Francium Trapping Facility

TRIUMF’s Francium Trapping Facility is using rare francium atoms to capture an ultra-precise fingerprint of atomic weak force symmetry breaking and potential beyond-Standard Model physics.

Among the four fundamental forces (along with electromagnetism, the strong force, and gravity) the weak force is the only one which violates symmetry: mirror image processes (parity); and time-reversed ones aren’t identical. Exploring these symmetry violations is a crucial area of research in numerous areas, including for understanding the universe’s most mysterious asymmetry: why does matter so thoroughly dominate antimatter?

The weak force only extends over tiny distances in the nucleus, less than the width of a proton, so must be studied indirectly. Enter francium (Fr) which has the ideal atomic structure for probing the nuclear weak force. As element 87, francium has a large nucleus, but like hydrogen it has the simplest possible outer electron structure, with just one electron in its outermost shell. This single valence electron is subtly influenced by its interactions with the large francium nucleus via the weak force.

Thus, the Francium Trapping Facility is building towards making the most precise spectroscopic measurements of the quantum transitions of francium valence electrons as a probe of weak force parity violation in the atom’s nucleus.

A key challenge is that francium is also the least stable of the first 103 elements of the Periodic Table; the francium-211(²¹¹Fr) used in Francium Trapping Facility experiments has a half-life of only three minutes. Thus, TRIUMF is one of only two locations worldwide (along with at CERN) capable of creating the necessary pure, high-intensity beams of short-lived francium atoms to enable the experiment.

Symmetry breaking weak force electron transitions have been measured in the cesium (Cs) atoms, but theory predicts that the observable effect in francium atoms will be 18-times larger. As a result, the increased precision of Francium Trapping Facility results could reveal potential beyond-Standard Model physics, for example, the existence of new weak force particles or a postulated super-weak force involving other types of bosons.

The Francium Trapping Facility is a collaboration between TRIUMF and researchers at the University of Manitoba, and researchers from the United States (University of Maryland and College of William and Mary) and Mexico (Universidad Autónoma de San Luis Potosí), including three graduate students, two postdoctoral fellows, and several undergraduate research students.

Housed in ISAC-I, the Francium Trapping Facility uses francium produced from a uranium carbide target and sent to the experiment via a dedicated beamline. Using lasers, a batch of up to millions of francium atoms is trapped in a volume smaller than a pen tip in the centre of an ultra-high vacuum chamber at a million-times colder than room temperature. Here, for 20 seconds, a laser is used to stimulate francium electrons and record the spectroscopic light of valence electron transitions.

The Physics

To explore weak force parity violation, the Francium Trapping Facility will use spectroscopy to observe the 7s to 8s orbital valence electron transition. In a purely electromagnetic world, the electric field oscillation of the laser light could not excite this 7s to 8s transition in francium; it is forbidden by the quantum mechanical parity selection rule.

However, the ever so feeble presence of the parity-violating weak interaction between the quarks in the nucleus and the electrons destroys the symmetry of the electronic orbitals, admixing tiny amounts of p-orbital character into 7s and 8s, in turn causing a slight violation of the selection rule, enabling a weak-interaction induced 7s to 8s transition.

In preparation for making the ultra-precise parity violating measurements, the Francium Trapping Facility has made a number initial first steps. These include successfully trapping francium atoms and measuring the hyperfine spectroscopic differences between a string of increasingly more massive francium isotopes.

The Francium Trapping Facility is composed of four key components: an electromagnetically shielded room; a francium capture foil; a francium collection trap; and a science chamber trap where the spectroscopic measurements are made.

The Faraday Room

To shield the experiment from surrounding electromagnetic noise, the Francium Trapping Facility is housed in a Faraday room, a room-like enclosure used to block electromagnetic fields. The bedroom-sized enclosure (about 6-by-4 meters) has walls made from two layers of steel sheet metal and a special door that seals tight, like a submarine hatch.

The Capture Foil: Embed, Neutralize, Release

The francium beam enters the Faraday enclosure through a transition vacuum system that drops the usual ISAC beam line vacuum to 10,000 times lower pressure. This is required to ensure that the francium atoms interact with as few other atoms as possible during the experiment.

In the Faraday room, the first step is to stop, collect and neutralize the francium beam by colliding the ions into a thin zirconium capture foil. The francium ions stop just several hundred angstroms, or atomic lengths, into the zirconium and interacting with the zirconium atoms, gain electrons and are neutralized. This is a crucial step since the atom must be neutral for the study of the valence electron. After 20 seconds, the foil, which has been upright like a door, is flipped down, and near instantaneously electrically heated to 850 °C, causing the embedded francium atoms to vaporize up from the metal’s surface into the capture trap. Then the foil is flipped back upright for 20 seconds to collect another batch of francium atoms while the current batch is used in the experiment.

The Collection Trap

The collection trap is the first of two magneto-optical traps (MOT), the first stacked above the second: the upper one, the capture trap, is used to initially cool and trap the francium atoms; the second to provide atoms for experiments in the science chamber. Both use a combination of laser light and magnetic fields to gather, condense, and cool the francium atoms into a millimeter-cubed size cluster.

