Cold atom fountains of Cs and Fr

Cs fountain development

Cs fountain EDM proof-of-principle experiment

Cs fountain EDM experiment
Fig. 1: First cold atom EDM experiment.

Figure 1 shows the world's first cold atom EDM experiment. The shiny cylinder to the right is the outer magnetic shield with openings to let in laser light (transported up along a small vertically-mounted optical table) and detect fluorescence. The image of the Cs cloud in the MOT can be seen on the TV screen at the left. See Electron electric-dipole-moment experiment using electric-field quantized slow cesium atoms, J.A. Amini, C.T. Munger, Jr., & H. Gould Phys. RevA75, 063416 (2007) Reprint here.

New Magnetic Fountain Prototype

Fig. 2: Magnetic fountain interaction region.

Our new fountains will use a 3D MOT loaded by a 2D MOT of our own design. Both are powered by Vescent DBR lasers through fiber optic cables and fiber optic beam splitters. The MOT telescopes are of our own design which makes extensive use of off-the-shelf Thorlabs 30 mm and 60 mm cage system parts.

Our fountain development starts with the cesium fountain shown on the left. It will be used to develop and test magnetic-field sensitive critical features of the experiment that do not involve electric fields:

It will have a glass column with optical windows (which may but need not be AR coated) and will be nearly free of metal to reduce magnetic Johnson noise. The three layers of magnetic shields are already on hand and have been used for magnetic shielding tests described here.

New Electric Fountain Prototype

Fig. 3 Interaction region of the Electric Fountain prototype.

The cesium electric prototype fountain shown in Fig. 3 will be used to develop and test atom beam optics using electrostatic lenses and electric field plates. Because these tests are insensitive to small magnetic fields, the apparatus will use a an ordinary stainless steel chamber, metal (later glass) electric field plates and electrostatic focusing elements. It needs no magnetic shielding.

In Fig. 3 the electrostatic triplet focusing lenses are labeled D1, F2, and D3 and the main electric field plates E4. Electrostatic lenses use slightly convex or concave surfaces to focus in one transverse direction while defocusing in the other.

Alternating lenses (alternating gradient focusing) results in a net focusing effect. For atoms traveling in the “z” direction and electric field in the “y” direction, the concave D lenses focus in the “y” direction and defocus in the transverse “x” direction. The F lenses do the opposite. Electrostatic focusing of a fountain of cesium atoms was demonstrated in Ref. 1.

Cesium Fountain EDM Null Experiment

A null Cs EDM experiment will make it unlikely that a francium electron EDM experiment will observe an effect in the absence of an electron EDM.

Cesium fountain apparatus
Figure 4: Interaction region of a Cs fountain “null” experiment.

After operating the magnetic and electric prototype fountains, we combine their features and many of their parts to make a fully functioning low magnetic-noise Cs-fountain electron EDM experiment.

Metal electric field plates are replaced with low-resistivity lithium-alumino-silicate glass electrodes to suppress magnetic Johnson noise and increase the breakdown electric field, and the metal vacuum chamber surrounding the electric field plates is replaced with a glass or ceramic vacuum chamber. The resulting apparatus interaction region is shown Fig. 4.

This is a null experiment in that, Cs is a factor of 8 to 16 less sensitive than Fr to an electron EDM, but a factor of 8 to 90 more sensitive than Fr to the major motional systematic effects. If we see no signal, in a Cs experiment, then we would expect to avoid a signal from a motional systematic effect in Fr for a factor of 64 to 720 below the Cs result.

Working with Francium

Francium decay properties

Francium, the last element to be discovered in nature, is the least stable of the first 102 elements, having no known isotope with a half life longer than 22 minutes: 211Fr and 221Fr, isotopes of interest for EDM experiments, have half-lives of 3.1 minutes and 4.8 minutes respectively.

Francium at TRIUMF

Fig. 4. Online isotope separator ISAC I at TRIUMF. Photo by B. Mier under Creative Commons license. Additional ISAC photos here

The EDM experiment will be performed at TRIUMF, where 500 MeV protons striking a uranium carbide or thorium carbide target produce a range of isotopes which are separated on-line to yield a beam of 211Fr.

In 2016, TRIUMF produced a beam of 1.9 x 109 atoms s-1 of 211Fr using a 10μA beam of protons on a uranium carbide target. Target development work, aimed at higher Fr intensities, is continuing at TRIUMF.

The francium is delivered as a 30keV beam of Fr+ with an emittance of 10 mm mr. The decay products of 211Fr do not include long lived isotopes of polonium or other easily vaporized elements.

Transition to Francium

221Fr alpha decay
Fig. 5: Spectrum from the alpha decay of 225Ac → 221Fr → 217At, → 213Bi which beta decays to 213Po which alpha decays to 209Pb, which beta decays to 209Bi which is stable. The longest lived 221Fr daughters are the isotopes that beta decay: 209Pb (3.3 h) and 213Bi (46 m). Spectrum taken by A. Ghiorso and H. Gould.

To convert the laboratory Cs fountain EDM experiment into a (TRIUMF) accelerator-based 211Fr fountain EDM experiment, we start, in the laboratory with 221Fr (t1/2 = 3.5 min) obtained from the alpha decay of commercially available 225Ac which has a ten-day half life. Collaboration members have already trapped 221Fr obtained from a 225Ac source[2,3] and developed a method of efficiently forming Fr beams from very small samples [4]. However the supply of 225Ac is insufficient to compete with TRIUMF-produced 211Fr.

221Fr is exceptionally well suited for use in a laboratory environment: the decay products of 221Fr are all short lived isotopes with no volatile element having a half-life longer than 32 ms. Once the 225Ac source is removed (and if there is no contamination from loose Ac or other precursors), radioactivity in the apparatus returns to near background levels in less than a week.