The extended track interaction model: supralinearity and saturation He-ion TL fluence response in sensitized TLD-100

The dependence of the thermoluminescence (TL) dose response on ionization density continues to be a subject of major importance from both theoretical and applied considerations. The supralinearity of heavy charged particle (HCP) TL-fluence response has been previously mathematically modelled in the framework of the track interaction model (TIM) and incorporated into the unified interaction model (UNIM) which treats all particle and radiation species in a unified mathematical and conceptual framework. In this paper we discuss an extended track interaction model (ETIM), developed to describe supralinearity, saturation, and sensitization in TL response to heavy charged particles (HCPs), and we apply the model to peak 5 in sensitized LiF : Mg, Ti (TLD-100). Track-segment HCP radial dose profiles for He ions slowing down in condensed phase LiF are calculated via Monte Carlo techniques. Defect occupation probabilities as a function of radial distance from the track axis are calculated from the appropriate optical absorption gamma dose response for the trapping centers (TCs) and sensitization gamma dose response for the luminescent centers (LCs). Saturation in HCP TL-fluence response is described as arising from the overlap of track cores in which the defect centers are heavily occupied. The track cores are characterized by two parameters, r100 and r50, defining the radial distance from the HCP track axis up to which defect centers are fully occupied and 50% occupied, respectively. From measurements of the He ion-TL fluence response for peak 5 in sensitized LiF : Mg, Ti (TLD-100), it is demonstrated that r50 equals 38 nm for 6.8 MeV He ions appropriate to a value of the dose filling constant of β=6x10-3 Gy-1 compared to values of β=0.9x10-3 and 3.1x10-3 Gy-1 for the TCs and LCs, respectively. This high value of β suggests that the TL fluence saturation of peak 5 arises from full occupancy of the available LCs and not the TCs. Further studies are required to analyze the possible dependence of the optical absorption dose filling constants on ionization density and on the level of occupation of the competing centers