Certified Health Physicist (CHP) — Study Guide

Certified Health Physicist (CHP) — Study Guide

Credential: Certified Health Physicist (CHP), "Comprehensive Certification" Issuing body: American Board of Health Physics (ABHP); the American Academy of Health Physics (AAHP) is the professional organization of CHPs Jurisdiction: United States (recognized internationally) Level: Professional certification — senior-level. Explicitly NOT a license to practice; it certifies competence, not legal authority. Prerequisites: Bachelor's degree in physical science, engineering, or biological science with at least 20 semester hours in physical science; 6 years of professional health-physics experience (master's may substitute 1 year, doctorate 2 years); reference from immediate supervisor plus at least two other individuals including one currently-certified HP; written professional report reflecting a health-physics effort. Part I may be taken early-admission (e.g., with an HP master's by June 30 of the exam year) before most other requirements are met. Exam format: Two parts. Part I: 150 multiple-choice questions, 3 hours, fundamentals of health physics; offered at Pearson VUE centers in the week before the Health Physics Society annual meeting. Historically the passing score has run around 92–97 correct (61–65%). Part II: 6-hour open-ended written examination in applied health physics, given the Monday of the HPS annual meeting (and simultaneously at other locations). Section 1: six mandatory core questions, 50 points each (personnel dosimetry internal/external, shielding and activation, measurements and instrumentation, biological effects/risk). Section 2: eight specialty questions, answer 4 of 8, 100 points each (Accelerators, Environmental, Fuel Cycle and Waste Management, Medical, Research and Power Reactors, University, General incl. nonionizing/emergency response). Total 700 points; historical passing score ~469 (67%). Part II must be passed within 7 years of Part I or both are retaken. A particularly poor Part II score bars re-taking the following year. (Sources: OSTI/ABHP radiation-disciplines document; Wikipedia summary of ABHP requirements; HPS chapter certification materials. The catalogue notes the official ABHP certification page could not be located via search — confirm current application deadlines and fees at the official ABHP page before relying on them.) Renewal: Continuing certification with continuing-education credits — exact current rules unverified; check ABHP. Flagged as estimated in catalogue.

Why this credential matters

The CHP is the senior professional credential for radiation protection — the person employers hire as Radiation Safety Officer, who signs off ALARA programs, and who is legally defensible as an expert witness. It is the credential that says "this person can do the calculations and defend the judgments," not just run a survey meter.

Knowledge domains

Domain 1 — Atomic structure, radioactivity, decay

Master the decay law A = A0 · e^(−λt), λ = ln2/t_1/2, and activity units (1 Ci = 3.7e10 Bq; 1 Bq = 1 disintegration/s). Branching ratios determine daughter-product yields. Parent-daughter equilibria: secular equilibrium (t_parent >> t_daughter, e.g., U-238 chain: daughter activity ≈ parent activity), transient equilibrium, and no equilibrium — derive from the Bateman equations. The exam's favorite: the radon-222 chain and working level (WL = any combination of short-lived Rn daughters in 1 L of air that releases 1.3e5 MeV of alpha energy; 1 WL = 100 pCi/L at equilibrium).

Specific activity: A/m = (N_A · ln2) / (M · t_1/2), in Bq/g. High specific activity means radiotoxic material in small mass — e.g., Pu-239 ≈ 2.3e9 Bq/g.

Domain 2 — Interaction of radiation with matter

Charged particles: stopping power, Bragg peak, range-energy relationships (alpha ranges: a few cm in air, microns in tissue; beta ranges scale with energy — rule of thumb: max beta range in water ≈ 0.5 cm per MeV). Photons: photoelectric effect (∝ Z^4–5/E^3.5), Compton scattering (dominant 0.1–10 MeV in tissue/water), pair production (>1.022 MeV). Buildup factor B: scattered photons add to the dose behind shielding — the uncollided-photon calculation underestimates dose, sometimes by 5–10× in broad-beam geometry. Neutrons: elastic scattering (moderation, hydrogen best), inelastic, capture — and the 1/v absorption cross-section behavior at thermal energies.

