The effect exerted by a magnetic field on moving charged particles is very widely used in technology.
For example, the deflection of an electron beam in television picture tubes is carried out using a magnetic field, which is created by special coils. In a number electronic devices a magnetic field is used to focus beams of charged particles.
In currently created experimental facilities to implement controlled thermonuclear reaction The effect of a magnetic field on the plasma is used to twist it into a cord that does not touch the walls of the working chamber. The circular motion of charged particles in a uniform magnetic field and the independence of the period of such motion from the particle speed are used in cyclic accelerators of charged particles - cyclotrons.
The Lorentz force is also used in devices called mass spectrographs, which are designed to separate charged particles according to their specific charges.
The diagram of the simplest mass spectrograph is shown in Figure 1.
In chamber 1, from which air has been pumped out, there is an ion source 3. The chamber is placed in a uniform magnetic field, at each point of which the induction \(~\vec B\) is perpendicular to the plane of the drawing and directed towards us (in Figure 1 this field is indicated by circles) . An accelerating voltage is applied between the electrodes A and B, under the influence of which the ions emitted from the source are accelerated and at a certain speed enter the magnetic field perpendicular to the induction lines. Moving in a magnetic field in a circular arc, the ions fall on photographic plate 2, which makes it possible to determine the radius R this arc. Knowing the magnetic field induction IN and speed υ ions, according to the formula
\(~\frac q m = \frac (v)(RB)\)
the specific charge of ions can be determined. And if the charge of the ion is known, its mass can be calculated.
Literature
Aksenovich L. A. Physics in high school: Theory. Assignments. Tests: Textbook. allowance for institutions providing general education. environment, education / L. A. Aksenovich, N. N. Rakina, K. S. Farino; Ed. K. S. Farino. - Mn.: Adukatsiya i vyakhavanne, 2004. - P. 328.
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FEDERAL STATE AUTONOMOUS EDUCATIONAL INSTITUTION OF HIGHER EDUCATION
"BELGOROD STATE NATIONAL
RESEARCH UNIVERSITY"
(National Research University "BelSU")
MEDICAL INSTITUTE
MEDICAL COLLEGE
CMC GENERAL EDUCATION DISCIPLINES
Abstract on the topic:
"Cyclotron. Mass spectrograph"
Vashchenko Nadezhda
Belgorod 2017
Introduction
2. Modifications of the cyclotron
3. Application of a cyclotron
Literature
Introduction
The Lorentz force is always perpendicular to the speed of a moving particle, so it does not change the magnitude of the speed, but only changes the direction of its movement, i.e. causes centripetal acceleration.
By bending the trajectory of a moving charged particle, it does no work, i.e. the kinetic energy of a charged particle moving in a magnetic field remains constant. Knowing the Lorentz force, it is possible to calculate the trajectory of particles and, therefore, control the flow of charged particles. This is used in various types of accelerators such as mass spectrograph and cyclotron.
1. Cyclotron. Device principle
Cyclotromn - a resonant cyclic accelerator of non-relativistic heavy charged particles (protons, ions), in which the particles move in a constant and uniform magnetic field, and a high-frequency electric field of constant frequency is used to accelerate them.
Two metal electrodes are placed in the gap of the electromagnet. These electrodes, called dees due to the similarity of their shape to the capital Latin letter"D", voltage is supplied from the generator high frequency. Near the center of the magnet, in the space between the dees, there is a source of positively charged ions. The entire system of electrodes and ion source is placed in a vacuum chamber, from which air is pumped out to a pressure of 10-5 mm Hg. Art. An ion emitted from the source at a time when electrode I has a negative potential will accelerate in the gap between the dees and enter the dee cavity. In it, the ion will describe a semicircle of constant radius, since there is no field in the dee cavity. If the generator frequency is chosen correctly, then by the time the ion leaves the cavity, direction I electric field will change to the opposite. Therefore, the ion will accelerate again and inside the cavity of dee II will describe a circle of a larger radius. Thus, moving in resonance with the high-frequency field, the ions will spiral toward the edge of the magnet pole. Their energy will increase after each particle passes through the accelerating gap between the dees. The acceleration process will continue until the particles reach the edge of the magnet poles. A target is placed in the path of the flow, and when the ions hit it they cause a nuclear reaction. More often, a beam of accelerated particles is removed from the chamber by means of a deflection electrode. A high negative potential is applied to this electrode, located at the edge of the chamber. Under the influence of an electric field, a beam of accelerated ions changes its trajectory, exits the chamber through a window covered with thin foil, and hits the target.
