A 25-MA ISOTOPE SHIFT IN B-I AT 2090-A - A POSSIBLE DIAGNOSTIC OF THE COSMIC B-11 B-10 RATIO

We present the first laboratory measurement of the isotope shift in a ground term transition of neutral boron, B I, performed with the VUV-Fourier Transform Spectrometer at Lund. The two diagonal transitions in the UV 2 multiplet of B I at 2090 angstrom have a net isotope shift of 25 mangstrom. This matches a spectral resolution of about 85,000, obtainable with the high-resolution spectrograph GHRS on board the Hubble Space Telescope. The oscillator strength of the strongest line at 2089.6 angstrom is about the same as for the weakest line in the UV 1 multiplet at 2496.8 angstrom, often used in abundance studies of boron. Furthermore, the lines at 2090 angstrom are outside the compact resonance region of singly ionized iron-group elements between 2300 and 2700 angstrom. A preliminary study of a synthetic spectrum in this region indicates that the 2089.6 angstrom line is free from blends of lines from high-abundance elements. We propose the use of the B I lines at 2090 angstrom for studies of the abundance and the cosmic isotope composition of neutral boron to test the theories for boron production by big bang nucleosynthesis and cosmic-ray spallation.