Search for the 6keV isomer excitation and decay in a $^{181}$Ta plasma produced with a high repetition rate laser

International audienceThe possibility to excite the 6.2 keV, 6.8$\mu$ isomer in $^{181}$Ta within a plasma was studied with the high repetition rate laser available at the CELIA facility at Universite de Bordeaux 1. The laser is a 10mJ, 1kHz, 30fs Tisapphire laser. Excitation of the Ta isomer can be effective through electron excitation or photon absorption by the Ta nuclei inside the plasma produced in the interaction of the laser with solid Ta targets. We have, in a first step characterized the phton production issued from the Bremssttrahlung of electrons for I=1-6 10$^{16}$ W.cm$^{-2}$ laser intensities. The photon spectrum extends up to 500 keV. The plasma expansionwas also studied. We have used several set-ups to optimize the direct detection of the 6.2 keV gamma line which is a definite signature of the Ta de-excitation. This 6.8µs isomer lifetime is short compared to nuclear physic electronic time scales and a special care is to be taken for the detection system in order to observe the gamma-line