RTEC A - WEEK 4 GENERAL SCIENCE REVIEW & X-RAY PRODUCTION IN THE TUBE

Objectives General Science review Atomic interactions in the tube

Atomic Models BOHR model of the atom. Electrons orbit around a nucleus (center) Bohr’s model of the atom. It looks like a miniature solar system in which the electrons are orbiting around the nucleus (center) at various energy levels. There are many other forms models of the atom, and the most accurate is the quantum mechanics model. But for the purposes of radiology the Bohr model better serves our purpose. Atom is the smallest quantity of an element.

ATOM The nucleus is positively charged, small and dense. In a normal atom the number of electrons is equal to the number of protons. The primary particles of the atom are : neutrons, protons and electrons. Neutrons are in the nucleus and is electrically neutral. Protons are also in the nucleus but they have a positive charge. Electrons orbit the nucleus and have a negative charge.

Differences in Binding Energy Difference in energy levels between the shells. Takes a 70kvp energy to knock out a K shell electron. In this element- which is tungsten. The filament and target material. It has 74 protons. Kev get smaller as the shells go out. This tells you the closer you are to the nucleus the more binding energy there is. The binding energy is what it will take to eject an electron from its home. SO in the k shell it would take a minimum energy (kVp) of 69.5 to remove an electron from a k shell. The P shell needs a much smaller amount. As it has less binding energy. The atoms are very structured. If a k shell electron is empty, it must be filled. As the holes are filled there is more photon energy released. They are always moving into the nucleus because they are negative and the nucleus is positive (law of attraction).

K L M Shells Electrons orbit around nucleus. The K,L,M are the shells we are interested in in radiology. Usually the shell of main importance is K shell.

Electrostatic Laws Repulsion/attraction Inverse square relationship Like charges repel Unlike charges attract Inverse square relationship Electrostatic force is very strong when objects are close together It decreases rapidly as objects separate Like charges repel, unlike charges attract Inverse square relationship is much like the inverse square law of x-ray intensity. The electrostatic force is directly proportional to the product of the electrostatic charges and inversely proportional to the square of the distance.

How “X-rays” are created TO PRODUCE X-RAYS YOU NEED: A SOURCE OF ELECTRONS A FORCE TO MOVE THEM QUICKLY SOMETHING TO STOP THEM SUDDENLY

How “X-rays” are created Power is sent to x-ray tube via cables mA (milliamperage) is sent to filament on cathode side. Filament heats up – electrons “boil off” Negative charge As electron kinetic energy is increased both the intensity (quantity) and energy (quality) of the x-ray beam are increased. All electrons have mass, electron kinetic energy is increased by raising the kVp. (Kinetic energy is the energy of motion) Distance between the filament and the anode target is only about 1cm.

How “X-rays” are created Positive voltage (kVp) is applied to ANODE Negative electrons = attracted across the tube to the positive ANODE. Electrons “slam into” anode – suddenly stopped. X-RAY PHOTONS ARE CREATED When the projectile electron hit the metal of the atoms of the x-ray tube target, they transfer their kinetic energy to the target atoms. The electrons can interact with orbital electrons or the nuclear field of target atoms. These interactions result in the conversion of electron kinetic energy into thermal energy (heat -99%) or electromagnetic energy (x-rays- 1%).

Electromagnetic Energy Spectrum Continuous range of energy Although there are precise ranges defined, they often overlap 3 most important to Radiologic technology: Visible light X-radiation Radiofrequency EM energy is a continuous range from lower energy AMFM radio to gamma rays. EM travels in the sine wave and oscillates from magnetic to electric fields If you get enough energy it can penetrate matter.These generally have shorter wavelengths and increased frequency. Each can be described as a bundle of energy consisting of various electric and magnetic fields traveling at the speed of light. The photons the EMS differ only in their wavelength and frequency.

Electromagnetic Radiation Photon is the smallest quantity of any type of EM radiation It is a small bundle of energy traveling at the speed of light Only visible light is naturally apparent to us May be described as wavelike fluctuations of electric and magnetic fields.

Electromagnetic Radiation These bundles of electric and magnetic fields travel at the same velocity: Travel at the speed of light 3 x 108 m/s or 186,400 miles per sec The Photons of EM radiation differ only in frequency and wavelength

General Characteristics of EMS X-ray photons: Have no mass or physical form Travel in a linear path (until interaction occurs) Dual nature: wave vs. particle Unaffected by electric or magnetic fields gravity Electromagnetic structure. Ionizing EM radiation is usually characterized by the energy of the photon. X-ray photons contains considerably more energy than visible light or radiofrequency. The energy of the photon is directly proportional to is frequency. As frequency increases: energy increases.

