A simple determination of the halo size from Be-10/Be-9 data

Context. The AMS-02 and HELIX experiments should soon provide Be-10/Be-9 cosmic-ray data of unprecedented precision. Aims. We propose an analytical formula to quickly and accurately determine L from these data. Methods. Our formula is validated against the full calculation performed with the propagation code USINE. We compare the constraints on L set by Be/B and Be-10/Be-9, relying on updated sets of production cross-sections. Results. The best-fit L from AMS-02 Be/B data is shifted from 5 kpc to 3.8 kpc when using the updated cross-sections. We obtained consistent results from the Be/B analysis with USINE, L = 3.8(-1.6) (+2.8) kpc (data and cross-section uncertainties), and from the analysis of Be-10/Be-9 data with the simplified formula, L = 4.7 +/- 0.6 (data uncertainties) +/- 2 (cross-section uncertainties) kpc. The analytical formula indicates that improvements on L thanks to future data will be limited by production cross-section uncertainties, unless either B-10/Be-9 measurements are extended up to several tens of GeV/n, or nuclear data for the production of Be-10 and Be-9 are improved; new data for the production cross section of O-16 into Be isotopes above a few GeV/n are especially desired.