5m. Our experimental scheme thus allows for imaging both the temporal and spatial evolution, along thepropagation axis, of the optical absorbance as thepulse propagates through thecell.

proton pulse in the water and the other without. By comparing the two images, the change in transmission induced by the interaction between the high energy protons and the water could be deduced (Fig. 2 proton pulse in the water cell to account for small fluctuations in the probe pulse intensity between the two recordings. For each set of data attained during the experiment, two recordings of the probe pulse were made: one with thepulse in theand the other without. By comparing the two images, the change in transmission induced by thebetween the highand thecould be deduced (Fig.). In the analysis process, the probe pulse intensities are set to be equal before the arrival of thepulse in thecell to account for small fluctuations in the probe pulse intensity between the two recordings.

A, of a material is given by the relation A = − log 10 ( T ) = ϵ c l , (1) where T is the transmittance, ϵ denotes the molar attenuation coefficient, c is the absorbant concentration and l is the path length. In our case, the absorbing species, at the probe pulse wavelength, is solvated electrons. The concentration of solvated electrons mainly depends on the amount of energy deposited into the water by the laser accelerated protons but also somewhat on the linear energy transfer, i.e., the yield of solvated electrons is halved for protons close to the end of their tracks. 6 109, 9393 (2005). 6. J. A. LaVerne, I. Stefanić, and S. M. Pimblott, J. Phys. Chem. A, 9393 (2005). https://doi.org/10.1021/jp0530303 energy spectrum of the proton beam in these experiments is exponentially decaying, meaning that the highest amount of deposited energy per volume unit (highest dose) is found where the protons enter the water cell. This means that c and by extension A are in fact functions of the position along the proton propagation axis in the water cell, x. The absorbance,, of a material is given by the relationwhereis the transmittance,denotes the molar attenuation coefficient,is the absorbant concentration andis the path length. In our case, the absorbing species, at the probe pulse wavelength, isThe concentration ofmainly depends on the amount ofinto theby the laserbut also somewhat on the lineari.e., the yield ofis halved forclose to the end of their tracks.Thespectrum of thebeam in these experiments is exponentially decaying, meaning that the highest amount ofper volume unit (highest dose) is found where theenter thecell. This means thatand by extensionare in fact functions of the position along thepropagation axis in thecell,

proton velocities. There is a variation in arrival time at the water cell with approximately 80 ps from the highest energy protons (∼10 MeV corresponding to ∼43 μm/ps) to the lowest energy protons (∼5 MeV corresponding to ∼32 μm/ps) that can penetrate through the 200 μm entrance window of Teflon. When the proton pulse arrives inside the water cell, it is found that the absorbance first rises rapidly over the duration of the proton pulse, as expected from the swift processes that solvates electrons. 21 51, 229 (1998). 21. V. Cobut, Y. Frongillo, J. P. Patau, T. Goulet, M.-J. Fraser, and J.-P. Jay-Gerin, Radiat. Phys. Chem., 229 (1998). https://doi.org/10.1016/S0969-806X(97)00096-0 deposited. The rising edge of the absorbance front, in other words the falling edge of transmission, was thoroughly investigated to confirm that the expansion velocity of the absorbance corresponds to the expectedvelocities. There is a variation in arrival time at thecell with approximately 80 ps from the highest(∼10 MeV corresponding to ∼43m/ps) to the lowest(∼5 MeV corresponding to ∼32m/ps) that can penetrate through the 200m entrance window of Teflon. When thepulse arrives inside thecell, it is found that the absorbance first rises rapidly over the duration of thepulse, as expected from the swift processes that solvatesThe absorbance varies over the different propagation depths as expected from the discussion above, with the highest absorbance found where the highest dose is

