New mass measurement of ${}^6\mathrm{Li}$ and ppb-level systematic studies of the Penning trap mass spectrometer TITAN

The frequency ratio of ${}^6\mathrm{Li}$ to ${}^7\mathrm{Li}$ was measured using the TITAN Penning trap mass spectrometer. This measurement resolves a $16$-ppb discrepancy between the ${}^6\mathrm{Li}$ mass of 6.015 122 795(16) u from the Atomic Mass Evaluation 2003 (AME03), which is based on a measurement by JILATRAP, and the more recent measurement of 6.015 122 890(40) u by SMILETRAP. Our measurement agrees with SMILETRAP and a more precise mass value for ${}^6\mathrm{Li}$ of 6.015 122 889(26) u is presented along with systematic evaluations of the measurement uncertainties. This result makes ${}^6\mathrm{Li}$ a solid anchor point for future mass measurements on highly charged ions with $m/q~6$.

Figure 1

The TITAN experimental setup that includes a RFQ, a high-precision Penning trap, an EBIT, an off-line ion source, and MCPs for beam diagnostics.

Figure 2

Illustration of the TITAN Penning trap formed from the hyperbolic ring (1) and end cap electrodes (2) that produces the harmonic potential, tube (3), and guard (4) correction electrodes that produce the harmonic potential. The RF is applied on (4).

Figure 3

${}^6\mathrm{Li}$${}^{+}$ cyclotron resonance taken with a 997-ms excitation time. The solid line is a fit of the theoretical line shape [29] to the data.

Figure 4

Decrease in the measured cyclotron frequency of ${}^6\mathrm{Li}$ and ${}^7\mathrm{Li}$ due to the relativistic correction as a function of ${\rho }_{-,\mathrm{ini}}$. When ${\rho }_{-,\mathrm{ini}}({}^7\mathrm{Li})=1.12(3)$ mm and ${\rho }_{-,\mathrm{ini}}({}^6\mathrm{Li})=0.96(2)$ mm (circle), the shift in cyclotron frequency for ${}^6\mathrm{Li}$ and ${}^7\mathrm{Li}$ are similar. For the same value of ${\rho }_{-,\mathrm{ini}}({}^7\mathrm{Li})$, if ${\rho }_{-,\mathrm{ini}}({}^6\mathrm{Li})=1.44(2)$ mm (square), the cyclotron frequency of ${}^6\mathrm{Li}$ shifts down by 6 ppb.

Figure 5

Deviation of the four ${\overline{R}}_{\mathrm{meas}.}$ and ${\overline{R}}_{\mathrm{corr}.}$ presented in Table with respect to ${\overline{R}}_{\mathrm{tot}.\mathrm{rel}.\mathrm{corr}.}$, represented by the error band. The relativistic effect can be effectively corrected using Eq. (8) as all ${\overline{R}}_{\mathrm{corr}.}$ agree within error including the case ${\rho }_{-,\mathrm{ini}}({}^6\mathrm{Li},{}^7\mathrm{Li})=[1.44(2)$ mm, 1.12(3) mm] for which its ${\overline{R}}_{\mathrm{meas}.}$ was inconsistent with the other measurement.

Figure 6

Upper plot shows the variation in the cyclotron frequency of ${}^7\mathrm{Li}$ over a period of 16 h. The lower plot gives the standard deviation of differences between measured and interpolated values for increasing time interval between calibration. The linear regression gives the systematic error due to the linear frequency interpolation.

Figure 7

${}^6\mathrm{Li}$ mass excess measured by the JILATRAP [15], SMILETRAP [13] groups and the present work. The TITAN value is in good agreement with that of Ref. [13].