Introduction to Francium Spectroscopy

What is francium?

   Francium is the heaviest of the naturally occuring alkali elements -- the left most column of the periodic table of the elements. It has a 87 protons and between 120 to 140 neutrons. Its official symbol is Fr.

   Francium is also the least stable of the first 103 elements. Compared to other elements, very little is known of the physical and chemical properties of francium. The longest lived francium isotopes are ²¹²Fr and ²²³Fr which have half-lives of about 20 minutes. ²²³Fr is produced naturally as part of the radioactive decay chain of uranium, though there is probably less than 30 g present in the entire Earth's crust at any one time.

   The low natural abundance of francium has severely constrained its investigation: M. Perey discovered francium in 1939 at the Curie Institute, and only several decades later in 1978 did the ISOLDE team at CERN observe the first optical transition in atomic francium.

How do you get francium?

   Naturally occuring francium is difficult to study since it must be quickly and efficiently extracted from the sample in which it is embedded. In practice, francium studies require its laboratory production. At Stony Brook, we produce francium in a heavy ion nuclear fusion reaction at the Stony Brook Nuclear Structure Laboratory's superconducting linear accelerator (LINAC).

We produce francium by colliding or fusing 100 MeV oxygen nuclei (¹⁸O) with gold nuclei (¹⁹⁷Au) in a stationary target:

Fusion reaction for producing francium.

The fusion reaction produces ²¹⁵Fr with a lot of internal energy, which is then released as neutrons. The number neutrons boiled off can be changed by tuning the energy of the incident ¹⁸O ions. Using this reaction, we can produce ²⁰⁸Fr, ²⁰⁹Fr, ²¹⁰Fr, and ²¹¹Fr. By substituting gold for platinum in the target, we can produce ²¹²Fr. In practice, we achieve the highest production rates with ²¹⁰Fr (with a half-life of 3 minutes).

   The neutrons produced during the fusion reaction pose a significant health hazard. In order to work in a neutron-free environment, we remove the francium from the target as an ion and transport it to a trapping room, located 10 m away behind a 1 m thick concrete wall. When the francium atoms diffuse out of the target, the gold surface work function strips the francium of its valence electron. The francium ion is then accelerated accross a 5 KeV potential and guided electrostatically to the trapping room:

Francium production and trapping apparatus.

The electrostatic optics of our transport beamline ensure a mass independent transport of all francium isotopes and even other alkalis, such as rubidium, which we use for testing most of our apparatus.

How do you study francium?

   We study francium by cooling and trapping it in a magneto-optical trap, and then subjecting it to a variety of laser pulses. We trap francium by neutralizing on a piece of heated yttrium and injecting it into a magneto-optical trap (MOT). The MOT is formed at the intersection of six laser beams shining through a glass cell vacuum chamber and an anti-Hemlholtz magnetic field, as depicted in the above apparatus figure.

   We use the MOT because it allows us to trap the francium in vacuum with no substrate in a volume less than 1 mm wide. The MOT also cools the trapped francium to below 100 µK, reducing the Doppler shift and broadening of spectroscopic lines to negligible levels.

   In 1995, the Francium Spectroscopy Group trapped 3,000 francium atoms in a MOT for the first time. In 2002, after redesigning and rebuilding the apparatus, the group succeeded in trapping francium with peak MOT populations of over 200,000 atoms, and an average MOT population of 50,000 atoms.

1995: 3,000 ²¹⁰Fr atoms in a MOT.

2002: 140,000 ²¹⁰Fr atoms in a MOT.