2. Modifications of the cyclotron
The disadvantage of a cyclotron is that charged particles in it cannot be accelerated to high energies, since for a relativistic particle the frequency of rotation begins to depend on energy.
If the synchronism condition is violated, the particles enter the accelerating gap out of the correct phase and stop accelerating. Thus, the cyclotron is significantly limited by the nonrelativistic energies of particles; in conventional cyclotrons, protons can be accelerated to 20-25 MeV. To accelerate heavy particles to significantly large values energy (up to 1000 MeV) use a modified installation, an isochronous cyclotron. In isochronous cyclotrons, to maintain a constant rotation frequency, a non-uniform magnetic field is created that increases along the radius. Another modification of the cyclotron is the synchrocyclotron (phasotron), in which the frequency of the accelerating electric field does not remain constant, but decreases synchronously with the frequency of rotation of the particles. However, it is clear that, unlike a classical cyclotron, which can operate in a continuous mode, a synchrocyclotron can only accelerate the beam pulsed. Finally, the cyclotron's most distant relative is the FFAG accelerator. In such an accelerator, the magnetic field does not have azimuthal symmetry, but during the acceleration of the beam it remains constant, and the frequency of the accelerating electric field varies.
3. Application of a cyclotron
The cyclotron turned out to be the most successful accelerator compared to previously built installations. Dozens of cyclotrons operate in various countries around the world, producing beams of protons, deuterons and b-particles of enormous intensity (up to 1016 particles per second).
Using cyclotrons, fluxes are also obtained fast neutrons. Of course, neutrons cannot be accelerated by an electric field, since they do not have electric charge. Beams of fast neutrons arise as a result of a nuclear reaction at a cyclotron target. To do this, the target is made from an element on which the previously described reaction with the emission of neutrons has high probability(for example, from beryllium).
There is another way to obtain fast neutrons, discovered a little later. This method is based on a completely different type of nuclear reaction. If a beam of fast deuterons is directed at a target, then high probability The following amazing process occurs.
As it passes by the nucleus, a deuteron (consisting of a proton and a neutron) “touches” the nucleus with a proton. The “stripped off” proton remains in the nucleus, and the deuteron neutron continues to move in the direction of the original deuteron beam with an energy of approximately equal to half deuteron energy.
IN recent years Cyclotrons began to accelerate multiply charged ions, for example, oxygen and nitrogen.
Cyclotrons are most widely used in studying the properties of nuclei; with their help it was possible to observe many new nuclear reactions on almost all elements periodic table elements. These experiments allowed physicists to make significant progress in understanding the laws that exist in the world of atomic nuclei.
An important application of the cyclotron is the production of radioactive isotopes. Before construction nuclear reactors only the cyclotron made it possible to prepare these isotopes in any significant amount. cyclotron spectrograph proton therapy
The simplicity of the cyclotron design, as well as the possibility of accelerating protons to energies of the order of 100 MeV, open up wide possibilities for their use in medicine. The new direction is called proton therapy (it must be said that cyclotrons are not the only type accelerators for proton therapy). The main goal of proton therapy is to defeat tumor cells with minimal damage to normal tissue. Beams of charged particles have a significantly better dose distribution in space compared to other radiation. These advantages are especially evident when a target of complex spatial configuration is irradiated and have crucial when exposed to radiation near vital human organs. To penetrate a proton beam to a depth of 5 cm, MeV energy is required; with an energy of up to 190 MeV, almost all tumors can be irradiated with maximum depth localization up to 24cm. Important characteristic accelerator complex is the ability to regulate the amount of energy and scan a beam of accelerated particles to form three-dimensional dose fields of a given shape. The required dose field can be formed both by adjusting the beam parameters in the accelerator and by a system of external scatterers and moderators. Currently, the energy of cyclotrons for proton therapy reaches 250 MeV Cyclotron in PSI (up to 590 MeV)
4. Mass spectrograph. Device principle
The charge of a particle is equal to one or more elementary charges. If it is known, then the mass of the particle can be calculated. This principle underlies the operation of an instrument called a mass spectrograph, which is used to measure the masses of the smallest charged particles - ions and electrons. The device is evacuated to high degree rarefaction vessel placed in a magnetic field, the lines of which are perpendicular to the plane of the drawing. Charged particles are emitted by source 1. The simplest source is electrical discharge in gas. The discharge is accompanied by intense ionization of the gas. At positive difference potentials between diaphragm 2 and the source slit, electrons will be “sucked out” from the discharge and negative ions, at negative difference potentials -- positive ions. By filling the source with various gases or vapors, ions of various elements can be obtained.
