Powder X-ray thermodiffraction study of mirabilite and epsomite dehydration. Effects of direct IR-irradiation on samples.

This paper investigates the thermal and irradiation-dependent dehydration and kinetics occurring in Na2SO4.10H2O (mirabilite) and MgSO4.7H2O (epsomite) at room conditions by using powder X-ray thermodiffraction. An improved version of a first optically stimulated X-ray diffractometer prototype was used. Specific software for the thermodiffraction study was developed (XPowder PLUS) and a filter inserted between the lamp (heating system) and the sample. The results show that these salts are thermal and irradiation sensitive. The temperature and kinetic rates of the salt conversions differed depending on direct exposure to high-intensity radiation (photodehydration) or whether the radiation was blocked by the filter (thermodehydration). In general, radiation-induced dehydration triggers the transformation at lower temperature and accelerates the kinetic reaction more than when the filter is used. Mirabilite dehydration starts with the initial radiation impacts, unlike epsomite. Thermodehydration and photodehydration of mirabilite is a non-isothermal reaction occurring through an amorphous-mediated step. Radiation damage in epsomite leads to isothermal dehydration, whereas non-isothermal dehydration occurs when epsomite is thermally damaged. In both cases, no amorphous material was observed. Because of the weaker bond between cation and oxygen atom in mirabilite, its thermal and radiation stability is lower than in epsomite. These results have important implications for the prevention of salt weathering of porous materials found in the cultural heritage.

Correlation between thermoluminescence and mechanoluminescence of gamma-irradiated Dy activated potassium and magnesium mixed sulphate.

Thermoluminescene (TL) and mechanoluminescence (ML) properties of gamma-irradiated Dy activated potassium and magnesium mixed sulphate have been studied. (K2Mg2:Dy) (SO4)3 samples having different concentrations of Dy were prepared by solid-state diffusion method. Two distinct peaks, the first approximately 130 degrees C and the second approximately 273 degrees C, are observed in the TL glow curve. It is also observed that TL intensities of both peaks decrease when TL glow curves were recorded after deforming the irradiated samples. Only one peak is observed in the ML intensity vs. time curve and the ML intensity decreases markedly with the post-irradiation annealing (to remove 130 degrees C TL peak) of the sample. Both ML and TL intensities have been observed optimum for 1 mol% of Dy in the mixed sulphate system. It is suggested that the recombination of electrons with the free radicals (anion radicals produced by gamma irradiation) released from the traps during the thermal or mechanical excitation is responsible for luminescence in this system.

Defect complexes in Re3+-doped magnesium sulphate phosphors.

A series of magnesium sulphate phosphors MgSO4:RE3+,X ( RE = Dy, Tm, Eu and X = P, Mn) have been prepared and studied. Based on the experimental results of thermoluminescence (TL) emission spectra and dose responses studies, it is proposed that in these phosphors large defect complexes are formed, which include intrinsic imperfections and dopants. These defect complexes were formed in the course of preparation of phosphors and could be regarded as basic elements in the TL multi-stage processes. This defect model might be applicable to the most of TL materials.

Non-linearity of TL dose responses in MgSO4:Dy CO-doped with Mn, P and Cu.

Thermoluminescence (TL) dose responses to gamma radiation in the dose range from 0.1 Gy to 20 kGy in MgSO4:Dy, MgSO4:Dy,Mn, MgSO4:Dy,P and MgSO4:Dy,P,Cu phosphors were obtained and fitted by the composite action dose response function to obtain non-linear characteristic parameters, one-hit factor R and characteristic dose D0, which indicate that the TL dose responses for individual TL glow peaks in MgSO4:Dy, MgSO4:Dy,Mn and MgSO4:Dy,P phosphors all are supralinear. The non-linearity of dose response for the main dosimetric peak in MgSO4:Dy,P,Cu depends critically on the concentration of doped Cu and sublinearity can be obtained in the dose responses of TL materials doped with a certain amount of Cu. Based on studying the TL glow curves, dose responses and three dimension emission spectra of these phosphors, it is supposed that the TL event is generated via a multi-stage process within a large defect complex in doped magnesium sulphate phosphors. Doping with Cu results in reformation of the defect complex in which there is a strong spatial association between a trapping centre and a recombination centre so that the probability of one-hit TL events increases. The non-linearity of the dose responses is usually a difficulty for measuring higher dose in TL dosemeters. It can be overcome by enlarging the linear range of the dose-response or by calibrating the dose-response curves using non-linear characteristic parameters in a dose response function, such as the composite action dose response function.

