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Ishchenko Alexander Faculty: Computer information technologies and automation Speciality: Scientific, analytical and ecological devices and systems Theme of master's work: "Construction method development of the selective control device of substance type" Scientific adviser: Prof. N. I. Chichicalo |
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Abstract Construction method development of the selective control device of substance type
Introduction The operating principle of the device is based on the method of nuclear quadrupole resonance. Last decades resonant methods have got widespread in the investigation of substance in gaseous, liquid and solid state. The set of these methods constantly replenishes through the opening of new types of resonance. Resonance types in solid bodies are especially diverse. The word "resonance" (from the fr. rosonance, lat. resono – sound in replay, called) means increase the response of a oscillation system on periodic exterior action in the convergence of the frequency action from one of the frequencies of natural oscillations. Most famous for the simplest type of mechanical resonance: the increase amplitude of oscillations of a pendulum with a periodic "pushing" with the frequency ν, when the frequency ν approaches the frequency of natural oscillations of pendulum ν. Oscillatory system, which are capable to resonate, can have very different nature. In substance such systems can be electrons, electron shell of atoms, magnetic and electric moments of atoms, molecules, impurity centres in crystals, etc. However, in all cases, the resonance overall picture remains: near to a resonance the amplitude of oscillations and energy which is transmitted to the oscillatory system from the outside increase. This increase stops when the energy losses compensate for its growth. Each substance has its own set of natural oscillation frequencies. Natural frequencies ν can be found in a wide frequency range (from 10^2 to 10^22 Hz). This set of frequencies is the substance card. If you investigate it, we can identify the chemical composition, structure, symmetry and other characteristics of the substance. The most convenient and widespread type of periodic exterior action is electromagnetic radiation. The frequencies of electromagnetic waves extends from10^2 Hz (radio waves) to 10^9 Hz (radio microwaves ), 10^13-10^14 Hz (infrared light), 10^15 Hz (visible light), 10^15-10^16 Hz (ultraviolet light), 10^17-10^20 Hz (X-rays) и 10^20-10^22 Hz (γ-radiation). Depending on the frequency range different methods of generation and detection are used. Thus, different fields of physics were formed: a radio spectroscopy, optical spectroscopy, X-ray spectroscopy etc. Though in all these fields resonant interaction of electromagnetic radiation with substance are investigate, the nature of such interaction is various. Resonant methods can be attributed to the most sensitive and exact methods for the investigation of substance. With their help was received by a large amount of valuable information on chemical composition, structure, symmetry and interactions between the structural units of substance.
General description of the resonance The classical description of the resonance is reduced to the solution of a problem of one or more linear harmonic oscillators. Natural oscillations of the oscillator caused by the initial impulse and the action returns a force that is proportional to deviation m from the equilibrium position. Natural oscillations can be described by the equation:
where m – the mass of the particle, which oscillates, ω0 – circular frequency of natural oscillations. The solution of this equation describes a harmonic oscillation.
If the oscillator is exposed
to activity
of periodic external force
The solution of this differential equation, that describes the oscillations, is
where the amplitude and phase are defined as
Dependence of the oscillation amplitude
x0 on
frequency
ω0 is
presented in Fig.1 for different values of a
quality factor
Fiugure 1 – Resonance curves x0(ω) for different values of quality factors Q Animation: GIF Animator, 10 frames, 5 cycles of recurrence, 45.3 КB
Nuclear quadrupole resonance Nuclear quadrupole resonance (NQR) is one of the most sensitive methods for the investigation of intramolecular structure of substance, its physical and chemical properties. NQR is widely used at investigation the nature of chemical bonds, intermolecular compounds, internal interior movements, phase transitions, defects in crystals, etc. This method of spectroscopy is one of the leaders. Specialists in a radiospectroscopy, whose works are covered in this section of physics, 12 Nobel Prizes have been awarded. Nuclear quadrupole resonance (NQR) is a resonance absorption of electromagnetic energy in crystals, caused by transitions between power levels, appearing as a result of interaction of kernels possessing an electric quadrupole moment with the electric crystalline field. NQR is the special case of nuclear magnetic resonance (NMR) in crystals. The so-called “pure” NQR is observed in the absence of the permanent magnetic field. The essence of the condition consists in the following. Some elements of Mendeleyev table are aspherical distribution of positive charge in the nucleus. The degree asphericity assessed special parameter, which is called the quadrupole moment. If round such nucleus is the local non-uniform electric field, which is created around this nucleus of electric charge, then there is the condition of NQR. At action on substance of an exterior electromagnetic field of a certain frequency resonant absorption of energy takes place. After graduating from the impact of the external electromagnetic field radiation of energy occurs at a frequency of NQR. Isotopes of following elements concern the quadrupole: "тitrogen-14", "chlorine-35", "chlorine-37", "boron -10", "boron -11", "iodine-127", "oxygen-17", "sodium-23", "copper-63", "copper-65" and many others. NQR is only possible when there are two factors: the quadrupole moment of the nucleus and local non-uniform electric field (the gradient). The substance can have more than one frequency NQR. Number of frequency and intensity of NQR signal depends on the number of nuclei of the quadrupole component in the formula for chemical compound, their chemical equivalence and the compound with atoms of other elements. Detectable substance can be distributed throughout the volume, or can be mechanically mixed with other substances. Impurities of other substances influence to the width of the spectral lines, and can slightly displace the frequency of NQR, however, a radical impact on the performance of detection, they do not have. Signal NQR, as a rule, is small enough, therefore a number of the special methods developed within the limits of a statistical radio engineering and a radiolocation is applied to accumulation of signal NQR and its allocation against noises. For the accumulation of the signal produced effects on the investigated object is not by one radio-frequency pulse, and special sequence of radio-frequency pulses. NQR signal is weakly dependent on the power of the probing signal, it is directly proportional to the number of resonant nuclei and, consequently, the mass of investigated sample. To detect a specific substance in the first place one needs to know its NQR frequency. NQR frequencies of different chemical compounds are in the range of hundreds of kilohertz to hundreds of megahertz.
Detection signals of NQR All existing methods for detecting NQR signals can be divided into stationary and pulse. Stationary methods directly allow to gain spectrum of frequency. Pulsed methods usually provide a temporary picture. To obtain the frequency dependence in the latter case, one needs to do the Fourier transform of signal induction. Pulsed methods require the use of more sophisticated equipment than the stationary. But the advantage of pulse methods in the investigation of spin-spin and spin-lattice interactions in solids obviously. The coefficients of absorption of radio waves in solids are small, and the nuclear resonance signals can be easily lost in noises amplifiers. Therefore high demands are made to equipment on sensitivity. In NQR spectroscopy most prevalent is method with two radio frequency impulses – a method of quadrupole spin echoes Han. The signal of the response of system is observed after action of impulses ("echo-signal").
Conclusion Resonance methods of materials investigation can be considered as the most informative and accurate. With their help, one can investigate the chemical composition, symmetry, structure, energy spectrum of the substance, the electrical, spin-orbital, magnetic, superthin interaction therein. These methods can successfully complement each other. They are widely used in physics, chemistry, biology and medicine. When this abstract was written, Master's work is not completed yet. Final completion: December, 2009.
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