Specific luminescence studies in plastic scintillators

Abstract

Using a thin-film scintillator detector direct measurements of relative specific luminescence ($\frac{\ensuremath{\Delta}L }{ \ensuremath{\Delta}x}$) and luminescence efficiency ($\frac{\ensuremath{\Delta}L }{ \ensuremath{\Delta}E}$) values have been obtained for an organic scintillator exposed to transiting light and heavy ions. These experimentally obtained luminescence data are presented as a function of ion energy, velocity, and stopping power for a wide variety of transiting ions ($Z=1,2,8,17,18,35,53$). The data emphasize the luminescence response in the low-energy range corresponding to maximum ion stopping power. Common trends, unique characteristics, and other interesting features pertinent to these graphical plots are noted and discussed. The double-valuedness of the specific luminescence $\frac{\ensuremath{\Delta}L }{ \ensuremath{\Delta}x}$ when plotted as a function of specific energy loss $\frac{\ensuremath{\Delta}E }{ \ensuremath{\Delta}x}$ is interpreted as arising mainly from the decreased effective charge of the ion as it picks up electrons in the slowing-down process. It is further emphasized that the specific energy-loss per se is an inappropriate parameter for characterizing (in a direct way) the luminescence response. A new model is formulated based on a conceptually different approach to the problem of luminescence production along and about the path of a transiting energetic ion. In essence, the number of electrons scattered into a thin disk of scintillator material (perpendicular to the ion trajectory) is taken, in the absence of saturation effects, as proportional to the luminescence response from that disk. Saturation effects are included by deriving an explicit relation for the number of electrons scattered per unit area of disk at a distance $r$ from the ion path. Above a critical number per unit area, no additional luminescence response is generated. These number-density profiles are plotted for various ions of different energies. Finally, a plot of the calculated specific luminescence for various heavy ions is shown to be in remarkably good agreement with experimental data.