water cell. The simulations are based on tabulated data from SRIM (The Stopping and Range of Ions in Matter) 22 22. See www.srim.org for the stopping and range of ions in matter. deposited energy into solvated electrons is taken from LaVerne et al. 6 109, 9393 (2005). 6. J. A. LaVerne, I. Stefanić, and S. M. Pimblott, J. Phys. Chem. A, 9393 (2005). https://doi.org/10.1021/jp0530303 et al. 23 111, 6869 (1999). 23. M. Assel, R. Laenen, and A. Laubereau, J. Chem. Phys., 6869 (1999). https://doi.org/10.1063/1.479979 solvated electrons at room temperature on the picosecond timescale, as presented by De Waele et al. and El Omar et al., 19,20 423, 30 (2006). 19. V. De Waele, S. Sorgues, P. Pernot, J.-L. Marignier, H. Monard, J.-P. Larbre, and M. Mostafavi, Chem. Phys. Lett., 30 (2006). https://doi.org/10.1016/j.cplett.2006.03.008 115, 12212 (2011). 20. A. K. El Omar, U. Schmidhammer, P. Jeunesse, J.-P. Larbre, M. Lin, Y. Muroya, Y. Katsumura, P. Pernot, and M. Mostafavi, J. Phys. Chem. A, 12212 (2011). https://doi.org/10.1021/jp208075v protons are also assumed to have an initial energy distribution, before propagating through the Teflon window, similar to what was measured during the experiment, i.e., an exponentially decaying proton energy spectrum from ∼ 10 11 protons/MeV/sr at 1 MeV to ∼ 10 9 protons/MeV/sr at 10 MeV. The results of these simulations are shown in Fig. 2 A model has been developed to simulate how the absorbance changes as a function of depth in thecell. The simulations are based on tabulated data from SRIM (The Stopping and Range ofin Matter)and the conversion efficiency fromintois taken from LaVerneThe molar attenuation coefficient, specific to our probe frequencies, is given in AsselThe bandwidth of the chirped probe pulse is only a few nanometers and therefore the coefficient is, as a good approximation, constant throughout the whole pulse. Furthermore, the small decay ofat room temperature on the picosecond timescale, as presented by De Waele. and El Omar.,is included in the model. Theare also assumed to have an initialdistribution, before propagating through the Teflon window, similar to what wasduring the experiment, i.e., an exponentially decayingspectrum fromprotons/MeV/sr at 1 MeV toprotons/MeV/sr at 10 MeV. The results of these simulations are shown in Fig.together with corresponding experimental recordings.