Why shielding math works the way it does: narrow-beam attenuation I = I0·e^(−μx) is only the starting point. Tenth-value layer (TVL) and half-value layer (HVL): HVL = ln2/μ = 0.693/μ. For photons: x = n · HVL ⇒ I = I0/2^n. For neutrons: removal cross-section approximation for hydrogenous shields, Σ_R with I = I0·e^(−Σ_R x). Skyshine and groundshine complicate real facilities.

Domain 3 — External dosimetry and shielding

Point-source gamma: Ḋ = Γ · A / r^2, where Γ is the specific gamma-ray constant (e.g., Co-60: ≈ 1.32 R·cm²/mCi·h ≈ 13.2 mR/h at 1 m per Ci — know the constants for Co-60, Cs-137 ≈ 3.3 R·cm²/mCi·h), A in mCi, r in cm → Ḋ in R/h. Convert exposure (R) to dose: for tissue, roughly 1 R ≈ 0.96 rad ≈ 0.96 rem for photons (quality factor Q=1 for photons/electrons, 20 for alpha). Effective dose E = Σ w_T · H_T with tissue weighting factors (ICRP 103/60). Operational quantities: H*(10) ambient dose equivalent, Hp(10) personal dose equivalent.

Inverse-square law governs survey and stay-time work. Beta dose at skin: surface dose rate from a beta source scales with source strength and the 70-μm depth dose — know the rule-of-thumb for Sr-90/Y-90 skin dose.

Domain 4 — Internal dosimetry

Committed effective dose E(50) = Σ_T w_T · H_T(50), integrating over 50 years for workers. Intake models: ICRP respiratory tract model (now HRTM — deposition fractions by AMAD, clearance classes F/M/S replacing the old D/W/Y), GI tract model, systemic biokinetic models. ALI (annual limit on intake) = 0.05 Sv / e(50) per unit intake; DAC (derived air concentration) = ALI / (2400 m³ breathing volume per work year: 1.2 m³/h × 2000 h). Tritium as HTO is the classic worked example: uniform whole-body distribution, 10-day biological half-life, DAC ≈ 8e-7 μCi/mL (older value 5e-6 μCi/mL pre-ICRP-30 revisions — watch which ICRP edition a question uses).

Bioassay: in-vivo (whole-body counting for gamma emitters), in-vitro (urinalysis for tritium, Pu via radiochemistry), fecal analysis for insoluble actinides. MDA (minimum detectable activity) and the decision level L_C vs detection limit L_D (Currie): L_C = 2.33·σ_B (95% confidence against false positives), L_D = 2.71 + 4.65·σ_B.

Domain 5 — Radiation biology and risk

Deterministic effects (threshold, severity ∝ dose: cataracts ~0.5 Gy lens, skin erythema ~2–3 Gy) vs stochastic effects (no threshold, probability ∝ dose: cancer, hereditary). LNT (linear no-threshold) is the regulatory model — know it's a policy assumption, not a proven biological fact, and the exam tests this distinction. Dose and dose-rate effectiveness factor (DDREF ≈ 2 in ICRP 103). BEIR VII risk coefficients (~5–6%/Sv lifetime cancer mortality for a population). Acute radiation syndrome phases (prodromal, latent, manifest, recovery/death) with dose ranges: hematopoietic 1–8 Gy, GI >8 Gy, neurovascular >20 Gy.

Domain 6 — Instrumentation and counting statistics

Gas-filled detectors (ion chambers, proportional counters, GM tubes — know the voltage regions on the gas multiplication curve), scintillators (NaI, plastic, ZnS for alpha), semiconductors (HPGe for spectroscopy, Si for charged particles). Counting statistics: Poisson, σ = √N; relative error = 1/√N. To halve the relative error, count 4× longer. Dead time correction: n = m/(1 − mτ), n true rate, m measured rate, τ dead time — know both paralyzable and non-paralyzable models qualitatively.

