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x-ray interaction
x-ray interaction with matter
| Question | Answer |
|---|---|
| three things that may occur with x-rays? | 1) it can pass through unaffected (transmitted) 2) it can change direction and loose energy (scattered) 3) it can be attenuated (absorbed) |
| What is an Attenuation? | it is the reduction in the number of x-ray photons in the beam, and the subsequent loss of energy, as the beam passes through matter |
| what kind of x-ray interaction with matter does happen at the atomic level? | attenuation and scatter |
| what does x-ray interact with? | - the whole atom (low energy) - one orbital electron (intermediate energy – diagnostic x-rays) – - the nucleus (high energy – radiation therapy) |
| what are the most common interactions within the diagnostic range of x-rays? | Interactions with orbital electrons |
| what is the binding energy of an electron? | it is the amount of energy required to remove the electron from its orbit *An atom has a positively charged nucleus surrounded by orbital electrons that have negative charges. These electrons are held in place by their binding energy |
| what determines the binding energy? | 1) the shell in which the electron resides (the closer to the nucleus, the greater: K-shell) 2) the atomic number of the atom (the higher the atomic number, the greater EB) |
| what is Electron volt (eV) ? | It is the kinetic energy acquired when an electron is accelerated by a potential difference of one volt. Electron volt (eV) is a measure of energy 1 eV = 1.6 x 10-19 Joules 1 keV = 1000 eV |
| what is the energy state of an electron? | Electrons that are further away from the nucleus have a higher total energy* than those close to the nucleus They are in a “higher energy state” When an outer shell electron drops into an inner shell, its total energy decreases |
| Five basic interactions between x-rays and matter | 1) coherent scattering 2) photoelectric effect 3) Compton scattering 4) pair production 5) photodisintegration |
| what is Coherent Scattering (classical=unmodified, Thompson) ? | a low E e interacts w/ atom, its E will be absorbed, but it will cause atom to vibrate,which will produce a second. x-ray w/same E and wavelength as the incident x-ray, but in a different direction. no energy is transferred and the atom is not ionized |
| condition for a coherent scatter occur? | occurs only with very low energy x-rays – below 10 keV |
| is atom ionized in Coherent interaction ? Is any energy transferred? | no to both |
| In Classical scattering interaction the INCIDENT XRAYS interact with the target atom. This causes what to occur? | he atom becomes excited and releases energy as a scattered xray with the same amount of energy as the incident xray. |
| Does Coherent scatter affect diagnostic imaging? why? when can it be a problem? | no. this occurs at such a low energy– most of the scatter does not hit the film. it can be a problem in mammography |
| Describe the Photoelectric Effect (PE) or Photoelectric Absorption? | the incident x-ray interacts with an inner (K or L) shell electron - the incident x-ray must have more energy than the binding energy of the orbital electron |
| When is the Photoelectric effect likely to occur? | when the incident x-ray has just slightly more energy than the electron |
| What happens to the photon in photoelectric effect? what happens to an atom? | 1. the x-ray is totally absorbed by the electron and the electron (photoelectron) is ejected from its orbit 2. the atom is ionized |
| how to calculate the kinetic energy of photoelectron in photel. effect interaction? | kinetic energy that is equal to the difference between the incident x-ray energy and the electron binding energy Ei = Eb + Eke - Ei = energy of the incident photon - Eb = binding energy of the electron - Eke = kinetic energy of the photoelectron |
| Can a photoelectron produce a secondary radiation? | yes. It can produce secondary radiation in other atoms, but it is usually very low energy. outer shell electrons drop into the vacated inner shell – cascade effect. energy is given off as a characteristic x-ray – secondary radiation |
| Differentiate between what happens to the incident xray photon in the PE and Compton interaction with matter. | PE - incident xray photon is completely absorbed giving all its energy to the atomCompton - incident xray photon changes direction and loses some of its energy |
| Can photoelectron ionize other atoms? | the photoelectron can ionize other atoms and create characteristic radiation elsewhere. - usually very low energy |
| why is the secondary radiation of PE a low energy radiation? | because of the low atomic number of most atoms in the body (hydrogen, oxygen, carbon) =>these x-rays are usually very low energy and in a different direction than the incident photon |
| Conditions/rules for PE to occur? | 1) the energy of the incident x-ray must be greater than the binding energy of the inner shell electron 2) when the x-ray photon energy and the electron binding energy are nearer to one another 3) if an electron is more tightly bound in its orbit |
| what are the chances of PE to occur if kev is high? | the higher the keV, the chance of interaction becomes MUCH less - photoelectric effect = 1/(energy)3 - at 50 kVp – 50.45% of the interactions are photoelectric (45.55% Compton scattering) at 130 kVp – only 24.78% /75.22% (Compton) |
| Describe relationships between keV and PE and scatter? | - the higher the kVp, the less the percentage of photoelectric and the greater the percentage of Compton scattering - the lower the kVp, the more of the photons are absorbed (photoelectric) and the less are scattered (Compton) |
| PE and atomic number? | - the probability of a photoelectric interaction increases dramatically as the atomic number increases - photoelectric effect = (atomic number)3 |
| Explain “differential absorption” | bone has a higher average atomic number than does water, so more x- rays will be absorbed in bone than in water (soft tissue) this gives us radiographic contrast |
