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X-ray
1. Use of X-raya. Medical diagnosis ……………………………………….1b. Industry …………………………………….……………….2
c. Chemistry ………………………………….……………….22. X-ray spectrum…………………………….…..…….6
3 . Emission Spectra…………………………………..8
4 . Production…………………………………………..11
5 . Properties…………………………………………..12
Guess what these are !
a. Medical diagnosis
X-ray penetrates flash but not bone. Bone will block most of the photons, and will appear white on developed film. Structures containing air will be black on film, and muscle, fat, and fluid will appear as shades of gray.
1. Use of X-ray:
For stomach or intestines, the patient must swallow barium (Ba, 鋇 ) which is opaque.
For blood, the patient must swallow Iodine (I2, 錪 )
The test is performed in a hospital radiology department or in the health care provider's office by an X-ray technologist. The positioning of the patient, X-ray machine, and film depends on the type of study and area of interest. Multiple individual
views may be requested
b. Industry
Use the penetrating power of X-rays to examine the inside of machine parts for cracks
c. Chemistry
Study about crystal structure by using the diffraction of X-rays
This application relies on the fact that the wavelength of X-rays is similar to the inter-atomic spacing in crystal.
Crystal DiffractionCrystal Diffraction
Who suggest “crystal diffraction ? The first proof of the wave-nature of X-rays was due to Laue in 1913. He suggested that the regular small spacing of atoms in crystals might provide a natural diffraction grating if the wavelengths of the rays were too short to be used with an optical line grating.
Experiments by Friedrich and Knipping showed that X-rays were indeed diffracted by a thin crystal, and produced a pattern of intense spots round a central image on a photographic plate placed to received them. The rays had thus been scattered by interaction with electrons in the atoms of the crystal. The diffraction pattern obtained gave information on the geometrical spacing of the atoms.
Who show “crystal diffraction ?
X-ray X-ray SpectrumSpectrumThe characteristic X-ray spectrum from a metal is usually superimp
osed on a background of continuous, or so-called 'white', radiation of small intensity. Figure 33.21 illustrates the characteristic lines, Kα, Kβ, of a metal and the continuous background of radiation for two values of p.d., 40000 and 32000 volts, across an X-ray tube. It should be noted that: (i) the wavelengths of the characteristic lines are independent of the p.d.—they are characteristic of the metal, and (ii) the background of continuous radiation has increasing wave lengths which slowly diminish in intensity, but as the wavelength diminish they are cut off sharply, as at A and B.
When the bombarding electrons collide with the metal atoms in the target, most of their energy is lost as heat. A little energy is also lost in the form of electromagnetic radiation. Here the frequencies are given E = hf with the usual notation, and the numerous energy changes produce the background radiation in Figure 33.21.
Emission SpectrumEmission Spectrum
When an energetic electron from the cathode strikes the atom, it can knock out an inner electron, leaving a vacancy in a lower level (line 1). An
electron from a higher energy level falls into
the vacancy (line 2); the energy difference is
carried away by a photo, which is usually in
the x-ray range. The wavelength of the X-ray thus emitted is obviously determined by the energy difference of the downward transition . Downward transitions to the K level give rise to the Kα, Kβ... lines; downward transitions to the L level give rise to the Lα, Lβ... lines, and so on., In fact, the K, L, M series are analogous to the Lyman, Balmer and Paschen series in the spectrum of hydrogen.
From: Text Book (2) P.377
What are x-rays?
They are a type of electromagnetic radiation produced whenever cathode rays (high-speed electrons) are brought to rest by matter.
Production
A focused beam of electrons is accelerated towards the tungsten target ( i.e. the anode)
On collision the electrons decelerate rapidly and x-rays are produced.
Over 99% of the KE of the electron goes into heat.
The rod is cooled by circulating oil through it or by the use of cooling fans.
The target is a high-melting point metal such as tungsten or molybdenum in a copper rod, the purpose of which is to conduct heat away from the target.
Maximum frequency for given tube potential
The continuous spectrum shows a well-defined minimum wavelength (maximum frequency). This corresponds to an electron losing all its energy in a single collision with a target atom.
The longer the wavelengths (smaller energies) corresponded to a more gradual loss of energy, which happens when the electron experiences several deflections and collisions and so is slowed down more gradually. All or some of the K.E. of the electron is converted into the energy of photon(s). This radiation is called bremsstrahlung (braking radiation). All targets show this continuous spectrum.
A continues spectrum
The kinetic energy of a bombarding electron = eV
Where V is the accelerating p.d.
Therefore, eV = hfmax
Where fmax is the frequency of the most energetic photon (processing all the initial kinetic energy of the colliding electron).
Greater current
The larger total kinetic energy of the electrons.
cathode
accelerating voltage
greater accelerating voltage
The larger kinetic energy of each electron colliding on the anode
Properties
i. They travel in straight lines at the velocity of light.
ii. They cannot be deflected by electric of magnetic fields. (This is convincing evidence that they are not charged particles.)
iii. They are electromagnetic radiation of very short wavelength about 10-10m.
iv. They penetrate matter, penetration is least with materials of high density and high atomic number.They can be reflected.
v. Refractive indices of all materials are very close to unity for x-rays so that very little bending occurs when they pass from one materials to another. They cannot be focused by lenses.
vi. They can be diffracted.
iv. They ionize gases through which they pass.
v. They affected photographic film.
viii. They can produce fluorescence.
ix. They can produce photoelectric emission.