Wavelength and Frequency Wavelength is the difference between: Crest to Crest Valley to Valley Frequency is the number of wavelengths passing a point of observation per second Wavelength & frequency are inversely proportional As Wavelength increases frequency decreases As wavelength decreases frequency increases

Wavelength and Frequency Frequency and wavelength are closely associated with the relative energy of electromagnetic radiations. More energetic radiations have shorter wavelengths and higher frequency.

Wavelength A second important characteristic of light waves, and all electromagnetic energy, is wavelength. That is the length of one wave measured from the top of one wave to the top of the next. The red line may be representative of light, with a longer wavelength. The blue line has a shorter wavelength and may be representative of x-rays. Courtesy of Mosby’s Radiography Online. (Elsevier)

Frequency Frequency refers to the number of waves that go by a point in 1 second. Remember that electromagnetic energy waves all travel at the same speed: the speed of light. Imagine two different waves traveling next to each other. You're timing them with a stopwatch and discover that one wavelength of the first wave goes by in 1 second. That's a frequency of one wave per second. The second wave has a wavelength half as long as the first, and because it is traveling at the same speed, two waves will go by in the 1 second. It has a frequency of two waves, or cycles, per second. Courtesy of Mosby’s Radiography Online. (Elsevier)

The shorter the wavelength – the higher the frequency Wavelength is the difference between crest to crest. Or valley to valley. High frequency or short wavelength is needed to penetrate tissue. Frequency how many crests pass per second.

Because all electromagnetic energy waves move at the same speed, there is a simple relationship between wavelength and frequency: the longer the wavelength, the lower the frequency. The shorter the wavelength, the higher the frequency

The Electromagnetic Spectrum X-rays have wavelengths much shorter than visible light, but longer than high energy gamma rays. As frequency increase the waves getter closer together indicating a higher energy. Frequency and wavelength are inversely proportional. Right past UV light (sunburn), cannot go all the way through but it does somewhat. (sunburn) X-ray is called a photon because it has properties of matter and energy. The scientists are unable to decide if it was matter or energy. The only difference between x-rays and gamma rays is their origin. X-rays are artificially simulated- emitted from an e- cloud of an atom. In electrical imaging systems. Gamma rays come from inside the nucleus of a radioactive atom. Spontaneously from radioactive material.

What is Ionization? Removal or addition of an electron. Electrons always move in a straight line. It is removed because another electron moved in and knocked out and replaced. X-rays can ionize atoms. In an atoms normal state it is electrically neutral, the electrical charge is zero. If an atom has an extra electron or is missing an electron it is said to be ionized.

When an electron is added or removed from the atom- it is ionized If we lose on it is positive. If we gain one it is negative. Loss of an negative electron makes it positive.

Kinetic energy Energy of motion The electrons KINETIC energy is converted to electromagnetic or PHOTON energy Taking kinetic energy and making it into photon energy.

X-ray production begins at the atomic level Energy (photons) are released when the electron collides with another electron, or passes close to the nucleus of the atom – the change in energy of the shells –produces photons This is not in the book.

X-ray Production in the TUBE

INTERACTIONS IN THE TUBE BREMS (Bremsstrahlung) CHARACTERISTIC HEAT 3 interactions with the tubes.

Tube Interactions Heat = 99% X-ray = 1% Bremsstrahlung (Brems) = 80% Characteristic = 20% Characteristic is better for contrast but the patient gets more exposure. Brems gets the job done but less exposure to patient.

Bremsstrahlung Radiation Heat & Characteristic produces EM energy by e- interacting with tungsten atoms e- of the target material Bremsstrahlung is produced by e- passing by closely with the nucleus of a target tungsten atom – the change in direction of the electron – releases a photon of energy

Heat Most kinetic energy of projectile e- is converted into heat – 99% Projectile e- interact with the outer-shell e- of the target atoms but do not transfer enough energy to the outer-shell e- to ionize Projectile electron from cathode to anode Not enough energy to kick a an electron out of its shell…but it excites the atom. Excitation is the release of heat 99% of the kinetic energy is converted into heat. This occurs when the projectile electron interacts with the outer-shell electron of the target. But they do not have enough force to ionize them. Therefore the electrons have an “excitation” phase in which heat is produced. Heat is directly proportional to increasing the x-ray tube current. Heat also increases when increasing kVp.

Heat

HEAT Projectile electrons from cathode Pass by the electrons in the target Causing the electrons to vibrate (excitation) Excitation produces small amounts of heat HEAT 8 p+ + 8e- = neutral atom e e

Heat is an excitation rather than an ionization Not enough energy to ionize….just excite.

Bremsstrahlung German word meaning slowed-down or braking radiation This occurs when a projectile electron loses some of its kinetic energy as a result of interacting with the nuclear field of the atom. The kinetic energy of the projectile electron is converted to electromagnetic energy (x-ray photons).

