A special mass-spectrometric method has been developed to determine the isotopic ratios and isotopic absolute cross sections of Li, Be, and B, formed by spallation of oxygen irradiated by protons of energy 135, 550, and 19 GeV. A brief description is given of the ion source of the mass spectrometer—of an ion sputtering type—which permits a very efficient ionization of beryllium and boron. The performances are given; they allow the analysis of ${10}^{-12\mathrm{}}$ to ${10}^{-13\mathrm{}}$ g of these elements. The oxygen target is made of very high purity water, and special handling techniques have been worked out to reduce contamination to an acceptable level. The determination of each of the isotopic ratios ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$/${}_{}{}^6{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$/${}_{}{}^7{}_{}{}^{}\mathrm{Be}$, ${}_{}{}^{10}{}_{}{}^{}\mathrm{Be}$/${}_{}{}^9{}_{}{}^{}\mathrm{Be}$, and ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$/${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$ requires a specific method, which is described. Isotope dilution techniques are used to obtain the absolute cross sections; values in millibarns for 135-MeV protons are as follows: ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$:10±2, ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$:8.0±2.5, ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$:1.7±0.5, ${}_{}{}^{10}{}_{}{}^{}\mathrm{Be}$:0.35±0.20, ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$:11±3, and ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$:25±8. There are no significant changes at the higher energies. The above cross sections give support to conclusions based previously on calculated values concerning the spallation origin of Li, Be, and B in the solar system, in stars, and in the cosmic radiation.

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