Electric discharge in gases, discharge tube, cathode rays, X-rays and their properties

Electric discharge in gases

Electrical discharge through gases is studied by using a specially designed glass tube commonly called as a discharge tube. It consists of a cylindrical glass tube having a side tube, and two metallic electrodes one at each end. These electrodes can be connected to the respective terminals of a high tension power supply. Air from inside the tube can be pumped out by connecting the side tube to a vacuum pump, and a desired pressure can be maintained inside the tube.

It was found that gases could not conduct electricity even when an electrical potential of about 10,000 volts was applied. But, it was discovered by William Crookes that gases could conduct electricity at low pressures. When, the pressures inside the discharge tube were reduced gradually, the following point was noted.

  • At about 10-2 atm pressure, a glow surrounding the cathode(negative electrode) leaves the electrode surface, and little space is left between it and the electrode. This is called Crooke ‘mass dark space. At this stage, electric current begins flow from one electrode to other.

At sufficiently low pressure (about 10-3 atm), this glow fills whole the tube. The color of the glow depends upon the nature of the gas in the tube, and on the color of the glass used for making the discharge tube.

  • When the pressure is lowered to about 10-5 atmosphere, lights emission by the residual air in the discharge tube opposite to the cathode start glowing. At this stage, a stream, called cathode rays is emitted from the cathode.

Thus, we see that when electrical discharge is passed through gasses at very low pressure, cathode rays are produced.

You must have seen bright advertising sign boards of different colors at the Shopping malls and around your city/town. These sign boards consist of many discharge tubes in which neon gas, or a mixture of neon gas with some other gas, is filled at a very low pressure. When very high electrical potential is applied across these tubes, the glows different colors are produced. The color of glow depends upon the nature of the gas in the tube, and the color of the glass used in making the discharge tube. For example, the colors obtained when neon gas or its mixture are used.

Sodium vapor street lamps and television tubes are also based only the same process of electrical discharge through gasses at low pressures.

 

Discharge tube

A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube; these include fluorescent lamps, metal-halide lamps, sodium-vapor lamps, and neon lights. Specialized gas-filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices.

The voltage required to initiate and sustain discharge is dependent on the pressure and composition of the fill gas and geometry of the tube. Although the envelope is typically glass, power tubes often use ceramics, and military tubes often use glass-lined metal. Both hot cathode and cold cathode type devices are encountered.

cathode rays

Cathode rays (also called an electron beam or an e-beam) are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite the negative electrode is observed to glow from electrons emitted from the cathode. Electrons were first discovered as the constituents of cathode rays. The image in a classic television set is created by focused beam of electrons deflected by electric or magnetic fields in cathode ray tubes (CRTs).

Cathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube. To release electrons into the tube, they must first be detached from the atoms of the cathode. The early cold cathode vacuum tubes, called Crookes tubes, used a high electrical potential between the anode and the cathode to ionize the residual gas in the tube. The electric field accelerated the ions and the ions released electrons when they collided with the cathode.

Modern vacuum tubes use thermionic emission, in which the cathode is made of a thin wire filament that is heated by a separate electric current passing through it. The increased random heat motion of the filament atoms knocks electrons out of the atoms at the surface of the filament and into the evacuated space of the tube. Since the electrons have a negative charge, they are repelled by the cathode and attracted to the anode. They travel in straight lines through the empty tube. The voltage applied between the electrodes accelerates these low mass particles to high velocities.

Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall of the tube, exciting the atoms of the glass and causing them to emit light—a glow called fluorescence. Researchers noticed that objects placed in the tube in front of the cathode could cast a shadow on the glowing wall, and realized that something must be traveling in straight lines from the cathode. After the electrons reach the anode, they travel through the anode wire to the power supply and back to the cathode, so cathode rays carry electric current through the tube.

X-rays and their properties

X-rays or x-radiation are part of the electromagnetic spectrum with shorter wavelengths (higher frequency) than visible light. X-radiation wavelength ranges from 0.01 to 10 nanometers, or frequencies from 3×1016 Hz to 3×1019 Hz. This puts the x-ray wavelength between ultraviolet light and gamma rays. The distinction between x-ray and gamma rays may be based on wavelength or on radiation source. Sometimes x-radiation is considered to be radiation emitted by electrons, while gamma radiation is emitted by the atomic nucleus.

German scientist Wilhelm Röntgen was the first to study x-rays (1895), although he was not the first person to observe them. X-rays had been observed emanating from Crookes tubes, which were invented circa 1875. Röntgen called the light “X-radiation” to indicate it was a previously unknown type. Sometimes the radiation is called Röntgen or Roentgen radiation, after the scientist. Accepted spellings include x rays, x-rays, xrays, and X rays (and radiation).

The term x-ray is also used to refer to a radiographic image formed using x-radiation and to the method used to produce the image.

Sources of X-Rays

X-rays may be emitted whenever sufficiently energetic charged particles strike matter. Accelerated electrons are used to produce x-radiation in an x-ray tube, which is a vacuum tube with a hot cathode and a metal target. Protons or other positive ions may also be used. For example, proton-induced x-ray emission is an analytical technique. Natural sources of x-radiation include radon gas, other radioisotopes, lightning, and cosmic rays.

How X-Radiation Interacts With Matter

The three ways x-rays interact with matter are Compton scattering, Rayleigh scattering, and photoabsorption. Compton scattering is the primary interaction involving high energy hard x-rays, while photoabsorption is the dominant interaction with soft x-rays and lower energy hard x-rays. Any x-ray has sufficient energy to overcome the binding energy between atoms in molecules, so the effect depends on the elemental composition of matter and not its chemical properties.

Most people are familiar with x-rays because of their use in medical imaging, but there are many other applications of the radiation:

In diagnostic medicine, x-rays are used to view bone structures. Hard x-radiation is used to minimize absorption of low energy x-rays. A filter is placed over the x-ray tube to prevent transmission of the lower energy radiation. The high atomic mass of calcium atoms in teeth and bones absorbs x-radiation, allowing most of the other radiation to pass through the body. Computer tomography (CT scans), fluoroscopy, and radiotherapy are other x-radiation diagnostic techniques. X-rays may also be used for therapeutic techniques, such as cancer treatments.

X-rays are used for crystallography, astronomy, microscopy, industrial radiography, airport security, spectroscopy, fluorescence, and to implode fission devices. X-rays may be used to create art and also to analyze paintings. Banned uses include x-ray hair removal and shoe-fitting fluoroscopes, which were both popular in the 1920s.

 

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