click below
click below
Normal Size Small Size show me how
241 Radiobio Unit 1
241 Radiobiology Unit 1
Question | Answer |
---|---|
ALARA stands for what | As Low As Reasonably Achievable |
BERT stands for | Background Equivalent Radiation Time |
Perceived Risk is | what an individual or group thinks the risk is |
Risk comparison is | the comparison between 2 or more activities |
Every use of radiation involves a potential _____ risk | small |
If radiation is underused it does not benefit the public health and is detrimental on the bell curve, True or False | True |
If radiation is overused it dose benefit the public health and is beneficial on the bell curve, True or False | False-Overuse is just as bad as under use of radiation. |
If a procedure has a Risk of +6 should the patient undergo the examination | Yes, when both the risk is low(positive) and the benefit is high(positive) then the patient will benefit the most, however if the Risk is high and the benefit is low (negative) then the patient should not have the exam |
Radiologic technologists are also called what | Radiologic science professionals |
What are the 3 radiologic science professional fields | Radiographer, Nuclear medicine technologists and radiation therapists |
What are the 3 common goals of radiologic professional | 1)Promote diagnostic/therapeutic efficacy of exams,2)Provide Radiation protection,3)provide the highest level of patient care possible |
Who is the best individuals to protect the public from radiation | Technologists |
GSD stands for | genetically significant dose |
Genetically significant dose (GSD) is the measure of the genetic exposure to the _____ from diagnostic and other forms of ionizing radiation | population- it indicates the genetic load on the population |
ESE stands for | Entrance skin exposure |
Entrance skin exposure (ESE) is determined how | without the patient by using technical and geometric factors it decreases with added filtration |
Apathetic technologists are what | technologists who avoid unnecessary radiation only as much as they are compelled to do so by mandate of law or department rules |
Actively involved technologists are what | technologists that recognize professional responsibility by practicing radiation protection to the benefit of the patient. They go above and beyond what is “necessary” |
Radiologists are not considered educators, True or False | False-Radiographers provide education to patients, other health care workers, the general public, and to Rad students |
Cyrile Houle developed a list of six orientations to learning what are they | 1)Oblivious person,2)Uninvolved person,3)Resistant person,4)Someday person,5)Focused person, and 6)Comprehensive person |
Laws are rules of behavior that do not have to be followed , True or False | False-Laws MUST be followed |
Ethics are rules of behavior that do not have to be followed however, they are based on acceptable behaviors(guidelines), True or False | True |
ASRT stands for | American Society of Radiologic Technologists |
TS/SNM stands for | Technologist Section/Society of Nuclear Medince |
Safe Medical Device Act of 1991 states | that technologists must report faulty/broken equipment |
Licensure is the process by which some competent authority grants permission to a qualified individual or entity to perform certain specified activities, True or False | True |
The goal of licensure is | to protect the public from excessive ionizing radiation, not to raise professional standing of practitioners |
Consumer-Patient Radiation Health and Safety Act of 1981 states | 1)provides standards for accreditation of programs for education individuals responsible for administering radiation to patients,2)licensing standards for radiologic technologists |
For negligence to be proven what 4 things must be satisfied | 1)A duty on the part of the professional exists,2)a breach of duty must be shown,3)the cause must be due to an action on the part of the professional,4)An injury must be proven |
As students we have a duty to ____attempt examinations until ready to do so | Not |
X-rays are a form of _______ ______ | electromagnetic Radiation |
Xrays are similar to ____ ____ but are of shorter wavelength | visible light |
Xrays have ____ mass or charge but behave as both ___ and ____ depending on how they are viewed | NO, waves and Particles |
A photon is what | smallest packet of energy |
Xrays are the ____ penetrating electromagnetic waves | MOST |
Xrays have the ability to pass though matter, True or False | TRUE |
Xrays are heterogeneous meaning | they contain many different wavelengths |
Xrays are polyenergetic meaning | they contain many different energies |
Differential absorption is dependent on what 2 things | thicknesses of the parts and atomic number of tissue |
Xray photons travel in what | straight divergent lines isotrophically (in all directions) |