The projectile electron has great kinetic energy as it approaches the nucleus. Because the nucleus has a positive charge and the electron has a negative charge, there is an electrostatic attraction between them. This pulls the electron closer to the nucleus, even though its momentum continues to carry it forward in a bending line. It loses kinetic energy due to the acceleration as its path changes. This is the energy that is emitted as an x-ray photon. Courtesy of Mosby’s Radiography Online. (Elsevier) The projectile electron completely avoids the electrons of the target atom. It is the close proximity to the positive nuclear field of the nucleus that it interacts with that creates a braking or slowing down of the electron and a x-ray photon. The closer the projectile e- gets to the nucleus the more it is influenced by the field of the nucleus.

Incoming electron is electrons coming in (straight line). Photons when they exit the tube ( wavy). Straight line- has to be happening in the tube. Wavy line is an x-ray photon in the sine wave form. As the projectile e- passes by the nucleus it is slowed down and changes its course. This change in direction causes a loss of kinetic energy. This loss of kinetic energy reappears as an x-ray photon. The e- can lose all of it’s energy or a fraction of it’s energy depending how close it is to the nucleus.

Bremsstrahlung Radiation If the projectile electron entering an atom in the metal of the anode does not strike any of that atom's electrons, it may continue toward the center of the atom and come near the nucleus. Remember that the electron has a negative charge and the nucleus has a positive charge. Therefore the passing projectile electron is attracted to the nucleus. This attraction slows the electron down as it passes the nucleus and alters the direction of the electron's path as the nucleus "pulls" on the electron. The slowing of the electron means that it loses kinetic energy—and this energy takes the form of a photon of x-ray energy being released. Courtesy of Mosby’s Radiography Online. (Elsevier) Brem’s accounts for 80% of the photons as it can be produced by any projectile e-. For this reason in diagnostic range, most x-rays are Brem’s radiation.

Creates a polychromatic spectrum – x-rays of different energies X-ray Photons – BREMS Creates a polychromatic spectrum – x-rays of different energies Brems is called braking radiation. Changes direction and this changes of direction causes the incident photon to lose energy and the energy is transferred to an photon. Electrons come across from cathode, they get so close to the nucleus they cause a change of energy. Hetero- because it has multi wavelengths. Why do you think it is 80% of reactions? Straight lines are electrons and photons are wavy line coming out. 38

Energy (photons) are released when the e passes close to the nucleus, then changes direction

BREMS RADIATION Electron Passes by nucleus Changes direction Energy released as a PHOTON The closer the electron is the to the nucleus the higher the energy will be. The tighter the wave- the higher the energy. Higher kVp causes a causes a higher energy photon. The change of energy becomes a photon (wavy) Most of the x-rays we create are this type.

Brem’s Radiation Animation http://www.coursewareobjects.com/objects/mrophysics_v1/mod08/0816a.htm

Characteristic Radiation Projectile e- with high enough energy to totally remove an inner-shell electron of the tungsten target All tube interactions result in a loss of kinetic energy from the projectile e- Characteristic x-rays are produced when outer-shell e- fills an inner-shell void All of the kinetic energy is changed, absorbed. If the projectile e- interacts with interacts with an inner shell e- of the target atom rather than with a outer-shell e-. Characteristic x-rays are produced. Characteristic x-rays result when the e- interaction has enough energy to ionize the target atom by totaling removing an inner-shell e-. Photons are emitted when an outer-shell e- fills the inner-shell void. K-shells are the only ones we are interested in, the rest of the shells have very low energy and they cannot penetrate the body. It is called characteristic because it is characteristic on tungsten target atoms.

Characteristic Radiation (Tube) Courtesy of Mosby’s Radiography Online. (Elsevier)

CHARACTERISTIC (in tube) Electron hits inner shell e in orbit – knocked out & creates a hole Other E’s want to jump in Energy released as PHOTONS Energy is created when the spaces in the shells are filled. The electron energy is converted in photon energy. With this interaction you get much more photons created, much more damaging to patients tissue. The only photons that get tot the patient are those in the K shell.

It is called characteristic because it is characteristic of the target element in the energy of the photon produced It is called characteristic because it creates the same energy photon as what was incoming. Its energy is characteristic of tungsten. The energy level is lower in the outer shells . L and down will not make it to through the patient. SO these are absorbed, creating more dosage to the patient.

Characteristic Radiation Animation http://www.coursewareobjects.com/objects/mrophysics_v1/mod08/0808a.htm

Tungsten Atom As this chart shows, the energy of x-rays that results from electrons farther from the atom's nucleus is greatly diminished. These x-rays have no value for diagnostic imaging, although they can still have effects within the body. Modern x-ray equipment is designed to minimize such low-energy x-rays. This is why a transformer is used in the x-ray machine to boost the voltage high enough to produce higher-energy x-rays. Courtesy of Mosby’s Radiography Online. (Elsevier)