Direct Mass Measurement of the Four-Neutron Halo Nuclide ${}^8\mathrm{He}$

A high-precision Penning trap mass measurement of the exotic ${}^8\mathrm{He}$ nuclide ($T_{1/2}=119\mathrm{ms}$) has been carried out resulting in a reduction of the uncertainty of the halo binding energy by over an order of magnitude. The new mass, determined with a relative uncertainty of $9.2\times {10}^{-8}$ ($\delta m=690\mathrm{eV}$) is 13 keV less bound than the previously accepted value. The mass measurement is of great relevance for the recent charge-radius measurement of ${}^8\mathrm{He}$ [P. Mueller et al., Phys. Rev. Lett. 99, 252501 (2007).]. The ${}^8\mathrm{He}$ mass is the first result from the newly-commissioned Penning trap: TITAN (TRIUMF’s Ion Trap for Atomic and Nuclear science) at the ISAC (Isotope Separator and Accelerator) radioactive beam facility at TRIUMF.

Figure 1

The TITAN mass spectrometer setup at the TRIUMF-ISAC radioactive beam facility. The EBIT was bypassed for the experiment presented here. The inset shows a typical off-line TOF resonance curve for ${}^6\mathrm{Li}^{+}$. The solid line is a fit of the theoretical curve [21] to the data points.

Figure 2

Time-of-flight resonances recorded for ${}^8\mathrm{He}^{+}$. The solid line is a fit of the theoretical curve [21] to the data.

Figure 3

Deviation of the TITAN mass excess (ME) results with respect to the Atomic Mass Evaluation AME2003 [28]. Left: the three individual ${}^8\mathrm{He}$ measurements. Center: ${}^4\mathrm{He}$ measurements with ${}^6\mathrm{Li}^{+}$ as reference. Right: verification of accuracy by measurements of ${}^6\mathrm{Li}^{+}$ with ${}^7\mathrm{Li}^{+}$ as reference. The TITAN results with statistical error are given as squares, and the hashed area represents their weighted mean and statistical error. The AME2003 value is given as the zero line with the error shown by the shaded area. The SMILETRAP [26] measurement for ${}^6\mathrm{Li}$ is indicated with “ST”. Note the different scales.

Figure 4

Previous mass measurements of ${}^8\mathrm{He}$ (references are given in the figure), as compared to the new TITAN Penning-trap result. The hashed area shows the mass resulting from the evaluation of all prior data as a two-sigma error band [28].