A new phase growth in LiF: Ti and LiF: Ti,Mg

Abstract

ITC measurements, optical absorption spectra in the wavenumber range 5.4 × 10⁴?200 cm^?1, microspectrophotometry, photoluminescence spectra, optical and electron microscopy and X-ray diffraction have been used to study the growth and the nature of a new phase occlusions in LiF: Ti³⁺ and LiF: Mg²⁺, Ti³⁺. They grow along the 100 direction as a consequence of high temperature heat treatments in moist atmosphere.     

Dose response of thermoluminescence emission spectra of LiF:Mg,Ti with different Mg, Ti impurity concentrations

The dose response of the TL emission spectra of an LiF:Mg,Ti (TLD-100) sample and three LiF:Mg,Ti samples with different impurity concentrations (0–6 ppm Ti and 80–100 ppm Mg) have been measured. At a dose less than 22 Gy the emission spectrum of the TLD-100 sample comprises one emission band at 420 nm. The sample without Ti shows also one emission band but now at 620 nm. The spectra of the other two samples comprises two emission bands at 420 nm and 620 nm of which the intensity of the 420 nm band increases with increasing Ti concentration. The dose response of the glow peaks is different for peaks at different temperatures and emission bands. From these observations it can be concluded than in LiF:Mg,Ti at least some of the traps and luminescent centers are coupled.     

Behaviour of LiF:Mg,Cu,P and LiF:Mg,Ti thermoluminescent detectors for electron doses up to 1 MGy

The behaviour of LiF:Mg,Cu,P and LiF:Mg,Ti detectors at ultra-high doses up to 1 MGy, has been investigated. The presence of the ultra-high-temperature peak (450 °C) of reproducible properties was observed in various batches of LiF:Mg,Cu,P, confirming earlier findings. The results indicate that this peak is not an effect of random impurities nor intrinsic effects of LiF, but it is rather connected with the doping.A parameter called ultra-high temperature ratio (UHTR) was defined in order to quantify the observed changes of LiF:Mg,Cu,P glow-curve shape at very high doses and very high temperatures. The use of this parameter allows to determine an absorbed dose in the range from 1 kGy to 1 MGy. This new method of high-dose dosimetry makes LiF:Mg,Cu,P a unique dosimeter, which is capable to cover at least 12 orders of magnitude of dose range: from a microgray to a megagray.     

Kinetic modeling of thermoluminescence,optical absorption and bleaching in LiF:Mg,Ti

In this paper we describe some aspects of our recent work which treats via kinetic simulations the experimentally observed linear/supralinear/saturating dose response of the thermoluminescence, optical absorption and behavior of optical bleaching at various photon energies in irradiated LiF:Mg,Ti (TLD-100) system in a physically realistic scenario. The results of the simulations are in good agreement with the experimental observations and this can be considered as a validation of the details of the proposed models.     

Dose reassessment by using PTTL method in MTS-N (LiF:Mg,Ti) thermoluminescent detectors

The thermoluminescent (TLD) method is one of the most commonly used in dose measurements in radiation protection dosimetry. Due to its many advantages this method is widely spread. However, TLD has especially one disadvantage which is very inconvenient: the dose information in already read detectors is erased and in routine standard way the dose can not be reassessed. The positive is that this shortcoming can be eliminated by applying UV radiation. After first readout the same detector can be subjected to UV exposure and then read once again to reassess the dose.This method for reassessment of dose is based on phototransferred thermoluminescence (PTTL). In an irradiated TL detector deep traps are not emptied during the first readout. During exposure to UV, electrons are transferred from deep traps to shallower dosimetric traps. This TL signal emerging during the second readout following UV illumination is called phototransferred thermoluminescence.A method for reassessment of dose in a previously readout TLD is presented in this work. Experiments show that the method works well within region of doses between 5 and 50 mGy, but could be applied for higher doses as well. The efficiency of dose reassessment reaches about 17 percent of the first readout.The method could be a noticeable improvement in TLD dosimetry, giving more opportunities for better control and reliability of measurements.     

