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Description
Resonant laser ionization is an efficient and highly selective method for producing radioisotopes. In the laser ion source of the ISOLDE – RILIS (Resonance Ionization Laser Ion Source), the laser interaction region is inside a metal tube, the so-called “hot cavity” which is heated to temperatures of up to 2200 degrees Celsius. In addition to providing a longitudinally confining electrostatic potential due to electron emission from the cavity material, this heating also induces surface ionization of elements with low (< 6 eV) ionization potential. If the overall ion load of laser and surface ionized species reaches a certain threshold the confining potential breaks down and the efficient extraction of these ions is compromised. In concrete terms this means that the extraction efficiency of laser ions which have a distinct and short time structure induced by the pulsed lasers reduces drastically whereas the surface ions, which have a constant mode of creation, remain unaffected (if the half-life is long enough). This effect is especially prevalent in facilities like MEDICIS which demand a high ion throughput and fast extraction for quick and efficient delivery of radioisotopes for medical applications (half-lives of ~ 5 days). This work aims to define the limits of the “classic” laser ion source which is currently used for medical radioisotope production at MEDICIS. The measurements were performed using the ISOLDE OFFLINE 2 facility to determine the operational parameters under high ion load. An outlook on possible alternative ion source geometries will be given, which might remedy the problem for future laser ion extraction.
