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BLD-NUCL84-05 · Lesson 1 · 24 clock hours · theory

Atomic Structure, Radioactive Decay, and Radiation Types

Objective. Students will identify the four major types of ionizing radiation, describe their decay mechanisms, and state the penetrating power and shielding requirements of each.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Atomic Structure and the Nucleus

Every atom consists of a dense central nucleus surrounded by orbiting electrons. The nucleus contains protons (positively charged) and neutrons (no charge). The number of protons defines the element; the total number of protons plus neutrons is the mass number. Atoms of the same element with different numbers of neutrons are called isotopes or nuclides. When a nucleus has an unstable proton-to-neutron ratio, it will spontaneously release energy or particles to reach a more stable configuration — this process is radioactive decay.

The Chart of the Nuclides (Knolls Atomic Power Laboratory edition) maps every known nuclide, showing its stability, decay mode, half-life, and decay products. You will use this chart throughout your training.

The Four Major Types of Ionizing Radiation

Alpha (α) Radiation

An alpha particle consists of two protons and two neutrons — identical to a helium-4 nucleus. It is emitted when a heavy nucleus sheds mass to become more stable. Alpha particles carry a +2 charge, making them highly ionizing but very weakly penetrating. A single sheet of paper or the outer layer of human skin stops alpha particles completely.

Key hazard: Alpha radiation becomes dangerous when alpha-emitting material is inhaled or ingested — internal contamination allows direct contact with sensitive tissue. External alpha exposure to intact skin is not a significant dose concern.

Shielding: A sheet of paper, a few centimeters of air, or intact skin.

Beta (β) Radiation

A beta particle is a high-speed electron (β⁻, the most common in plant operations) or positron (β⁺) ejected from the nucleus during decay. Beta-minus decay occurs when a neutron converts to a proton, releasing an electron and an antineutrino. Beta particles are more penetrating than alpha but less penetrating than gamma rays.

Key hazard: Beta particles can penetrate the outer skin and cause radiation burns. Internal contamination is also a concern. Use low-atomic-number (low-Z) shielding such as plastic or acrylic — do not use lead as the primary beta shield, because high-Z materials produce bremsstrahlung (secondary X-rays).

Shielding: Plastic, acrylic, or aluminum; typically a centimeter or less of material.

Gamma (γ) Radiation

Gamma rays are high-energy photons emitted when a nucleus transitions from an excited energy state to a lower one, often following alpha or beta decay. They carry no charge and no mass. Gamma rays are the most penetrating common radiation type and the primary external dose concern in nuclear plant work.

Key hazard: Gamma rays penetrate the entire body, depositing dose in internal organs. Time, distance, and shielding are all critical controls.

Shielding: Dense, high-Z material — lead is standard; thick concrete and steel are used in plant construction. Dose decreases with the square of the distance from the source (inverse-square law).

Neutron Radiation

Neutrons are released during fission and from certain activation reactions. Because they carry no charge, neutrons penetrate deeply and can make stable materials radioactive through neutron activation — a major source of plant contamination (e.g., Co-60 from cobalt impurities in steel). Neutrons also deposit dose through recoil of hydrogen nuclei in tissue.

Key hazard: High biological effectiveness and activation of structural materials. Primary concern near the reactor core and during refueling.

Shielding: Hydrogen-rich materials (water, polyethylene) slow (moderate) neutrons; borated water or borated polyethylene then absorb them. Concrete also provides effective neutron shielding.

Common Plant Nuclides

| Nuclide | Decay Mode | Primary Radiation | Half-Life |

|---------|-----------|------------------|-----------|

| Co-60 | Beta, then Gamma | β, γ (two gamma lines) | ~5.27 years |

| Cs-137 | Beta, then Gamma | β, γ (0.662 MeV) | ~30.2 years |

| I-131 | Beta, then Gamma | β, γ | ~8.02 days |

Radioactive Decay and Half-Life

Decay is a random process described statistically. The decay constant (λ) is the probability per unit time that a given nucleus will decay. The activity (A) — the number of decays per second, measured in becquerels (Bq) or curies (Ci) — follows the equation:

A = A₀ × e^(−λt)

where A₀ is the initial activity, t is elapsed time, and e is Euler's number (~2.718). The half-life (t½) is the time for activity to fall to half its original value. The relationship between λ and t½ is:

λ = ln(2) / t½ ≈ 0.693 / t½

Always carry units through every calculation and verify that time units for λ and t are consistent.

