Laser spectroscopy

The unique TRIUMF-based Collinear Fast-Beam Laser Spectroscopy facility is using the smallest changes in electrons’ quantum-level jumps to map the extreme frontier of nuclear structure in rare isotopes.

The core challenge in studying these extreme nuclei, ones with lopsided ratios of neutrons and protons, is that they’re very short-lived. The rare isotope must be produced, transported to the experiment and measured usually in just thousandths of a second.

The Laser Spectroscopy facility captures nanosecond “snapshots” of these short-lived rare isotopes by creating a head-on collision between a rare-isotope beam moving at about a million meters a second (the “fast” in the technique’s name) and laser light. The result of thousands of photon-atom collisions is a light fingerprint that provides scientists with a fine-grained reflection of the atom’s nuclear structure, including its size, shape and electromagnetic characteristics.

Importantly, TRIUMF’s ability to produce a range of pure, intense rare-isotope beams enables researchers to watch the evolution in nuclear structure over a range of an element’s increasingly heavier isotopes. For example, rubidium-97 (⁹⁷Rb) undergoes a dramatic shape change when it gains an extra neutron becoming rubidium-98 (⁹⁸Rb).

The Laser Spectroscopy facility’s experimental results are essential for testing nuclear theories of extreme nuclei and important for exploring fundamental symmetries. They’re also giving astrophysicists a better view of our stardust origins: extreme nuclei are critical in element formation in stars and subtle changes in nuclear structure can help explain differences in the ratios of different elements observed in stellar environments.

Collaborators include more than a dozen researchers and students from eight universities in five countries: Canada; United Kingdom; Jordan; Japan; and the United States.

Housed in ISAC-I, the Laser Spectroscopy facility is supplied by a dedicated beamline which can also serve MTV, Osaka and TITAN.

Spectroscopy records the light absorbed or emitted by an atom’s valence electrons producing a characteristic light fingerprint of that particular atom. For example, when an electron absorbs a photon it can “jump” to a higher energy orbital. Similarly, when the excited electron decays to a lower, more stable orbital, it emits a photon that contains the exact amount of energy between the two orbitals.

The structure of these electron orbitals is a result of the interaction between electrons and the atom’s nucleus. Subtle changes in the nuclear structure of an element from one isotope to another result in tiny, parts-per-billion changes in the electron orbital structure, known as hyperfine splitting. As a result, an atom’s hyperfine spectroscopic fingerprint is a remarkable window into the atom’s nuclear structure, including its average diameter, shape and magnetic and electric moments, or poles.

This hyperfine interaction is used in reverse in TRIUMF’s creation of polarized rare isotope beams. For experiments, including Osaka and MTV, the Laser Spectroscopy facility’s laser manipulates electrons in order to polarize the orientation of nuclei in a beam.

To capture an atom’s hyperfine light fingerprint the Laser Spectroscopy facility has four main components: a rare isotope trap and buncher; a beam accelerator and neutralizer; the laser; and the detectors.

Collect, Cool and Bunch

 The rare isotopes used in an experiment are created in an ISAC target and transported as ions by beamline to the Laser Spectroscopy facility. In order to increase the measurement sensitivity and interaction with the laser, the rare isotope ion beam must be converted from a continuous flow into a beam of ion bunches.

To create this bunched beam, the beam is channelled into TRIUMF’s TITAN facility’s radio-frequency quadrupole buncher. The buncher is the equivalent of a bucket collecting beam ions using a four-directional radio frequency field to slow, cool and trap desired ions in the centre of the trap.

An electric “kick” pushes the bunched ions out of the trap and back into the beamline as a bunched beam. This experimental setup is the first instance of a bunched ion beam being reverse-extracted from such a trap.

Re-accelerate and Neutralize

Laser spectroscopy requires neutral atoms all moving at the same speed. Thus, an adjustable voltage gradient in the beamline is used to accelerate the bunched ions so that they are all moving at approximately the same speed, up to a million meters per second. This limits Doppler broadening, or the obscuring of the hyperfine lines caused by light emitted from atoms moving at different speeds. Then the bunched beam is sent through a low-pressure vapour of hot sodium where the rare isotope ions pick-up electrons creating neutral atoms.

The Laser

The neutral atoms are hit with the beam from a tunable titanium: sapphire laser. This laser spectroscopy is roughly analogous to tuning a radio to a particular frequency at which the sound is clearest (though in this case there will be several hyperfine signal areas). When the laser is resonant with the atomic transition resonance, electrons absorb photons, are excited to a higher orbital and then immediately decay to a more stable orbital by emitting a photon of light at the resonant wavelength. By measuring the intensity of the emitted light, while slowly varying the frequency of incident photons, researchers determine the precise energy levels of hyperfine states and extrapolate this to gain a view of nuclear structure.

Notably, rather than adjust the frequency of the laser, its frequency is locked and an adjustable voltage gradient is used to slightly decelerate the atomic beam, Doppler-tuning it to change the laser photon frequency experienced by the atoms.

Detectors

The photons emitted by decaying electrons are detected by a photomultiplier tube mounted at right angle to the laser and rare-isotope beam. The detector suppresses background signals by only accepting light signals whose timing coincides with the presence of a bunch.