Introduction to Francium Spectroscopy

What is francium?

Francium in the Periodic Table of the Elements

   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:

Francium fusion reaction
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:

Stony Brook francium production and trapping apparatus
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 ľK, 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.

3,000 francium atoms in 1995
1995: 3,000 210Fr atoms in a MOT.

140,000 francium atoms in 2002
2002: 140,000 210Fr atoms in a MOT.

The Francium Spectroscopy Group has trapped and studied 5 francium isotopes 208-212Fr:

  1. 1st generation MOT apparatus was used for laser spectroscopy studies of the 7P 1/2, 7P3/2, 7D 3/2, 7D5/2, 8S 1/2, and 9S1/2 levels, as well as the hyperfine anomaly.

  2. 2nd generation MOT apparatus has been used for laser spectroscopy studies of the 8P1/2 and 8P3/2 levels, as well as more in depth studies of the 8S1/2 and 9S1/2 levels.

What is interesting about francium?

Francium is an interesting atom because of its usefulness in testing fundamental theories of nature.

The francium atom is a useful laboratory for testing the standard model and quantum field theory in the following three areas:

  1. Experimental tests of the standard model of the electro-weak interaction through parity non-conservation (PNC).
  2. The parity-violating electromagnetic anapole moment of the nucleus.
  3. Precision tests of relativistic atomic physics.

Francium's usefulness over other atoms stems from its simplicity as an alkali atom and its large atomic mass and number:

  1. Simplicity: As an alkali atom, francium can be considered as an inner core and a single valence electron. This means that electron-electron interaction, while non-negligible, play much less of a role than in non-alkali atoms. This simplicity means that the electronic wave functions of francium can be computed to high accuracy. The high accuracy of the calculations is essential for accurately extracting nuclear anapole and standard model weak force parameters from PNC measurements.

  2. Heavy atom: The large nucleus and relativistic velocity of the valence electron enhance both the standard PNC effect and the anapole moment both of which are expected to be an order of magnitude larger than in cesium, the heaviest of the stable alkali atoms. The accuracy of atomic physics calculations can be tested in the relativistic limit.
Web page updated: January 13, 2007.