Abstract

1. Schematic layout of the equipment Raw Data n.7 n.9 n.1 n.14 n.19 Nuclear data sheets value for the half-life of 196Au: 6.17 days. Very Large! High quality electron beams A LASER DRIVEN ELECTRON SOURCE FOR THE PRODUCTION OF RADIONUCLIDES Andrea Gamucci, Marco Galimberti, Danilo Giulietti, Leonida A. Gizzi, Luca Labate, Gianluca Sarri, Paolo Tomassini, Antonio Giulietti Intense Laser Irradiation Laboratory, IPCF, CNR Campus, Pisa, Italy & INFN, Sezione di Pisa, Italy Nicolas Bourgeois, Jean-Raphaël Marquès Laboratoire pour l'Utilisation des Lasers Intenses, CNRS UMR 7605, Ecole Polytechnique 91128 Palaiseau, France Tiberio Ceccotti, Sandrine Dobosz, Pascal Monot, Pascal D'Oliveira, Horia Popescu, Fabrice Réau, Philippe Martin CEA-DSM/DRECAM/SPAM Gif sur Yvette Cedex, France David Hamilton, Jean Galy Institute for Transuranium Elements, Karlsruhe, Germany Abstract We present the results of an experiment performed with 10 TW laser pulses focused onto a helium gas-jet operating at different backing pressures. The accelerated electrons, impinging on a tantalum radiator, undergo bremsstrahlung radiation emission and are capable of radio-activating a gold sample put behind the radiator. Electron bunches with energy peaks in the 10 – 50 MeV region and angular divergences of few tens mrad with a high-efficiency (≈1010 electrons of energy > 8 MeV per Joule of laser energy) were produced and gave rise to an absolute reaction rate of photoactivations of 1.46 x 106 per Joule of incident laser energy. Bunches of this kind can be employed for a variety of nuclear studies, e.g. to perform measurements of the differential photonuclear cross section on radioisotopes, or to measure the polarization of the electron bunches using a technique known as Compton transmission polarimetry. The Source Production of energetic electrons with a non extremely high-intensity laser A suitable “radiator” converts the laser-plasma accelerated electrons via Bremsstrahlung process in -radiation able to activate a sample with the production of radioactive nuclei. The experimental set-up 10 TW UHI-10 Ti:Sa • 65 fs CPA pulses • Energy up to 0.8 J • 0 = 800 nm • power Contrast Ratio > 106 • Radiator • 2mm Tantalum • Sample • 4mm 197Au • F/5 OAP • w0  10 m • I 8.5x1018 W/cm2 • a0 2 The Sample Radioactivation The first step consist in irradiation and activation of the gold foil • To obtain the electron spectrum, their angular distribution and their number, 2 diagnostics have been used independently • For photon energies in the range 10 – 15 MeV, the cross section for 197Au (,n) 196Au reaction grows relatively due to a resonance in the nuclear photo-absorption amplitude, known as Giant Dipole Resonance (GDR) • Several nozzles with different aperture sizes (from 2 to 6 mm) have been employed at a wide range of He backing pressures. The best data have been obtained with the 4mm nozzle @ 25 bar He pressure. The energetic electrons produced in the interaction have been carefully characterized • A magnetic spectrometer before a scintillating Lanex screen has been used to get the shot to shot spectrum with 1D angular resolution along the slit axis • SHEEBA (Spatial High Energy Electron Beam Analyzer): a set of radiochromic films spaced out by layers of different material and thickness: Electron spectra for the 2mm @ 8 bar and 4mm @ 25 bar nozzles • SHEEBA detector has been used on sets of 10 shots. The data show a high degree of collimation even after this integration (angular divergence is evaluated better than 100 mrad). The produced electron energy fits very well in this spectral interval! M. Galimberti et al., Rev. Sci. Instrum. 76, 053303 (2005) Only 106 laser shots to induce a non-negligible amount of photonuclear reactions • Ne (E>3.2 MeV)  3.07875 1011 The Data Analysis • From the number of 196Au nuclei produced by the photo-activation process, we can calculate the bremsstrahlung flux that originated these reactions. • The comparison is made with a Monte Carlo simulation. Once we know the bremsstrahlung flux, then we can go back to the corresponding electron beam flux, if the electron spectrum and the 197Au (,n) 196Au reaction cross section are known. The next steps of the electron beam flux measurement concern the post-irradiation gamma spectroscopy and a detailed analysis of the data coupled with dedicated Monte Carlo calculations. • As a further self-consistency check the experimentally determined half-life has been calculated from the exponential count growth over the measurement period. • A value of 6.17 days was taken from the nuclear data sheets for the half-life of 196Au. • After irradiation, the decay photons from the gold sample have been detected in a high-purity germanium detector cooled to  80 K. (Efficiency at these photon energies: 0.018  0.001) • Post-irradiation period of 143 hours. • The primary radioactive decay channel for this nuclide is via isomeric transition with the emission of two primary photons (333 keV and 355 keV) • The code also accounts for the experimental geometry Results and Discussion Thank You! If you want, you can leave here your email address • A nuclide with a well-known cross section (197Au) was used as an activation sample and irradiated in a bremsstrahlung flux generated from electrons produced in a laser driven accelerator. • There a lot of possible way to efficiently employ this source: • The goal of the Monte Carlo calculation is the experimentally determined (,n) reaction yield. After the computation, that accounts also for the underlying physics processes, results for the entire data set of 106 laser shots and for corresponding values for a single laser shot are obtained. • Given the laser energy of  0.8 J, the absolute reaction rate is 1.46106 per Joule of incident laser energy • It’s worth noting that these results are fully consistent with the other diagnostics’ ones (see SHEEBA!) • Perform experiments on radioisotopes on a day-by-day basis • Nuclear physics studies • Photonuclear cross section measurements • Biomedical employment of radioactive samples Calculation uncertainties are below 0.1 % • Furthermore, this source can be used for measurements of Compton transmission polarimetry, that for photons of a few MeV, is a well established method that relies on the fact that the transmission of a photon beam through iron depends on the polarization of the beam photons as well as on the magnetization of the iron target. Reversing the polarity of the magnetic field in iron results in an asymmetry of the transmission signal at the percent level.