X-Rays & Their Properties Review & Overview of X-Rays & Their Properties X-Rays were discovered in 1895 by German physicist Wilhelm Conrad Röntgen. They were called “X-Rays” because their nature was unknown at the time. He was awarded the Physics Nobel Prize in 1901. The 1st X-Ray photograph taken was of Röntgen’s wife’s left hand. Wilhelm Conrad Röntgen (1845-1923) Bertha Röntgen’s Hand 8 Nov, 1895

Review of X-Ray Propertıes X-Rays are invisible, highly penetrating Electromagnetic Radiation of much shorter wavelength (higher frequency) than visible light. Wavelength (λ) & frequency (ν) ranges for X-Rays: 10-8 m ~ ≤ λ ~ ≤ 10-11 m 3 × 1016 Hz ~ ≤ ν ~ ≤ 3 × 1019 Hz

X-Ray Energies λx-ray ≈ 10-10 m ≈ 1 Ǻ  E ~ 104 eV In Quantum Mechanics, Electromagnetic Radiation is described as being composed of packets of energy, called photons. The photon energy is related to its frequency by the Planck formula: We also know that, in vacuum, the frequency & the wavelength are related as: Combining these gives: λ = Wavelength ν = Frequency c = Speed of Light λx-ray ≈ 10-10 m ≈ 1 Ǻ  E ~ 104 eV

are produced by the movement This released energy is in X-Ray Production Visible light photons, X-Ray photons, & essentially all other photons are produced by the movement of electrons in atoms. We know from Quantum Mechanics that electrons occupy energy levels, or orbitals, around an atom's nucleus. If an electron drops to a lower orbital (spontaneously or due to some external perturbation) it releases some energy. This released energy is in the form of a photon The photon energy depends on how far in energy the electron drops between orbitals.

Schematic Diagram of Photon Emission Incoming particles excite an atom by promoting an electron to a higher energy orbit. Later, the electron falls back to the lower orbit, releasing a photon with energy equal to the energy difference between the two states: hν = ΔE Remember that this figure is a schematic “cartoon” only, shown to crudely illustrate how atoms emit light when one of the electrons transitions from one level to another. It gives the impression that the electrons in an atom are in Bohr-like orbits around the nucleus. From Quantum Mechanics, we know that this picture is not valid, but the electron wavefunction is spread all over the atom. So, don’t take this figure literally!

X-Ray Tubes X-Rays can be produced in a highly evacuated glass bulb, called an X-Ray tube, that contains two electrodes: an anode made of platinum, tungsten, or another heavy metal of high melting point, & a cathode. When a high voltage is applied between the electrodes, streams of electrons (cathode rays) are accelerated from the cathode to the anode & produce X-Rays as they strike the anode. Evacuated Glass Bulb Cathode Anode

Monochromatic & Broad Spectrum X-rays X-Rays can be created by bombarding a metal target with high energy (> 104 eV) electrons. Some of these electrons excite other electrons from core states in the metal, which then recombine, producing highly monochromatic X-Rays. These are referred to as characteristic X-Ray lines. Other electrons, which are decelerated by the periodic potential inside the metal, produce a broad spectrum of X-Ray frequencies. Depending on the diffraction experiment, either or both of these X-Ray spectra can be used.

X-Ray Absorption X-Rays The atoms that make up our body’s tissue absorb visible light photons very well. The energy level of the photon fits with various energy differences between electron states. Radio waves don't have enough energy to move electrons between orbitals in larger atoms, so they pass through most materials. X-Ray photons also pass through most things, but for the opposite reason: They have too much energy. You will never see something like this with Visible Light!!  X-Rays

Generation of X-rays (K-Shell Knockout) not to be taken literally! An electron in a higher orbital falls to the lower energy level, releasing its extra energy in the form of a photon. It's a large drop, so the photon has high energy; it is an X-Ray photon. Another schematic cartoon diagram, not to be taken literally! A “free” electron collides with a tungsten atom, knocking an electron out of a lower orbital. A higher orbital electron fills the empty position, releasing its excess energy as an X-Ray Photon

Similar Viewpoint: Generation of Bremsstrahlung Radiation “Braking” Radiation: Electron deceleration releases radiation across a spectrum of wavelengths. Atom of the Anode Material Electron Orbits Electron (slowed & changed direction) Bremsstrahlung radiation means “braking” radiation. Electron deceleration releases radiation across a spectrum of wavelengths. The braking radiation represents a continuum (white radiation). Nucleus Fast Incident Electron X-ray