Domain 7 — Regulations and standards

10 CFR Part 20: occupational dose limits (5 rem/yr TEDE, 50 rem/yr to skin/extremities, 15 rem/yr lens of eye), public limits (100 mrem/yr), ALARA as a required program element, surveys, postings (CAUTION vs GRAVE DANGER thresholds), effluent limits. 10 CFR Part 35 (medical use), 10 CFR Part 71/49 CFR (transport: A1/A2 values, Type A vs B packages, TI — transport index = dose rate at 1 m in mrem/h), 10 CFR Part 61 (low-level waste classification: Class A/B/C by radionuclide concentration tables). OSHA ionizing radiation standard 29 CFR 1910.1096 vs NRC jurisdiction — know which applies (NRC/Agreement State licensees vs others). EPA: 40 CFR 190 (fuel cycle, 25 mrem/yr), 40 CFR 61 Subpart H (NESHAPs radionuclides). ICRP/ICRU/NCRP guidance vs regulation.

Domain 8 — Specialty areas (Part II Section 2)

Pick four of eight: accelerators (skyshine, photoneutrons, activation), environmental (atmospheric dispersion — Gaussian plume model; liquid effluent dilution), fuel cycle (criticality safety basics: k_eff < 0.95 administrative limit, double contingency; UF6, MOX, reprocessing), medical (10 CFR 35, therapy misadministration criteria, HDR brachytherapy), reactors (containment, effluent tech specs, fuel handling), university (broad-scope licenses, diverse isotopes), general (nonionizing: lasers per ANSI Z136, RF/microwave, UV; emergency response).

Key equations with units and assumptions

• A = A0·e^(−λt); λ = ln2/t_1/2 (s^-1); A in Bq or Ci.
• Specific activity (Bq/g) = 4.17e23 / (M · t_1/2[s]) — assumes pure radionuclide.
• Point gamma: Ḋ(R/h) = Γ(R·cm²/mCi·h) · A(mCi) / r(cm)² — point source, no shielding, no buildup.
• HVL = 0.693/μ; TVL = 2.303/μ; broad-beam: Ḋ = Ḋ0·B·e^(−μx)/... — buildup B ≥ 1 required for broad beams.
• Inverse square: Ḋ2 = Ḋ1·(r1/r2)².
• DAC (μCi/mL) = ALI(μCi) / 2.4e9 mL; assumes light work breathing 1.2 m³/h, 2000 h/yr.
• Currie: L_C = 2.33σ_B, L_D = 2.71 + 4.65σ_B (counts); MDA = L_D/(ε·t·K...) with detector efficiency, count time, conversion factors.
• Dead time: n = m/(1 − mτ), all rates in s^-1, τ in s (non-paralyzable).
• Gaussian plume centerline: χ = Q/(π·σ_y·σ_z·u) (Bq/m³), Q release rate (Bq/s), u wind speed (m/s), σ in m — assumes continuous release, flat terrain, no deposition.

Worked examples

Example 1 — Decay and activity. 100 mCi of Mo-99 (t_1/2 = 66 h) at 08:00 Monday. Activity at 08:00 Thursday (72 h)? λ = 0.693/66 = 0.0105 h^-1. A = 100·e^(−0.0105×72) = 100·e^(−0.756) = 100 × 0.4696 ≈ 47 mCi. Rule-of-thumb check: 72 h is ~1.09 half-lives → slightly less than half remains. ✓.

Example 2 — Point-source dose rate. 5 Ci Co-60 source, worker at 2 m. Γ(Co-60) ≈ 13.2 mR·m²/Ci·h (equivalently 1.32 R·cm²/mCi·h). Ḋ = 13.2 × 5 / 2² = 66/4 = 16.5 mR/h ≈ 16.5 mrem/h (photons, Q=1). At 4 m: 16.5/4 ≈ 4.1 mrem/h. Note: this is exposure rate; dose-equivalent rate in tissue is within ~5% for Co-60 gammas — state the approximation.

Example 3 — Shielding. Reduce the 16.5 mrem/h at 2 m to ≤ 0.5 mrem/h using lead. TVL for Co-60 in lead ≈ 4 cm (HVL ≈ 1.2 cm). Reduction needed: 16.5/0.5 = 33. Number of TVLs: log10(33) ≈ 1.52 → thickness ≈ 1.52 × 4 cm ≈ 6.1 cm of lead, using narrow-beam attenuation. With buildup in broad-beam geometry, add margin — a CHP states the narrow-beam result and the buildup caveat explicitly; examiners award the caveat.

