In situ observation of the Yb2+ emission in the radiodarkening process of Yb-doped optical preform.

This Letter relates the clear evidence of Yb2+ formation under 2.5 MeV electron irradiation in optical fiber preforms showing a darkening of the core. We thus detected by in situ photoluminescence measurements the green emission of divalent Yb2+ under the 355 nm excitation. Moreover, we showed the existence of two types of Yb2+ ion species with different stabilities. We demonstrated that the radiodarkening mechanism is based on a pair association of Yb2+ with aluminum oxygen hole center point defects.

Kerma rate effects on thermoluminescent response of CVD diamond.

The thermoluminescence (TL) response of chemical vapour deposition diamond is studied after X-irradiation (45 kV) at room temperature for various air kerma rates ranging from 0.7 to 100 Gy.min(-1). For a given air kerma, the TL signal as a function of air kerma rate exhibits large variations and is higher for low air kerma rates than for higher ones. In order to explain these results, the simplest energy-level scheme has been considered, that is, with only one electron trap and one recombination centre. Kinetic equations are numerically solved. Calculations are made for various electron-hole generation rates and show that the trap filling as a function of electron-hole generation rate follows, actually, the same evolution as the one of experimental TL response. The kerma rate effect on TL response is explained by the competitivity between the recombination and the trapping processes during irradiation.

Optical bleaching, TSL and OSL features of CVD diamond.

Luminescence and optical features of chemical vapour deposition (CVD) diamond have been studied in view of the potential application of this material in ionising radiation dosimetry field. For this purpose, thermally stimulated luminescence (TSL) and optically stimulated luminescence (OSL) techniques have been used. A large amount of work has emphasised the excellent dosimetric properties of CVD diamond. Nevertheless, TSL measurements showed that after irradiation, this material is extremely sensitive to ambient light and the stored dose information is drastically affected by optical bleaching. From OSL analysis, it follows that both types of processes (TSL and OSL) were characterised by the same excitation and emission spectra and that optical bleaching originated from a broad stimulation band lying from visible to near infrared with a continuous character.

The electron energy dependence of the TL response for CVD diamonds.

The thermoluminescent (TL) response of CVD diamond is investigated after bombarding a sample with monoenergetic electrons (at 6 and 21 MeV). Irradiations are performed at room temperature with a medical linear electron accelerator PRIMUS (KD2-Siemens). The TL curve shows a peak at approximately 540 K. The area of this peak as a function of fluence exhibits saturation behaviour at high energies. Moreover, there is no significant difference between the TL response at 21 MeV electrons and that at 6 MeV. In order to explain these experimental results, calculations of the ionising dose have been performed with the code MCNPX (Monte Carlo N-Particle) for various incident energies from 0.1 MeV up to 100 MeV.

Thermoluminescence properties of CVD diamond for clinical dosimetry use.

The application of diamond to dosimetry is desirable because of its tissue equivalence, chemical inertness and small size, but this has not been commercially viable owing to the non-reproducible response of natural diamond. The chemical vapour deposition (CVD) of diamond permits controlled, reproducible and large-scale production of this material at potentially low cost. An investigation of some clinically relevant features like the depth-dose distribution as well as the absorbed dose profile, obtained using thermoluminescence (TL), is reported for several CVD diamond films. The TL characterisation presented here shows that CVD diamond films should be excellent TL-mode detectors in instances of radiotherapy and in vivo radiation dosimetry.

The incorporation site of Er in nanosized CaF2.

The incorporation site of Er dopants inserted at high and low concentration (respectively 5 and 0.5 mol%) in nanoparticles of CaF2 is studied by x-ray absorption spectroscopy (XAS) at the Er L III edge. The experimental data are compared with the results of structural modeling based on density functional theory (DFT). DFT-based molecular dynamics is also used to simulate complete theoretical EXAFS spectra of the model structures. The result is that Er substitutes for Ca in the structure and in the low concentration case the dopant ions are isolated. At high concentration the rare earth ions cluster together binding Ca vacancies.