Particles passing through slot 3 enter the magnetic field at the speeds imparted to them by the potential difference accelerating them. All particles with a given ratio q/m acquire equal speeds and will describe circles of the same radius in a magnetic field. After deflecting by 180°, the particle beam hits the photographic plate; a dark stripe will appear at the point where the beam hits after developing the plate. The distance AB (Fig. 351) is equal to twice the radius r of the circle along which the particle moved. The value of r depends on the speed of the particle. To find the speed, we use the fact that the particle flies into a magnetic field with kinetic energy Wк=mv2/2, obtained due to the work of the electric field, equal to qU. Thus,
Mass spectrograph diagram: 1 - ion source (gas discharge tube), 2 - diaphragm with slit 3, 4 - photographic plate, U - voltage accelerating ions
Substituting into this formula known values q, B, U and the radius r obtained by measurement, we can calculate the mass of particles that hit point B of the plate.
If the beam emitted by the source contains particles with different relationships charge to mass, several parallel stripes will appear on the photographic plate. The streak closest to the slit is caused by particles moving in a circle smallest radius. These particles have the highest charge to mass ratio. If the charges of all particles in the beam are the same, then the strip closest to the slit corresponds to the particles of the smallest mass.
By analogy with optics, the image obtained on a photographic plate is called a spectrum. An optical spectrograph provides the spectrum of wavelengths of a light beam, i.e., the distribution spectral lines by wavelength. A mass spectrograph provides the mass spectrum of a beam of particles, i.e., the distribution of particles by mass (more precisely, by ratio q/m).
In an experiment to measure the mass of an electron using a mass spectrograph, only one stripe is detected on a photographic plate. Since the charge of each electron is equal to one elementary charge, we conclude that all electrons have the same mass.
5. Application of a mass spectrograph
The mass spectrograph can be used both for research purposes and for production control during various methods obtaining gasoline containing hydrocarbons up to C inclusive.
Undoubtedly, the use of a mass spectrograph for the determination of small amounts of impurities in graphite and many other materials is very interesting and promising. When using stable isotopes their detection and quantification usually carried out using a mass spectrograph and only in rare cases (for example, when working with heavy hydrogen) by determining specific gravity combustion products. If organic compound contains radioactive isotopes, then the determination can easily be made by measuring the radioactivity of the corresponding substance (for example, using a Geiger-Muller counter).
In subsequent years, intensive work was carried out to establish the isotopic composition of elements using a mass spectrograph. However, to determine the relative content of isotopes, it was necessary to increase the accuracy of measurements, which was achieved by using an electrometric lamp connected to a galvanometer as a recorder.
To determine gas-forming impurities in gallium arsenide, the vacuum melting method for determining oxygen and hydrogen, as well as the mass spectrometric method using a mass spectrograph with a spark ion source, are recommended. The latter method determines carbon, nitrogen, oxygen, as well as lithium, magnesium, sulfur and silicon.
This chapter describes such methods, which can be called physical methods of gas analysis. This includes, in particular, various optical methods, as well as gas analysis using a mass spectrograph. It should be noted, however, that the identification of these physical methods in separate group has, of course, a conditional character, since here too in a number of cases it is necessary to combine these physical methods using certain chemical reagents.
