The researchers will investigate aspects of the weak interaction, the force responsible for radioactivity, inside the nucleus of the least stable naturally occurring element, francium. Unlike the other three fundamental forces, the weak interaction does not act symmetrically in all circumstances. It behaves one way in one set of spatial coordinates, and another way when those coordinates are exchanged for a mirror image, a phenomenon called parity violation. The experiments will explore manifestations of this intrinsic handedness in different isotopes of francium confined in a laser trap and excited with microwaves. "Forbidden" (extremely rare) transitions by the atoms are linked to the presence of a peculiar shape of the current inside the nucleus, the anapole moment, that follows the surface of a bagel. To date this has only been observed in one isotope of cesium. Because the anapole moment is strongly affected by the number of neutrons that contribute to the parity-violating weak interaction, changing the isotope and repeating the experiment will allow measurements of parameters that characterize the weak interaction in the nucleus.
This international effort, taking place at TRIUMF, the accelerator in Vancouver Canada, will allow two graduate students to be educated in the boundary between atomic, molecular and optical physics and nuclear physics. Local undergraduates will participate during the academic year; and in the summer Mexican students will take part through a program coordinated by the Sociedad Mexicana de FÃsica.
The anapole moment arises from the intrinsic handedness of the electrical currents inside an atomic nucleus. The handedness is a signature that there are processes that involve the weak force, the one responsible for the beginning of the solar cycle. Francium, with 87 protons and 123 neutrons is expected to have a large anapole moment. Its measurement will provide unique parameters to understand how the weak force operates inside a nucleus. This grant allowed the development of an apparatus to trap and cool francium atoms on line with the TRIUMF accelerator in Vancouver, Canada. More than a million atoms (see figure) will be interrogated with techniques tested during this grant period that have achieved enough sensitivity to extract the anapole moment of a series of francium isotopes. The apparatus for the anapole measurement has required the development of a special trap for francium, based on a single-focused-laser that rotates off axis forming a cigar shaped region of darkness. This trap minimizes the disturbance to the atoms. The disturbance measured is well under control and should not affect the anapole work. Another important development necessary for the anapole measurement has been the fabrication and characterization of a microwave resonator, a Fabry-Perot, where the microwaves can reflect many times between two mirrors and interact with the francium atoms for a long time. The technology includes using of sub-wavelength structures patterned of metal that permit engineering of the transmission of the mirrors. Graduate and undergraduate students have been trained in the disciplines of atomic, optical, nuclear, and particle physics. The efforts are part of a larger international group of scientists, the FrPNC collaboration that includes participants from the United States, Canada, and Mexico.
