Luminescence Properties of NaI:Tl and CeBr3 with New Codopants

The effect of new dopants on the luminescence properties of two types of scintillator crystals have been investigated. First, the luminescence properties of sodium iodine (NaI) crystals doped with thallium (Tl) and co-doped with Sulpher (S) and selenium (Se) were determined. Tl occupies cation sites whereas S and Se are supposed to occupy anion sites in the NaI crystal lattice. Doping with both S and Se led to colouring of the crystals and were not dissolved into the NaI crystal. The light yield and resolution of these crystals were reduced compared with NaI doped with only Tl (NaI:Tl). A fast light intensity decay component was observed for low temperatures (77 – 150 K) when exciting the NaI:Tl crystal with relatively low energy X-rays ($<$40 keV). This fast component may be responsible for the increase in light yield when going from 150 to 77 K. Next, the luminescence properties of cerium bromide (CeBr$_3$) crystals co-doped with cadmium (Cd), yttrium (Y), lutetium (Lu), and double co-doped with strontium (Sr) and sodium (Na), and strontium (Sr) and calcium (Ca). All co-dopants when dissolved occupy cation sites in the CeBr$_3$ crystal lattice. The Ce$^{3+}$ cations act as a luminescence centre. The CeBr$_3$ with isovalent co-dopants (Y, Lu), showed perturbation of the Ce$^{3+}$ doublet and light traps where this was not expected. The improvement of the light yield at low temperatures is stronger for the CeBr3:Lu crystal compared with that of the undoped and the Y co-doped CeBr$_3$ crystals, suggesting that Lu is better dissolved than Y into the CeBr$_3$ crystal. The divalent co-dopant Cd results in a strong perturbation of the Ce$^{3+}$ doublet and so-called deep traps in the CeBr$_3$ crystal. This causes a weak thermoluminescence intensity as well as low light yield, which strongly improve with decreasing temperature. The double co-doped CeBr$_3$:Sr,Ca crystal showed the strongest perturbation of the Ce$^{3+}$ doublet and highest thermoluminescence below 200 K. This is reflected by the decrease of the light yield in the temperature range of 77 to 200 K. The double co-doped CeBr3:Sr,Na crystal show a weak perturbation of the Ce$^{3+}$ doublet and a weak thermoluminescence due to a low trapping probability for electrons. The CeBr$_3$ crystal with Sr,Ca displays the most proportional behaviour and the highest resolution as well as light yield. The crystal with Sr,Na as co-dopants, on the contrary, has a low resolution and low light yield. The isolvalent co-dopants Y and Lu both showed a moderate proportional behaviour and a low resolution, but Y resulted in a low whereas Lu in a high light yield. The CeBr$_3$ with Lu as co-dopant shows an anomaly at 8 keV in the non-proportionality response curve (proportionality equals 40\%). Cd as a co-dopant displayed the lowest light yield and a low resolution, but a reasonable proportionality. The low resolution of all co-dopants except for Sr,Ca were caused by inhomogeneity of the crystals.Applied SciencesApplied Physic