M. M. Aléonard

Response functions of imaging plates to photons, electrons and 4He particles.

Imaging plates from Fuji (BAS-SR, MS, and TR types) are phosphor films routinely used in ultra high intensity laser experiments. However, few data are available on the absolute IP response functions to ionizing particles. We have previously measured and modeled the IP response functions to protons. We focus here on the determination of the responses to photons, electrons, and (4)He particles. The response functions are obtained on an energy range going from a few tens of keV to a few tens of MeV and are compared to available data. The IP sensitivities to the different ionizing particles demonstrate a quenching effect depending on the particle stopping power.

High flux of relativistic electrons produced in femtosecond laser-thin foil target interactions: Characterization with nuclear techniques

We present a protocol to characterize the high energy electron beam emitted in the interaction of an ultraintense laser with matter at intensities higher than 10(19) W cm(-2). The electron energies and angular distributions are determined as well as the total number of electrons produced above a 10 MeV threshold. This protocol is based on measurements with an electron spectrometer and nuclear activation techniques, combined with Monte Carlo simulations based on the GEANT3 code. The method is detailed and exemplified with data obtained with polypropylene and copper thin solid targets at a laser intensity of 2x10(19) W cm(-2). Special care is taken of the different sources of uncertainties. In particular, the reproducibility of the laser shots is considered.

NATALIE: A 32 detector integrated acquisition system to characterize laser produced energetic particles with nuclear techniques

We present a stand-alone system to characterize the high-energy particles emitted in the interaction of ultrahigh intensity laser pulses with matter. According to the laser and target characteristics, electrons or protons are produced with energies higher than a few mega electron volts. Selected material samples can, therefore, be activated via nuclear reactions. A multidetector, named NATALIE, has been developed to count the β(+) activity of these irradiated samples. The coincidence technique used, designed in an integrated system, results in very low background in the data, which is required for low activity measurements. It, therefore, allows a good precision on the nuclear activation yields of the produced radionuclides. The system allows high counting rates and online correction of the dead time. It also provides, online, a quick control of the experiment. Geant4 simulations are used at different steps of the data analysis to deduce, from the measured activities, the energy and angular distributions of the laser-induced particle beams. Two applications are presented to illustrate the characterization of electrons and protons.

Prospects for nuclear physics with lasers

The development of high intensity lasers has opened up new opportunities for nuclear physics studies in extreme conditions which cannot be reached with conventional particle accelerators. A laser is a unique tool to produce plasma and very high fluxes of photon and particle beams in very short duration pulses. Both aspects are of great interest for fundamental nuclear physics studies. In plasma the electron?ions collisions may modify atomic and nuclear level properties. This is of prime importance for the population of isomeric states and the issue of energy storage in nuclei. Nuclear properties in the presence of very high electromagnetic fields, nuclear reaction rates or properties in hot and dense plasmas are new domains of investigation. Our group has launched an experimental program to evaluate the possibilities for such nuclear physics studies at high intensity laser facilities. This program and its first results are presented.

Particle characterization for the evaluation of the 181mTa excitation yield in millijoule laser induced plasmas

The particles (ions, atoms, x-rays and high-energy electrons) resulting from the interaction of a high repetition rate laser with a tantalum target are characterized at laser intensities ranging between 3 × 1015 and 6 × 1016 W cm−2 using plasma and nuclear physics techniques. A simple model is developed to estimate, from these data, the 181Ta isomeric state excitation yields in the ablated matter and in the remaining target. Both nuclear photoexcitation and electron inelastic scattering processes have been taken into account in the calculations. A maximum of a few 10−3 excitations per laser shot are predicted in the investigated intensity range.

Absolute energy distribution of hard x rays produced in the interaction of a kilohertz femtosecond laser with tantalum targets

Previous reports have indicated the anomalous excitation rate for the 6.2keV nuclear level of Ta181 in a plasma produced with a femtosecond laser. A detailed characterization of the electrons and x-ray sources produced in such a plasma is required to interpret these results. In a preliminary work, the continuous energy distribution of hard x rays (10–500keV) produced in the interaction of a kilohertz femtosecond laser beam with a tantalum solid target is investigated in the 3×1015–6×1016W∕cm2 range of intensity. A sodium iodide detector with appropriate shielding is used. Strong collimation and absorption filters are used to avoid the pileup of photons in the detector. The response function of this setup is calculated with the GEANT3 simulation code. We demonstrate the necessity to quantify the Compton scattered events in the raw spectra in order to restore the absolute x-ray energy distribution.

Energetic electrons produced in the interaction of a kiloHertz femtosecond laser with tantalum targets

The electrons produced in the interaction of a high repetition rate laser with a thick tantalum target generate a continuous distribution of photons via the bremsstrahlung process occurring mainly in the target. The photon energy distributions, between 50 and 500 keV, are unfolded to obtain the true X-ray energy distributions. These distributions are used in a Monte Carlo simulation to infer the initial electron energy distributions. These properties have been studied at laser intensities ranging from 3 × 1015 to 6 × 1016 W cm− 2. The electron energy distributions are different above ≃2 × 1016 W cm− 2 as compared to lower intensities. This is evidence for a different laser plasma interaction regime.