Narrow beam emission of slow positrons from negative affinity surfaces

We have measured the angular distribution of the slow positrons (e⁺·) from negative work function (φ⁺) surfaces of Cu and Al bombarded by keV e⁺ in ultrahigh vacuum. In analogy with √-point electrons emitted from negative electron affinity surfaces, the majority (>50%) of the slow e⁺ leave the surface with energy ≈ φ⁺ and velocity within ∼20° of the surface normal. This agrees qualitatively with the predictions of a simple 1D model.