An x-ray fluorescence analysis technique is described which allows for the direct determination of the weight fraction of zinc in biological specimens from the male reproductive tract of several mammalian species. It can also be used for absolute determinations of zinc content in small biological specimens with a limit of detestability of about 4 × 10−9 gm in a 5-min integration period. The same instrumentation, with modifications, can be applied to the determination of a number of elements above atomic number 11 found in biological materials.

 

 

 

 

CHAPTER ONE
1.0                                                        INTRODUCTION
X-ray fluorescence is the emission of characteristic “secondary” (or fluorescent) X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemical analysis, particularly in the investigation of metals, glass, ceramics and building materials, and for research in geochemistry, forensic science, archaeology and art objects such as paintings and murals.
When a primary x-ray excitation source from an x-ray tube or a radioactive source strikes a sample, the x-ray can either be absorbed by the atom or scattered through the material. The process in which an x-ray is absorbed by the atom by transferring all of its energy to an innermost electron is called the “photoelectric effect.” During this process, if the primary x-ray had sufficient energy, electrons are ejected from the inner shells, creating vacancies. These vacancies present an unstable condition for the atom. As the atom returns to its stable condition, electrons from the outer shells are transferred to the inner shells and in the process give off a characteristic x-ray whose energy is the difference between the two binding energies of the corresponding shells. Because each element has a unique set of energy levels, each element produces x-rays at a unique set of energies, allowing one to non-destructively measure the elemental composition of a sample; measurement is typically done with a solid state detector, most commonly called Si-PIN, SDD or CdTe. The process of emissions of characteristic x-rays is called “X-ray Fluorescence,” or XRF. Analysis using x-ray fluorescence is called “X-ray Fluorescence Spectroscopy.” In most cases the innermost K and L shells are involved in XRF detection. A typical x-ray spectrum from an irradiated sample will display multiple peaks of different intensities once the signals are processed through a digital pulse processor.
The characteristic x-rays are labeled as K, L, M or N to denote the shells they originated from. Another designation alpha (a), beta (b) or gamma (g) is made to mark the x-rays that originated from the transitions of electrons from higher shells. Hence, a Ka x-ray is produced from a transition of an electron from the L to the K shell, and a Kb x-ray is produced from a transition of an electron from the M to a K shell, etc. Since within the shells there are multiple orbits of higher and lower binding energy electrons, a further designation is made as a1, a2 or b1, b2, etc. to denote transitions of electrons from these orbits into the same lower shell.
The XRF method is widely used to measure the elemental composition of materials. Since this method is fast and non-destructive to the sample, it is the method of choice for field applications and industrial production for control of materials. Depending on the application, XRF can be produced by using not only x-rays but also other primary excitation sources like alpha particles, protons or high energy electron beams.
Sometimes, as the atom returns to its stable condition, instead of emitting a characteristic x-ray it transfers the excitation energy directly to one of the outer electrons, causing it to be ejected from the atom. The ejected electron is called an “Auger” electron. This process is a competing process to XRF. Auger electrons are more probable in the low Z elements than in the high Z elements.

1.2                                               OBJECTIVE OF THE STUDY
X-ray fluorescence is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by bombarding with high-energy X-rays. The aim of this study was to evaluate the effect of the modulation of the radiation spectrum with the use of X-ray on copper and zinc.

1.3                                           SIGNIFICANCE OF THE STUDY
X-Ray fluorescence is particularly well-suited for investigations that involve:

• bulk chemical analyses of major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P) in rock and sediment
• bulk chemical analyses of trace elements (>1 ppm; Ba, Ce, Co, Cr, Cu, Ga, La, Nb, Ni, Rb, Sc, Sr, Rh, U, V, Y, Zr, Zn) in rock and sediment

1.4                                            LIMITATIONS OF THE STUDY
In theory the XRF has the ability to detect X-ray emission from virtually all elements, depending on the wavelength and intensity of incident x-rays. However

• In practice, most commercially available instruments are very limited in their ability to precisely and accurately measure the abundances of elements with Z<11 in most natural earth materials.
• XRF analyses cannot distinguish variations among isotopes of an element, so these analyses are routinely done with other instruments.
• XRF analyses cannot distinguish ions of the same element in different valence states, so these analyses of rocks and minerals are done with techniques such as wet chemical analysis or Mossbauer spectroscopy.

1.5                                                   SCOPE OF THE STUDY
When materials are exposed to short-wavelength X-rays or to gamma rays, ionization of their component atoms may take place. Ionization consists of the ejection of one or more electrons from the atom, and may occur if the atom is exposed to radiation with an energy greater than its ionization potential. X-rays and gamma rays can be energetic enough to expel tightly held electrons from the inner orbitals of the atom. The removal of an electron in this way makes the electronic structure of the atom unstable, and electrons in higher orbitals "fall" into the lower orbital to fill the hole left behind. In falling, energy is released in the form of a photon, the energy of which is equal to the energy difference of the two orbitals involved. Thus, the material emits radiation, which has energy characteristic of the atoms present. The term fluorescence is applied to phenomena in which the absorption of radiation of a specific energy results in the re-emission of radiation of a different energy.

 

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