Recently, nanoparticles have been considered as a method of providing radiation dose enhancement in tumors. In order to quantify this affect, a dose enhancement factor (DEF) is defined that represents the ratio of the dose deposited in tumor with nanoparticles, divided by the dose deposited in the tumor without nanoparticles. Materials with atomic numbers (Z) ranging from 25 to 90 are considered in this analysis. In addition, the energy spectrum for a number of external beam x-ray sources and common radionuclides are evaluated. For a nanoparticle concentration of 5 mg/ml, the DEF is < 1.05 for Co-60, Ir-192, Au-198, Cs-137, 6, 18, and 25 MV x-rays for all materials considered. However, relatively large increases in the DEF are observed for 50, 80, 100, and 140 KVp x-rays as well as Pd-103 and I-125. The DEF increases for all sources as Z varies from 25-35. From Z = 40-60, the DEF plateaus or slightly decreases. For higher Z materials (Z>70), the DEF increases and is a maximum for the highest Z materials. High atomic number nanoparticles coupled with low energy external beam x-rays or brachytherapy sources offer the potential of significantly enhancing the delivered dose.
Magnetization measurements were performed on single crystals of YBa2Cu307 -& to study the effect of fast neutron irradiation on flux pinning and the critical magnetization current. Neutron irradiation up to a fluence of 8.16X10' n/cm systematically enhanced the magnitude and reduced the anisotropy of the critical magnetization current compared to unirradiated samples. The largest increases in the critical magnetization current occurred at 77 K. There was a slight linear decrease in the transition temperature with neutron fluence.A major problem in developing applications of the high-T, superconductors La2 "M"Cu04(M =Ca, Sr, Ba) and YBa2Cu307-s is the low superconducting critical current found in polycrystalline samples of these materials as they are commonly prepared. ' The low critical currents are often attributed to the granular nature of the sintered polycrystalline samples. To make full use of the high transition temperature available in these materials, applications must be carried out near liquid-nitrogen temperatures. At these temperatures, the critical current in bulk polycrystalline and single-crystal samples has been found to be lower by two orders of magnitude than its value at liquid-helium temperatures. ' ' In singlecrystal samples the critical current has been found to be highly anisotropic with the a-b plane supporting currents a factor of 10 greater than those in the c direction. For applications, it is desirable to achieve isotropic critical currents of the order of 10s A/cm at liquid-nitrogen temperatures. To this end we have investigated the effect of fast neutron irradiation on single crystals of YBa2Cu307q where the intrinsic behavior, uncomplicated by the question of granularity, can be studied. We find a systematic increase in the critical magnetization current with increasing neutron fluence accompanied by a slight decrease in T,. The anisotropy usually found in singlecrystal samples is reduced.The single-crystal samples used in this experiment were prepared by the same technique as those used for our previous study of the anisotropy of the critical magnetization current. The crystals ranged in mass from 175-497 pg with typical dimensions of 0.4 mm in the a-b plane and 0.2 mm along the c direction. The c direction was easily recognized with optical microscopy and confirmed by x-ray diffraction. The transition temperature of these crystals as determined by shielding-effect measurements described below is above 91 K without oxygen annealing. The four crystals used in this study were not of equal quality. Although their transition temperatures were all above 91 K before irradiation, their shielding curves showed varying degrees of structure with de-creasing temperatures. Typically, the shielding curve showed a sharp superconducting transition at about 91.5 K followed by a shoulder at approximately 75 K and a second decrease at around 60 K. This structure is present for both irradiated and unirradiated samples. Typical transition curves for irradiated samples are shown in Fig.
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