3 3(b) proton energy spectra, both considering variations in maximum energy (20%) and the number of protons for each energy interval (50%). For lower doses, all the recordings fall within the error bars. For the highest doses, i.e., where the protons enter the water cell, there is however a clear discrepancy. One possible explanation for this is that the model does not take into account any intertrack effects. A high energy proton passing through water will deposit energy along its track in the so called spurs of radiolytic species. In this experiment, the track radius is in the order of a few nm 24 51, 245 (1998). 24. Y. Frongillo, T. Goulet, M.-J. Fraser, V. Cobut, J. P. Patau, and J.-P. Jay-Gerin, Radiat. Phys. Chem., 245 (1998). https://doi.org/10.1016/S0969-806X(97)00097-2 protons enter the water cell, is approximately 5000 protons/μm2, indicating that the track structures of 5 nm radius would fill nearly half the water volume. Kreipl et al. 25 48, 349– 359 (2009). 25. M. S. Kreipl, W. Friedland, and H. G. Paretzke, Radiat. Environ. Biophys., 349–(2009). https://doi.org/10.1007/s00411-009-0234-z solvated electrons in the tracks of 20 MeV protons propagating through water, when the protons are in close proximity of one another. This effect increases with the temporal and spatial density of the proton tracks. Since the linear energy transfer for the protons of lower energy increases, giving an even higher density of radiolytic species in their tracks, it seems reasonable to assume that intertrack reactions would be at least as important for the protons of 10 MeV or below. This may explain why the difference in absorbance between the lowest and the highest dose is slightly smaller in the experiment than that predicted by the model, suggesting that for sufficiently high dose rates the yield of solvated electrons is decreased. Given the linear relation between absorbance and concentration of solvated electrons (see Eq. 3(b) solvated electrons for high dose rates (kGy/80 ps) could be as large as a factor of two. A comparison between the model and the experimental findings (see Fig.) shows reasonable agreement for the level of absorbance for the expected doses, especially for the lower doses. In Fig., the error bars in the model represent likely shot-to-shot fluctuations in thespectra, both considering variations in maximum(20%) and the number offor eachinterval (50%). For lower doses, all the recordings fall within the error bars. For the highest doses, i.e., where theenter thecell, there is however a clear discrepancy. One possible explanation for this is that the model does not take into account any intertrack effects. A highpassing throughwillalong its track in the so called spurs of radiolytic species. In this experiment, the track radius is in the order of a few nmat the times considered. The area density, where theenter thecell, is approximately 5000 protons/, indicating that the track structures of 5 nm radius would fill nearly half thevolume. Kreiplshow through simulations a decrease in the yield ofin the tracks of 20 MeVpropagating throughwhen theare in close proximity of one another. This effect increases with the temporal and spatial density of thetracks. Since the linearfor theof lowerincreases, giving an even higher density of radiolytic species in their tracks, it seems reasonable to assume that intertrack reactions would be at least as important for theof 10 MeV or below. This may explain why the difference in absorbance between the lowest and the highest dose is slightly smaller in the experiment than that predicted by the model, suggesting that for sufficiently high dose rates the yield ofis decreased. Given the linear relation between absorbance and concentration of(see Eq. (1) ), the comparison in Fig.between our experimental data and the model, extrapolating data from the previous publications, indicates that the correction in the yields offor high dose rates (kGy/80 ps) could be as large as a factor of two.

proton pulse duration is approximately 80 ps when entering the water, but also at later times the absorbance continues to slowly increase (see Fig. 3(a) electrons 21 51, 229 (1998). 21. V. Cobut, Y. Frongillo, J. P. Patau, T. Goulet, M.-J. Fraser, and J.-P. Jay-Gerin, Radiat. Phys. Chem., 229 (1998). https://doi.org/10.1016/S0969-806X(97)00096-0 19,20,26 423, 30 (2006). 19. V. De Waele, S. Sorgues, P. Pernot, J.-L. Marignier, H. Monard, J.-P. Larbre, and M. Mostafavi, Chem. Phys. Lett., 30 (2006). https://doi.org/10.1016/j.cplett.2006.03.008 115, 12212 (2011). 20. A. K. El Omar, U. Schmidhammer, P. Jeunesse, J.-P. Larbre, M. Lin, Y. Muroya, Y. Katsumura, P. Pernot, and M. Mostafavi, J. Phys. Chem. A, 12212 (2011). https://doi.org/10.1021/jp208075v 63, 47 (1977). 26. W. G. Burns, R. May, G. V. Buxton, and G. S. Tough, Faraday Discuss., 47 (1977). https://doi.org/10.1039/dc9776300047 3(a) solvated electrons from longer times and lower dose rates does not fully describe the scenario. Thepulse duration is approximately 80 ps when entering thebut also at later times the absorbance continues to slowly increase (see Fig.), in contrast to what would be expected from the swift solvation process ofand the decay kinetics involved.Towards 300 ps, this increase levels out. To avoid shot-to-shot fluctuations affecting the results in Fig., the data from each of the different times are taken from one single recording. This further confirms that an extrapolation of yields offrom longer times and lower dose rates does not fully describe the scenario.

In conclusion, we have presented experimental data of how picosecond bunches of high energy protons interact with water. This enables further understanding of how ultrahigh dose rates affect the energy deposition into water. Our experimental scheme provides a unique tool to study high energy proton/water interactions and gather information that was previously primarily accessible through simulations.