Thermoluminescence of MgSO4 doped with Eu and P impurities.

CaSO4:Eu, MgSO4:Eu and MgSO4:Eu,P phosphors have been prepared and their thermoluminescence (TL) characteristics were studied. A main glow peak due to Eu2+ ions is seen at approximately 146 degrees C and 440 nm and glow peaks at approximately 145 degrees C, approximately 190 degrees C, approximately 260 degrees C and approximately 360 degrees C for 590 nm and 625 nm wavelengths are identified as Eu3+ ion emissions in MgSO4:Eu. Emission spectra in MgSO4:Eu and the MgSO4:Eu,P show that the MgSO4:Eu3+ glow peak at 260 degrees C for 590 nm and 625 nm shifts to 280 degrees C with enhanced intensity while the Eu2+ ion glow peak at 146 degrees C remains but with reduced intensity. The main glow peak at approximately 146 degrees C and 440 nm from Eu2+ ions shows significant difference from the characteristic glow peaks of Eu3+ ions. It is observed that the wavelength of the Eu2+ ion glow peak is inversely proportional to the radius of the cation of the host sulphate in alkaline-earth sulphate phosphors. By contrast the wavelengths of the Eu3+ ion glow peaks remain unchanged in different sulphates. Besides, the glow curve at approximately 146 degrees C obtained using a conventional blue sensitive reader shows simply the first order kinetics. It is concluded that the luminescence centres and distribution of traps related to Eu2+ ions are different from that of Eu3+ ions in MgSO4:Eu and MgSO4:Eu,P phosphors.

ESR dosimetry using inorganic materials: a case study of Li2CO3 and CaSO4:Dy as prospective dosimeters.

The CO2- radical ion, detected by ESR technique in bones and teeth enamel, was proved to be invaluable in high level and retrospective dosimetry. In these matrices, impurity carbonate (at phosphate sites) was the precursor to CO2-. With a view to investigate the possibility of using inorganic materials such as lithium carbonate as ESR dosimeters, studies were carried out on gamma-irradiated Li2CO3. The intensity of radiation-induced ESR signals of Li2CO3 at g = 2.0036 (CO3-) and g = 2.0006 (CO2-) was followed as a function of gamma dose in the low dose range of 1-1350 Gy. It was observed that the intensity of the ESR signal at g = 2.0036 (CO3-) was in a linear relation with the radiation dose in the dose range 10-800 Gy and the signal at g = 2.0006 (CO2-) showed linear response in the dose range 5-800 Gy. The lowest dose that could be detected in the present studies using the signal of CO2- in Li2CO3 powder samples (approximately 50 mg) is 3.2 Gy. ESR studies were also carried out on the widely used TL dosimetric material CaSO4:Dy and in pure CaSO4 after gamma irradiation. The TL materials were used in powder as well as pellet forms. The linearity of ESR response with dose for powder and pellet forms of CaSO4: Dy was also studied using the signals at g = 2.0030 (SO3-) and at g = 2.0139 (SO4-). It was observed that the range of linearity of dose response extended between 20 and 1200 Gy, for SO3- signals. The results of dosimetric study indicate that the ESR-Li2CO3 system could be used in dosimetric applications in radiotherapy. However, for the actual applications further advancement is needed to lower the detection limit. The TL phosphor, CaSO4:Dy in powder and pellet forms, could be used as ESR dosimeter in the dose range 20-600 Gy.

Experimental simulations of the photodecomposition of carbonates and sulphates on Mars.

There is indirect spectroscopic evidence for the presence of sulphates and carbonates on the martian surface, and such minerals are also found in SNC meteorites, which are thought to be of martian origin. But although carbonates are expected to be abundant in the martian regolith, attempts to detect them directly have been unsuccessful. Here we report laboratory studies of the decompostion of calcium carbonate and magnesium sulphate under ultraviolet irradiation, which mimic the conditions under which photodecomposition of surface minerals by solar ultraviolet light might occur on Mars. We find that, even for a low abundance of carbonate minerals in the martian regolith, the rate of CO2 release due to photodecomposition is higher than the rate of CO2 loss from the atmosphere by solar-wind-induced sputtering processes, making this process a potential net source of atmospheric CO2 over time. SO2 is also released from the sulphate, albeit more slowly. The rate of carbonate degradation is high enough to explain the apparent absence of these compounds at the martian surface.