Abstract: A method of investigating the response of an instrument to gamma emissions using a source package includes providing a source package on one side of a collimator including an aperture and a detector on the other side of the collimator relative to the source package, the collimator and detector positions defining an operating axis for the instrument. The position of the source package is then moved relative to the collimator. The response of the detector is measured for two or more of these different positions of the source package. The aperture in the collimator has a first dimension measured in a first direction perpendicular to the operating axis. The source package includes one or more individual gamma emitting sources provided in a container. During use, the individual source(s) have an overall extent in the first direction, the overall extent, during at least 10 seconds of use, being two or more times the first dimension of the collimator in that first direction.