Apparatus for the half‐life measurements of x rays from fission fragments is described. The fission fragment detector was an ultrathin plastic scintillator and the x‐ray detector was a 1 mm NaI (Tl) crystal. Both detectors were mounted on fast (56 AVP) photomultipliers. The details of the design of the plastic scintillator are given and curves showing the double humped fission fragment kinetic energy spectrum are illustrated. A systematic study of the x‐ray energy detection characteristics of the NaI (Tl) on a 56 AVP is given. The pulse heights were determined to be linear with x‐ray energy and the spectra were shown to follow Poisson's statistics. The number of photoelectrons produced at the photocathode was determined to be 3.2 per kiloelectron volt. The time measuring device was a conventional time to amplitude converter using pulse overlap methods. The prompt response of this converter could be predicted from the results gained in the energy detector study. Both the width and the apparent half‐life were consistent with the assumption that the time to amplitude converter was triggered on a single photoelectron.