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A mass measurement of the short-lived halo nucleus ¹¹Li with the TITAN Penning trap spectrometer Smith, Mathew Jonathon

Abstract

New measurements of the masses of the isotopes⁸,⁹,¹¹Li were made using recently commissioned TITAN Penning trap mass spectrometer at TRIUMF. The measurement of the halo nucleus ¹¹Li represents a new standard in Penning trap mass spectrometry, as it is the shortest lived, t₁/₂ = 8.8 ms, isotope ever weighed using this technique. Low energy, E = 20 keV, beams of these radioactive isotopes were produced using the ISAC facility. These were subsequentlycooled and bunched using a square-wave-driven Radio- Frequency Quadrupole (RFQ) ion guide, which was filled with hydrogen gas. The cooled ion bunches were then passed into a Penning trap where the mass measurements were made. A description of the RFQ in the ISAC hall is given along with some results from the commissioning of the device. A new set of harmonic deceleration optics is presented which have been successfully used to inject ions into the RFQ. Cooling of lithium ions with high DC efficiencies of 20%, in helium, and 40%, in hydrogen, are shown. Extraction of extremely short ion bunches, 30 ns FWHM, is also demonstrated. Storage times for stable lithium ions in helium and hydrogen were investigated. It was found that lithium ions could be cooled in hydrogen for up to 30 ms without significant losses whereas cooling in helium lead to exponential losses with a half-life of 5.7(1)ms. The TITAN Penning trap is described and the ⁸,⁹,¹¹Li data presented. Final values for the mass excess of ∆(⁸Li) = 20945.70(38) keV, ∆(⁹Li) = 24954.80(60) keV and ∆(¹¹Li) = 40728.1(12) keV are obtained. The ⁹,¹¹Li results are then used to obtain a new value for two neutron separation energy of ¹¹Li, S₂n = 369.3(1.3) keV. This agrees with the recent measurement from the MISTRAL spectrometer, 376(5) keV, at the two sigma level, but shows over three standard deviations from the most recent atomic mass evaluation, 300(20) keV

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