Practice Exam Atomic Structure & Interactions

Which statement best explains why a neutral atom has no overall electric charge?

• Neutrons cancel proton charge

• The nucleus has equal numbers of protons and neutrons

• Electrons equal the number of protons

• Electrons have less mass than protons

Electrons equal the number of protons.

Which quantity determines the identity of an element?

• Number of neutrons

• Mass number

• Number of electrons

• Atomic number

Atomic number

Two isotopes of the same element differ in:

• Number of protons

• Number of neutrons

• Electron charge

• Chemical behavior completely

Number of neutrons

An atom in a ground state:

• Must have a completely full outer shell

• Has no neutrons

• Has all inner shells filled appropriately

• Cannot form molecules

Has all inner shells filled appropriately

The chemical properties of an atom are primarily determined by:

• The nucleus mass

• Number of outer shell electrons

• Number of neutrons

• Binding energy

Number of outer shell electrons

Which statement about mass-energy equivalence is correct?

• Energy cannot become mass

• Mass and energy are unrelated

• Mass may be transformed into energy

• Only energy can become mass

Mass may be transformed into energy

The mass defect of a nucleus represents:

• Lost electrons

• Missing neutrons

• Energy used to bind nucleons together

• Gamma energy only

Energy used to bind nucleons together

A nuclide has mass number 238 and atomic number 92. How many neutrons does it contain?

• 92

• 146

• 238

• 330

146

(238 - 92 = 146)

During beta-minus decay:

• A proton changes into a neutron

• A neutron changes into a proton

• Two neutrons are emitted

• The mass number decreases by 4

A neutron changes into a proton

Which decay changes neither the atomic number nor mass number?

• Alpha decay

• Beta-minus decay

• Positron emission

• Gamma emission

Gamma emission

Electron capture most commonly involves which shell electron?

• N shell

• M shell

• L shell

• K shell

K shell

Which radiation has the greatest specific ionization?

• Gamma rays

• Beta particles

• Alpha particles

• X-rays

Alpha particles

Beta decay produces a continuous spectrum of energies primarily because:

• Beta particles collide with electrons immediately

• Gamma rays are emitted simultaneously

• Energy is shared with neutrinos/antineutrinos

• Beta particles change mass in flight

Energy is shared with neutrinos/antineutrinos

Which radiation interaction absorbs the entire gamma photon energy?

• Pair production

• Elastic scattering

• Compton scattering

• Photoelectric effect

Photoelectric effect

Pair production requires gamma energy of at least:

• 0.511 MeV

• 1.022 MeV

• 2.044 MeV

• 10 MeV

1.022 MeV

Why are dense high-Z materials generally avoided for beta shielding?

• They absorb too few beta particles

• They increase neutron activation

• They increase bremsstrahlung production

• They become fissile

They increase bremsstrahlung production

Thermal neutrons typically have energies near:

• 20 MeV

• 10 keV

• 0.025 eV

• 1 MeV

0.025 eV

Water is an effective neutron shield primarily because:

• Oxygen strongly absorbs all neutrons

• Hydrogen efficiently slows neutrons through elastic scattering

• Water blocks gamma rays completely

• Water prevents activation products

Hydrogen efficiently slows neutrons through elastic scattering

Inelastic neutron scattering differs from elastic scattering because:

• No kinetic energy is transferred

• The nucleus becomes excited

• The neutron is absorbed

• No photons are emitted

The nucleus becomes excited

Which statement best describes fission?

• Electrons split the nucleus

• A nucleus spontaneously loses only gamma energy

• A nucleus splits into smaller nuclei after neutron absorption

• A proton changes into a neutron

A nucleus splits into smaller nuclei after neutron absorption

Explain the difference between ionization and excitation.

Ionization: Radiation ejects an electron completely from an atom, creating an ion pair.

Excitation: Radiation raises an electron to a higher energy level without ejecting it.

Why are alpha particles considered a serious internal hazard but usually not an external hazard?

Alpha particles have:

• very high specific ionization/high LET

• very short range in tissue

They cannot penetrate dead skin externally, but if inhaled/ingested they deposit large amounts of energy into living tissue and can cause severe damage internally.

Compare alpha particles and beta particles in terms of:

• mass

• charge

• penetration

• track behavior in matter

Alpha Particles:

• Mass: ~4 AMU

• Charge: +2

• Penetration: Low

• Track: Straight

Beta Particles:

• Mass: 1/1832 AMU

• Charge: -1 or +1

• Penetration: Moderate

• Track: Jagged/irregular

Explain why isotopes of the same element have similar chemical properties.

Chemical properties depend mainly on the number of outer-shell electrons. Isotopes have the same number of protons/electrons, so they behave chemically similarly.

Describe how neutron leakage can create radioactive materials outside a reactor core.

Escaping neutrons can activate nearby materials through neutron absorption, producing radioactive isotopes such as Ar-41.

Explain why increasing count time improves statistical accuracy in radiation measurements.

