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Physics- Units 6-8
Exam 2
| Question | Answer |
|---|---|
| T or F. X and gamma rays transfer energy to a medium. | True |
| What is the initial step in energy transfer? | ejection of electrons |
| Once electrons are ejected, how do they transfer energy? | via ionization/excitation of atoms along their path |
| What destroys a cell's reproductive capacity? | energy deposition |
| Most of absorbed energy is converted into ________. | heat |
| This occurs when a neutral atom requires a + or - charge. | Ionization |
| What are some examples of directly ionizing radiation. | electrons, protons, and alpha particles (particles associated with charge) if energy is high enough |
| If energy is not high enough to cause ionization, this will cause ______________. | excitation |
| Indirectly ionizing radiation is also known as? | electromagnetic radiation |
| Indirectly ionizing radiation includes what 2 types of particles? | neutral particles and photons |
| What 3 ways do photons interact? | -photoelectric effect -Compton scatter -Pair production |
| What 2 things decrease the intensity of an xray beam? | attenuation and 1/r^2 effect |
| Photodisintegration is the emission of _____________. | neutrons |
| Photodisintegration is only for photons below or equal to _____ MeV. | 10 |
| Name all 5 interactions of photons with matter. | -Photodisintegration -Coherent scattering -Compton Scatter -Photoelectric effect -Pair Production |
| Coherent scatter is also known as... | classical or Rayleigh scattering |
| Explain how coherent scatter is produced. | Photon approaches an electron and causes the electron to oscillate, and this reradiates the photon at same frequency and energy. |
| In coherent scatter, is energy transferred from the photon? | No |
| Coherent scatter is __________ angle scatter. | small |
| What kind of material is coherent scatter prevalent in? | high atomic number materials at low photon energy |
| Is coherent scatter applicable for rad therapy? | No |
| PE is dominant in _________________ energy ranges. | diagnostic (keV) |
| Explain how PE occurs. | A photon interacts with and ejects and bound electron, and all energy is transferred to this electron which creates a photoelectron. |
| PE usually occurs in what shells? | K,L,M, or N |
| After the ejection of the electron in the PE, what happens? | a vacancy is open in that shell and the atom is in an excited state which causes the cascade effect |
| The probability of PE to occur depends on photon ________. | energy |
| PE is proportional to _________. | 1/E^3 |
| What does the PE depend strongly on? | atomic number of material |
| PE is proportional to _______. | Z^3 |
| PE is __________ proportional to E^3 and ____________ proportional to Z^3. | inversely;directly therefore, PE is proportional to Z^3/E^3 |
| The greatest effect of PE is with ________ atomic number and _________ energy. | high;low |
| With the compton effect, a photon interacts with a _________ electron. | free |
| Free electrons | means the binding energy of the electron is less than the photon energy |
| How does the compton effect occur? | the electron gets some of the photon energy and is emitted at a small angle, and the photon is scattered at a small angle with less energy than it originally had. |
| **Look at the special cases of compton effect. Direct hit , grazing effect, and the 90 degree photon scatter. I was not sure on how to include this in the cards. Page 10-11 of Unit 6 and TB pg. 68-69 ** | ... |
| T or F. With PE, the photon energy is approximately the binding energy of the electron. | True |
| With compton effect, the photon energy is ______ than the binding energy of the electron. | greater than |
| Compton effect decreases with _____________ photon energy. | increasing |
| Compton effect depends on electron ___________. | density |
| In pair production, the photon energy is greater than _______ MeV. | 1.02 (this is bc resting energy is .511 and then multiply that by 2 bc it is a pair = 1.02) |
| Explain how pair productions occurs. | A photon interacts and gets an e- and e+ pair which tends to be ejected in forward direction relative to the photon. |
| In pair production, energy is changed to _______. | mass |
| Charged particle interactions occur when? | via ionization and excitation by collisions |
| With heavies, as a particle slows down, the rate of energy loss __________ and therefore, energy absorbed in medium (dose) ____________. | increases;increases |
| Bragg peak | peaking of dose near end of particle range |
| What is an example of bragg peak? | protons |
| Electron Particle Interactions. *this is listed from the slides. | -greater multiple scattering and motion direction changes due to small mass -no Bragg peak -via ionization or excitation |
| What 2 processes can neutron particles interact? | -recoiling protons from H and other nuclei -nuclear disintegrations |
| What is an example of a "qualitative" method that was historically used to determine the radiation dose? | skin erythema dose |
| In 1928, ICRU designated this unit for measuring X and gamma ray exposure. | Roentgen (R) |
| The measurement of ionization produced in air by photons. | Exposure |
| 1 R=... | 2.58x10^-4 C/kg air |
| Electronic Equilibrium | ionization loss is compensated by ionization gained |
| With the FIA chamber, the definition of Roentgen is ____________. | satisfied |
| FIA chambers are confined to what? | national standards labs |
| What 4 conditions need to be met in the FIA chamber? | 1. electrons produced must spend all energy by ionization of air btwn the plates 2. electron range < plate sep. distance 3. beam intensity must stay constant 4. diaphragm-ion coll. region distance > electron range in air |
| What 4 corrections to measurements does the FIA chamber make? | 1. air attenuation correction 2. recombination of ions correction 3. T, P, humidity corrections 4. correction factor for ionizations from scattered photons |