Neutron absorbed dose determination by calculations of recoil energy.

The aim of this work is to calculate the absorbed dose to matter due to neutrons in the 5-150 MeV energy range. Materials involved in the calculations are Al2O3, CaSO4 and CaS, which may be used as dosemeters and have already been studied for their luminescent properties. The absorbed dose is assumed to be mainly due to the energy deposited by the recoils. Elastic reactions are treated with the ECIS code while for the non-elastic ones, a Monte Carlo code has been developed and allowed to follow the nucleus decay and to determine its characteristics (nature and energy). Finally, the calculations show that the absorbed dose is mainly due to non-elastic process and that above 20 MeV this dose decreases slightly with the neutron energy.

Influence of the irradiation temperature on the dosimetric and high temperature TL peaks of Al2O3:C.

The TL glow curves of Al2O3:C crystals have been investigated as a function of the irradiation temperature. The nature of the observed TL peaks has been studied by optical annealing. The filling of traps was found strongly dependent on the irradiation temperature in the case of UV exposure, which has been explained by the temperature dependence of the photoionisation of F centres. This latter phenomenon could have a part in the luminescence quenching and UV bleaching of F centres.

Characterisation of thermoluminescent LiF:Mg,Ti (MTS-N) detectors in the 85-550 K temperature range.

Miniature LiF:Mg,Ti (MTS-N) pellets, specially designed for dosimetry in proton radiotherapy, were studied in the 85-550 K temperature range. TL glow curves for differently Mg- or Ti-doped detectors were recorded, showing the preponderant influence of Mg concentration on the intensity of peak 5. The presence of a three-peak complex was also confirmed near 140 K; spectral emission of this peak consists of two bands whose maxima occur at 290 and 420 nm. TSEE glow curves above 300 K similar to those of TL were observed. The behaviour of TSEE and TL peak 5 following an X ray exposure from 33 to 500 Gy was studied: its intensity is a linear function of air kerma in both cases.

Thermoluminescent properties of Ni and Co doped synthetic, high pressure, high temperature diamonds: application to ionising radiation dosimetry.

An investigation of the thermoluminescence (TL) properties of high pressure, high temperature (HPHT) synthetic diamond crystals grown under diluted nickel or cobalt as solvent catalysts is reported. After a study of TL properties of 6 different samples, it is shown that a crystal grown with Ni+2%Ti and annealed at 2100 K presents an intense glow peak at around 490 K. This peak is characterised by a broad emission band centred at 530 nm (2.34 eV). This crystal presents a significant, reproducible and linear TL response relative to the absorbed dose up to an X ray air kerma of 10 Gy. All these features make this material suitable for ionising radiation dosimetry. A similar study is made on another crystal grown from pure Co, and a comparative review of the results does show that for dosimetry work, Ni-containing diamonds are more appropriate than those grown from Co catalyst.

On the luminescence properties of CaSO4:Ce.

The luminescent properties of cerium doped calcium sulphate are studied: fluorescence and excitation spectra, optical absorption and thermoluminescence (TSL). It is known that, in rare earth doped CaSO4, only cerium induces a strong 400 degrees C TSL peak. In CaSO4:0.2%Ce samples synthesised under oxidising conditions, the recovery step of Ce3+ fluorescence is correlated with the 400 degrees C TSL peak readout, as mentioned by Nair er al. Our results indicate that an oxidation of Ce3+ ion does occur under X-irradiation (Ce3+ --> Ce4+), followed by a complete return to the trivalent state after thermal annealing at about 500 degrees C; our results confirm the hypothesis of Nair et al that Ce3+ ions are oxidised by ionising irradiation. So, a pure redox reaction seems the most probable for the 400 degrees C TSL peak of CaSO4:Ce. Moreover, the use of the 400 degrees C TSL peak for high temperature dosimetry applications is discussed.