Towards Laser Spectroscopy of Boron-8

The BOR8 experiment aims at the determination of the nuclear charge radius of boron-8 with high-resolution laser spectroscopy. 8B is perhaps the best candidate of a nucleus exhibiting an extended proton wave-function or one-proton-halo. Strongest evidence for the halo character of the proton comes from the unusually large quadrupole moment of this proton-rich nucleus as measured by β-NMR [1]. The most decisive observable, the nuclear charge radius, which is directly correlated with the extent of the proton wave function, can be extracted in a model-independent way from the measured isotope shift along the boron isotopic chain. Atomic theory calculations of the five-electron system, which were recently carried out [2], pave the way for targeting neutral boron atoms, whose spectroscopic properties are well suited for such measurements. 8B is produced in-flight via the 6Li(3He,n)8B reaction in a liquid-nitrogen cooled 3He gas target, and subsequently thermalized in a Helium gas catcher system. This technique is already established at Argonne National Laboratory (ANL), where the the primary 6Li beam at 45 MeV is provided by the Argonne Tandem Linac Accelerator System (ATLAS). We will employ a collinear laser spectroscopy technique to measure the isotope shifts. The secondary low-energy (~10 keV) 8B beam extracted from the gas catcher will be overlapped with two laser beams to correct for the large Doppler shifts inherent in the collinear technique and to extract the boron isotope shift with the required precision. In a first off-line experiment at TU Darmstadt, the isotope shift of the stable isotopes 10,11B has been measured with resonance ionization mass spectrometry. This provides a valuable test not only of atomic theory, but also of experimental equipment which will later be used at ANL. This work is supported by the U.S. DOE, Office of Science, Office of Nuclear Physics, under contract DE-AC02-06CH1135, and by the Deutsche Forschungsgemeinschaft through Grant SFB 1245. [1] Sumikama, T., et al.: Phys. Rev. C 74, 024327 (2006) [2] Puchalski, M., et al.: Phys. Rev. A 92, 062501 (2015)