Spectral shaping of laser generated proton beams

The rapid progress in the field of laser particle acceleration has stimulated a debate about the promising perspectives of laser based ion beam sources. For a long time, the beams produced exhibited quasi-thermal spectra. Recent proof-of-principle experiments demonstrated that ion beams with narrow energy distribution can be generated from special target geometries. However, the achieved spectra were strongly limited in terms of monochromacity and reproducibility. We show that microstructured targets can be used to reliably produce protons with monoenergetic spectra above 2 MeV with less than 10% energy spread. Detailed investigations of the effects of laser ablation on the target resulted in a significant improvement of the reproducibility. Based on statistical analysis, we derive a scaling law between proton peak position and laser energy, underlining the suitability of this method for future applications. Both the quality of the spectra and the scaling law are well reproduced by numerical simulations

A vision for laser induced particle acceleration and applications

High intensity lasers interacting with solid and gas targets can produce energetic beams of electrons, protons, heavy ions, photons and neutrons. There is the potential for producing "table top" sources of radiation for many different applications

High-power laser production of PET isotopes

Recent experiments have demonstrated that laser-solid interactions at intensities greater than 1019 W/cm2 can produce fast electron beams of several hundred MeV [1], tens of MeV γ-rays [2, 3], up to 58MeV proton beams [4, 5], and heavier ions [6] of up to 7MeV/nucleon. One of the potential applications of the high-energy proton beams is the production of radioactive isotopes for positron emission tomography (PET). PET is a form of medical imaging requiring the production of short-lived positron emitting isotopes 11C, 13N, 15O, and 18F, by proton irradiation of natural/enriched targets using cyclotrons. PET development has been limited because of the size and shielding requirements of the nuclear installations. Recent results have shown when an intense laser beam interacts with solid targets, tens of MeV protons capable of producing PET isotopes are generated [7, 8, 9]

Laser induced nuclear physics and applications

Laser induced beams of protons, neutrons and gamma rays using short pulse lasers is currently important principally because of the potential applications e.g. isotope production, transmutation studies, laser induced fission, heavy ion fusion reactions, neutron production and radiography, fast ignitor and spallation studies of neutrons. Although the Strathclyde group in collaboration with teams from Imperial College and the Rutherford Appleton Laboratory have been at the forefront of many of these applications this presentation only deals with laser induced PET isotope production and laser transmutation studies. These two applications will be dealt with in separate sections

Femtosecond ionization and dissociation of laser desorbed nitro-PAHs

Mass spectra of six nitro-PAHs, 5-nitroacenaphthene, 9-nitroanthracene, I -nitropyrene, 3-nitrofluoranthene, 6-nitrochrysene, and 3-nitrobenzanthrone, have been investigated using laser desorption/femtosecond laser mass spectrometry (LD/FLMS). A prominent parent ion was observed for each molecule along with the structurally-characteristic [M - NO](+) and [M - NO2](+) fragments. The consistent observation of the [M - NO](+) and [M - NO - CO](+) ions, in the mass spectra of all the nitro-PAHs, along with the presence of certain doubly charged fragments, is thought to be indicative of a molecular rearrangement. Although this photorearrangement may be occurring within the pulse duration (<80 fs), it is thought that it is more likely to be taking place within the low intensity regions of the laser pulse. In addition to this, an abundance of doubly charged polyatomic ions were observed for the first time in the mass spectra of these molecules under laser irradiation. It was found that only the two smallest molecules (5-nitroacenaphthene and 9-nitroanthracene) were able to generate an observable dication, with the highest-mass doubly charged species corresponding to the [M - NO2](2+) fragment in the remaining molecules. An investigation has also been conducted into the effect of varying the molecular structure of the analyte and the position of the focussed beam. It was found that variation of molecular structure had little effect on the observed fragmentation pathways. However, movement of the focussed beam was found to exert a considerable influence over the observed mass spectra

Interaction mechanism of some alkyl iodides with femtosecond laser pulses

The interaction of 1-iodopropane, 2-iodopropane, 1-iodobutane, 2-iodobutane, and 1-iodopentane with (5 × 1013-5 × 1015 W/cm2) femtosecond laser fields is studied by means of a time-of-flight mass spectrometer. It is found that multiphoton ionization (MPI) and field ionization (FI) processes are involved in the molecular ionization. The contribution of these processes can be distinguished using the peak profile of the ions in the mass spectra. Thus, from the mass spectra of 2-iodoropane and 2-iodobutane, it is concluded that MPI processes are taking place even for Keldysh parameter values γ 0.3. The field ionization process depends on the characteristics of the molecular binding potential well and leads to an asymmetric charge distribution of the transient multiply charged parent ions. In the case of 1-iodobutane, the MPI processes lead to a stable doubly charged parent ion production with a laser intensity threshold higher than that found for I2+ ions. In addition, the isomers studied exhibit distinct differences in their mass spectra and their origin is discussed in detail

Femtosecond laser time-of-flight mass spectrometry of labile molecular analytes: laser-desorbed nitro-aromatic molecules

Femtosecond laser time-of-flight mass spectra of solid samples of trinitrobenzene (TNB), trinitrotoluene (TNT) and trinitrophenol (TNP) have been recorded. Desorption of the solid samples was enacted by the fourth harmonic output (266 nm) of a 5 ns Nd:YAG laser. Subsequent femtosecond post-ionisation of the plume of neutral molecules was achieved using 800 nm laser pulses of 80 fs duration. Mass spectra have been recorded for desorption laser intensities from 2-6 × 109 W cm-2 with ionisation laser intensities between 2 × 1014 and 6 × 1015 W cm-2. Femtosecond laser ionisation has been shown to be capable of generating precursor and characteristic high-mass fragment ions for labile nitro-aromatic molecules commonly used in high-explosive materials. This feature is critical in the future development of femtosecond laser-based analytical instruments that can be used for complex molecular identification and quantitative analysis of environmentally important labile molecules. Furthermore, a comparison of femtosecond post-ionisation mass spectra with standard 70 eV electron impact data has revealed similarities in the spectra and hence the fragmentation processes

Molecular hydrogen ion elimination from alkyl iodides under strong laser beam irradiation

The elimination of H2+ from alkyl iodides under strong (up to 5 × 1015 W cm−2) laser irradiation is studied by means of time-of-flight mass spectrometry. The study has been performed by using 60 fs (λ = 800 nm) and 35 ps (λ = 1064, 532, 355 and 266 nm) laser pulses. It is concluded that the H2+ ions are ejected from ionic states via Coulomb explosion processes. The molecular rearrangement leading to H2+ formation is attributed to a tunneling process through a H transfer barrier. For the case of methyl iodide, about 10% of the doubly charged parent ions undergo molecular rearrangement. From a comparison of the H2+/H+ ion yield ratio of the studied molecules, it turns out that the H2+ formation from H atoms bonded to a terminal carbon atom is more efficient than that arising from H atoms bonded to central C atoms of the molecular chain