WADE: Weak transition Amplitudes DEtection
RN: Luca Tomassetti
The WADE experiment is a collaboration among University of Ferrara and INFN, University of Siena, University of Pisa, University of Trento and INFN LNL. It fits into the fundamental physics research area devoted to the study of symmetries in atomic systems. In particular the objectives of the project are committed to the development of experimental techniques able to allow in the future high precision measurements of atomic parity non-conservation (APNC) as a test of the Standard Model.
At the same time important achievements towards a better knowledge of the Francium atoms through preliminary spectroscopic measurements of not yet observed transitions and a deeper understanding of the trapping processes are expected.
A primary 18O beam, with intensities of about 0.2 p−μA, is accelerated to kinetic energies Elab =100÷105 MeV from the LNL Tandem-XTU accelerator and delivered on a thick 197Au target. Francium nuclei (mainly 210Fr) are produced via fusion-evaporation reactions. The target is placed on a tungsten rod and heated to T ≃ 1000 ◦C. After diffusion through the gold, desorption and ionization from the surface, francium is focused with a conical electrode and accelerated to 3 keV. An 8-m electrostatic beam line transports the ions to the Magneto – Optical Trap (MOT).
In the MOT cell the ions hit a small heated yttrium foil, where they are neutralized and then released. The MOT has a standard configuration, with six laser trapping and repumping beams counterpropagating in the three directions of space, and a magnetic-field gradient (about 10 G/cm). The trapping light, slightly red-detuned with respect to the 7S − 7P3/2 cycling transition at 718 nm, is provided by a Ti:Sa laser. The repumping beam comes from a diode laser tuned to the 7S − 7P1/2 transition at 817 nm: it prevents the accumulation of Fr atoms in the ”wrong” ground hyperfine state out of the cycling transition. In order to prevent sticking on the pyrex surface of the MOT cell and favor multiple passes in the trapping area, we apply a Dryfilm coating on the cell walls. The coating greatly reduces the adsorption coefficient of the cell walls; the atoms are either trapped by the twelve laser beams crossing at the cell center or lost through the input aperture.
Light detection of atom fluorescence is realized with a cooled CCD camera, which has been calibrated and extensively tested with Rb both from vapor and ionic beam sources. A short focal length objective has been coupled to the camera to locate a dark region behind the trap. A real– time differential detection algorithm (weighted background subtraction) is implemented in order to acquire uniform images and to compensate for laser intensity fluctuations. The CCD system is calibrated in power and number of atoms present in the cloud. Tests with Rb ions and cold neutralizer show a sensitivity limit smaller than 50 atoms (5 fW).
We could observe as much as 1100 210Fr trapped atoms in a steady-state regime. It is possible to operate also in a pulsed regime. We let francium accumulate on cold yttrium for three half-lives: when we turn on the neutralizer, francium atoms are released altogether. With this technique we managed to detect a pulsed 210Fr trap of more than 8000 atoms and we obtained the first measurement of diffusion parameters of francium ions in Yttrium. We also trapped in the steady-state regime 209Fr (270 atoms) and 211Fr (180 atoms), produced in the usual fusion-evaporation reactions with lower rates. We performed precision measurements of the trapping frequencies obtaining an accuracy of 5 MHz, about a factor 20 better than older results.