The idea of using a mass spectrograph for gas analysis purposes (in particular, for the analysis hydrocarbon gases) was put forward after the invention of this device, which was originally used for the separation and determination of isotopes.
However, the complexity and high cost of the mass spectrograph limit its use in gas analysis. It should also be taken into account that many developed in lately In our Union, instruments for gas microanalysis are superior to a mass spectrograph in their relative sensitivity, and even more so in their simplicity. These devices require analysis more gas, however for most practical problems Obtaining gas samples with a volume of 0.2-1.0 liters or even more is usually not difficult.
Of course, medicine cannot do without mass spectrometry. Isotope mass spectrometry of carbon atoms is used for direct medical diagnosis of human infection with Helicobacter pylori and is the most reliable of all diagnostic methods. Also, mass spectrometry is used to determine the presence of doping in the blood of athletes.
Difficult to imagine the area human activity, where there would be no place for mass spectrometry. Let's limit ourselves to simply listing: analytical chemistry, biochemistry, clinical chemistry, general chemistry And organic chemistry, pharmaceuticals, cosmetics, perfumes, food industry, chemical synthesis, petrochemistry and oil refining, control environment, production of polymers and plastics, medicine and toxicology, forensics, doping control, control narcotic drugs, control alcoholic drinks, geochemistry, geology, hydrology, petrography, mineralogy, geochronology, archaeology, nuclear industry and energy, semiconductor industry, metallurgy.
Literature
· https://ru.wikipedia.org
· http://nuclphys.sinp.msu.ru/experiment/accelerators/ciclotron.htm
· http://physiclib.ru/books/item/f00/s00/z0000039/st007.shtml
· http://chem21.info/info/1608949/
· http://www.physel.ru/2-mainmenu-73/mainmenu-74/708-s-198--.html
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Write down the numbers in your answer, arranging them in the order corresponding to the letters:
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Answer:
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Note.
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Metal ball 1, mounted on a long insulating handle and having a charge, is brought alternately into contact with two similar balls 2 and 3, located on insulating stands and having, respectively, charges − q and + q.
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The spring stiffness of the dynamometer shown in the figure is equal to
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Select two statements from the proposed list that correspond to the results of the conducted experimental observations. Indicate their numbers.
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In a mass spectrograph
1) electric and magnetic fields serve to accelerate a charged particle
2) electric and magnetic fields serve to change the direction of movement of a charged particle
3) the electric field serves to accelerate a charged particle, and the magnetic field serves to change the direction of its movement
4) the electric field serves to change the direction of motion of a charged particle, and the magnetic field serves to accelerate it
Mass spectrograph
Where U B- magnetic field induction; m And q
Answer:
When the magnetic induction increases by a factor of 2, the radius of the circle along which a given charged particle moves,
1) will increase by times
2) will increase 2 times
3) will decrease by half
4) will decrease by 2 times
Mass spectrograph
A mass spectrograph is a device for separating ions based on their charge-to-mass ratio. In its simplest modification, the device diagram is shown in the figure.
Test sample special methods(evaporation, electron impact) is converted into gaseous state, then the resulting gas is ionized in source 1. Then the ions are accelerated by an electric field and formed into a narrow beam in accelerating device 2, after which they enter chamber 3 through a narrow entrance slit, in which a uniform magnetic field is created. The magnetic field changes the trajectory of particles. Under the influence of the Lorentz force, the ions begin to move along a circular arc and fall on screen 4, where the location of their impact is recorded. Registration methods can be different: photographic, electronic, etc. The radius of the trajectory is determined by the formula:
Where U - electrical voltage accelerating electric field; B- magnetic field induction; m And q are the mass and charge of the particle, respectively.
Since the radius of the trajectory depends on the mass and charge of the ion, different ions fall on the screen at different distances from the source, which makes it possible to separate them and analyze the composition of the sample.
Currently, numerous types of mass spectrometers have been developed, the operating principles of which differ from those discussed above. For example, dynamic mass spectrometers are manufactured, in which the masses of the ions under study are determined by the time of flight from the source to the recording device.
