COLLAPS is the oldest active experiment at ISOLDE-CERN, operating since 1978.

Collinear laser spectroscopy is a powerful tool to measure the properties of ground and isomeric states of short-lived isotopes far from stability. These properties include the nuclear spin, electromagnetic moments and mean-square charge radii, which can help us understand the shape and size of the different atomic nuclei. Such measurements have been used to discover and investigate a wide range of phenomena in the complex many-body system that is the nucleus. By comparison with nuclear theoretical calculations, such measurements guide theoretical modelling and provide stringent benchmarking of modern ab-initio theory. Ultimately, this enables us to gain a stronger understanding of the nucleon-nucleon interaction.

At COLLAPS we observe the tiny changes in atomic energy levels which are due to the specific properties of the nucleus. Consequently, our work brings together expertise in atomic and nuclear physics, as determination of nuclear properties from these tiny changes requires a good understanding of the atomic system. To observe these tiny changes in atomic energy levels we use extremely narrow linewidth lasers to probe the relevant atomic transitions. Typically, the linewidth of these lasers is in the rage of 1/1 000 000 000 of the energy of the transition. To reach this level of stability for the full range of atomic elements studied requires our third area of expertise - laser systems.

All methods used in the COLLAPS experiment are based on the technique of the collinear laser spectroscopy. This method was developed in mid-1970s by S.L. Kaufman: see S.L. Kaufman, Opt. Comm. 17, 309-312 (1976)

COLLAPS is an experiment located at the “radioactive isotope factory” ISOLDE at CERN. The radioactive ions produced by ISODLE are delivered to the COLLAPS beamline. To access appropriate atomic transition for the wide range of short-lived nuclei produced by ISOLDE, the COLLAPS laser systems routinely produce extremely narrow linewidth laser light in the full ~210 nm to 1000 nm range.

Although ISOLDE as a general rule provides an ionic beam in the 1+ charge state, it is frequently better to perform laser spectroscopy on the neutral atom. In order to neutralise the ions from ISODLE, the ionic beam is passing through a charge exchange cell (CEC) filled with alkaline vapours where by collisional charge transfer the ions become atoms. In other cases, we perform spectroscopy directly on the ion. Recent examples include beryllium, magnesium, calcium and scandium.

A scan of the atomic transition is performed by reaccelerating (retarding) the incoming ions just before they enter the Charge exchange cell. This is achieved by applying a variable voltage to a set of retardation lenses in the range of ±10kV. The change in speed of the ions results in a frequency shift of the laser frequency in the rest frame of the atoms due to the Doppler effect.

The detection system is located at the end of the beam line. It consists of 8 large-diameter aspheric lenses which transfer light generated by fluorescent de-excitation to 4 photomultiplier tubes (PMTs).

In addition to optical detection, a range of other non-optical detection techniques are used to study radioactive isotopes with extremely low production rates.