Xrays can be focused by lenses, True or False | FALSE |
Xrays are electrically neutral, True or False | True |
Xrays have mass and particles associated with it, True or False | False- Xrays have no mass or particle they are considered “pure energy” |
Xrays produce secondary and scatter radiation when interacting with ______ | Matter |
Secondary Radiation is radiation produced inside an object by interactions between the photon and the atoms inside the object, True or False | TRUE |
Scatter Radiation is produced when xray interacts with matter and have their direction and or energy levels ______ | Change |
Xrays cause _____ to fluoresce | crystals |
Xrays affect ______ film | photographic |
Xrays ____ all matter including gases | Ionize |
Xrays do not cause biological changes, True or False | FALSE |
What are the 6 medical uses of Xrays | 1)Plain Film radiography,2)Fluoroscopy,3)Tomography,4)Mammography,5)Computed radiography,6)Computed tomography |
What are the 3 components of an xray tube | 1)vacuum,2)Cathode,3) Anode |
mAs is the measurement of the number of electrons moving from the ____ to the _____ | cathode to Anode |
The electrons, depending on the kVp set can travel at half the speed of light, True or False | TRUE |
kVp determines the ____energy of the xrays produced and the _____ ability of them | Maximum, penetrating |
The Radiation produced in the tube is called _____ _____ | Primary Radiation |
What are the 2 types of radiation produced | Bremsstrahlung and Characteristic |
Brems Radiation is known as _____ _____ | braking radiation |
Characteristic radiation is produced when what happens | an inner shell electron is knocked out and then an outer shell electron drops into its place |
Leakage radiation is what | radiation that does not pass thought the window and is of diagnostic use (leaks out of the tube housing) |
Portions of the primary beam that pass though the patient without interacting are called what | Remnant radiation |
Radiation that is produced in the patient is called | secondary radiation |
Alternating current is what | electrons that alternate directions in a circuit |
_____ is the processes of modifying the AC to DC | Rectification |
3 types of rectification are | single phase rectification, three phase rectification, and High frequency rectification |
In single phase rectification the ______ pulses are converted to _____ pulses | Negative, Positive |
The U.S. uses _____Hertz power | 60 |
The problem with single phase rectification is that ______ is not constant | Voltage-It goes from 0 to 100% then back to 0% |
Root mean square value is what | a measure of the average energy in the beam |
In single phase rectification you are only getting about ____ % of the peak voltage set | 70%,- to find the effective voltage (RMS) you need to multiply the peek by 0.707, and to find the maximum from the effective multiply it by 1.41 |
______ phase is the most commonly used type of rectification | Three |
Voltage never goes all the way to 0 when using three phase, True or False | True |
Three phase six pulse equipment you get about ____% of your peek voltage | 92% |
The RMS value of a 3phase 12 pulse is ____% | 97 |
In High frequency generators the incoming 60Hz power is changed to as much as ______Hz power | 6000 |
The RMS value of High frequency generators is ___ to ___% | 97-98% |
____ ____ generators the mA is not constant but is allowed to decrease during the exposure | Falling Load |
The main purpose of falling load generators is to reduce ___ ___ and permit a better use of the ___ ____ | exposure time, tube limits |
In Falling load generators the technologist can only set ____ | mAs |
What 2 types of generators are used in mobile equipment | 1)Capacitor-discharge, 2) Battery powered |
______ _____ is dynamic imaging or the imaging of motion | Fluoroscopic imaging |
_____ _____ is a piece of equipment that emits light when struck by xrays in fluoroscopy | Fluorescent screens |
Intensification screens do what | improves the brightness of the image |
The weakest link in fluoroscopic imaging process is the ____ _____ _____ | TV monitor resolution |
Resolution is measured in _______ | line pairs per mm (lp/mm) |
The total brightness gain(in image intensifiers) is a product of the gain due to making the image smaller times the flux gain, True or False | True |
The minification gain is calculated by the ratio of the area of the imput phosphor (Screen) to the output screen squared, True or False | TRUE |
______is a general term used to describe the process of emitting radiant energy in the form of waves or particles | Radiation |
Radiation can be defined as ____in ____ | energy in transit |
When did Roentgen discover xrays | November 8th 1895 |
