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Question 1Radiation Safety Fundamentals and Radiation Physics

The number of protons contained in the nucleus of an atom is called the:

  1. neutron number
  2. atomic mass
  3. atomic number Correct answer.
  4. mass number

Explanation

The atomic number (Z) equals the number of protons in the nucleus and fixes the identity of the element. The mass number is the total count of protons plus neutrons, so it cannot equal the proton count alone.

Question 2Radiation Safety Fundamentals and Radiation Physics

A radioisotope has a half-life of 10 days. What fraction of its initial activity remains after 30 days?

  1. one sixth
  2. one eighth Correct answer.
  3. one sixteenth
  4. one third

Explanation

Thirty days represents three half-lives of 10 days each. Each half-life halves the remaining activity, so the surviving fraction is (1/2)^3 = 1/8.

Question 3Radiation Safety Fundamentals and Radiation Physics

The SI unit of radioactivity that is defined as one disintegration per second is the:

  1. curie
  2. gray
  3. roentgen
  4. becquerel Correct answer.

Explanation

One becquerel (Bq) is exactly one nuclear transformation per second and is the SI unit of activity. The curie is the older non-SI activity unit equal to 3.7 x 10^10 disintegrations per second.

Question 4Radiation Safety Fundamentals and Radiation Physics

If a radiographer doubles his distance from a small gamma source, the exposure rate becomes:

  1. one half of the original value
  2. one quarter of the original value Correct answer.
  3. one eighth of the original value
  4. unchanged

Explanation

For a point source the exposure rate follows the inverse square law, varying as 1/d^2. Doubling the distance multiplies the rate by 1/2^2, which is one quarter.

Question 5Radiation Safety Fundamentals and Radiation Physics

Gamma rays differ from x-rays in that gamma rays:

  1. are always lower in energy than x-rays
  2. are emitted by the nucleus of a radioactive atom Correct answer.
  3. are produced when electrons fall into inner-shell vacancies
  4. are produced by bremsstrahlung in an x-ray tube

Explanation

Gamma rays originate from energy changes within an excited nucleus, whereas x-rays come from electron shell transitions or from the deceleration of electrons in a target. The two radiations are identical in nature and may overlap in energy, so energy alone cannot distinguish them.

Question 6Radiation Safety Fundamentals and Radiation Physics

The interaction in which a gamma photon transfers part of its energy to an outer-shell electron and is scattered in a new direction is called:

  1. photoelectric absorption
  2. coherent scattering
  3. pair production
  4. Compton scattering Correct answer.

Explanation

In Compton scattering the photon gives up only part of its energy to a loosely bound electron, which is ejected, and the degraded photon is deflected at an angle. The scattered photon always leaves with less energy than the incident photon.

Question 7Radiation Safety Fundamentals and Radiation Physics

In the photoelectric effect, an incident gamma photon:

  1. passes through the atom without interacting
  2. is scattered with essentially no loss of energy
  3. transfers all of its energy to an inner-shell electron Correct answer.
  4. splits into an electron-positron pair

Explanation

Photoelectric absorption occurs when a photon interacts with a tightly bound inner-shell electron and transfers its full energy, ejecting the electron. The resulting vacancy is filled by an outer-shell electron that emits characteristic x-radiation.

Question 8Radiation Safety Fundamentals and Radiation Physics

A steel shield has a half-value layer of 1.5 cm. What fraction of the incident gamma intensity is transmitted through 4.5 cm of steel?

  1. one half
  2. one quarter
  3. one sixteenth
  4. one eighth Correct answer.

Explanation

The 4.5 cm thickness contains 4.5 / 1.5 = 3 half-value layers. Each half-value layer removes half the intensity, so the transmitted fraction is (1/2)^3 = 1/8.

