Recent studies on radiation induced F-center and colloid particle formation in synthetic NaCI and natural rock salt

Radiation induced F-center and Na metal colloid formation have been studied, at temperatures between 100 and 350°C, in melt growth synthetic NaCl crystals and natural rock salt samples, using equipment for making optical measurements during 1–3 MeV electron irradiation. In both types of NaCl the damage formation kinetics are qualitatively similar. However, there are large quantitative differences which depend on irradiation temperature, dose rate, total dose, inherent strain, strain applied prior to irradiation and other factors. Natural rock salt samples from different localities exhibit markedly different radiation induced colloid formation rates. An extrapolation of the current data to the doses expected in radioactive waste depositories indicate that natural salt immediately adjacent to the planned waste canisters could develop between 0.1 and 50 percent colloidal sodium in 50 to 400 years.     

Effect of gamma irradiation on the thermal,mechanical and structural properties of chlorinated polyvinyl chloride

Non isothermal studies were carried out using thermogravimetry (TG) and differential thermogravimetry (DTG) to obtain the activation energy of decomposition for chlorinated polyvinyl chloride (CPVC) before and after exposure to gamma doses at levels between 5.0 and 50.0 KGy. Thermal gravitational analysis (TGA) indicated that the CPVC polymer decomposes in one main breakdown stage and a decrease in activation energies was observed followed by an increase on increasing the gamma dose. The variation of melting temperatures with the gamma dose has been determined using differential thermal analysis (DTA). Also, mechanical and structural property studies were performed on all irradiated and non-irradiated CPVC samples using stress-strain relations and X-ray diffraction. The results indicated that the exposure to gamma doses at levels between 27.5 and 50 KGy leads to further enhancement of the thermal stability, tensile strength and isotropic character of the polymer samples due to the crosslinking phenomenon. This suggests that gamma radiation could be a suitable technique for producing a plastic material with enhanced properties that can be suitable for high temperature applications and might be a suitable candidate for dosimetric applications.     

Concentrations of radiation defects with almost isoenergetical levels in silicon

Isochronous annealing of radiation defects with almost isoenergetical levels (indistinguishable from each other by means of temperature dependence of the majority charge carriers concentration) in silicon is studied. The concentrations of G- and A-centres with a level E _c?0.17 eV in n-Si and some (unidentified) vacancy-type and V₂+B complexes with a level E _v+0.22 eV in p-Si are determined.     

Radiation effects in KMgF 3 crystals

Effects of x, g and UV irradiation on Tb 3+ and Eu 2+ doped KMgF 3 crystals were studied. Thermoluminescence (TL) excited by VUV radiation was compared to that induced by x and g rays. TL excitation spectra showed maxima at 125, 140, 155 and 180 v nm. The main TL peaks appeared after VUV irradiation at about the same temperatures and with the same thermal activation energies as after x or g irradiation, indicating that these peaks are due to the same trapping levels. In samples, which had previously been exposed to ionizing radiation, TL could also be excited with longer wavelengths that could not excite any TL in un-irradiated crystals; this is attributed to a process of phototransfer (PTTL). The x and UV excited luminescence was measured at LNT and RT and showed essentially the same main emission bands that appeared in the TL and PTTL peaks of these crystals. The dose dependence of the 730 v K TL peak in of KMgF 3 :Eu 2+ was linear up to about 2000 v Gy. The TL sensitivity of KMgF 3 :Eu 2+ was by an order of magnitude higher than that of KMgF 3 :Tb 3+ , which was by a factor of two greater than that of the known TLD-l00 phosphor.     

Optical Transmittance and Irradiation Resistance of Rare-Earth (Ce3☎, Tb3☎, Pr3☎) Doped Heavy Germanate Glasses

In this work we investigated the possibility of using scintillating rare earth Ce^3?, Tb^3?, and Pr^3? ions to optically activate germanate glass matrices. Glasses were characterised in terms of radiation resistance, UV-VIS transmittance and luminescence spectra. Their radiation induced response was found to be dependent on glass composition and these trivalent rare earth ions turned out to be efficient in suppressing formation of colour centres absorbing light in the visible range.     