Safety Reminders

Key terms10

Radioactive decayThe spontaneous process by which an unstable nucleus releases energy or particles to reach a more stable state, resulting in a different nuclide or energy level.
Alpha particleA helium-4 nucleus (2 protons, 2 neutrons) emitted during decay of heavy nuclides; highly ionizing but stopped by a sheet of paper or intact skin.
Beta particleA high-speed electron (β⁻) or positron (β⁺) emitted from the nucleus during decay; more penetrating than alpha, shielded by low-Z materials such as plastic.
Gamma rayA high-energy photon emitted when a nucleus transitions between energy states; the most penetrating common radiation type, requiring dense shielding such as lead or thick concrete.
NeutronAn uncharged nuclear particle released during fission; penetrates deeply, can activate materials, and is shielded by hydrogen-rich materials and borated water.
Half-life (t½)The time required for the activity of a radioactive sample to decrease to half of its initial value; a characteristic constant for each nuclide.
Decay constant (λ)The probability per unit time that a given radioactive nucleus will decay; related to half-life by λ = 0.693 / t½.
Activity (A)The rate of radioactive decay of a sample, measured in becquerels (Bq, one decay per second) or curies (Ci); calculated using A = A₀e^(−λt).
Neutron activationThe process by which a stable nucleus absorbs a neutron and becomes radioactive; a major source of contamination in nuclear plants (e.g., Co-60 from steel components).
ALARAAs Low As Reasonably Achievable; the guiding principle in radiation protection requiring that occupational doses be kept as low as practicable using time, distance, and shielding controls.

Worked examples4

1. A sample of I-131 has an initial activity of 8.0 × 10⁶ Bq. The half-life of I-131 is 8.02 days. What is the activity after 24.06 days?
  1. Step 1 – Determine the number of half-lives elapsed: n = t / t½ = 24.06 days / 8.02 days = 3.00 half-lives.
  2. Step 2 – Calculate the decay constant: λ = 0.693 / 8.02 days = 0.08641 day⁻¹.
  3. Step 3 – Apply the decay equation: A = A₀ × e^(−λt) = 8.0 × 10⁶ Bq × e^(−0.08641 × 24.06).
  4. Step 4 – Evaluate the exponent: −0.08641 × 24.06 = −2.0794 ≈ −ln(8) (since 3 half-lives = reduction by factor of 2³ = 8).
  5. Step 5 – Calculate: A = 8.0 × 10⁶ Bq × e^(−2.0794) = 8.0 × 10⁶ × 0.1250 = 1.0 × 10⁶ Bq.
  6. Step 6 – Unit check: Bq × (dimensionless) = Bq. ✓
Answer: A = 1.0 × 10⁶ Bq after 24.06 days (three half-lives reduce activity by a factor of 8).
2. A Co-60 source has an initial activity of 3.20 × 10⁵ Ci. The half-life of Co-60 is 5.27 years. What is the activity after 10.54 years?
  1. Step 1 – Determine half-lives elapsed: n = 10.54 years / 5.27 years = 2.00 half-lives.
  2. Step 2 – Calculate decay constant: λ = 0.693 / 5.27 years = 0.13151 year⁻¹.
  3. Step 3 – Apply the decay equation: A = 3.20 × 10⁵ Ci × e^(−0.13151 × 10.54).
  4. Step 4 – Evaluate the exponent: −0.13151 × 10.54 = −1.3861 ≈ −ln(4) (2 half-lives = reduction by factor of 2² = 4).
  5. Step 5 – Calculate: A = 3.20 × 10⁵ Ci × 0.2500 = 8.0 × 10⁴ Ci.
  6. Step 6 – Unit check: Ci × (dimensionless) = Ci. ✓
Answer: A = 8.0 × 10⁴ Ci after 10.54 years.
3. A Cs-137 source (t½ = 30.2 years) registers an activity of 5.0 × 10⁴ Bq today. What was its activity 60.4 years ago?
  1. Step 1 – Determine half-lives elapsed: n = 60.4 years / 30.2 years = 2.00 half-lives.
  2. Step 2 – Rearrange the decay equation to solve for A₀: A₀ = A × e^(+λt) (we are going backward in time).
  3. Step 3 – Calculate decay constant: λ = 0.693 / 30.2 years = 0.02295 year⁻¹.
  4. Step 4 – Evaluate exponent: +0.02295 × 60.4 = +1.3862 ≈ ln(4).
  5. Step 5 – Calculate: A₀ = 5.0 × 10⁴ Bq × e^(1.3862) = 5.0 × 10⁴ × 4.000 = 2.0 × 10⁵ Bq.
  6. Step 6 – Unit check: Bq × (dimensionless) = Bq. ✓
Answer: A₀ = 2.0 × 10⁵ Bq sixty years ago.
4. A plant technician needs to know when a measured activity of 1.6 × 10⁶ Bq of I-131 (t½ = 8.02 days) will decay to 2.0 × 10⁵ Bq for safe handling. How many days will this take?
  1. Step 1 – Write the decay equation rearranged for t: t = −(1/λ) × ln(A / A₀).
  2. Step 2 – Calculate λ: λ = 0.693 / 8.02 days = 0.08641 day⁻¹.
  3. Step 3 – Calculate the ratio: A / A₀ = 2.0 × 10⁵ / 1.6 × 10⁶ = 0.125 = 1/8.
  4. Step 4 – Take the natural log: ln(0.125) = ln(1/8) = −ln(8) = −2.0794.
  5. Step 5 – Solve for t: t = −(1 / 0.08641 day⁻¹) × (−2.0794) = 2.0794 / 0.08641 = 24.07 days.
  6. Step 6 – Unit check: (day) × (dimensionless) = days. ✓ (This equals exactly 3 half-lives, confirming the answer.)
Answer: Approximately 24.1 days (3 half-lives) must elapse before activity falls to 2.0 × 10⁵ Bq.