_____discovered that certain naturally occurring substances emitted radiation who was he | Becquerel |
Becquerel discovered 3 different types of radiation that were later named by Ernest Rutherford, what are their names | Alpha, Beta, and Gamma |
______ Radiation produces positively and negatively charged particles as it passes though matter | Ionizing |
There are 2 ways of creating radiation what are they | Natural and Man made |
Atomic number or ____ number represents the # of protons in an atom | Z |
Changing the number of neutrons in the nucleus of the atom often makes it ______ | Radioactive |
____radiation is made up of four particles and does not penetrate matter easily | Alpha |
Alpha radiation is the ______moving radiation | slowest |
______ radiation is made up of 2 protons and 2 neutrons and has a mass of ___amu(atomic mass units) | Alpha, 4 |
Alpha particles are produced by _____ ____ primarily of heavy atoms such as plutonium | nuclear disintegration |
Naturally occurring alpha particles can travel ___ to ___ cm | 4 to 8 |
Alpha particles can penetrate a piece of paper, True or False | False |
Personnel monitoring devices are sensitive to Alpha particles, True or False | False |
Alpha particles are Helium nuclei that have no electrons, True or False | True experiments show that if a gas filled vessel is irradiated for some time that the alpha particles bind with electrons creating neutral helium atoms |
_____ _____ is only produced in or near the nucleus of the atom and essentially consists of high speed electrons | Beta radiation |
Beta particles can travel approx 3 meters/MeV of energy in air and up to several ______ in tissue | Millimeters |
Beta particles are used in medical treatments, True or False | True-Radioactive phosphorus is used to reduce fluid accumulation in the serosal cavities resulting from metastatic carcinoma |
When beta rays travel though a _____ ____ their course is deflected or changed | magnetic field—this suggests that they consist of streams of charged particles (Electrons) |
_____ and ____ are similar to visible light, radio waves, and microwaves | Xrays and Gamma rays |
All electromagnetic waves travel with the same velocity what is it | 10x108 meters per second or 186000miles per second |
_____ and _____rays have the highest frequency and the shortest wavelength | Gamma, X |
Gamma rays are identical to Xrays in 5 ways name them | energy, wavelength, frequency, no mass, no charge |
Gamma rays originate from _____ the nucleus | within |
Most radioactive materials used in medicine emit gamma rays, True or False | True |
Brachytherapy means what | short distance irradiation of cancer close to the surface of the body or within the body |
Teletherapy means what | irradiation of cancer lesion with the source of radiation distant from the body |
There are 2 different units used to measure radiation one the world uses and one the U.S. uses what are they | Systeme International (SI) and Traditional (AKA Customary units) |
When measuring radiation in air the traditional unit used is the ____ and the SI unit is _____ | roentgen(R), coulombs per kilogram(C/kg) |
R or C/kg is a measure of the ionization of air produced by xradiation and gamma radiation below ___ million electron volts | 3MeV |
_____ ____ is a doubly ionized helium atom | Alpha particle |
Alpha particles have a charge of ____ | +2 |
Beta particle Negatron have a charge of ____ and Positrons have a _____ | -1, +1 |
The absorbed dose of a patient is measured in ____ Traditional and ____ SI | RAD(Radiation Absorbed Dose) and Gray(Gy) |
KERMA stands for | Kinetic Energy Released in Matter |
A Rad is defined as an energy transfer of 100ergs (small unit of energy) per gram of any absorbing material, True or False | True |
1 Rad = ___R | 1 |
The ratio between the number of ______ and the number of ____ is called the f-factor | Roetgens(exposure), Rads(energy transfer) |
____are determined by multiplying the absorbed dose(RAD) times a quality factor and are expressed as dose equivalences | REM |
REM stands for | Radiation equivalent man |
____ and ____ are units of biological effect | Rem and Sievert |
____ ____ is a measure of the biologic impact or damage of a particular type of radiation | Quality Factor |
Rems are the most commonly used to measure personnel does, True or False | True |
When measuring Radioactivity the traditional unit is the ____ and the SI unit is the ____ | Curie, Becquerel |
Curie and Becquerel measure rate of nuclear disintegration (Decay) of a material, True or False | True |
Curie is defined as ________disintegrations per second of any radioactive substance | 3.7x1010 |
The Becquerel is defined as one decay per second of any radioactive substance, True or False | True |
The Curie is larger than the Becquerel, True or False | True |
The activity of a radiation source does not remain constant but tends to decrease with time this is known as | Radioactive Decay |
