A campaign of pure helium discharges in the EFDA-JET tokamak equipped with the MarkII GB divertor has been performed. This paper describes some of the experimental observations of divertor detachment obtained in L-mode density ramp He discharges and presents a selection of results from the application of B2.5-Eirene code modelling to a JET He plasma. Detachment is very different from that observed in equivalent D discharges. Particle fluxes remain attached up to higher densities than in comparable D discharges. With decreasing input power, particle detachment occurs earlier in density, but the low measured target Te at all but the lowest ne, leads to an earlier decrease in power flux. Simulations show that detachment is caused by the escape of He neutrals from the target vicinity where, together with the He+ ions, they radiate along the separatrix and above the X-point area starving regions downstream of power.