Question 9Radiation Safety Fundamentals and Radiation Physics

Bremsstrahlung radiation is produced when:

  1. high-speed electrons are decelerated in the electric field of a nucleus Correct answer.
  2. a nucleus emits an alpha particle
  3. a positron annihilates with an electron
  4. an outer-shell electron fills an inner-shell vacancy

Explanation

Bremsstrahlung, or braking radiation, results when fast electrons are deflected and slowed by the Coulomb field of a nucleus, converting kinetic energy into a continuous x-ray spectrum. Its maximum photon energy equals the kinetic energy of the incident electron.

Question 10Radiation Safety Fundamentals and Radiation Physics

A dose equivalent of 0.05 Sv is equal to:

  1. 0.5 rem
  2. 500 rem
  3. 5 rem Correct answer.
  4. 50 rem

Explanation

One sievert equals 100 rem, so a dose equivalent in sieverts is multiplied by 100 to convert to rem. Thus 0.05 Sv x 100 = 5 rem.

Question 11Radiation Safety Fundamentals and Radiation Physics

Which of the following radiations is stopped by a sheet of paper or by the dead layer of skin?

  1. neutrons
  2. high-energy beta particles
  3. alpha particles Correct answer.
  4. gamma rays

Explanation

Alpha particles are heavy, doubly charged particles that lose energy very rapidly and travel only a few centimeters in air. A sheet of paper or the dead outer layer of skin is enough to stop them, although they are a serious internal hazard if inhaled or ingested.

Question 12Radiation Safety Fundamentals and Radiation Physics

When a nucleus emits a beta-minus particle, its atomic number:

  1. remains unchanged
  2. decreases by two
  3. increases by one Correct answer.
  4. decreases by one

Explanation

Beta-minus emission occurs when a neutron in the nucleus converts into a proton, an electron, and an antineutrino. The proton count therefore rises by one, so the atomic number increases by one while the mass number is unchanged.

Question 13Radiation Safety Fundamentals and Radiation Physics

The roentgen is a unit that expresses:

  1. the activity of a radioactive source
  2. absorbed energy per unit mass of tissue
  3. exposure, the ionization produced in air Correct answer.
  4. the biological effectiveness of different radiations

Explanation

The roentgen is defined as the quantity of gamma or x-radiation that produces 2.58 x 10^-4 coulomb of charge in one kilogram of dry air. It therefore measures exposure, not absorbed dose or activity.

Question 14Radiation Safety Fundamentals and Radiation Physics

An absorbed dose of one gray means that:

  1. one rad has been delivered to the material
  2. one coulomb of charge has been produced per kilogram of air
  3. one disintegration per second has occurred in the material
  4. one joule of energy has been absorbed per kilogram of material Correct answer.

Explanation

The gray is the SI unit of absorbed dose and equals one joule of absorbed energy per kilogram of material. One gray is equivalent to 100 rad.

Question 15Radiation Safety Fundamentals and Radiation Physics

The linear attenuation coefficient of a material whose half-value layer is 2.0 cm is approximately:

  1. 0.347 per cm Correct answer.
  2. 0.693 per cm
  3. 0.173 per cm
  4. 1.386 per cm

Explanation

For exponential attenuation the half-value layer equals ln 2 divided by the linear attenuation coefficient, so the coefficient equals 0.693 divided by the half-value layer. Dividing 0.693 by 2.0 cm gives approximately 0.347 per cm.

Question 16Radiation Safety Fundamentals and Radiation Physics

The three basic methods of reducing a radiographer's radiation dose are:

  1. surveying, training, and recordkeeping
  2. shielding, calibration, and monitoring
  3. time, distance, and shielding Correct answer.
  4. filtration, collimation, and intensification

Explanation

Occupational dose is controlled by limiting the time spent near the source, maximizing distance from it, and placing appropriate shielding between the source and the person. The inverse square law makes distance especially powerful.

Question 17Radiation Safety Fundamentals and Radiation Physics

Pair production can occur only when the incident photon energy exceeds:

  1. 0.511 MeV
  2. 2.044 MeV
  3. 0.662 MeV
  4. 1.022 MeV Correct answer.

Explanation

Pair production requires at least the rest-mass energy of an electron plus a positron, which is 2 x 0.511 MeV = 1.022 MeV. Any energy above this threshold appears as kinetic energy of the created pair.