Increasing count time increases the total number of detected events, reducing statistical uncertainty and decreasing standard deviation.

Describe the relationship between half-life and radioactive decay.

Half-life is the time required for radioactive material to decay to half its original activity. Each radionuclide has a fixed half-life.

Explain why gamma spectroscopy can identify specific nuclides.

Each isotope emits unique discrete gamma energies, allowing identification through gamma spectroscopy.

Describe how bremsstrahlung radiation is produced.

Bremsstrahlung occurs when a beta particle is decelerated or bent near a nucleus, converting kinetic energy into an x-ray photon.

Explain how the Chart of Nuclides predicts likely decay modes.

• Nuclides above the line of stability tend toward β+ decay/electron capture.

• Nuclides below the line of stability tend toward β− decay.

Complete the following equation. Include missing atomic numbers, mass numbers, and emitted particles.

²²⁶₈₈Ra → ? + α

²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂α

Complete the following equation. Include missing atomic numbers, mass numbers, and emitted particles.

¹⁴₆C → ? + β^-

¹⁴₆C → ¹⁴₇N + β^-

(Antineutrino also emitted)

Complete the following equation. Include missing atomic numbers, mass numbers, and emitted particles.

⁵⁷₂₈Ni → ? + β⁺

⁵⁷₂₈Ni → ⁵⁷₂₇Co + β⁺

(Neutrino also emitted)

Complete the following equation. Include missing atomic numbers, mass numbers, and emitted particles.

¹²⁵₅₃I + e^- → ?

¹²⁵₅₃I + e^- → ¹²⁵₅₂Te

Complete the following equation. Include missing atomic numbers, mass numbers, and emitted particles.

⁶⁰₂₇Co → ⁶⁰₂₈Ni + ?

⁶⁰₂₇Co → ⁶⁰₂₈Ni + β^- + γ

A nuclide undergoes alpha decay. Describe how the atomic number and mass number change.

Alpha Decay:

• atomic number decreases by 2

• mass number decreases by 4

A nuclide undergoes beta-minus decay. Describe how the atomic number and mass number change.

Beta-Minus Decay:

• atomic number increases by 1

• mass number unchanged

A nuclide undergoes positron emission. Describe how the atomic number and mass number change.

Positron Decay:

• atomic number decreases by 1

• mass number unchanged

A Co-60 source has an initial activity of 80 Ci. The half-life is 5.3 years.

How much activity remains after 15.9 years?

Use:

A = A₀(1/2)^t/T_1/2

15.9 years = 3 half-lives

A = 80(1/2)³
A = 80 • (1/8)
A = 10 Ci

An Ir-192 source starts at 240 Ci. Its half-life is 74 days.

What activity remains after 222 days?

222 days = 3 half lives

A = 240(1/2)³
A = 240 • (1/8)
A = 30 Ci

A radionuclide undergoes 5 half-lives. What fraction of the original activity remains?

(1/2)⁵ = (1/32)

Remaining Fraction: 1/32

A sample decreases from 96 Ci to 12 Ci.

How many half-lives have passed?

96 → 48 → 24 → 12

3 half-lives passed.

If a source has a half-life of 10 years, what percent of the original activity remains after 30 years?

30 years = 3 half-lives

(1/2)³ = 1/8 = 12.5%

A background count rate is 400 cpm with a standard deviation of 20 cpm.

A sample measurement gives 435 cpm.

Would this likely be statistically distinguishable from background at the 2σ level? Explain.

2σ range:
400 ± (2 •20)
400 ± 40

Range:
360 to 440

435 cpm falls within this range, so it would not likely be distinguishable from background at the 2σ level.

A beta particle and an alpha particle have the same kinetic energy. Which would generally travel farther in tissue and why?

The beta particle would generally travel farther because it has much smaller mass and lower specific ionization than the alpha particle.

Why do gamma photons not have a finite range while charged particles do?

Charged particles continuously lose energy through interactions with matter and eventually stop. Gamma photons may travel long distances without interacting, so they do not have a finite range.

Explain why fast neutrons are less likely to be absorbed than thermal neutrons.

Fast neutrons move too quickly to be readily absorbed. Slowing them to thermal energies increases absorption probability.

Describe how neutron activation produces Co-60 in nuclear plants and why it is important radiologically.

Co-60 is produced when Co-59 absorbs a neutron:

⁵⁹₂₇Co + n → ⁶⁰₂₇Co

Co-60 is important because it emits high-energy gamma radiation and contributes significantly to plant dose rates.

Why does pair production become more likely in high atomic number materials?

High-Z nuclei create stronger electric fields, increasing the probability that a gamma photon will interact near the nucleus and undergo pair production.

A detector system has difficulty distinguishing a source from background radiation. List at least four ways to reduce the Minimum Detectable Activity (MDA).

Methods to reduce MDA:

• Reduce background radiation

• Shield the detector

• Increase count time

• Use a more efficient detector

• Increase sample size

• Improve detector/sample geometry