Microstructural changes induced in borosilicate glasses by 1 MeV electrons

Radiation damage to borosilicate glass has been studied using a high voltage electron microscope to simultaneously generate and image structural changes At low doses and dose rates (φ ? 5 × 10²⁴ electrons m^-2, F ? 5 × 10²² el m^-2 s^-1), ionic depletion generates a new crystalline phase rich in S₁O₂ For incubation doses exceeding 5 × 10²⁴ electrons m^-2, gas bubbles are observed High damage rates are necessary for bubble nucleation though not for their subsequent growth The critical nucleation flux increases rapidly with irradiation temperature, whereas the gas precipitation efficiency remains constant above 300 K.     

Thermo- and photostimulated luminescence in LiF : Mg,Ti single crystals irradiated by ions and VUV light

A coordinated study of the relaxation of optical absorption induced by vacuum ultraviolet radiation, x-rays, and α-particles, as well as of photo- and thermostimulated luminescence (TSL) of LiF : Mg, Ti crystals (TLD-100) in the 295–750-K interval, has revealed that TSL regions characterized by activation energies E _a = 2.2–2.4 eV and anomalously high frequency factors p ₀ = 10²¹–10²² s^?1 alternate with regions where E _a = 1.5 eV and p ₀ = 10¹²–10¹⁴ s^?1. The relative intensities of the TSL peaks produced by UV illumination (10–17 eV) differ strongly under the conditions of selective photon-induced generation of anion excitons, free electrons and holes, or near-impurity electronic excitations. The latter are responsible for the high efficiency of tunneling radiative (involving titanium centers) or nonradiative (involving hydroxyl ions) recombination. The analysis of TSL peaks of LiF: Mg, Ti and LiF took into account two-step processes, namely, thermal dissociation of three-fluorine F ₃ ^? molecules and recombination of the products of their decay (V _K and V _F centers, H interstices).     

Study of Optically Stimulated Luminescence of LiF:Mg,Ti for beta and gamma dosimetry

Modern advances in radiation medicine – radiodiagnosis, radiotherapy and interventional radiography – each present dosimetry challenges for the medical physicist that did not exist previously. In all of these areas a constant balance has to be made between the treatment necessary to destroy the tumor and the unnecessary exposure of healthy tissue. Innovative applications of OSL dosimetry are now appearing in each of these areas to help the medical physicist and oncologist design the most effective, and least deleterious, treatment for their patients. High sensitivity, precise delivery of light, fast readout times, simpler readers and easier automation are the main advantages of OSL in comparison with TLD.This work aimed to study the application of OSL technique using lithium fluoride dosimeters doped with magnesium and titanium (LiF:Mg,Ti) for application in beta and gamma dosimetry.     

LiF:Mg,Ti (MTT) TL detectors optimised for high-LET radiation dosimetry

The properties of LiF:Mg,Ti (distributed as, e.g., TLD-100 or MTS-N), the most frequently used thermoluminescent detector, have been optimised for measurements of sparsely ionising radiation (gamma rays), typically encountered in radiation protection or clinical dosimetry. However, these detectors need also to be applied in conditions of mixed-field dosimetry with a high-LET component, such as those encountered in heavy ion beams or in space.

At the Institute of Nuclear Physics in Kraków a new type of LiF:Mg,Ti detector (named MTT) has been recently developed through modification of its dopant composition. This composition is intended to increase the detection efficiency after a dose of high-LET radiation. The concentration of dopants in the MTT material is: C_Mg=50 ppm, and C_Ti=120 ppm, i.e. about a three times less of magnesium and about 10 times more of titanium content, compared with the standard MTS-N. The MTT TL detectors feature an increased relative efficiency to high-LET radiation, which for 5 MeV alpha-particles is about twice that of standard LiF:Mg,Ti. The response of MTT detectors has been studied in charged particle beams of the HIMAC accelerator in Chiba, Japan and in Dubna, Russia. The main foreseen application of MTT detectors are dose measurements in space. The dose after high-LET exposure can be estimated from the difference of the response of MTS and MTT detectors. In the near future MTT detectors will be applied in the “Matroshka” experiment. Within this experiment a specially constructed human phantom will be exposed in free space (outside the International Space Station) for 1 year. The phantom will incorporate a few thousand measuring points enabling radiation doses to particular organs to be determined.     

