Naturally occurring erbium (68Er) is composed of 6 stable isotopes, with 166Er being the most abundant (33.503% natural abundance). 39 radioisotopes have been characterized with between 74 and 112 neutrons, or 142 to 180 nucleons, with the most stable being 169Er with a half-life of 9.4 days. It is used as a radiopharmaceutical. All of the remaining radioactive isotopes have half-lives that are less than 3.5 hours, and the majority of these have half-lives that are less than 4 minutes.
30 minute measurement with a High Purity Germanium (HPGe) radiation detector.