or another relatively short range beta emitter such as 14 c or 35 s10 and 110 significantly larger or denser scintillant latex particles 11 and 111 are represented before and after centrifugation the antigen or antibody coatings 12 and 112 and the reactant under assay 13 and 113 are also represented before and after centrifugation the arrows 14 and 114 represent photons produced in the scintillation particles by radiation from the attached radioactive particles photons whose paths terminate within the medium or do not otherwise arrive at the detectors 15 or 115 will of course not be recorded the arrows 16 and 116 represent background photons arising from the residual fluorescence still present even in the tinuvin p doped polystyrene particles described in u s pat no 4 388 296 the arrows 17 and 117 represent the bottom region of the counting vial since the radioactive latex particles are much smaller than the scintillant latex particles a higher speed of centrifugation is required to spin down those radioactive particles which are freely suspended in the medium than the larger scintillant particles if after appropriate incubation the counting vial is centrifuged at a speed sufficient to spin down the scintillant particles but insufficient to spin down the freely suspended radioactive particles it follows that essentially only those radioactive particles which are bound to scintillant particles will be concentrated in region 117 measurement of the photons arriving at the detectors 115 from the restricted region 117 results in several improvements in sspa first of all since the signal producing dimers are now localized within the relatively small region 117 the system detectors can be grouped around this region maximizing the geometric efficiency of signal detection secondly the effect of optical absorption within the medium is grossly reduced.