From the course on electricity, we know that a charged particle moving in a magnetic field is acted upon by a force called the Lorentz force. The Lorentz force is perpendicular to the magnetic field and to the velocity of the particle, and its direction is determined by the left-hand rule (Fig. 349). The magnitude of this force is proportional to the charge of the particle, its speed, the magnetic induction of the field and the sine of the angle between the vectors and. If the direction of velocity is perpendicular to the direction of induction, then the modulus of the Lorentz force is expressed by the formula
where is the charge of the particle in coulombs, is its speed in meters per second, is the induction in tesla, and is the force in newtons. The acceleration imparted by the Lorentz force, like any force in general, is directly proportional to the force and inversely proportional to the mass of the particle.
Rice. 349. Direction of the Lorentz force acting on a charge moving in a magnetic field with a speed. The case of a positive charge is depicted. For negative charge force is directed in the opposite direction
Let us consider the motion of a particle in a uniform magnetic field directed perpendicular to the particle velocity. Since the Lorentz force and, therefore, acceleration are perpendicular to the speed, the particle will move in a circle; in this case, the velocity module remains unchanged, because, as is known from mechanics, perpendicularity of acceleration and velocity is characteristic of uniform motion in a circle. The acceleration of a particle during uniform motion in a circle is equal to , where is the radius of the circle. Thus, the particle acceleration
,
The less, the larger radius particle trajectory for given and B (Fig. 350). Knowing and and measuring the radius of the trajectory, we can determine the ratio of the particle’s charge to its mass. The charge of a particle is equal to one or more elementary charges. If it is known, then the mass of the particle can be calculated. This principle underlies the operation of an instrument called a mass spectrograph, which is used to measure the masses of the smallest charged particles - ions and electrons.
Rice. 350. Trajectories of charged particles with equal initial speeds in a uniform magnetic field: 1 – small ratio, 2 – large ratio; 1 and 2 – negatively charged particles; 3 – positively charged particle. The magnetic field lines are perpendicular to the drawing plane and directed towards us
The diagram of a mass spectrograph with a uniform magnetic field is shown in Fig. 351. The device is a vessel evacuated to a high degree of vacuum, placed in a magnetic field, the lines of which are perpendicular to the plane of the drawing. Charged particles are emitted by source 1. The simplest source is an electrical discharge in a gas. The discharge is accompanied by intense ionization of the gas. With a positive potential difference between diaphragm 2 and the source slit, electrons and negative ions will be “sucked out” from the discharge; with a negative potential difference, positive ions will be “sucked out”. By filling the source with various gases or vapors, ions of various elements can be obtained.
Rice. 351. Diagram of a mass spectrograph: 1 - ion source (gas discharge tube), 2 - diaphragm with slit 3, 4 - photographic plate, - ion accelerating voltage
Particles passing through slot 3 enter the magnetic field at the speeds imparted to them by the potential difference accelerating them. All particles with a given ratio acquire equal speeds and will describe circles of the same radius in a magnetic field. After being deflected, the particle beam hits the photographic plate; a dark stripe will appear at the point where the beam hits after developing the plate. The distance (Fig. 351) is equal to twice the radius of the circle along which the particle moved. The value depends on the speed of the particle. To find the speed, we use the fact that the particle flies into a magnetic field with kinetic energy obtained due to the work of the electric field, equal to . Thus,
(198.2)
From (198.1) and (198.2) we have
By substituting the known values and the radius obtained by measurement into this formula, we can calculate the mass of particles that hit the point of the plate.
If the beam emitted by the source contains particles with different charge-to-mass ratios, several parallel stripes will appear on the photographic plate. The streak closest to the slit is caused by particles moving along a circle of smallest radius. These Particles have the greatest; ratio of charge to mass. If the charges of all particles in the beam are the same, then the strip closest to the slit corresponds to the particles of the smallest mass.
By analogy with optics, the image obtained on a photographic plate is called a spectrum. An optical spectrograph provides the spectrum of wavelengths of a light beam, i.e., the distribution of spectral lines over wavelengths. A mass spectrograph provides the mass spectrum of a particle beam, i.e., the distribution of particles by mass (more precisely, by ratio).
