Proton extraction by laser ablation of transition metals L. Velardi a,⇑ , D. Delle Side a , J. Krása b , V. Nassisi a a Department of Mathematics and Physics, INFN and LEAS Laboratory, University of Salento, Italy b Institute of Physics, ASCR, Prague, Czech Republic article info Article history: Received 24 September 2013 Received in revised form 29 October 2013 Accepted 1 November 2013 Available online xxxx Keywords: Proton beams Laser ion source Laser ablation abstract A study on the proton beams extraction from a plasma generated by pulsed laser ablation by targets con- taining transition metals is presented. The targets used were pure disks of titanium and tantalum and disks of TiH 2 , obtained by compression of TiH 2 powder. The plasma was produced by means of a nano- second excimer KrF laser operating at low irradiance (10 9 –10 10 W/cm 2 ). The proton and ions emission was analyzed by the time-of-flight technique using a Faraday cup as ion collector. Studies on the pro- duced protons and ions at different laser irradiances from 2 to 15 GW/cm 2 were performed. The charac- terization showed that it is possible to obtain good proton fluxes from these targets, up to 10 11 proton/ pulse. The results obtained are very interesting if compared with those available in literature where pro- ton fluxes per pulse ranging from 10 8 to 10 9 by hydride targets were obtained, at the same laser irradiances. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Recently, proton production from high power laser interaction with thin metal targets [1] has attracted great interest in the scien- tific community, due to its possible applications in Ion Beam Ther- apy (IBT) and it has been suggested as a potential, cost saving alternative to common accelerator devices. The laser driven accel- eration (LDA) has in fact, the advantage of obtaining energetic ion beams with a very good emittance from the target, but the beam qualities are far from being comparable with those required for IBT (high energy and flux, low energetic spread). The physical mechanisms involved in LDA at high values of laser irradiance (>10 18 W/cm 2 ) are described by two different frameworks TNSA [2] (Target Normal Sheath Acceleration) and RPA [3] (Radiation Pressure Acceleration), depending on the experimental parameters. Other laser-matter interaction frame- works such as Pulsed Laser Ablation (PLA) (which works at low- er irradiances of the order of 10 8 –10 10 W/cm 2 ) allow to obtain ions of moderate energy, which can be increased up to few hundreds of keV by means of post acceleration systems [4,5]. These beams have a wide range of applications, from ion implantation for material modification to thin film deposition for optoelectronic devices [6–8] or as injector source for common particle accelerators [9]. In this work, we present the results regarding proton generation by an UV laser ion source (LIS), the PLATONE device. The solid tar- gets used to generate protons were disks of titanium and tantalum, pure at 99.99% and disks of titanium hydride. It is widely known in literature that some of the transition metals, such as Ta, Nb, Pd and Ti, are good hydrogen adsorbers [10,11]. Despite this advantageous characteristic, these elements are not usually considered as targets for the production of protons in LIS. Moreover, hydrogen rich materials have recently attracted attention as proton sources in LIS [12–15]. Our aim is to study and characterize these targets and possibly to enlarge the set of materials usable for the production of proton beams by PLA. 2. Experimental setup The laser ion source (LIS) employed in this experiment is the PLATONE accelerator available at the LEAS Laboratory in Lecce, Italy. It consists of a Compex 205 KrF excimer laser (k = 248 nm, s FWHM = 23 ns) which works at laser irradiances of 10 8 –10 10 W/cm 2 , and an interaction chamber (GC) where the plasma is produced by PLA and diagnosed by the use of ion col- lectors. In Fig. 1 it is shown the sketch of the PLATONE setup. The laser beam entered in GC with an angle of 70° with respect to the target normal. It was focused by a thin lens onto the target surface, resulting in a spot of 0.005 cm 2 . The targets were mounted on a cylindrical support T and were irradiated in high vacuum (10 À6 mbar) at different laser irradiances (2.2, 4.4, 8.8 and 14.6 GW/cm 2 ). As targets we used disks of tantalum and titanium, pure at 99.99% and compressed disks of TiH 2 powder, pure at 99%. The lat- ter were obtained by compression of 230 mg of TiH 2 powder at a pressure of 10 4 kg/cm 2 for 30 min. 0168-583X/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.11.029 ⇑ Corresponding author. Tel.: +39 0832297554. E-mail address: luciano.velardi@le.infn.it (L. Velardi). Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb Please cite this article in press as: L. Velardi et al., Proton extraction by laser ablation of transition metals, Nucl. Instr. Meth. B (2014), http://dx.doi.org/ 10.1016/j.nimb.2013.11.029