The irradiator assembly

The UV radiation output of the lamp will deteriorate after a time, which is typically in excess of 500 h in our experience. It is essential that the performance of the lamp is monitored routinely, and empirically this can be done by comparing the decomposition of thiourea standards relative to equivalent DOC concentration standards of the photo-chemically labile potassium oxalate. 

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Figure 1. Schematic diagrams of the irradiation assembly. For clarity the liquid nitrogen reservoir has been omitted from the front view and the inclined mirror from the side view...

The irradiation assembly must provide the same neutron flux for both sample and standard, or appropriate correction factors must be determined. In addition to variations in the absolute magnitude of the thermal-neutron flux as a function of the position inside the nuclear reactor, the ratio of thermal-neutron and fast-neutron fluxes changes appreciably with position. This may result in serious errors caused by unwanted threshold reactions. When employing 14 MeV generators, time-depen-dent variations in the neutron flux may also become significant. 

The concrete block walls of the cell housing the generator tube and associated components are 1.7 meters thick. The facility also includes a Kaman Nuclear dual-axis rotator assembly for simultaneous transfer and irradiation of reference and unknown sample, and a dual Na iodide (Nal) scintillation detector system designed for simultaneous counting of activated samples. Automatic transfer of samples between load station to the rotator assembly in front of the target, and back to the count station, is accomplished pneumatically by means of two 1.2cm (i.d.) polyethylene tubes which loop down at both ends of the system and pass underneath the concrete shielding thru a pipe duct. Total one-way traverse distance for the samples is approx 9 meters. In performing quantitative analysis for a particular element by neutron activation, the usual approach is to compare the count rates of an unknown sample with that of a reference standard of known compn irradiated under identical conditions... 

Self-assembled nanorods of vanadium oxide bundles were synthesized by treating bulk V2O5 with high intensity ultrasound [34]. By prolonging the duration of ultrasound irradiation, uniform, well defined shapes and surface structures and smaller size of nanorod vanadium oxide bundles were obtained. Three steps which occur in sequence have been proposed for the self-assembly of nanorods into bundles (1) Formation of V2O5 nuclei due to the ultrasound induced dissolution and a further oriented attachment causes the formation of nanorods (2) Side-by-side attachment of individual nanorods to assemble into nanorods (3) Instability of the self-assembled V2O5 nanorod bundles lead to the formation of V2O5 primary nanoparticles. It is also believed that such nanorods are more active for n-butane oxidation. 

Figure 4. SEC profiles of SI42 and SI44 before and after 1 h UV irradiation in w-heptane as well as of SI42 after 1 h UV irradiation in THF without the self-assembly.[46]...

Figure 4 shows typical SEC profiles of two PS-3-PI diblock copolymers before and after the photodimerization reaction. [46] The appearance of a peak after the UV irradiation at a short retention time with a doubled molar mass indicates the formation of PS-3-PI-C-PI-3-PS copolymer chains. In contrast, the irradiation of SI 42 in THF without the self-assembly only leads to a slight increase of the molar mass. It confirms that the self-assembly helps to concentrate and expose the reactive ends of precursor diblock copolymers on the periphery of the core-shell micelles, which greatly increases the coupling efficiency of the photodimmerization reaction between two coumarin end groups. 

SEM of the calcined sample 11-IV are shown in Fig. 3. With the increase of microwave irradiation time, the morphologies shows transition from mesopore materials to the fine small particles of round-like type of silicalite-1 having ca. 3-10 pm size of cubic phase surrounding with small mesoporous particles. Fig. 3c reveals the combined assemblies of MF1 and mesoporous type materials. The silicalite-1 is partly covered with... 

This study demonstrated that the micro-mesoporous composite materials could be synthesized with two-step treatment by microwave using two different templates system with TPABr and MTAB. This formation was controlled by the self-assembly formation of supramolecular templates between MTA micelles and SiO /TPA gels. As varying microwave irradiation time of micro-mesoporous materials, gradually transition from the mesophase to micro-mesophase was occurred. These materials have higher dm spacing of mesoporous materials and lead to transition from mesophase to micro-microphase by an increment of synthetic time, while the calcined products is formed with bimodal and trimodal pore size distribution under microwave irradiation within 3 h. From TG-DTA and PL analysis, the self-assembly formation of supramolecular templates between MTA+ micelles and SiO /TPA+ gels were monitored. 

Generally speaking, the research reactors are small, on the order of 1 MW of thermal power, and are optimized to provide intense neutron fluxes for the irradiation of samples. These reactors are fueled with a few kilograms of enriched uranium (20-90% 235U) in fuel rods that are clad with a zirconium alloy or with aluminum. The entire core assembly of a 1-MW research reactor is on the order... 

Experimental. All photodimerizations were carried out in a stainless steel fixed volume cell (1.75 cm ID with a 1.0 cm path length) with sapphire windows under the irradiation of a Hanovia medium pressure mercury lamp filtered through water and Pyrex for a 13.5 hour exposure. The cell and lamp assembly have been described previously (31). For selected runs a custom built 0.9 mL variable-volume pump was connected to the cell and the pressure was varied to determine the exact location of the phase boundary, based on light scattering measured in a Cary 2290 UV-Vis spectrophotometer (Varian Inst.). The spectrophotometer was also used to measure the concentrations of the monomeric cyclohexenone before and after reaction. 

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