Stored energy in irradiated NaCl

In this paper we describe the development of stored energy in pure and doped NaCl samples as well as natural rock salt during irradiation. The dependencies of the stored energy value on the irradiation temperature and the presence of dopants have been studied. Irradiation experiments at controlled temperatures between 50 and 150 degrees C and doses up to 150 Grad have been carried out. For pure NaCl we have found that the damage is formed only in a relatively narrow range of irradiation temperatures. In almost all cases the effect of doping is that the stored energy value is larger and the temperature range where stored energy develops is wider as compared to pure NaCl

AN IRRADIATION FACILITY FOR RADIATION-DAMAGE INVESTIGATIONS

An irradiation facility developed to irradiate many different samples simultaneously is described. As a radiation source an electron Van de Graaff accelerator (0.5-2.5 MeV and 0-250-mu-A) is used. The beam is deflected by means of a magnetic X-Y deflection, including a cycle slipping facility. The area of the target is 18 x 18 cm2. The target consists of an array of 15 subtargets, which can be operated at different temperatures between 20 and 175-degrees-C. In each of the subtargets 24 samples can be accommodated, i.e 360 samples can be irradiated simultaneously. Our first differential scanning calorimetry (DSC) and optical absorption results for pure NaCl are presented

RADIATION-DAMAGE IN NACL .3. MELTING PHENOMENA OF SODIUM COLLOIDS

We report measurements on the melting behavior of colloids produced in irradiated NaCl. With differential scanning calorimetry (DSC) several latent-heat peaks near the melting temperature of pure bulk sodium metal have been detected. It appears that the total latent heat in these peaks is an accurate measure of the amount of damage in the crystal. Peak temperature and shape provide more detailed information about the properties of the colloids. The different melting temperature can be explained by differences in the typical sizes of the colloid, based on the theory for melting of small particles. This DSC technique provides a method to evaluate the production of radiation damage in detail without changing the properties of the damaged crystal

RADIATION-DAMAGE IN NACL .4. RAMAN-SCATTERING

Raman-scattering experiments on heavily irradiated pure and doped NaCl crystals are described. The experiments have been performed at room temperature and at approximately 25 K. The crystals had been irradiated up to a maximum dose of 95 Grad by means of electrons from a Van de Graaff accelerator. The Raman spectra show the set of phonon peaks corresponding to the NaCl modes. At low frequencies the reduced Raman intensity shows a clear power-law dependence on the frequency, I(red)(omega) is-proportional-to omega(nu), with an exponent nu congruent-to 1.4, indicating that the colloids in heavily irradiated samples exhibit a fractal structure. The change in the melting peak pattern reveals that the initial form of the sodium particles is strongly affected by the performance of a Raman experiment at room temperature. At low measuring temperatures two unknown phonon peaks at 330 and 560 cm-1 are observed. These peaks are located well above the phonon cutoff frequency of NaCl at 260 cm-1, and are associated with local modes due to the ultrafine structure of the colloids

RADIATION-DAMAGE IN NACL .2. THE EARLY-STAGE OF F-CENTER AGGREGATION

We study the early stage of aggregation of F centers into colloids in pure NaCl under irradiation. The crystals have been electron irradiated with a dose rate of 2 Mrad/h up to doses of 1500 Mrad and measured by optical-absorption spectroscopy. The major bands, the F, M, and the colloid band, are analyzed qualitatively as well as quantitatively. We have observed a relationship between the concentrations of the F and the M centers, which changes from quadratic to linear. The colloid band appears to peak at two distinct wavelengths, indicating that two types of colloids are formed during the nucleation stage. The defect concentrations are determined as a function of the dose and the irradiation temperature and are discussed in terms of models which describe the kinetics of defect formation