Question 18Radiation Safety Fundamentals and Radiation Physics

Two curies of radioactive material contain how many becquerels? (1 Ci = 3.7 x 10^10 Bq)

  1. 7.4 x 10^10 Bq Correct answer.
  2. 2.0 x 10^10 Bq
  3. 1.85 x 10^10 Bq
  4. 3.7 x 10^10 Bq

Explanation

Each curie equals 3.7 x 10^10 becquerels, so two curies equal twice that value. Multiplying 2 by 3.7 x 10^10 gives 7.4 x 10^10 Bq.

Question 19Radiation Safety Fundamentals and Radiation Physics

Neutron radiation is shielded most effectively by:

  1. water or polyethylene Correct answer.
  2. aluminum
  3. depleted uranium
  4. lead

Explanation

Neutrons are uncharged and interact mainly with nuclei, so they are slowed most efficiently by elastic collisions with light nuclei of similar mass. Hydrogen-rich materials such as water, polyethylene, and concrete are therefore good neutron shields.

Question 20Radiation Safety Fundamentals and Radiation Physics

A material has a mass attenuation coefficient of 0.100 cm^2/g and a density of 11.3 g/cm^3. Its linear attenuation coefficient is:

  1. 11.3 per cm
  2. 0.0089 per cm
  3. 1.13 per cm Correct answer.
  4. 0.100 per cm

Explanation

The linear attenuation coefficient equals the mass attenuation coefficient multiplied by the density. Multiplying 0.100 cm^2/g by 11.3 g/cm^3 gives 1.13 per cm.

Question 21Radiation Safety Fundamentals and Radiation Physics

Which of the following is electromagnetic radiation?

  1. alpha particles
  2. neutrons
  3. beta particles
  4. gamma rays Correct answer.

Explanation

Gamma rays are packets of electromagnetic energy with no mass and no charge. Alpha particles, beta particles, and neutrons are all particulate radiation.

Question 22Radiation Safety Fundamentals and Radiation Physics

A gamma source produces an exposure rate of 40 mR/h at 2 m. At what distance will the rate be 10 mR/h?

  1. 8 m
  2. 16 m
  3. 1 m
  4. 4 m Correct answer.

Explanation

The rate must fall to one quarter of its original value, and by the inverse square law this requires doubling the distance. Doubling 2 m gives 4 m.

Question 23Radiation Safety Fundamentals and Radiation Physics

An x-ray tube is operated at 150 kVp. The maximum photon energy in the emitted spectrum is:

  1. 15 keV
  2. 300 keV
  3. 150 keV Correct answer.
  4. 75 keV

Explanation

The Duane-Hunt limit states that the highest-energy x-ray photon has an energy equal to the kinetic energy of the incident electron. For a peak voltage of 150 kV, that maximum energy is 150 keV.

Question 24Radiation Safety Fundamentals and Radiation Physics

A gamma ray with an energy of 0.662 MeV has an energy of:

  1. 662 keV Correct answer.
  2. 6620 keV
  3. 6.62 keV
  4. 66.2 keV

Explanation

One million electron volts equals one thousand kiloelectron volts, so the numerical value is multiplied by 1000 when converting MeV to keV. Thus 0.662 MeV equals 662 keV.

Question 25Radiation Safety Fundamentals and Radiation Physics

Lead attenuates gamma radiation more effectively per centimeter than aluminum mainly because lead:

  1. has a higher atomic number and density, which increase the probability of interaction Correct answer.
  2. has a lower atomic number, which favors Compton scattering
  3. emits secondary gamma rays that cancel the primary beam
  4. is a better moderator of fast neutrons

Explanation

Attenuation per unit thickness increases with the number of atoms per unit volume and with the probability of interaction per atom, both of which rise with density and atomic number. Lead's high atomic number strongly favors photoelectric absorption at typical radiography energies.