| What is the limit that the FIA chamber can't go over? | 3 MeV |
| Thimble chambers are calibrated against what? | standard FIA chamber |
| With thimble chambers, the wall thickness must be greater than or equal to what? | max range of electrons liberated in wall |
| The inner surface of the chamber wall of a thimble chamber is __________ ____________. | electrically conductive |
| What does the rod in the thimble chamber act as? | other plate (electrode) |
| For the thimble chamber wall to be the same as a FIA chamber, what must happen? | thimble wall must be air equivalent |
| What are the typical wall materials used for a thimble chamber? | graphite, bakelite, and other plastics |
| What must be equivalent in the thimble chamber in order to be overall equivalent to air? | wall and inner electrode combined |
| There are 6 desired chamber characteristics of a thimble chamber. Try to list them. **will be 2 separate cards** | 1. min. change in response over range of energies 2. volume suitable to range of exposures 3. min. change in sensitivity w/ direction of incident radiation 4. min. stem leakage 5. calib. vs stand. for rad. to be msrd 6. min. losses due to ion recomb. |
| How can there be a minimal change in sensitivity with direction of incident radiation in a thimble chamber? | use same chamber configuration/geometry as specified under calibration conditions; "black stripe towards beam" |
| What is stem effect? | irradiation of stem contributes to measurements and needs to be accounted for (taken out) |
| How does stem effect occur? | -ionization in stem body -ionization in air between end of chamber and metal cap |
| What 2 types of thimble chambers are there? | condensor chamber and farmer chamber |
| Condensor chamber | + condensor (up to 2 MeV) condensor stores charge have to discharge chamber after use (not practical) |
| Farmer chamber | less stem effect and flatter energy response compared to condensor type chambers |
| A device used to measure charge. | electrometer |
| What is the chamber connected to? | electrometer |
| What does the amplifier do to the electrometer? | increases amount of charge measured in chamber |
| What is extrapolation chamber used for? | to measure surface dose by varying space between electrodes |
| The parallel plate chamber is the same as the extrapolation chamber except that it can't? | change spacing between electrodes |
| What is the parallel plate chamber used for? | electrons since no perturbance of field |
| With ion collection efficiency, what percent do we want to stay under for losses due to recombination? | 1% |
| Polarity effects | -compton current -extracameral current -measures at both + and - potentials to get measurement that minimizes this effect |
| Compton current | can add or reduce to collecting current depending on + or - biased voltage |
| Extracameral current | current collected outside sensitive volume of chamber |
| Environmental conditions are affected by what 2 things? | temperature and pressure |
| Environmental conditions are ____________ proportional to temperature and _______________ proportional to pressure. | directly;inversely |
| Radiation detectors are ______ filled. | gas |
| Which type of radiation detector can discriminate between type of radiation? | proportional counters |
| With radiation detectors, recombination ___________ voltage and the number of ion pairs collected is _________. | decreased;small |
| What is the plateau region for an ion chamber? | up to about 200 V |
| What occurs in a proportional counter? | gas multiplication |
| In a proportional counter, the number of ions collected in the pulse is proportional to what? | the original number produced |
| Can a proportional counter measure the energy of alpha particles? | Yes |
| What is the limit of the proportional counter? | up to 700 V |
| This radiation detector causes voltage to increase enough that gas atoms are directly ionized and tube continually discharges. | discharge |
| This radiation detector uses crystals which transform photons measured into light. | scintillation detectors |
| In scintillation detectors, the amount of light is _____________ to energy of photons. | proportional |
| Neutron dosimeters are used for energy under _______ MV. | 10 |
| KERMA | kinetic energy released in matter |
| Rad represents? | energy absorbed per unit mass 1 rad= 100 erg/g = 1 cGy |
| F-Factor | a conversion from exposure in air to dose in tissue |
| Dose equivalent | relates different radiation types to xrays in terms of biological effect |
| What does quality of radiation refer to? | penetrating ability of the radiation |
| What is the ideal way to describe quality? | spectral distribution |
| We use HVL for __________ energies. | decreased |
| We use peak energy for ___________ energies. | increased |
| Average energy is about ______ of peak energy. | 1/3 |
| What is an example of a combination filter? | Thoraeus filter |
| What are 2 direct methods used to measure peak voltage? | voltage divider and sphere-gap method |
| Voltage divider | access increased tension leads; invasive |
| Sphere-gap method | measure distance to create arc |
| What are 3 indirect methods to measure peak voltage? | -fluorescence method -attenuation method -penetrameter |
| Fluorescence method | using materials w/ different k-absorption edges |
| Attenuation method | slop of transmit curve depends on peak kV |
| Penetrameter | comparison of transmission through materials; compare OD's of film |
| Effective energy | the photon energy in monoenergetic beam which is attenuated at same rate as the radiation in question |
| Measuring MV energy | -Percent depth dose (PDD) and comparison to published data -Photoactivation ratio (irradiate foils and determine spectra) |
Created by:
lheard
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