M. Tarisien

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.

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.

Absolute energy distributions of Al, Cu, and Ta ions produced by nanosecond laser-generated plasmas at 1013 W cm−2

The charge state and energy distributions of ions produced by a pulsed 1 J, 9 ns Nd:YAG laser focused onto solid aluminum,copper, and tantalum targets were measured with an electrostatic analyzer coupled with a windowless electron multiplier detector. Special attention was paid to the detector response function measurements and to the determination of the analyzer transmission. Space charge effects are shown to strongly affect this transmission. Measured absolute energy distributions are presented for several charge states. They follow Boltzmann-like functions characterized by an effective ion temperature and an equivalent accelerating voltage. These parameters exhibit power laws as a function of I λ2 which open the possibility to predict the expected shape of the relative energy distributions of ions on a large range of laser intensities (106–1016 W cm−2 μm2).

Energy distributions of electrons emitted by a biased laser-produced plasma at 1013 W cm−2

In this paper, we present the electron bunches extracted from a dense plasma produced by the interaction of a ns laser at an intensity of 1013 W cm−2 with a solid target, when this plasma expands in an electric field. The plasma expansion dynamics measured in the previous works [M. Comet et al., J. Appl. Phys. 119, 013301 (2016)] is used to determine the electron charge density profile at different instants after the laser shot. When applying the electric field, a few 1013 electrons are extracted with continuous energy distributions. Particle-In-Cell simulations are run to understand the extraction process and reproduce the measured energy distributions, with the electron charge density profiles used as inputs. These simulations show that the electron extraction proceeds from the plasma front edge throughout the plasma expansion.

Electron extraction from an expanding laser induced plasma cathode

Summary form only given. The development of a bright and pulsed source of low energy (multi-keV) electrons is essential to measure cross sections of nuclear excitation by inelastic electron scattering which are expected to occur in hot and dense plasmas1-4. Due to the small values of the calculated cross sections of this excitation process (10-33/10-30 cm2), bunches of electrons with up to 1013-1014 particles in a few tens of ns are necessary with energies ranging from 10 keV to 100 keV. We are developing an original electron source based on the capacity of laser heated plasmas to produce large numbers of free electrons at low energies (a few eVs). These electrons could be extracted and post-accelerated by traditional electrostatic optics. First studies have been performed to characterize the dynamics of the expanding plasma produced with a ns laser pulse at 1013 W.cm-2 in which 1015 free electrons are produced5. Using electrostatic devices, electron bunches have been extracted from the expanding plasma in an energy range of a few keV. Several characteristics of these electrons have been measured and confronted with PIC simulations in order to better understand the dynamics of the extraction, and to identify key parameters to optimize the number of extracted electrons.

Half-life of the first excited state of {sup 201}Hg

The lifetime of the first excited state of {sup 201}Hg, populated by the {sup 201}Tl electron capture decay and subsequent {gamma}-ray transitions, has been measured for the first time. This measurement has been carried out using a coincidence between an internal conversion electron and a {gamma}-ray. The half-life of 81{+-}5 ns has been obtained and B(E2) and B(M1) values were deduced and compared to previous estimates. With these reduced matrix elements, the excitation rate of the first excited state of {sup 201}Hg in plasma have been calculated in the frame of a Nuclear excitation by electronic transition (NEET) process.