Effect of radiation-induced emission of schottky defects on the formation of colloids in alkali halides

Formation of vacancy clusters in irradiated crystals is considered taking into account radiation-induced Schottky defect emission (RSDE) from extended defects. RSDE acts in the opposite direction compared with Frenkel pair production, and it results in the radiation-induced recovery processes. In the case of alkali halides, Schottky defects can be produced as a result of the interaction of extended defects with excitons, as has been suggested by Seitz in 1954. We consider a model that takes into account excitonic mechanisms for the creation of both Frenkel and Schottky defects, and which shows that although the contribution of the latter mechanism to the production of primary defects may be small, its role in the radiation-induced evolution of microstructure can be very significant. The model is applied to describe the evolution of sodium colloids and the formation of voids in NaCl, which is followed by a sudden fracture of the material, presenting a potential problem in rock salt-based nuclear waste repositories. The temperature, dose rate and dose dependence of colloid growth in NaCl doped with different types of impurities is analyzed. We have found that colloid growth may become negative below a threshold temperature (or above a threshold dose rate), or below a certain impurity concentration, which is determined by the RSDE, that depends strongly on the type and concentration of the impurities. The results obtained with the model are compared with experimental observations.     

Photo- and thermo-stimulated luminescence of CsI—Tl crystal after UV light irradiation at 80 K

Abstract

Thermo- and photo-stimulated luminescence are studied for CsI—Tl crystal after the irradiation with the UV light at 80 K. Creation spectrum of the photostimulated luminescence coincides with the D absorption band of Tl⁺ ions. Nature of the defects created by UV light at low temperatures is discussed basing on the correspondence between the thermostimulated glow curve peaks and thermal evolution of the photostimulation spectra observed after irradiation in the D absorption band. Three bands at 1400, 950 and 580 nm have been observed in the stimulation spectrum at 80 K. The 1400 and 950 nm stimulation bands are presumably explained as the optical transitions in the Tl⁰ centre forming the spatially correlated defect pair with V_k centre while the 580 nm stimulation band is connected with the unperturbed Tl⁰ centres. It is concluded that the Tl⁺ luminescence at low temperature is connected with the electron recombination with the Tl²⁺ centre.     

Proton and Gamma Ray Induced Gain Degradation in Bipolar Transistor

This paper describes the results of the effect of 24 MeV proton and ⁶⁰Co γ-irradiation on the collector characteristics and forward current gain of commercial bipolar transistor (npn 2N2219A). The transistor has been exposed to these radiations in the biased condition and the collector characteristics and forward current gain have been measured as a function of proton fluence and γ-dose. The observation is that both the proton and γ-irradiation induce significant gain degradation in the transistor. The results are discussed in terms of displacement damage produced by energetic protons and γ-radiation in the bulk of the semiconductor.     

Interaction of plasma facing materials for fusion devices with low energy hydrogen and helium particles

Abstract

Radiation damage formed in metal specimens exposed to long pulse tokamak plasmas of TRIAM-1M was examined by transmission electron microscopy. By comparing these results with those of low energy hydrogen ion irradiation it was concluded that the charge exchange energetic neutrals of hydrogen emitted from the core plasma caused remarkable displacement damage. The flux of the neutrals in the energy range of 0.5–3 keV which were responsible for displacement damage, was estimated to be about 1.5–3 × 10¹⁸ H/m²/s. These energetic neutrals cause not only material degradation at the sub-surface region but also change bulk properties of plasma facing components in a plasma confinement device. Effect of helium plasma was also discussed with emphasis on very strong effects on damage accumulation. Damage by He is a serious issue of plasma facing materials.     

Theoretical analysis of the evolution from ignition kernel to flame ball and planar flame

Dynamics of flame kernel evolution with and without external energy addition has been investigated analytically and numerically. Emphasis is placed on the effects of radiation heat loss, ignition power and Lewis number on the correlation and transition between the initial flame kernel, the self-extinguishing flame, the flame ball, the outwardly propagating spherical flame and the propagating planar flame. The present study extends previous results by bridging the theories of the non-adiabatic stationary flame balls and travelling flames and allowing rigorous consideration of radiation heat losses. The results show that the effects of radiation heat loss play an important role in flame regimes and flame transition and result in a new isolated self-extinguishing flame. Furthermore, it is found that radiation heat losses significantly increase the critical ignition radius and result in three different dependences of the minimum ignition power on the Lewis number. Comparisons between the results from the transient numerical simulation and those from the quasi-steady state analysis show a good agreement. The results suggest that prediction of flame initiation without appropriate consideration of radiation is not acceptable.