The math in this lesson2

Learners model radioactive decay as an exponential function of time.
F-LE.1F-LE.4
Brush up in the math halls at newSchoolWorks.com:
In class: Lecture and annotated diagram exercise: students label alpha, beta, gamma, and neutron radiation on decay-chain charts for common plant nuclides (Co-60, Cs-137, I-131) and write one shielding recommendation for each. · Card-sort activity: students match decay type to detector type, shielding material, and typical source in a nuclear plant, then justify choices in a paired discussion. · Short-answer quiz on half-life calculations using the decay equation A = A₀e^(−λt), requiring at least five worked problems with units checked.
Materials: Nuclide chart (Knolls Atomic Power Laboratory edition), whiteboard, scientific calculators, NRC Regulatory Guide 8.29 (Instruction Concerning Risks from Occupational Radiation Exposure), Chart of the Nuclides
Check: Written 20-question quiz on radiation types, decay equations, and shielding; passing score 70% or higher.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Radiation Protection — NRC Non-Licensed Operator Prep / NRRPT
1. Which type of ionizing radiation is stopped by a single sheet of paper and poses its greatest hazard through internal contamination?
Radiation Protection — NRRPT / NUCP Uniform Curriculum
2. A technician must shield a strong beta source. Which material is the BEST primary shield choice to minimize secondary radiation?
Radiation Protection — NRC Non-Licensed Operator Prep / NUCP Uniform Curriculum
3. Co-60 is a significant radiation source in nuclear plant piping systems. What is the PRIMARY reason Co-60 is present in plant components?
Radiation Protection — NRC Non-Licensed Operator Prep / NRRPT
4. Using the decay equation A = A₀e^(−λt), an initial activity of 4.0 × 10⁸ Bq decays for exactly two half-lives. What is the resulting activity?
Radiation Protection — NRRPT / NRC Reactor Operator License
5. Which shielding material is MOST effective at slowing fast neutrons before they are absorbed?
Radiation Protection — NUCP Uniform Curriculum / NRRPT
6. During beta-minus decay, which particles are released from the nucleus?
Radiation Protection — OSHA 10 / NRC Non-Licensed Operator Prep
7. A worker is entering a high-radiation area where Cs-137 (gamma emitter) is the primary source. According to ALARA principles, which combination of controls is MOST effective?
Radiation Protection — NRRPT / NUCP Uniform Curriculum
8. The decay constant (λ) of a nuclide with a half-life of 5.0 days is approximately:
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