Molybdenum emitters

Initially the molybdenum fluxes in [55] measured via atomic Mo-lines (379.8 nm, 386.4 nm and 390.3 nm) led to unreasonably high flux values. This was the case because for the calculation of the respective excitation rates the formula of van Regemorter was used [56], as these lines are optically coupled to the ground state (resonance lines). Therefore, experiments were performed to measure the excitation and ionization rates directly both in a linear plasma machine [57] and in a crossed beam experiment with a thermal molybdenum emitter [58], The results can be seen in Fig. 6.17 and show that the experimental values for excitation are about a factor of 5 larger than those from the van Regemorter formula, which leads to a reduction of the flux by the same order according to the smaller S/XB. More refined R-Matrix calculations have later confirmed the same factor and are included in Fig. 6.3. 

There are various applications of DLC in the electronics industry too. For example, DLC coating has been demonstrated to lead to an increase in the electron emission of molybdenum emitters. 

The Noddacks also claimed to have detected element 43 and named it masurium after Masuren in Prussia. This claim proved to be incorrect, however, and the element was actually detected in 1937 in Italy by C. Perrier and E. Segre in a sample of molybdenum which had been bombarded with deuterons in the cyclotron of E. O. Lawrence in California. It was present in the form of the emitters Tc and Tc... 

In this design, a mulhple emitter assembly with heated tungsten filaments placed in parallel to the product s direction is used. The beam current is controlled by molybdenum grids held at a common potential. A planar screen... 

The mechanism by which the sublimed oxygen-containing Mo collector leads to better performances is not well understood. It is commonly assumed that Cs reduces the molybdenum oxide formed during the sublimation, the resulting volatile CS2O then dissociating on the emitter surface. To bypass the molybdenum oxide reduction step, the first approach tried was to load commercial "cesium oxide" (composition close to that of CSO2) in hollow collectors. 

A schematic diagram of the power generation test apparatus is shown in Figure 2. The emitter, which was a disc with a diameter of 16 mm and a thickness of 5 mm was joined using ruthenium-molybdenum braze to a support made of tantalum, and was placed facing the molybdenum collector at a distance of 400 im. The emitter was heated to 1400 1900 K by electron bombardment Then the collector was cooled by radiative cooling to about 1000 K. 

Based on these results, a (0001) oriented rhenium thermionic emission layer ( 1(X)// m ), and a tungsten diffusion prevention layer (5(X) /im) were formed on a molybdenum substrate, and emitters with a composition gradient at each interface were produced. 

A gradient structure emitter, in which the molybdenum substrate is covered by a ((XX)1) rhenium layer with a three-dimensional surface and an intermediate tungsten layer was fabricated, and the work function and thermionic power generation characteristics were evaluated. The following results were obtained. 

Among the long-lived technetium isotopes only the p -emitter Tc with a half-life of 2.13-10 a is obtained in vveighablc amounts, either by neutron irradiation of highly purified natural molybdenum or by induced fission of with thermal neutrons. Because of the high fission yield of 6.13 atom%, appreciable quantities of Te ean be isolated from uranium fission product mixtures. Nuelear reactors with a power of 3500 MWth produce about 100 g of Tc per day or 6 TBq ( 10 kg) c/GWn, per year. 

Rhodium, vanadium, cobalt, and molybdenum have been used as emitters for SPNDs. Since rhodium SPNDs are the main in-core instruments for the determination of power distribution in pressurized-water reactors (PWR), they are discussed first and in greater detail than the others. 

Nuclear applications of nanocapsules are related to the emitting physical properties of the encapsulated material. Emitted radiation can be electromagnetic of high energy (y), electrons or positrons (/3), alpha particles (" He nucleus), or fission products [67]. These emitters can be in themselves radioactive or can be activated by a nuclear reaction, usually a neutron capture. The particular advantage of carbon nanocapsules in nuclear applications is related to the protective characteristics that the carbon capsule confers to the interior product. Experiments on irradiation of fullerenes have shown that knocked carbon atoms from one cage are foimd in another fuUerene and even form dimers and trimers by a recoil-implantation mechanism [68]. The observed major damage of capsules in nanoencapsulated molybdenum irradiated in a nuclear reactor was produced by... 

The products are neutron-rich jS-emitters, and the precursors of Tc are the refractory elements zirconium, niobium, and molybdenum ... 

The P" emitter Rb (Ti/2 = 1.3 min) has found application in myocardial blood flow studies with PET. The possible production methods for the parent Sr include the He- and Qt-particle induced reactions on natural krypton, the (p,4n) reaction on Rb and the spallation of molybdenum with high energy protons. In practice, however, the " Rb(p,xn) Sr process is commonly used, and the major quantity of Sr is presently produced at intermediate energy accelerators (cf. Waters and Goursey 1987 Philipps et al. 2000). 

Since that time nuclear reactors around the world have produced hundreds of isotopes of most of the elements in the periodic table. As discussed in Chapter 13, some of these isotopes are used daily in nuclear medicine. Technetium-99m is a gamma ray emitter used daily in hospitals. Tc-99m is produced from the beta decay of molybdenum-99 as shown in the following reaction ... 

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