The characteristic time of radioactive decay is known as the | Half life- the time it takes a radioactive material to decay to 50% of the original activity |
____ are used in dosimetry and measure biologic effects | Rems |
1R x ______=1 C/kg | 2.58x10-4 |
1Rad x _____= 1gray | 0.01 |
1Rem x _____= 1 sievert | 0.01 |
1 Ci x ______=1 becquerel | 3.7x1010 |
______is the determination by scientific methods of the amount, rate, and distribution of radiation emitted from a source of ionizing radiation | Dosimetry |
____is a device used to detect and measure exposure to radiation | Dosimeter |
____is a person who plans an optimal radiation treatment dosage pattern or who establishes a radiation distribution pattern for Radiation therapy | dosimetrist |
2 general classes of instruments are used to detect ionizing radiation what are they | Field survey instruments and Personnel monitors |
Field survey instruments come in 3 basic types name them | Ionization chambers, Geiger-Mueller detector and the portable scintillation detector |
_____ _____ determines the amount of radiation by collecting ions in a chamber filled with gas | ionization chamber |
Ionization chambers are sensitive to a wide range of radation energies from about ___ to ____ and are portable | 10 mR to several hundred R |
Ionization chambers are good at detecting what 3 types of radiation | xrays, gamma rays and high energy beta |
Ionization chambers are not well suited to detecting alpha radiation or low levels of radioactive contamination, True or False | True |
Geiger-Mueller counters (GM) determine the amount of radiation by collecting ions in a gas, housed in the detector probe, True or False | True |
GM counters can detect what 3 types of radation | Alpha , Beta and Gamma, however, not low levels of them |
GM counters are most useful in detection rather than measurement of radiation sources, and should not be used for exposure or rate measurements, True or False | True |
_______ ______ uses a sodium iodide or cesium iodide crystal that produces small flashes of light upon exposure to radiation | Scintillation counter |
A _____ ____ converts flashes of light into electric impulses which can be measured | photomultiplier tube |
Scintillation counter is the most sensitive detector to _____ and ____ radiation | xray and gamma |
Name the 3 basic types of personnel monitors | Film Badges, Thermoluminescent dosimeters, and pocket ionization chambers |
Name the 5 desirable characteristics in personnel monitors | 1)portability, 2) Ruggedness, 3) sensitivity, 4) reliability and 5) low cost |
Personnel monitoring is preformed in any situation in which an individual is expected to receive 10% of the effective dose equivalent, True or False | True |
____ ____ are the most common and are used to measure whole body irradiation | Film badges |
Film badges are sensitive down to about ____ | 10mRem |
Film badges are most sensitive to an energy of ____keV | 50 |
Film badges are made up of a film holder that has a number of ____ over the film in order to estimate the amount and type of radiation making the exposure. | filters |
Film badge reports are more accurately called a ___ ____ ____ because it can report on doses from both Film badges and TLDs | Radiation dosimetry report |
The badge reports normally take ___ to ___ weeks to arrive back at the department after reaching the vendor | 4-6 |
Radiation dosimetry reports are normally measured in ______ | millirems |
_____ _____ stores radiation energy by alternation s in its crystalline structure | Thermoluminescent dosimeter |
In TLDs the energy is released when the crystals is heated to several hundre degrees, True or False-True | |
TLD’s are less sensitive than film badges, True or False | False, TLDs are sensitive to about 5 mrem |
TLD’s biggest disadvantage is what | They do not provide a permanent record, if the report is incorrect then there is no way to go back and check it again |
Ring Badges are a film badge, True or False | False they are a type of TLD |
____ ____ ____ have the advantage of providing immediate readings but are not wide in use because they must be recalibrated daily and must be read immediately as the electrical charge inside the dosimeter bleeds off. | Pocket Ionization chambers |
Pocket Ionization chambers are not easily damaged by mechanical trauma, True or False | False dropping or bumping it can cause the reading to be incorrect |
Pocket ionization chambers only measure what 2 types of radiation | Xray and Gamma |
Pocket Ionization chambers that can be held to the light and read are called ____ _____ | self reading dosimeter |
When xray or gamma photons in the primary beam pass though matter they undergo ____ or ___ | absorption or scatter |
Absorption and Scatter are methods of ____ | attenuation |
Attenuation is the reduction in the number of photons as they pass though matter, True or False | True |
