Laboratory Astrophysics Research Group

The Fast-Ion-Beam Laser Lab at Western

Astronomy needs atomic data:

Astrophysicists study the process of nucleosynthesis in which the elements are created by nuclear reactions in stars and supernovas. The elements beyond ⁵⁶Fe, the most stable nucleus, are produced by neutron capture. In regions of intense neutron flux (supernovas) the “rapid” r-process creates unstable neutron-rich nuclei which beta decay to more stable isotopes, while the rare p-process creates proton-rich isotopes by photodisintegration. In less intense regions the “slow” s-process produces a sequence of relatively stable isotopes. The lanthanide elements (rare earths) are particularly suited to unraveling the history of nucleosynthesis. Samarium, for example, has seven stable isotopes: ¹⁴⁸Sm and ¹⁵⁰Sm are produced by the s-process, ¹⁴⁴Sm by the p-process, ¹⁵⁴Sm by the r-process, and ¹⁴⁷Sm, ¹⁴⁹Sm, and ¹⁵²Sm by both the r- and s-processes. These studies require that the abundances of the elements be determined from observations.

Stellar astrophysicists can only study stellar interiors indirectly by observing the visible outer surface, called the photosphere. In stars like our Sun, the abundance distribution of the chemical elements is representative of the bulk abundance in the whole star; however, in the “chemically peculiar” CP stars, the surface abundances can differ by orders of magnitude from the bulk abundances. This chemical fractionation is produced by radiation pressure selectively driving certain species of atoms toward the surface. In some stars, convection and turbulent mixing restores a homogenous distribution, but not in CP stars; thus the surface abundances contain valuable information on what is happening in the unseen interior of the star.

The determination of chemical abundances from astronomical observations requires atomic data: the probabilities for absorption of light by each species of atom at its set of characteristic wavelengths. These probabilities are usually given in a form called, for historical reasons, the “oscillator strength.”

The Fast-Ion-Beam Laser Lab at Western:

We produce a 10-keV beam of ions traveling in vacuum in our accelerator facility

A continuous-wave dye laser (pumped by an argon-ion laser) excites the ions to a particular unstable state, from which they can decay to various possible states of lower energy, emitting a photon while still traveling at high speed (1.5 x 10⁵ m/s)

From the curve of light emitted as a function of distance “downstream” from the excitation region (which corresponds to time since excitation) we determine the average lifetime of the unstable state, typically tens to hundreds of nanoseconds

From spectroscopic measurements of the relative intensities of the light emitted in the various possible transitions, together with the lifetime of the upper state, we then determine the probability per second of emitting a photon in each of the possible transitions. This quantity is called the “Einstein A coefficient.”

A simple relationship between the probabilities for spontaneous emission and for absorption, first derived by Einstein, allows us to calculate the quantities needed by astrophysicists: the oscillator strengths.