K.W.D. Ledingham

Proton beams generated with high-intensity lasers: Applications to medical isotope production

Proton beams of up to 10 MeV have been obtained by the interaction of a 10 Hz “table-top” laser, focused to intensities of 6×10^19 W/cm^2, with 6-μm-thin foil targets. Such proton beams can be used to induce 11B(p,n)11C reactions, which could yield an integrated activity of 13.4 MBq (0.36 mCi) after 30 min laser irradiation. This can be extended to GBq levels using similar lasers with kilohertz repetition rates, making this positron-emission tomography isotope production scheme comparable to the one using conventional accelerators.

Observation of a highly directional γ-ray beam from ultrashort, ultraintense laser pulse interactions with solids

Novel measurements of electromagnetic radiation above 10 MeV are presented for ultra intense laser pulse interactions with solids. A bright, highly directional source of γ rays was observed directly behind the target. The γ rays were produced by bremsstrahlung radiation from energetic electrons generated during the interaction. They were measured using the photoneutron reaction [63Cu(γ,n)62Cu] in copper. The resulting activity was measured by coincidence counting the positron annihilation γ rays which were produced from the decay of 62Cu. New measurements of the bremsstrahlung radiation at 1019 W cm−2 are also presented.

Production of radioactive nuclides by energetic protons generated from intense laser-plasma interactions

Nuclear activation has been observed in materials exposed to the ablated plasma generated from high intensity laser–solid interactions (at focused intensities up to 2×1019 W/cm2) and is produced by protons having energies up to 30 MeV. The energy spectrum of the protons is determined from these activation measurements and is found to be consistent with other ion diagnostics. The possible development of this technique for “table-top” production of radionuclides for medical applications is also discussed.

HIGH INTENSITY LASER MASS SPECTROMETRY : A REVIEW

Abstract Resonance Enhanced Multiphoton Ionization (REMPI) and Resonance Ionization Mass Spectrometry (RIMS) with their non-resonant equivalents are sensitive laser-based analytical techniques which have been used for many years to great effect. The lasers are conventionally pulsed, with typically nanosecond pulse widths. Although nanosecond REMPI and RIMS have been notably successful, especially RIMS for the detection of atomic species, there are two areas of research where these techniques have been less effective. First, during the detection of molecular species, the absorption of one or more photons may reach dissociative states below the ionization limit which cause the production of neutral species. This can sometimes be used to great effect for molecular dissociative studies but for analytical purposes it often leads to small or even no parent ion production and hence ambiguous analysis. This problem is especially true for the analysis of nitro-compounds, organometallic molecules and some of the biomolecules. The second area of difficulty for the nanosecond regime is surface analysis. Surface analysis by laser ionization (SALI) has been shown to be very sensitive for a number of elements but has poor sensitivity for elements with high ionization potentials. It is often of great value to carry out simultaneous analysis of elements with similar efficiencies. With the advent of picosecond and femtosecond lasers in the past few years, a number of laboratories have addressed the above two problems using these ultra-fast laser systems. For the molecular dissociative problem, the dissociative states can often be “defeated” leading to large molecular parent ion formation especially for thermally labile molecules. This leads to increased molecular sensitivity and selectivity. In the surface analytical problem, atoms ionise rapidly when subjected to intense laser fluxes regardless of the ionization potential and the laser wavelength with the potential for similar ionization efficiencies. Since these ultra-fast lasers have now been generally available for a number of years, sufficient time has elapsed to evaluate the potential for high intensity laser mass spectrometry.

High power laser production of short-lived isotopes for positron emission tomography

Positron emission tomography (PET) is a powerful diagnostic/imaging technique requiring the production of the short-lived positron emitting isotopes 11C, 13N, 15O and 18F by proton irradiation of natural/enriched targets using cyclotrons. The development of PET has been hampered due to the size and shielding requirements of nuclear installations. Recent results show that when an intense laser beam interacts with solid targets, megaelectronvolt (MeV) protons capable of producing PET isotopes are generated. This report describes how to generate intense PET sources of 11C and 18F using a petawatt laser beam. The work describing the laser production of 18F through a (p,n) 18O reaction, and the subsequent synthesis of 2-[18F] is reported for the first time. The potential for developing compact laser technology for this purpose is discussed.

Characterization of a gamma-ray source based on a laser-plasma accelerator with applications to radiography

The application of high intensity laser-produced gamma rays is discussed with regard to picosecond resolution deep-penetration radiography. The spectrum and angular distribution of these gamma rays is measured using an array of thermoluminescent detectors for both an underdense (gas) target and an overdense (solid) target. It is found that the use of an underdense target in a laser plasma accelerator configuration produces a much more intense and directional source. The peak dose is also increased significantly. Radiography is demonstrated in these experiments and the source size is also estimated.

Laser accelerated ions and electron transport in ultra-intense laser matter interaction

Since their discovery, laser accelerated ion beams have been the subject of great interest. The ion beam peak power and beam emittance is unmatched by any conventionally accelerated ion beam. Due to the unique quality, a wealth of applications has been proposed, and the first experiments confirmed their prospects. Laser ion acceleration is strongly linked to the generation and transport of hot electrons by the interaction of ultra-intense laser light with matter. Comparing ion acceleration experiments at laser systems with different beam parameters and using targets of varying thickness, material and temperature, some insight on the underlying physics can be obtained. The paper will present experimental results obtained at different laser systems, first beam quality measurement on laser accelerated heavy ions, and ion beam source size measurements at different laser parameters. Using structured targets, we compare information obtained from micro patterned ion beams about the accelerating electron sheath, and the influence of magnetic fields on the electron transport inside conducting targets.

Fast particle generation and energy transport in laser-solid interactions

The generation of MeV electron and ion beams using lasers with intensities of up to 1020 W cm−2 is reported. Intense ion beams with high energies (up to 40 MeV and to 3×1012 protons >5 MeV) are observed. The properties of these particle beams were measured in considerable detail and the results are compared to current theoretical explanations for their generation.

Laser-driven photo-transmutation of 129I - a long-lived nuclear waste product

Intense laser–plasma interactions produce high brightness beams of gamma rays, neutrons and ions and have the potential to deliver accelerating gradients more than 1000 times higher than conventional accelerator technology, and on a tabletop scale. This paper demonstrates one of the exciting applications of this technology, namely for transmutation studies of long-lived radioactive waste. We report the laser-driven photo-transmutation of long-lived 129 I with a half-life of 15.7 million years to 128 I with a half-life of 25 min. In addition, an integrated cross-section of 97±40 mbarns for the reaction 129 I(γ ,n) 128 I is determined from the measured ratio of the (γ ,n) induced 128 I and 126 I activities. The potential for affordable, easy to shield, tabletop laser technology for nuclear transmutation studies is highlighted.

Characterization of 7Li(p, n) 7Be neutron yields from laser produced ion beams for fast neutron radiography

Investigations of 7Li(p,n)7Be reactions using Cu and CH primary and LiF secondary targets were performed using the VULCAN laser [C.N. Danson et al., J. Mod. Opt. 45, 1653 (1997)] with intensities up to 3×1019 W cm−2. The neutron yield was measured using CR-39 plastic track detector and the yield was up to 3×108 sr−1 for CH primary targets and up to 2×108 sr−1 for Cu primary targets. The angular distribution of neutrons was measured at various angles and revealed a relatively anisotropic neutron distribution over 180° that was greater than the error of measurement. It may be possible to exploit such reactions on high repetition, table-top lasers for neutron radiography.