Laser Spectroscopy of Niobium Fission Fragments: First Use of Optical Pumping in an Ion Beam Cooler Buncher

Abstract

A new method of optical pumping in an ion beam cooler buncher has been developed to selectively enhance ionic metastable state populations. The technique permits the study of elements previously inaccessible to laser spectroscopy and has been applied here to the study of Nb. Model independent meansquare charge radii and nuclear moments have been studied for 90;90 m;91;91 m;92;93;99;101;103 Nb to cover the region of the N ¼ 50 shell closure and N % 60 sudden onset of deformation. The increase in mean-square charge radius is observed to be less than that for Y, with a substantial degree of softness observed before and after N ¼ 60. Collinear laser spectroscopy has for many years been used to provide the (sole) model-independent probe of charge distributions of radioactive nuclei [1]. Efficient optical measurements are typically limited to transitions from atomic (or ionic) ground states or naturally populated low-lying metastable states. However, elements around Z ¼ 40 such as Y, Nb, and Mo present a variety of challenges to such spectroscopic approaches. In this Letter, we report a new, general technique of wide applicability, in which radio ions are deliberately prepared in metastable states by a rapid and efficient method of optical pumping. The technique is exploited here in the spectroscopic study of the A ¼ 100 region.The well-known onset of deformation in the neutronrich A % 100 region has been of long standing interest and the subject of recent mass [2,3], -ray spectroscopic [4,5], and optical measurements [6,7]. Collinear laser spectroscopy of Y isotopes was recently performed [6] in order to extract quadrupole moments in the region for comparison with charge-radii systematics. It was seen that with increasing neutron number from the N ¼ 50 shell closure, the nuclear deformation becomes increasingly oblate and increasingly soft. At N ¼ 60, the transition to a strongly deformed rigid prolate shape occurs, but prior to this, although the nuclear deformation is increasing with N, a proportionate increase in softness is also observed. Mass measurements [2,3] have indicated that the shape change, which is most pronounced for 39 Y, weakens with increasing Z, becoming almost undetectable in the 42 Mo chain. Further detailed model-independent optical analysis of the mean-square charge radii, nuclear moments and values of spin has so far, however, been impossible in the region because of experimental atomic physics constraints.Production of radioactive beams in this region, which are of a refractory nature, is presently limited to relatively small yields, and efficient spectroscopy is therefore required. Such elements are produced for optical study at the JYFL IGISOL facility, University of Jyväskylä, Finland [8]. Following mass selection at this on-line separator, the emittance of the ion beam is reduced in an ion beam cooler [7], which is a gas-filled rf quadrupole, held at a potential just below the $30 kV of the ion source. If desired, a trapping potential can accumulate the ions at the end of the device for ...

Isotope shifts from collinear laser spectroscopy of doubly charged yttrium isotopes

“…In the Z ≈ 40 region of the nuclear chart, a sudden onset of nuclear deformation occurs at N ≈ 60 [5,[14][15][16]. This dramatic shape transition is accompanied by a sharp increase in mean-square nuclear charge radius.…”

“…More recently, optical pumping has been used to enhance the population of a selected metastable state, from which collinear laser spectroscopy can be performed. Due to the long exposure time to the laser, efficient optical pumping can be achieved within the cooler-buncher [5], where optical access to the central axis is permitted by a twin quadrupole bend arrangement. Pulsed, broadband lasers are readily able to produce a range of wavelengths through frequency multiplication, with sufficient power to saturate a transition for an ionic ensemble in the trapping region.…”

“…Such an approach was used to avoid J l = 0 → J u = 1 ground-state transitions in order to determine the nuclear spin of 100m Y [23] and in niobium isotopes [5], where in the latter case the ground-state line (used for optical pumping) was additionally too weak for efficient collinear laser spectroscopy. For manganese, the optical pumping scheme utilized a ground-state line too weak for spectroscopy (but nevertheless could be saturated in the cooler-buncher), providing access to a more favorable line in terms of spectroscopic efficiency and sensitivity to the nuclear electric quadrupole moment [18,33].…”

“…One must therefore rely on multiconfiguration Dirac-Fock (MCDF) calculations, or semiempirical estimates, of both F and M. Similar problems have recently arisen in studies of niobium [5], scandium [17] and manganese [18,19], and, to a lesser extent, also copper [20] and gallium [21], which each have two stable isotopes. In these latter cases, (nuclear model dependent) nonoptical data could at least be used to correct the mass shift factor, which is the more challenging factor of the two to calculate due to the stronger dependence on electron correlations.…”

“…The most common high-resolution form of such spectroscopy is collinear laser spectroscopy, in which a laser beam and a radioactive ion beam are overlapped in a parallel geometry. Collinear laser spectroscopy is used at many different facilities; examples include ISOLDE (CERN, Switzerland) [4], JYFL (Jyväskylä, Finland) [5], NSCL (Michigan, USA) [6], and TRIUMF (Vancouver, Canada) [7], with complementary techniques for heavy elements [8,9]. For this work, beams of neutron-rich isotopes of yttrium are uniquely available at the JYFL IGISOL IV facility.…”