Question 26Radiation Safety Fundamentals and Radiation Physics

The half-life of a radionuclide is the time required for its activity to:

  1. decrease to one-tenth of its initial value
  2. decrease to one-half of its initial value Correct answer.
  3. increase to twice its initial value
  4. decrease to zero

Explanation

Half-life is defined as the time in which the activity of a given quantity of radionuclide falls to half of its starting value. Radioactive decay is exponential, so each successive half-life removes another half of whatever activity remains. The activity never rises and never reaches zero in a finite time.

Question 27Radiation Safety Fundamentals and Radiation Physics

One becquerel is defined as an activity equal to:

  1. 2.58 x 10^-4 coulomb per kilogram per second
  2. 3.7 x 10^10 disintegrations per second
  3. one disintegration per second Correct answer.
  4. one gray absorbed per second

Explanation

The becquerel is the SI unit of activity and equals one nuclear transformation (disintegration) per second. The curie is the older unit and equals 3.7 x 10^10 becquerels. The coulomb-per-kilogram-per-second quantity relates to exposure rate, and the gray per second relates to absorbed dose rate.

Question 28Radiation Safety Fundamentals and Radiation Physics

The recommended annual whole-body effective dose limit for an occupationally exposed radiation worker is:

  1. 500 mSv (50 rem)
  2. 5 mSv (0.5 rem)
  3. 1 mSv (0.1 rem)
  4. 50 mSv (5 rem) Correct answer.

Explanation

Occupational radiation workers are limited to an effective dose of 50 mSv (5 rem) per year averaged over the whole body. The 1 mSv figure applies to individual members of the public. Limits are set well below levels at which deterministic effects occur and are supported by the ALARA principle.

Question 29Radiation Safety Fundamentals and Radiation Physics

A photon has a frequency of 3.0 x 10^18 Hz. Its energy is closest to:

  1. 12.4 keV Correct answer.
  2. 4.14 keV
  3. 124 keV
  4. 1.24 keV

Explanation

Photon energy is given by E = hf, where h is Planck's constant (4.136 x 10^-15 eV·s). Multiplying gives about 1.24 x 10^4 eV, which is 12.4 keV. The energy of a photon is directly proportional to its frequency.

Question 30Radiation Safety Fundamentals and Radiation Physics

An absorbed dose of 2 mGy is received from alpha particles, which have a radiation weighting factor of 20. The equivalent dose is:

  1. 0.1 mSv
  2. 2 mSv
  3. 40 mSv Correct answer.
  4. 400 mSv

Explanation

Equivalent dose is obtained by multiplying absorbed dose by the radiation weighting factor: 2 mGy x 20 = 40 mSv. The weighting factor accounts for the greater biological harm produced by densely ionizing radiations such as alpha particles. For beta, gamma, and x-rays the factor is 1, so the numerical values would be equal.

Question 31Radiation Safety Fundamentals and Radiation Physics

When a positron annihilates with an electron, the two photons produced each have an energy of approximately:

  1. 0.511 MeV
  2. 1.33 MeV
  3. 1.022 MeV Correct answer.
  4. 0.662 MeV

Explanation

The rest-mass energy of an electron or positron is 0.511 MeV, so the pair holds 1.022 MeV in total. On annihilation this energy appears as two photons emitted in opposite directions, each carrying half of the total, or 0.511 MeV. This 0.511 MeV annihilation radiation is a characteristic feature of positron emitters.

Question 32Radiation Safety Fundamentals and Radiation Physics

For 1 MeV gamma photons interacting with aluminum, the predominant interaction is:

  1. coherent (Rayleigh) scattering
  2. photoelectric absorption
  3. Compton scattering Correct answer.
  4. pair production

Explanation

Compton scattering dominates at intermediate photon energies in low-atomic-number materials such as aluminum. Photoelectric absorption prevails at low energies and in high-Z absorbers, while pair production requires energies above 1.022 MeV and a high-Z nucleus to be significant. Coherent scattering is a minor contributor at this energy.

Question 33Radiation Safety Fundamentals and Radiation Physics

An x-ray photon has a wavelength of 0.05 nm. Its energy is approximately:

  1. 2.48 keV
  2. 12.4 keV
  3. 248 keV
  4. 24.8 keV Correct answer.

