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 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-rays ξ16-1 The generate of x-rays 1.Device for generating X-ray 1. the condition 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 0.05—100Å. An electron 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. Let 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-rays The numbers of the emitted x ray photons and the total energy is approximately proportional to the square of the atomic number of the target material. High voltage applied to the tube V: kv current across the tube i: mA

2. X ray generator A 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 16-1 Am 10 2 kv cathodeanode

Chapter 16 X-rays The 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 given up as X rays, whose emission accompanies the transition. 3.Actual focus area and effective focus area usually 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 medical treatment.

2. The intensity and hardness of x ray (1)x ray intensity where N i is the number of the x ray photon with energy hν i, two way to increase intensity I: a. tube current i ↑ → N i ↑ → I ↑(convenient to be measured) b. Tube voltage V ↑ → νi↑ → I ↑ (hard to be measured) So we use tube current i to marking x ray intensity with the unit mA.

Chapter 16 X-rays (2) x ray hardness x ray hardness means x ray’s penetrating ability, and only depends on the wavelength of the x ray. That is :V ↑ → νi↑ → penetrating ability ↑. So we use tube voltage V to marking x ray’s hardness with the unit kv.

Hardness tube voltage （ kv ） λmin （ Å ） usage very soft 5 ∼ ∼ 0.62 photography and skin treatment soft 20 ∼ ∼ 0.12 photography and fluoroscopy hard 100 ∼ ∼ 0.05 deeper tissue treatment very hard ≥250 ≤0.05 deep tissue treatment

Chapter 16 X-rays ξ16-2 x ray spectrum The total x-ray emission is composed of two separate parts. The wavelength of 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)principle :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 possible 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-rays kinetic energy of electron Combining these results, we can write a formula for the wavelength of the most energetic photon. h =Planck’s constant c =velocity of light e = electron charge U=voltage across the tube

2. characteristic x-ray The notation(symbol) for named X rays is shown in Fig Shell designation O N α β M M series α β γ L L series α β γ δ K K series Figure 16-2

The notation(symbol) for named X rays is shown in Fig 16-2, Shell designation O N α β M M series α β γ L L series α β γ δ K K series Figure 16-2

Each 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 upper 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 (α,β,γand so on)indicate the position of the line in the series.

Chapter 16 X-rays The K α line is the first line in the K series, which result from L shell electron making a transition to a vacancy in the K shell. The K β line result from M shell electron making a transition to a K shell vacancy, and so on.

Chapter 16 X-rays Extensive research on X rays by the English physicist Henry Moseley shown that the wavelength of K α and K β line could be calculated from empirical formulas similar to Rydberg’s formula except for the addition of the factor (Z-1) 2 in the K α wavelength formula and the factor (Z-7.4) 2 in the K β wavelength formula.

Moseley’s formula for the wavelength of K α and K β X rays lines: R=Rydberg constant Z = atomic number for target material

Assignment: P ; 13-7;13-8 Chapter 16 X-rays

Intensity Wavelength