The capture MOT is a central glass cell surrounded by an electromagnet, all of which is contained in a stainless-steel cube with openings on its six faces to permit the entrance of laser light. To prevent the francium atoms from sticking to the glass cell, its inner walls are covered with silane-based coating, which acts as a non-stick surface for francium atoms, akin to Teflon in a frypan.

The trap is surrounded by three pairs of lasers (six in total) resting on vibration-insulated optics tables, their light transmitted to the MOTs by optical fiber cables. Each of the six titanium: sapphire tunable ring laser beams is nearly 5 cm in diameter, together almost filling the MOT cell entirely, and they’re driven by the same type of green laser used in many laser pointers, but with about a thousand times more power.

The francium atoms are photon-corralled into the centre of the trap using laser cooling. Every element’s atoms have a characteristic resonant frequency – a frequency at which they will absorb, and be energized by, photons at a particular wavelength of light. When an atom absorbs a photon, it gets a momentum kick in the direction the photon was travelling. Atoms absorb more photons closer to the resonant frequency. The six lasers are set to shine light at a frequency just below francium’s resonant frequency that if an atom is moving upwards, its upward momentum causes it to experience the laser light hitting it from above as slightly Doppler-shifted to the blue, or to a higher frequency.

Thus, an upward-moving francium atom absorbs more of these resonant photons, than photons coming from below, and slows down. In this way, francium atoms moving in any direction are dramatically slowed or cooled, and corralled towards the centre of the trap.

To tightly cluster the slowed atoms, the MOT uses a finely calibrated quadrupole magnetic field around the trap. Just as the Earth has poles, every atom has a slight magnetic moment. The collection trap magnets are arranged so that the magnetic field is weakest at a tiny point in the very centre of the trap, a magnetic well. Based on the physics of resonance frequencies, the further an atom is from the well, the more photons it absorbs, and is thus be pushed back towards the magnetic low point.

In this way, every twenty seconds about one million francium atoms are trapped in a 1 mm diameter ball at the centre of the MOT.

However, precise measurements of francium electron transitions can’t be made here because the francium beam hitting the foil contained a soup of rare elements whose decay creates radioactive background noise in the detectors, and the heat from the capture foil would also influence the results.

So, for just a fraction of a second, the six trap laser beams are turned down and a seventh laser beam shines in from the top pushing just the tight cluster of trapped francium atoms, like corks on a river, through the opening previously covered by the zirconium flap (now upright and collecting francium ions) and down into the science chamber.

Science Chamber

It’s in the science chamber that precise spectroscopic measurements of francium are made. In FTF’s use of spectroscopy, electrons are stimulated using laser light and then the light from the electrons recorded as they fall back to their ground state. The emitted light becomes a detailed spectroscopic fingerprint revealing the influence of the weak force on the electrons.

The science trap uses the same combination of six lasers and a magnetic field as the capture trap to force the atoms into a suspended, still, cluster of about one-millimeter cubed. The atoms are cooled to about 1/10,000 K, at which temperature an atom is moving at about half-a-meter-per-second; the same francium atom at room temperature would be moving at about 200 meters-per-second. In addition, the science trap, is sided with transparent electric field plates, similar material to that used for a smartphone touch screen, capable of producing a precise electric field while still transmitting the laser beams for the MOT.

A spectroscopy laser excites the francium valence electrons using a blue-green light, at a 506 nm wavelength, just the right frequency to boost a 7s valence electron to the 8s orbital. However, once at 8s, the electron quickly decays back to 7s. This decay process is multistep, emitting photons each step of the way, photons which are recorded by a photomultiplier tube, creating a light fingerprint which reveals the precise influence of francium’s weak force on the electron, or atomic parity violation.

The spectroscopic measurements are made in a milliseconds-long series of on-off measurements, in order to keep the atoms trapped and also make spectroscopic measurements.  In a millisecond, the trapping quadrupole magnetic is turned off, and a homogenous north-south magnetic field is turned on along with the spectroscopy laser for several milliseconds, during which the atoms as in free fall in earth’s gravitational potential. Then the trapping magnetic field and lasers are turned back on for several milliseconds re-trapping the atoms. Each atom’s electron transition is measured many times during a 20-second experimental session, and the measured result is the sum of all the millions of observed transitions.

In order to test for atomic symmetry breaking, Francium Trapping Facility researchers control the motion of the francium atoms in three dimensions, or x, y and z-axes, using magnetic, electric and laser fields. Reversing any one of the axes, for example flipping the direction of the north and south poles of the science chambers magnetic field, is the equivalent of creating a mirror image system in which parity can be tested. Thus, Francium Trapping Facility researchers will flip and compare the magnetic, electric and laser fields and observe whether the spectroscopic 7s-8s fingerprint is identical, or as expected, breaks parity with a theorized difference of one-in-10,000. The ability to test in all three dimensions gives FTF the ability to create checks-and-balances against an error in the measurement in any single direction.