Explanation

Photon energy in keV can be found from E = 1.24 / wavelength in nanometers. Substituting 0.05 nm gives 1.24 / 0.05 = 24.8 keV. Energy and wavelength are inversely related.

Question 34Radiation Safety Fundamentals and Radiation Physics

A sample contains 4.0 x 10^12 atoms of a radionuclide with a half-life of 10 days. Its activity is approximately:

  1. 3.2 x 10^5 Bq
  2. 4.0 x 10^12 Bq
  3. 3.2 x 10^6 Bq Correct answer.
  4. 8.0 x 10^-7 Bq

Explanation

Activity equals the decay constant multiplied by the number of atoms: A = lambda x N. The decay constant is ln2 divided by the half-life in seconds, 0.693 / 864000 s = 8.0 x 10^-7 per second. Multiplying by 4.0 x 10^12 atoms gives about 3.2 x 10^6 disintegrations per second, or 3.2 x 10^6 Bq.

Question 35Biological Effects of Radiation and Radiation Protection

The SI unit used to express absorbed dose is the:

  1. sievert
  2. becquerel
  3. coulomb per kilogram
  4. gray Correct answer.

Explanation

Absorbed dose is the energy deposited per unit mass of material, and its SI unit is the gray, where 1 Gy equals 1 joule per kilogram. The sievert applies weighting for biological effect, and the becquerel measures activity.

Question 36Biological Effects of Radiation and Radiation Protection

A dose equivalent of 1 sievert is numerically equal to how many rem?

  1. 1000 rem
  2. 10 rem
  3. 100 rem Correct answer.
  4. 1 rem

Explanation

The conversion between SI and traditional units is fixed: 1 Sv equals 100 rem, because 1 rem equals 0.01 Sv. This relationship is used whenever old dosimetry records are compared with SI values.

Question 37Biological Effects of Radiation and Radiation Protection

Under NRC regulations, the annual total effective dose equivalent limit for an adult radiation worker is:

  1. 1 rem
  2. 5 rem Correct answer.
  3. 50 rem
  4. 0.5 rem

Explanation

The occupational whole-body limit in 10 CFR 20 is 5 rem (50 mSv) per year for adults. Separate, higher limits apply to extremities, skin, and the lens of the eye.

Question 38Biological Effects of Radiation and Radiation Protection

The annual effective dose limit for a member of the general public from licensed operations is:

  1. 5 rem
  2. 0.1 rem Correct answer.
  3. 0.5 rem
  4. 1 rem

Explanation

The NRC public dose limit is 0.1 rem (1 mSv) per year total effective dose equivalent. This is one-fiftieth of the adult occupational limit.

Question 39Biological Effects of Radiation and Radiation Protection

Which of the following is classified as a stochastic effect of radiation?

  1. Cancer induction Correct answer.
  2. Cataract formation
  3. Acute radiation sickness
  4. Skin erythema

Explanation

Stochastic effects are assumed to have no threshold, and their probability of occurrence rises with dose while severity does not. Cancer induction is the classic example of a stochastic somatic effect.

Question 40Biological Effects of Radiation and Radiation Protection

The ALARA philosophy requires that occupational exposures be:

  1. eliminated entirely in all cases
  2. measured with a film badge
  3. kept below the legal limit only
  4. reduced below limits where practical Correct answer.

Explanation

ALARA means doses are kept as low as reasonably achievable, weighing social and economic factors, even when regulatory limits are already met. Compliance with a limit is a floor for performance, not the goal.

Question 41Biological Effects of Radiation and Radiation Protection

The three fundamental methods for controlling external radiation exposure are:

  1. distance, shielding, monitoring
  2. time, temperature, shielding
  3. shielding, filtration, collimation
  4. time, distance, shielding Correct answer.

Explanation

Exposure is controlled by minimizing time near the source, maximizing distance from it, and placing appropriate shielding between the source and the person. These three controls apply to every external exposure situation.

