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1、Chapter 16 X-rays X-rays were discovered late in the nineteenth century by the German physicist Wilhelm Roentgen. Roentgen was studying low-pressure discharges in a gas and noticed that the anode of his discharge tube was the source of energetic and penetrating radiation. The penetrating properties
2、of x-rays are of great practical important and have been utilized by medical profession almost since the year of their discovery.X-ray are produced when high-velocity electrons strike a solid target. (usually metal)Chapter 16 X-rays16-1 The generate of x-rays1.Device for generating X-ray1. the condi
3、tion for generating X-ray are:(1). High-velocity electrons beam (2). the metal target The metal target resist the motion of the electrons and transfer their kinetic energy to the X-rays energy. X-rays are high-frequency electromagnetic wave whose possess the wavelength between some 0.05100. An elect
4、ron discharge through a low-pressure gas makes relatively small energies available for excitation. The outmost atomic electron is excited and the resulting spectrum mainly in the visible and infrared region. A mode of excitation that can eject electrons from inner levels result in x-rays emission. L
5、et us briefly describe the usual method of exciting inner shell electrons. And then interpret The situation in term of quantum energy levels.Chapter 16 X-raysThe numbers of the emitted x ray photons and the total energy is approximately proportional to the square of the atomic number of the target m
6、aterial. High voltage applied to the tube V: 100-102kvcurrent across the tube i:102-103mA2. X ray generatorA typical X-ray tube is illustrated in figure below. A beam of electrons emitted from a hot cathode in a vacuum is accelerated by high-voltage and strike a metal target anode. Fig 16-1Am102kvca
7、thodeanodeChapter 16 X-raysThe impact energy of the beam of electrons eject some electrons from the inner shells of the atoms in the target, enabling electrons from the outer shells to make transitions to the available inner shells vacancies. The energy lost by electrons that make the transitions is
8、 given up as X rays, whose emission accompanies the transition.3.Actual focus area and effective focus areausually effective focus area is smaller than Actual focus area,and is approximately shape to a rectangle. (see Fig 16-2) smaller focus area for photography and larger smaller focus area for med
9、ical treatment. 2. The intensity and hardness of x ray(1)x ray intensitywhere Ni is the number of the x ray photon with energy hi,two way to increase intensity I:a. tube current i Ni I (convenient to be measured)b. Tube voltage V i I (hard to be measured)So we use tube current i to marking x ray int
10、ensity with the unit mA.Chapter 16 X-rays(2) x ray hardnessx ray hardness means x rays penetrating ability, and only depends on the wavelength of the x ray. That is :V i penetrating ability .So we use tube voltage V to marking x rays hardness with the unit kv.Hardness tube voltage（kv） min（） usagever
11、y soft 5 20 2.50 0.62 photography and skin treatmentsoft 20 100 0.62 0.12 photography and fluoroscopyhard 100 250 0.12 0.05 deeper tissue treatmentvery hard 250 0.05 deep tissue treatmentChapter 16 X-rays16-2 x ray spectrumThe total x-ray emission is composed of two separate parts. The wavelength of
12、 the characteristic x-ray that depend on the material of target. The continuous part of the X-ray radiation, known as bremsstrahlung(German for “braking radiation”). Does not depend on target material, but only on the magnitude of the high voltage applied to the tube. 1. Continuous spectrum(1)princi
13、ple :it was known as braking radiation.As the name implies, bremsstrahlung has its origin in the braking effect of the target on the electrons in the incident beam. The target slow these electrons, and some of the energy lost is given off directly as X ray. The most energetic X-ray photon that is po
14、ssible has a quantum energy equal to the kinetic energy of an electron in the beam. The kinetic energy in turn arise from the transport of an electron charge through the high voltage potential difference. We therefore have: maximum photon energy Chapter 16 X-rayskinetic energy of electron Combining
15、these results, we can write a formula for the wavelength of the most energetic photon. h =Plancks constantc =velocity of lighte = electron charge U=voltage across the tube2. characteristic x-ray The notation(symbol) for named X rays is shown in Fig 16-2. Shell designation O N M M series L L series K
16、 K series Figure 16-2The notation(symbol) for named X rays is shown in Fig 16-2, Shell designation O N M M series L L series K K series Figure 16-2Each series of X ray lines in named for the shell in which the excitation process causes the vacancy. The K series results from the an electron in an upp
17、er shell making a downward transition to a K shell vacancy, The L series from a downward transition to a L shell vacancy, and so on. A Greek letter subscript (,and so on)indicate the position of the line in the series. Chapter 16 X-raysThe K line is the first line in the K series, which result from
18、L shell electron making a transition to a vacancy in the K shell. The K line result from M shell electron making a transition to a K shell vacancy, and so on.Chapter 16 X-raysExtensive research on X rays by the English physicist Henry Moseley shown that the wavelength of K and K line could be calcul
19、ated from empirical formulas similar to Rydbergs formula except for the addition of the factor (Z-1) 2 in the K wavelength formula and the factor (Z-7.4) 2 in the K wavelength formula.Moseleys formula for the wavelength of K and K X rays lines:R=Rydberg constant Z = atomic number for target materialAssignment: P327 13-6; 13-7;13-8 Chapter 16 X-raysIntensityWavelength
