This paper intends to evaluate the uptake of cadmium ions from aqueous solution by 21 hydroxyapatite samples which have been synthesized in different conditions. It has been determined thatthe variation on the hydroxyapatite sorption capacity is neither related to sample solubility nor to hydroxyapatite Ca/P molar ratio. Cd2+ sorption is controlled by sample BET surface area, which shows a direct dependence on the hydroxyapatite crystallite dimensions. The hydroxyapatite pore distribution presented modes at 1000 and 60,000 A, corresponding to intracrystallite voids and voids between the agglomerate of these crystallites, respectively. Pores belonging to the former mode immobilize the major part of Cd2+. The influence of sample thermal treatment on Cd2+ sorption efficiency has been studied using hydroxyapatite samples calcined at temperatures ranging from 500 to 1140 degrees C. Similarly to nonthermally treat samples, the Cd2+ sorption on calcined hydroxyapatite could be described by Langmuir isotherms. The results showed that the maximum sorption capacity decreased from 0.631 mmol g(-1) for the noncalcined sample to 0.150 mmol g(-1) for the one calcined at 900 degrees C. This drop in the sorption capacity could also be explained by a reduction in its specific surface area, which is induced bythe increase of the crystal size.

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A b s b d . Monochromatic photon beams have been used to investigate lhe photofission of "AI at 69 MeV with plastic nuclear-track detectors. The measured fissility was found to he (5.7 * 1.1) X lo-'. Wis result, along with the available data for "' Sm and '74Yb nuclei taken at the same energy, has been interpreted within the frameaork of a two-step model for the photofission reaction. According to this model the incoming photon is considered to he absorbed by a neutron-proton pair, and then followed by an evaporation-fission competition for the exiled residual nucleus The measured fissility values are seen IO be in satisfactory agreement with the calculated ones, and the predictions from the model are confirmed, specially in the region of light fissioning systems.

In this work, the synthetic hydroxyapatite (HAP) was studied using different preparation routes to decrease the crystal size and to study the temperature effect on the HAP nano-sized hydroxyapatite crystallization. X-ray diffraction (XRD) analysis indicated that all samples were composed by crystalline and amorphous phases . The sample with greater quantity of amorphous phase (40% of total mass) was studied. The nano-sized hydroxyapatite powder was heated and studied at 300, 500, 700, 900 and 1150 °C. All samples were characterized by XRD and their XRD patterns refined using the Rietveld method. The crystallites presented an anisotropic form, being larger in the [001] direction. It was observed that the crystallite size increased continuously with the heating temperature and the eccentricity of the ellipsoidal shape changed from 2.75 at 300 °C to 1.94, 1.43, 1.04 and 1.00 respectively at 500, 700, 900 and 1150 °C. In order to better characterize the morphology of the HAP the samples were also examined using atomic force microscopy (AFM), infrared spectrometry (IR) and thermogravimetric analysis (TGA).

Experimental fissility data from photofission reactions of Th-232, U-238, and U-235 nuclei at 52 MeV have been obtained using monochromatic and polarized photon beams and dielectric fission-track detectors. These data, along with literature data for Np-237, U-235,U-238 Th-232, Bi-209, Pb-208, and Yb-174 nuclei have been analyzed within the framework of a simple two-step model for photofission reactions, i.e., absorption of the incident photon by a neutron-proton pair followed by an evaporation-fission competition mechanism for the excited nucleus. For nuclei in the Ta-Np region the trend of calculated fissilities clearly shows shell effects in the vicinity of Pb-208. In the case of U-238 the effect of photon polarization on fission direction has also been studied, and isotropy was observed in the fragment azimuthal distribution