Question 42Biological Effects of Radiation and Radiation Protection

A radiographer doubles the distance from a small sealed source. The exposure rate at the new position becomes:

  1. one quarter Correct answer.
  2. the same
  3. one half
  4. one eighth

Explanation

Exposure rate from a point source follows the inverse square law, so doubling the distance divides the rate by 2 squared, which is 4. Distance is often the fastest control available because its effect is large.

Question 43Biological Effects of Radiation and Radiation Protection

After passing through three half-value layers of shielding, the intensity of a beam is reduced to what fraction of its original value?

  1. 1/3
  2. 1/6
  3. 1/16
  4. 1/8 Correct answer.

Explanation

Each half-value layer removes half the remaining intensity, so after three layers the transmitted fraction is 1/2 raised to the third power, or 1/8. Half-value layers compound multiplicatively rather than additively.

Question 44Biological Effects of Radiation and Radiation Protection

A survey meter reads 400 mR/h at 1 meter from a point source. Ignoring attenuation in air, the reading at 4 meters is:

  1. 50 mR/h
  2. 6.25 mR/h
  3. 25 mR/h Correct answer.
  4. 100 mR/h

Explanation

Distance increases by a factor of 4, so the inverse square law reduces the rate by 4 squared, or 16. Dividing 400 mR/h by 16 gives 25 mR/h.

Question 45Biological Effects of Radiation and Radiation Protection

A declared pregnant radiographer's occupational exposure to the embryo/fetus is limited to what value over the entire gestation period?

  1. 0.5 rem Correct answer.
  2. 0.1 rem
  3. 5 rem
  4. 1.5 rem

Explanation

The NRC limit for the embryo/fetus of a declared pregnant worker is 0.5 rem (5 mSv) for the full gestation period. Once the pregnancy is declared, the licensee must track and limit fetal dose accordingly.

Question 46Biological Effects of Radiation and Radiation Protection

For acute whole-body exposure without aggressive medical support, the LD50/60 for humans is approximately:

  1. 2000 rad
  2. 100 rad
  3. 1000 rad
  4. 400 rad Correct answer.

Explanation

The lethal dose for 50 percent of an exposed population within 60 days is roughly 400 rad (4 Gy) when only supportive care is given. Modern medical treatment can raise survival well above this level.

Question 47Biological Effects of Radiation and Radiation Protection

Which characteristic defines a deterministic effect of radiation?

  1. A threshold dose exists Correct answer.
  2. The effect is inherited by offspring
  3. Probability rises with dose and no threshold applies
  4. Severity is independent of dose

Explanation

Deterministic effects occur only after a threshold dose is exceeded, and their severity then increases with further dose. Examples include erythema, cataract, and acute radiation syndrome.

Question 48Biological Effects of Radiation and Radiation Protection

The NRC annual dose equivalent limit for the lens of the eye is:

  1. 50 rem
  2. 0.5 rem
  3. 5 rem
  4. 15 rem Correct answer.

Explanation

The lens dose equivalent limit in 10 CFR 20 is 15 rem (150 mSv) per year. The lens is more radiosensitive than most tissues but less so than the whole-body effective-dose basis.

Question 49Biological Effects of Radiation and Radiation Protection

The annual limit for the extremities and skin of an adult radiation worker is:

  1. 50 rem Correct answer.
  2. 15 rem
  3. 5 rem
  4. 500 rem

Explanation

Extremities and skin are limited to 50 rem (500 mSv) per year, ten times the whole-body limit. This recognizes that localized doses are less likely to produce systemic harm.

Question 50Biological Effects of Radiation and Radiation Protection

A radionuclide has a physical half-life of 8 days and a biological half-life of 8 days in the body. Its effective half-life is:

  1. 16 days
  2. 8 days
  3. 2 days
  4. 4 days Correct answer.

Explanation

Physical and biological removal act in parallel, so effective half-life is found from the reciprocal sum: 1/Te = 1/Tp + 1/Tb. With both values at 8 days, Te equals 4 days.