Disintegration of the deuteron by 4 to 11 MeV electrons

The ratio of photodisintegration to electrodisintegration yields was measured in the 4 to 11 MeV region. The results are in good agreement with a virtual photon calculation.     

Angular distributions of planar-channeled 4 MeV electrons

Abstract

A method of calculation of angular distributions of channeled electrons is proposed in the paper. The depth dependence of the angular distribution is investigated. At large penetration depths, the planar channeled particles are seen to spread widely parallel to the atomic planes, while the transverse motion is more confined.     

A study on thermal degradation of LiF:Mg,Cu,P and LiF:Mg,Cu,Si

We investigated the thermal degradation of LiF:Mg,Cu,P (NTL-250) and LiF:Mg,Cu,Si (MCS) for the development of TL sheet. By thermogravimetry and differential scanning calorimetry (TG-DSC), the exothermic reaction was observed between 320 °C and 400 °C in MCS as well as NTL-250. The heat value of MCS was twice as large as that of NTL-250. This ratio corresponded with that of Mg amount in these TL materials measured by ICP-OES (inductively-coupled plasma optical emission spectrometry). X-ray diffraction (XRD) measurements were also carried out, and the peaks of MgF₂ phase were also observed in degraded MCS sample as well as NTL-250. Moreover, X-ray absorption near-edge structures (XANES) of Cu in these LiF TLDs were measured. The valences of Cu did not change before and after degradation. It indicates that the thermal degradation is caused by not Cu but Mg ion state change. The exothermic reaction is possible caused by the stabilization reactions, and then it was expected to correspond with MgF₂ precipitation. From these results, we concluded that the thermal degradations of these LiF TLDs are caused by the precipitation of MgF₂.     

Radioluminescence of ionized electron color centers in LiF crystals

Photoluminescence (PL) and temporal variation of optical absorption and radioluminescence (RL) of LiF crystals after irradiation with an electron impulse are investigated by pulse spectrometry methods using different irradiation regimes and different degrees of initial radiation damage. The difference in the RL and PL characteristics of ionized F ₂ ⁺ and F ₃ ⁺ centers is revealed. Several mechanisms for inducing these centers by irradiation, which differ in energy and kinetic parameters and in temperature dependence, are proposed. It is established that the ionized centers in the radiation active state are created due to the interaction of the respective neutral centers with holes of different thermalization extent. A mechanism for the excitation of these radiation-active centers is proposed.     

Investigation of the optical absorption characteristics of slow-cooled LiF:Mg,Ti (TLD-100)

The optical absorption (OA) spectrum of LiF:Mg,Ti has been studied as a function of dose at two different cooling rates following the 400 °C pre-irradiation anneal in order to further investigate the role of cooling rate in the thermoluminescence (TL) mechanisms of this material. “Slow-cooling” following the pre-irradiation 400 °C anneal substantially decreases the OA bands at 3.25 eV and 4.0 eV, in agreement with the overall loss in TL peaks 2–5 intensity using slow-cooling routines. Slow-cooling appears to shift the maximum intensity of peak 5 to lower temperatures (a behaviour which has been attributed to an enhanced intensity of peak 5a), however, no difference in the shape of the 4.0 eV OA band is detected following “slow-cooling”. Apparently the OA band related to peak 5a is too close in energy to the peak 5 OA band to be observed due to lack of sufficient resolution and spectral deconvolution process or it is not present at room temperature (RT) and formed during heating of the sample. The intensity of the 4.0 eV OA band does not change if the sample (prior to irradiation to a standard dose of 200 Gy) is irradiated to 4 kGy followed by a 500 °C/1 h post-irradiation anneal. This result demonstrates that the loss of intensity at high levels of dose (so-called radiation damage) of TL glow peak 5 results from alteration of the LCs or to the creation of additional competitive centers and is not correlated with the dose behaviour of the TCs.