Irradiation source material

The neutrons in a research reactor can be used for many types of scientific studies, including basic physics, radiological effects, fundamental biology, analysis of trace elements, material damage, and treatment of disease. Neutrons can also be dedicated to the production of nuclear weapons materials such as plutonium-239 from uranium-238 and tritium, H, from lithium-6. Alternatively, neutrons can be used to produce radioisotopes for medical diagnosis and treatment, for gamma irradiation sources, or for heat energy sources in space. 

This section will deal briefly with some aspects of expls safety peculiar to neutron activation analysis expts. We are concerned here with a) the possible effect of the ionizing radiation dose on the energetic material which will cause it to be more sensitive or hazardous to normal handling as an expl, and b) the potential direct expl hazards involved in the physical and mechanical transportation of samples to and horn the irradiation source and in a nuclear counting system... 

Electron beam physical vapor deposition (EB-PVD) is accomplished by directing a narrow beam of high-powered ( 20 kV, 500 mA) electrons at the source material. The energy released by these electrons when they are absorbed by the source material causes local melting in the source material. The entire source material is not melted, only the portion irradiated by the electron beam. This production of a small pocket of melted source material contained within solid source material is sometimes referred to as skulling. Hence, the vapor produced does not suffer contamination from the vessel containing the source material. Singh and Wolfe (2005) reviewed the use of electron beam—physical vapor deposition for the fabrication of nano- and macro-structured components. 

The presentation of samples within the test chamber can have a significant effect on the outcome of the photostability study. Factors of importance are alignment of the samples relative to the irradiation source sample form and layer thickness and selection of protective material (Table 7.2). 

This special volume Polymers and Light deals with very recent developments of photon interactions with polymers, in areas outside the scope of the familiar photoresist technique and optical lithography. Recent developments in microlithography still apply the same processing steps (irradiation of the photoresist through a mask followed by a subsequent wet chemical development step), but with new photoresist materials, and new irradiation sources, i.e. excimer lasers that emit in the UV, e.g. at 157, 193, and 248 nm. Excimer lasers are now the main photon sources for microlithography in many research laboratories and in industry. 

Figure 2-28. Schematic drawing of a growth model for a-axis-oriented YBCO films grown by laser-assisted MOCVD a) partial decomposition and chemical interaction of source materials by UV irradiation, b) formation of nuclei for a-axis-oriented growth, and c) the a-axis-oriented grains become dominant. (From Ushida et al. [159].)...

In photocathodes or photomultipliers the incident photons force electrons to leave the material. This external photoeffect can be calibrated and amplified by acceleration and multiplication of the electrons in multi-electrode arrangements (dynodes). These devices have very short response times and can be successfully used to control the stability of a light source. Therefore such devices are frequently included in commercially available set-ups. An example is given in Fig. 4.32 combining an irradiation source with a measurement set-up. This is commercially available [119] and allows simple control of a photoreaction. However, due to geometry and inhomogeneity of the sensitive layer of the photodiode, non-homogeneous irradiation can cause errors. 

Radioactive contaminants in filters, planchets, detectors, and shields may be primordial radionuclides and their progeny in aluminum (e.g., thorium), lead (e.g., uranium progeny), and filters C K). Materials with low radionuclide content should be selected from carefully screened supplies. Man-made radionuclides have contaminated steel and other metals during processing (from airborne fallout radionuclides) or reprocessing (from radioactive tracers or medical irradiation sources such as °Co, Cs, or Ra). 

Activation analysis methodology is quite similar to other instrumental analysis methods that use energy sources of either light, heat, X rays, or electricity to irradiate a material to bring about the emission of characteristic radiations. The detection and measurement of these radiations can then be used to indicate the amount of an elemental species in the material. Activation analysis requires a source of nuclear particles, such as neutrons, charged particles, or gamma rays, to bombard (or irradiate) the sample material to make it radioactive. 

In addition to the nuclear fuel rods with the cladding as discussed above, the reactor core also contains structural materials such as spacers, springs, bolts etc., as well as the fuel assembly upper and lower end structures, in which radionuclides are also produced by neutron activation reactions. Quite in contrast with the fuel itself, no generally valid data on the activity inventories of these structural materials can be established. The main reason for this are the differences in composition of the materials used by different manufacturers, in particular as regards impurities such as cobalt (which is the source element of Co which is frequently the predominant radionuclide in irradiated structural materials). Another important fact is that the standard codes for activity calculations such as Origen, Korigen or Anisn usually are quite accurate for the active zone of the reactor core, whereas in the outer regions (where the fuel assembly end pieces are located) only approximate values can be obtained, due to the steep axial decrease in neutron flux. 

The irradiation source and irradiation conditions influence the structure of the resulting hybrid materials [114,115,132,133]. Microdifractograms (Fig. 19) and X-ray images show that copper, silver, nickel, and palladium NPs can be successfully obtained via reduction of the correspraiding metal ions in the IPEC [PAA-PEI] films, using electron accelerators as well as X-ray and y-radiation sources [81, 114, 115, 132, 133]. 

The original techniques employed for exposure of resists used simple shadow printing techniques where the resist-coated substrate was flood-exposed with ultra-violet light through a patterned mask which was close to, or in intimate contact with, the resist surface. These systems are still used for production of small numbers of devices where their simplicity and cheapness are a great advantage. They also make best use of the whole spectral output of the irradiation source which is normally limited only by the absorption of the mask substrate material. 

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