Lifetime of a Negative Helium Ion

The lifetime of a negative ion of helium produced from He⁺ by charge exchange in helium gas is measured and found to be 18.2±{}2.7 {μ}sec. This is consistent with the lower limit of 10 {μ}sec previously determined. Total loss cross-section measurements in the energy range 20-70 keV are reported and compared with a previous published value at 17.5 keV. An investigation is also made of the destruction of the negative ions by thermal photons; this is found to be negligible at the temperatures involved, enabling an upper limit of 2.0×{}10^-15 cm² to be set on the photodetachment cross section of He^-.