COLLAPS experiment and spectroscopy beam lines in the ISOLDE facility at CERN
TheCOLinear LAser SPectroscopy (COLLAPS) experiment is located in theISOLDE facility at CERN. The purpose of the experiment is to investigateground andisomeric state properties of exotic, short livednuclei, includingspins, electro-magnetic moments andcharge radii.[1] The experiment has been operating since the late 1970s, and is the oldest active experiment at ISOLDE.[2][3]
The technique of collinearspectroscopy was developed in the mid-1970s by S.L. Kaufman.[4] This describes a method of obtaining narrowabsorption lines, specifically providing a sensitivity ideal for experiments on short-livedisotopes.
Two beams are used in the technique: alaser beam sent through the sample, and a probe beam. The alignment of both beams collinearly (along the same path) allow for control of the time and spatial overlap.[5] This enables investigation into the nuclear properties of the sample simultaneously.[6]
Optical detection region at COLLAPS in the ISOLDE facility
COLLAPS is located within the ISOLDE facility at CERN, giving it access to the radioactive ions produced by ISOLDE's resonance ionisation laser ion source (RILIS).[7] The ions are delivered to the COLLAPSbeamline and are excited usingtunablecontinuous-wave lasers through the technique of collinear spectroscopy.[8][9] The laser systems produce laser light in the 210 nm to 1000 nm range with a narrowlinewidth. The systems allow access to the atomic transitions necessary for the short-lived nuclei produced by ISOLDE.[3]
Laser spectroscopy is better performed on a neutral atom, and therefore a charge exchange cell (CEC) is needed to neutralise the ionic beam from ISOLDE.[7] A CEC neutralises the ions by causing the ionic beam to collide with the alkali vapours in the cell and transfer charge. Prior to entering the CEC, the ions are reaccelerated (retarded) and a scan of the atomic transition is taken using Doppler-tuning electrodes.[10] Laser spectroscopy is then performed on the neutral atom, however can also be performed directly on the ion. The detection system, located at the end of the beamline, consists of eight large-diameter aspheric lenses.[3] The atoms de-excite and release fluorescent light, which is transferred to the four photomultiplier tubes (PMTs) by the lenses.
The following are some notable results from the COLLAPS experiment.[11][3]
Charge radii, spins and electro-magnetic moments for various isotopes, including magic isotopes
Developing the laser spectroscopy setup to examine very exotic isotopes[12]
Found first direct evidence for the importance of proton and neutron excitations across shell gaps in ground state wave functions of neutron-rich Mn isotopes[13]