Primary Radiation – attenuation = _______ | remnant or exit radiation |
Gamma radiation is produced outside the nucleus of atoms and are considered monoenergetic | False- they are produced INSIDE the nucleus of atoms |
HVL means what | Half value layer-the amount of attenuator necessary to remove half of all the photons in the beam |
Photons will pass though matter and not interact this is called _____ | Transmission |
Photons can interact with the entire atom, the nucleus or the electrons, True or False | True |
What shell in an atom has the highest binding energy | K |
Inner shell electrons have a high binding energy but a low ____ energy | potential |
Potential energy is the ability to do work , True or False | True |
What 4 factors affect attenuation | 1)Atomic Density, 2)Atomic number, 3) Energy of beam is increased, 4)Electons per gram |
Name the 5 interactions of xrays and gamma rays with matter | 1) Photoelectric effect, 2) Compton Scatter, 3) Coherent scatter, 4) Pair production, 5) Photodisintegration |
______ _____ all of the energy of the incoming photon is totally transferred to the atom. | Photoelectric effect |
Photoelectric effects occurs when the incoming photon excites an inner shell electron to the point of breaking its binding energy and causing it to eject from the atom, True or False | True, the energy needs to be at or slightly above the binding energy of the electron |
Lead has a z number of ___ and a K shell energy of __ | 82, 88.0 |
Barium has a Z number of ___ and a K shell energy of ___ | 56, 33.2 |
Tungsten has a Z number of ___ and a K shell energy of ___ | 74, 70 |
What are the 2 basic rules of Photoelectric interactions | 1) are more likely to occur if the photon’s energy is greater than but close to the binding energy of the electron and 2) have a greater likelihood of occurrence when the electron is more tightly bound in its orbit |
The energy of the photons coming from the tube is approximately ____ the maximum energy set on the control panel | 1/3 |
The likelihood of photoelectric interactions is approx ___ | Z3 |
An atom that has lost an electron and the electron that is lost are known as ___ ___ | Ion Pairs |
Electrons ejected from photoelectric interactions are known as ______ | photoelectrons or Recoil electrons |
One photoelectric interaction may cause many more ion pairs to be formed because why | because 1 ion pair can interact with other atom’s and electrons causing them to become ionized |
The atom that has had its inner shell electron ejected because of a photoelectric interaction has its ___ shell electrons drop into the inner shell hole and causes ______ ____ | outer, Characteristic cascade |
Characteristic photons are also called ____ ____ | Secondary Radiation |
___ ____ is a process in which a photon is partially absorbed by an outer shell electron being ejected from its atom while the rest of the photon exits the atom. | Compton scatter |
Compton scatter is not a hazard to personnel, True or False | False Compton scatter is the largest hazard to personnel |
The electron that is ejected from a Compton Interaction is called a ______ | Compton or Recoil electron |
The angle of deflection refers to the degree of change in the photon when it has been scattered, it can range from ___ to ___ degrees | 0 to 180 degrees |
If a photon changes directions 180 Degrees it is called _____ | Backscatter |
Coherent scatter has 3 other known names what are they | Classic, Rayleigh, or Thompson |
_____ ____ the photon causes excitation in the over all energy of the atom rather than ionization, the energy is then released in a different direction but has the same energy and wavelength as it went into the atom with. | Coherent Scattering |
____ _____ occus when the incoming photon has energy of above 1.02 MeV | Pair Production |
Pair production is an interaction is a good example of the conversion of energy into matter and matter back into energy, True or False | True |
When an Incident photon strikes the nucleus of an atom in pair production, 2 new electrons are produced what are they called | Negatron and Positron |
Negatrons are not easily absorbed by other tissues, True or False | False Negatrons are negatively charged electrons |
Positrons are a form of antimatter, True or False | True, they are a positively charged electron, and are not able to exist freely in nature |
When a positron interacts with another electron both particles are destroyed while energy equaling half the incident photon is released from the interaction, what is this called | annihilation reaction |
As Z number decreases the probability of pair production increases, True or False | False, Z number needs to Increase |
______ occurs above 10MeV, a high energy photon is absorbed by the nucleus and becomes excited, causing it to become radioactive, to revert to a stable state the nucleus ejects a neutrons, protons, alpha particles, clusters of fragments and or gamma rays | |
Alpha radiation is _______energetic | Mono |
