From direct to absolute mass measurements: A study of the accuracy of ISOLTRAP

Abstract

For a detailed study of the accuracy of the Penning trap mass spectrometer ISOL-TRAP all expected sources of uncertainty were investigated with respect to their contributions to the uncertainty of the final result. In the course of these investigations, cross-reference measurements with singly charged carbon clusters 12 C + n were carried out. The carbon cluster ions were produced by use of laser-induced desorption, fragmentation, and ionization of C 60 fullerenes and injected into and stored in the Penning trap system. The comparison of the cyclotron frequencies of different carbon clusters has provided detailed insight into the residual systematic uncertainty of ISOLTRAP and yielded a value of 8 · 10 −9 . This also represents the current limit of mass accuracy of the apparatus. Since the unified atomic mass unit is defined as 1/12 of the mass of the 12 C atom, it will be possible to carry out absolute mass measurements with ISOLTRAP in the future. The ISOLTRAP ExperimentThe ISOLTRAP experiment is a Penning trap mass spectrometer for accurate highprecision measurements of atomic masses. It is installed at the on-line isotope separator ISOLDE at CERN (Geneva, Switzerland) [1], and it is mainly intended for mass measurements of short-lived radioactive nuclides. The masses of close to two hundred radionuclides have already been measured with the ISOLTRAP apparatus [2].ISOLTRAP consists of a radiofrequency quadrupole (RFQ) ion beam cooler and buncher [3] and two Penning traps, each located in the field of a superconducting magnet [4]. The RFQ ion beam cooler and buncher (RICB) serves the purpose of decelerating, accumulating, cooling, and bunching the 60-keV ISOLDE ion beam. The first Penning trap is a cylindrical trap which is used for the cooling and isobaric cleaning of the ion bunches. The actual mass measurement is carried out in the second Penning trap, a hyperboloidal trap. It is based on a determination of the cyclotron frequency ν c of an ion with charge q unknown mass m ion in a magnetic field of magnitude B [4]:

High-precision QEC -value measurement of the superallowed β+ emitter Mg22 and an ab

“…For the determination of the resonance frequency ratios, only cycles with 1 detected ion/cycle were used in order to reduce effects on the measurement which may result from ion-ion interactions (δ i−i ), which was the largest systematic uncertainty in this work. The error estimate for multiple ion interactions in the Penning trap during RF excitation was determined through a count-class analysis [37]. The measured frequency ratios, as well as the error budgets for each systematic in the TITAN system, are given in Table I, based largely on the studies of Refs.…”

Binding Energy of Strongly Deformed Radionuclides

“…where c is a parameter around 0.9, which slightly depends on the quality of the experimental data [Kel03], N is the number of events and T RF is the length of the quadrupole excitation pulse. Neither of these three factors is problematic for the measurement of the cyclotron frequency of the reference ion species, because the reference ions used at ISOLTRAP are radioactively stable, abundantly produced and can be easily purified by the available techniques, which will be discussed in the following section.…”

“…The first component of this drift is linear over a measurement's duration and is due to the decay of the current flowing through the superconducting magnet. The second component of the drift is a fluctuation due to changes of the ambient temperature and of the pressure of the helium inside the cryostat housing the magnet [Kel03]. To correct for the linear component of this drift, two reference TOF-ICR measurements are performed, one before and one after the TOF-ICR of the ion of interest.…”

Isobar separation and precision mass spectrometry of short-lived nuclides with a multi- reflection time-of-flight analyzer

“…Exotic ions are delivered from the online mass separator ISOLDE [28] as quasi-continuous beams with up to 60 keV kinetic energy (for singly charged ions). Typically, the ions are decelerated, accumulated, bunched, and cooled in a linear radio-frequency quadrupole (RFQ) Paul trap [29], transferred to the MR-ToF component for a first mass selection, then send to a first Penning trap [30] for further mass selection (if necessary) and for preparation [31], and finally forwarded to a second Penning trap for time-of-flight ion-cyclotron-resonance (ToF-ICR) mass measurements [32,33] with relative uncertainties down to 8 × 10 -9 [34].…”