Generation of Characteristic Radiation K L K K L M Emission Photoelectron Electron Incoming electron knocks out an electron from the inner shell of an atom. Designation K,L,M correspond to shells with a different principal quantum number. Here the electrons suddenly decelerate upon colliding with the metal target. If enough energy is contained within the electron, it is able to knock out an electron from the metal atom’s inner shell. This is an unstable state, and this vacancy is quickly filled by an electron from a higher shell. This process releases a “quantum” or photon of radiation that has a wavelength (or energy) characteristic of the energy difference between shells. The designations K, L, and M correspond to the quantum number q = 1, 2, 3 α and β indicates the shell that the “filling” electron is from relative the vacancy shell. Ka indicates the photon that is released from an electron transition from the L shell to the K shell

Generation of Characteristic Radiation Bohr`s model Not every electron in each of these shells has the same energy. The shells must be further divided. K-shell vacancy can be filled by electrons from 2 orbitals in the L shell, for example. The electron transmission and the characteristic radiation emitted is given a further numerical subscript. In reality, the picture a little more complicated. All electrons in each of these shells do not have exactly the same energy. The shells themselves must be further divided to indicate this. So, in this example a K-shell vacancy can be filled by 2 different L-shell electrons with slightly different energies. These shells are designated 1 and 2. So when we talk about Kα radiation here we are actually talking about both the Kα1 and Kα2 wavelengths (and sometimes more).

KKK000 Generation of Characteristic Radiation Energy levels (schematic) of the electrons M The L level is actually made up of 3 different energy levels . Intensity ratios KKK000 L K K K K K

Emission Spectrum of an X-Ray Tube Braking = continuous spectra Characteristic = line spectra At 8.5 kV, no characteristic line is produced. As accelerating voltage is increased, characteristic lines appear and grow in intensity, but so does the braking radiation. Although the characteristic radiation is much stronger, all of these wavelengths can participate in XRD and at the very least cause undesirable background.

Emission Spectrum of an X-Ray Tube: Close-up of Ka As we will see later, we must have a source of monochromatic X-rays for X-ray diffraction. We must try to eliminate white radiation and Kβ radiation to use only the Kα. Cullity, B.D. and Stock, S.R., 2001, Elements of X-Ray Diffraction, 3rd Ed., Addison-Wesley

Sealed X-ray Tube Cross Section The tube itself is evacuated and sealed to insulate the anode and cathode, prevent contamination, and generally extend the tube’s lifetime. The cathode is a tungsten filament, and the anode is a metal target (Cu, Co, Cr, Fe, or Mo). The filament current of ~ 3 amps produces electrons that are accelerated to the target by the voltage across the cathode and anode. Tube current is a measure of the flow of electrons from the filament to the target. X-rays are emitted in all directions, but they only exit the tube at 2 or more Be windows. The windows maintain the evacuated environment, but are very transparent to X-rays. This process produces considerable heat, which necessitates water cooling. Cullity, B.D. and Stock, S.R., 2001, Elements of X-Ray Diffraction, 3rd Ed., Addison-Wesley Sealed tube Cathode / Anode Beryllium windows Water cooled 16

Characteristic Radiation for Common X-ray Tube Anodes Ka1 (100%) Ka2 (50%) Kb (20%) Cu 1.54060 Å 1.54439 Å 1.39222 Å Mo 0.70930 Å 0.71359 Å 0.63229 Å Note how anodes made of a different material have Ka lines with different wavelengths. To get a different wavelength, simply change the composition of the anode!!

Modern Sealed X-ray Tube Tube made from ceramic Beryllium window is visible. Anode type and focus type are labeled. Tube now made of ceramic. Beryllium windows

Sealed X-ray Tube Focus Types: Line and Point Target Filament Spot Take-off angle The X-ray beam’s cross section at a small take-off angle can be a line shape or a spot, depending on the tube’s orientation. The take-off angle is the target-to-beam angle, and the best choice in terms of shape and intensity is usually ~6°. A focal spot size of 0.4 × 12 mm: 0.04 × 12 mm (line) 0.4 × 1.2 mm (spot) The filament is shaped to produce a line focus on the target material. The beam’s cross section viewed at a small take-off angle can be line- or spot-shaped, depending on the tube’s orientation with respect to the focal spot. The take-off angle is the target-to-beam angle. Best angle is typically 6 degrees in terms of shape and X-ray beam intensity. Focal spot size = 0.4 × 12 mm results in beam cross-section of 0.04 × 12 mm for line and 0.4 × 1.2 mm for point. Target Line

X-Ray Absorption A larger atom is more likely to absorb an X-Ray Photon in this way than a smaller one because larger atoms have greater energy differences between orbitals.  The energy level difference then more closely matches the energy of an X-Ray Photon. Smaller atoms, in which the electron orbitals are separated by relatively low energy differences, are less likely to absorb X-Ray Photons. The soft tissue in our bodies is composed of smaller atoms, & so does not absorb X-Ray Photons very well. The calcium atoms that make up our bones are large, so they are better at absorbing X-Ray photons.