Scintillating nanoparticles (NPs) in combination with X-ray or γ-radiation have a great potential for deep-tissue cancer therapy because they can be used to locally activate photosensitizers and generate singlet oxygen in tumours by means of the photodynamic effect. To understand the complex spatial distribution of energy deposition in a macroscopic volume of water loaded with nanoscintillators, we have developed a GEANT4-based Monte Carlo program. We thus obtain estimates of the maximum expected efficiency of singlet oxygen production for various materials coupled to PS, X-ray energies, NP concentrations and NP sizes. A new parameter, ηnano, is introduced to quantify the fraction of energy that is deposited in the NPs themselves, which is crucial for the efficiency of singlet oxygen production but has not been taken into account adequately so far. We furthermore emphasise the substantial contribution of primary interactions taking place in water, particularly under irradiation with high energy photons. The interplay of all these contributions to the photodynamic effect has to be taken into account in order to optimize nanoscintillators for therapeutic applications.
The role of charge carrier trapping
in determining radioluminescence
(RL) efficiency increase during prolonged irradiation of scintillators
has been studied by using YPO4:Ce,Nd as a model material.
The Nd3+ ions act as efficient electron traps minimizing
the role of intrinsic defects. Different Nd contents were considered
in order to point out the correlation between the trap concentration
and the detected RL efficiency dose dependence. RL measurements as
a function of temperature clarified the role of the trap thermal stability
in determining the shape and the magnitude of such effect. We propose
also a model based on trap filling which is able to describe accurately
the complex processes which are involved.
A model of energy relaxation in alkali
halide scintillators doped with Tl-like activators is presented. Interaction
between thermalized charge carriers, their diffusion, and capture
by traps are considered. The model of energy relaxation suggested
in the work includes essential electron excited states in alkali halides
doped with Tl-like activators. Self-trapping of holes occurs in alkali
halides at LNT, giving rise to creation of self-trapped excitons (STEs).
Thallium-like activator impurity can act both as an electron or a
hole trap. Once both of the charge carriers are trapped by the dopant,
activator recombination channel comes to action. The model is verified
using CsI classical scintillation crystals doped with thallium and
indium ions in a range of concentrations from 10–4 to 10–1 mol %. Temperature dependences of the
STE and the activator-induced emission yield are measured as a function
of the activator concentration under continuous X-ray excitation.
A system of rate equations is used to simulate the applicability of
the model under different excitation conditions. Evaluation of the
parameters of the system is done for a numerical solution. The model
of energy relaxation suggested allows to explain energy losses in
CsI:A scintillators in a 10–300 K temperature range.
Full concentration range of Lu 2x Gd 2-2x SiO 5 (LGSO:Ce) crystals was grown by the Czochralski method. Dependence of scintillation properties on composition (х) in the range of solid solutions is established. It was determined that LGSO:Ce scintillation yield increases in the range 0.3<х< 0.8 and reaches 29000 phot/MeV at 60% of Lu in the host (x=0.6), and energy resolution improves up to 6.7 % at 662 KeV. The observed light yield increase, surprisingly high Ce
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