| What is Compton Scattering? | Intermediate E incident photon knocks electron out of outer shel (Recoil e)l. Incident photon isnt absorbed (bc contains more energy)& changes directionw/less E = Scattered ; Compton/recoil electron continues with less energy. Scattered xr will hit IR |
| What does happen to the recoil e after it is ejected from the outer shell? | it may interact with other atoms, and as a result, it will produce Bremsstrahlung and Characteristic xrays |
| How to calculate the energy of incident xray in Compton interaction? | Energy transfer: Ei = Es + Eb + Eke Ei = energy of the incident photon Es = energy of the Compton scattered photon Eb = binding energy of Compton electron Eke = kinetic energy given to the Compton electron |
| What is angle of deflection? | The amount of energy retained by the scattered photon is dependent on the “angle of deflection” The greater the angle, the more energy is given to the Compton electron and the less is retained by the scattered photon |
| 0 degrees of deflection 180 degrees | 0 degrees = no energy transfer 180 degrees = maximum energy transfer – this is called “back scatter” Compton scattering contributes the vast majority of scatter reaching the film & is the primary cause of occupational exposure to the radiographer |
| Pair Production | Only occurs with very high energy photons (over 1.02 MeV) The photon interacts with the nuclear field surrounding the nucleus of an atom |
| Explain pair production? | The energy of the photon is absorbed and creates two pieces of matter. Two electrons – one negatively charged (negatron) and one positively charger (positron) – are created/ SCATTERED |
| What will happen to negatron? | is absorbed by another atom |
| What will happen to positron? | The positron will interact with an electron and will undergo the “annihilation reaction” which will turn matter into energy |
| what happens when negatron and positron interact with each other? | P is an antimatter of negatron with the same amount of charge (0.51MeV). When they interact, they blow up. |
| Where is the pair production interaction used? | PET scan and rad. therapy |
| Explain Photodisintegration? | A very high energy photon (above 10 MeV) will interact with the nucleus of an atom. The photon is absorbed and a piece (fragment) of the nucleus is ejected. The nuclear fragment can ionize other atoms and cause biological damage Used in rad. therapy |
| Why does the probability of PE interaction decrease with increasing kVp? | As kVp increases you have increased penetration which would not allow absorption. |
| Describe PE interaction. | Incident xray photon is absorbed by atom. Atom releases innershell electron because of absorption of incident photons energy. It is now a photoelectron. Outershell electron fills innershell to make stabile (electron cascading) |
| Differentiate between scatter and secondary radiation. | Scatter(incident xray photon is slowed and deflected in a diff. direction when interacting with an outershell e-). Secondary - incident xray photon is absorbed by an atom and creates a new e- |
| X-radiation is a form of __________ radiation. | ionizing (electromagnetic) |
| In order for pair production to occur, the incident photon must have energy of at least _____. When the energy photon approaches the nucleus it ____ giving up a ___ and ____. What is meant by annihilation? | 1.02 mev, positron, negatron, Annihilation is the destruction of a photon creating new particles (coincidence radiation) |
| Roentgen (C/kg) | exposure in air of x or gamma up to 3 mev |
| Rem (Sievert) | the biological effect caused by radiation exposure |
| Gray (rad) | radiation absorbed dose; patient dose |
| Name types of particulate radiation (particles) can ionize matter | - alpha particles - beta particles - electrons - protons -neutrons - heavy nuclei |
| Alpha particles | -has two protons and two neutrons -have a positive charge -emitted from the nucleus of very large elements that undergo radioactive decay -have a lot of energy (because of their size) |
| Beta particles | - identical to electrons -come from the nucleus of radioactive material -negatively charged - have less energy than alpha particles |
| what are two sources of ionizing radiation | - natural - man-made |
| Natural sources of ionizing radiation | 1) cosmic rays 2) terrestrial radiation - uranium, plutonium, thorium, radon, etc. 3) radionuclides - potassium-40 |
| Man-made sources of radiation: | 1) Major source: medical x-rays (about 60%) 2) nuclear medicine 3) consumer products (watch dials, smoke detectors, TVs, airport x-ray machines, etc.) 4) nuclear power plants 5) industrial sources |
| rem | radiation equivalent man |
| rad | radiation absorbed dose |
| What are the only electromagnetic waves that have the proper energy to ionize matter | X-rays and gamma () rays |
| What happens to the incident xray photon after it interacts with the innershell? | It is completely absorbed giving up all its energy to the atom |
| What effect does compton have on an image? | Decreases contrast, produce fog, no useful information |
| At kVp greater than 70, ______ is characeristic and ______ is brems. | 10-20%, 80-90% |
| Describe the interaction of characteristic. | Incident electron has enough energy to interact with inner shell electron.Innershell e- is knocked out and becomes a free e-.Outer shell e- cascades to fill the innershell e- to make stable.Lost energy is characteristic |
| Describe the interaction of Brems. | the radiation which is emitted when electrons are decelerated or "braked" when they are fired at a metal target. Incident electron does not have enough energy to overcome shell. E- slows and changes direction. Lost energy is Brems. |
| Half Value Layer (HVL) | thickness of absorving material necessary to reduce he xray intensity to half of its original value. |
| What is the technical factor that affecs attenuation? | kVp |
| What is the purpose of filtration? | hardens the beam and removes low energy, long wavelengths xrays resulting in an xray beam with higher energy, greater penetrability and higher quality ex. aluminum and copper |
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