Beta particles and positrons are ______energetic | Poly |
2 main types of interactions of charged particles with matter are ___ and ____ | elastic and inelastic |
Elastic interactions there is no change in the total kinetic energy of the interacting particles as the energy is transferred from one particle to another, True or False | True |
Inelastic interactions the total energy is changed after the interaction because some of the kinetic energy is transformed into other types of energy such as xrays, True or False | TRUE |
Bremsstrahlung is a type of inelastic interaction, True or False | TRUE |
LET stands for | Linear Energy Transfer |
SI (Not units of measure) stands for | Specific Ionization |
LET is the radiation absorption rate of an attenuator, True or False | True it is the rate at which energy is deposited in the form of charged particles as it travels though matter |
Specific Ionization is not closely related to LET, True or False | False as SI increases so does LET |
Specific Ionization (SI) is the number of ion pairs produced per u nit of distance traveled | |
Xrays are high LET/SI, True or False | False, Xrays are considered low because they do not give off energy easily but they have high penetrability |
_____ ____ is the actual measure of radiation received by a portion of the patients skin and is measured by ______ | Skin Dose, TLDs |
_____ _____ _____ is an estimated value of exposure by using technical factors and geometry of an exposure, the patient is not needed. | Entrance skin Exposure |
____ ____ measures the percentage of skin dose found at a certain depth within a patient | Depth Dose |
_____ _____ is the dose received by a particular organ | Organ Dose |
____ ____ is sometimes used instead of Organ Dose | Average Dose |
___ ____ total amount of energy absorbed by a specific mass of tissue | Integral dose |
When calculating the radiation dose from radioactive substances in the body what 5 factors must be known | 1)amount of the radionuclide in curies 2)Physical half life ofthe radionuclide 3)mixture of radiation such as alpha,beta and emitted,4)Fraction ofthe substance in each organ ofthe body 5)the biologic half life of the material |
____ ____ is the length of time the substance remains in each organ of the body | Biologic half life |
Higher Atomic Number elements have a lower binding energy, True or False | False |
Photoelectric effects happen at what KVP lvl | 30-15 |
Compton scatter happens at what voltage level | 150kv-3MV |
Coherent scatter happens at what voltage level | 10-20 kev |
Pair Production happens at what voltage level | 1.02 |
Photodisintegration happens at what voltage level | 10MV |
Negative pi-meson are what | negative particles produced by linear accelerators |
____ is a nucleus of “heavy hydrogen | with 1 proton and 1 neutron |
Protons are Hydrogen nuclei, True or False | True |
What was the first visible unit of measurement for radiation | Skin Erythema Dose |
F-Factor depends on energy of phton, atomic # , density, and thickness of tissue, True or False | True |
The F- Factor is another indicator of absorbed dose, True or False | True |
The ratio between the # of R and the # of Rads is the _______ | F-Factor |
Xrays have a QF of | 1 |
Protons have a QF of | 2 |
Thermal neutrons less than 10kev has a QF of | 5 |
Fast neutrons 100keV-2Mev has a QF of | 20 |
Alpha particles have a QF of | 20 |
MCP unknown energy have QF of | 20 |
DE(Rem)= | Rad times QF |
DE(Sv)= | Gray times QF |
______ _____ Radiation is measured indirectly by measuring effect radiation has on a medium such as air or film or crystals | Detection instruments |
Occupational Exposure for medicine is what | 150mRem |
Industry occupational exposure is what | 240mRem |
Nuclear power occupational exposure is what | 560mRem |
Government occupational exposure is what | 120mRem |
Miscellaneous occupational exposure is what | 160mRem |
Other worker occupational exposure is what | 180mRem |
Other(visitors) Occupational Exposure is what | 9mRem |
OSL stands for | Optically stimulated luminescence dosimeter |
OSL use what phosphor | Aluminum Oxide |
OSL luminescence occurs when the dosimeter is struck by ____ ____ | laser light |
The luminescence of an OSL is proportional to the amount of _____ _____ received | radiation exposure |
Ionization chambers are also known as ____ ____ | cutie pie |
Ionization chambers used most often to determine ____ ____ | exposure rate |
Proportional counters are used in labs to detect what ____ and ____ | alpha and beta |
____ ____ is a device used to accelerate subatomic particles for radiation therapy, radionuclide production and physics research | Linear Accelarator |
Linear Accelarators produces megavoltages of ___ to ___ | 4-40 |