Kinetics of phase transformation in nanostructured Ga-Se-Te glasses.

Bulk samples of Ga25Se(F5-x)Te(x) chalcogenides were prepared by melt quenching technique. The glassy nature of these alloys was verified by X-ray diffraction. The morphology of these as-prepared alloys was studied using field emission scanning electron microscopy. Kinetics of crystallization in these glassy alloys was studied under non-isothermal conditions at different heating rates (5, 10, 15, 20 and 25 K/min) using differential scanning calorimetry. The value of glass transition and crystallization temperature was found to be composition and heating-rate dependent. The value of order parameter indicates that the crystallization was due to volume nucleation with two dimensional growths. The activation energy of crystallization and activation energy of glass transition were determined from the heating rate dependence of glass transition and crystallization temperature. Both the activation energy for glass transition and the activation energy of crystallization were found to decrease from 130.05 to 102.99 kJ/mole and from 130.05 to 102.99 kJ/mole respectively with increasing Te content in Ga-Se system.

Assessment of background hydrogen by the Monte Carlo computer code MCNP-4A during measurements of total body nitrogen.

The use of a hydrogen internal standard to enable the estimation of absolute mass during measurement of total body nitrogen by in vivo neutron activation is an established technique. Central to the technique is a determination of the H prompt gamma ray counts arising from the subject. In practice, interference counts from other sources--e.g., neutron shielding--are included. This study reports use of the Monte Carlo computer code, MCNP-4A, to investigate the interference counts arising from shielding both with and without a phantom containing a urea solution. Over a range of phantom size (depth 5 to 30 cm, width 20 to 40 cm), the counts arising from shielding increased by between 4% and 32% compared with the counts without a phantom. For any given depth, the counts increased approximately linearly with width. For any given width, there was little increase for depths exceeding 15 centimeters. The shielding counts comprised between 15% and 26% of those arising from the urea phantom. These results, although specific to the Swansea apparatus, suggest that extraneous hydrogen counts can be considerable and depend strongly on the subject's size.

Monte Carlo simulation of prompt gamma neutron activation analysis using MCNP code.

Prompt gamma neutron activation analysis (PGNAA) is the most direct method of measuring total-body nitrogen. In combination with internal hydrogen standardisation, it is possible to reduce the dependence on body habitus. The uniformity of activation and detection, however, cannot be optimised sufficiently to eliminate the dependence entirely, and so further corrections are essential. The availability of the powerful Monte Carlo code MCNP(4A) has allowed a more accurate analysis of the activation facility, and yields corrections for body habitus and superficial fat layers. The accuracy of the correction is retained as the source-to-skin distance is reduced, although the activation uniformity is thereby degraded. This allows the use of a 252Cf source with lower activity and hence reduces the running cost of the facility.