A low noise soft x‐ray appearance potential spectrometer has been constructed. It consists of a liquid nitrogen‐cooled silicon surface‐barrier diode that detects the variations in the emitted photon flux. A 500 Å thick Al window acts as a discriminator for low energy photons (visible light and the tail of the bremsstrahlung). The performance of the detector arrangement has been tested on clean Ni films and chemisorbed O on these films. It is shown that the detector behavesideally, i.e., the signal‐to‐noise ratio is limited by theshot noise in the photon fluxfor primary electron beam intensities in the range tested (30–600 &mgr; A) and for electron energies, 300–900 eV.