The GALLEX experiment at the Gran Sasso laboratory measured the ve flux from the Sun by counting the electrons produced in the reaction v_e + ⁷¹Ga → ⁷¹Ge + e^–. Its energy threshold is E_th = 233 keV. From the solar luminosity one finds the expected neutrino flux Φ = 6 x 10¹⁴m^-2s^-1. For a rough calculation, assume the whole flux to be above threshold and the average cross-section to be σ = 10^-48 m². Assuming the detection efficiency ε = 40%, how many ⁷¹Ga nuclei are necessary to have one neutrino interaction per day? What is the corresponding ⁷¹Ga mass? What is the natural gallium mass if the abundance of the ⁷¹Ga isotope is a = 40%? (The measured flux turned out to be about one half of the expected one. This was a fundamental observation in the process of discovering of neutrino oscillations.)