Example 4 — DAC and stay time. Tritium (HTO) area reads 2 DAC. A worker without respiratory protection may not exceed 1 ALI in a year; 2000 DAC-h = 1 ALI. Allowable: 2000 DAC-h / 2 DAC = 1000 h/yr in that area. If the worker is already at 60% of ALI, remaining = 800 DAC-h → 400 h. (DAC-hour bookkeeping is the operational heart of internal-dose control.)

Example 5 — Counting statistics. A 10-minute background count gives 100 counts (10 cpm, σ_B = 10 counts). Sample counts 10 min: 145 counts. Is the sample distinguishable from background at 95% confidence? L_C = 2.33 × 10 = 23.3 counts. Net = 145 − 100 = 45 counts > L_C → detected (95% confidence against false positive). L_D = 2.71 + 46.5 = 49.2 counts — the net is below L_D, so a true activity at the detection limit would be missed 5% of the time; report "detected" but note it's near the detection limit.

Standards/regulations cross-reference

• 10 CFR Part 20 — the core exam regulation: dose limits (§20.1201/1301), ALARA (§20.1101), surveys and monitoring (§20.1501/1502), postings and labels (§20.1901–1905), waste disposal (§20.2001+). A CHP must know the numeric limits cold.
• 10 CFR Part 35 — medical use; written directives, misadministration/event criteria.
• 10 CFR Part 71 + 49 CFR 173 — transport: A1/A2, package types, TI, placarding.
• 10 CFR Part 61 — LLW classification tables (know Class A/B/C logic, not every number).
• 40 CFR 190, 40 CFR 61 Subpart H — EPA fuel-cycle and air-emission standards.
• ANSI/ANS, ICRP 103/60/30, NCRP reports — guidance the exam treats as examinable science even where not law.
• ASME NQA-1 / 10 CFR 50 App. B — QA context for the fuel-cycle/reactor specialty.
• ANSI Z136.1 — laser safety (nonionizing specialty).

Pitfalls and trap patterns

1. Exposure vs dose vs dose equivalent vs effective dose. The exam's most reliable distractor set. R (exposure, air ionization) ≠ rad (absorbed dose) ≠ rem (dose equivalent) ≠ Sv-effective-dose. State units at every step.
2. ICRP edition traps. ICRP 26/30 (Q factors, D/W/Y classes, old DACs) vs ICRP 60/66/103/130-series (w_T, radiation weighting, F/M/S, HRTM). A question mixing old ALI values with new models is a classic.
3. Buildup factor omission. Any shielding question in broad-beam geometry that ignores B is wrong; if the question gives you a buildup table, you must use it.
4. Part II time trap. Six mandatory + four specialty in 6 hours = ~36 min per question. The mandatory six are deliberately comprehensive — candidates who perfect one specialty question and starve the mandatory set fail.
5. "Which limit applies" questions. Occupational vs public vs embryo/fetus (500 mrem/gestation) vs lens vs skin. Read the receptor.
6. Background subtraction with paired counts. L_C applies to the net count with proper propagation (σ_net = √(N_s + N_b) for equal count times), not the gross count.

Exam strategy

• Memorize: dose limits (Part 20 table), key constants (Γ for Co-60/Cs-137/Ir-192), HVL/TVL values for lead/concrete for common energies, WL definition, DAC-hour rule (2000 DAC-h = 1 ALI), Currie formulas, tissue weighting factors.
• Derive: everything in decay, shielding, statistics, and internal dosimetry from the five or six core equations — the exam rewards first-principles setup over memorized special cases.
• Part II: outline each answer before writing; graders award partial credit generously for correct setup with arithmetic slips, and zero for a confident wrong model. Always state assumptions (point source, no buildup, which ICRP edition).
• Weighting: Part I domains are roughly: Measurement/Instrumentation ~25%, Standards/Requirements ~20%, Hazards Analysis/Control ~20%, Operations/Procedures ~20%, Fundamentals/Education ~15% (per HPS chapter materials).

Practice problems

1. 250 mCi of I-131 (t_1/2 = 8.02 d). What activity remains after 24 days? Express in mCi and MBq.
2. A Cs-137 point source gives 40 mR/h at 30 cm. (a) Dose rate at 1.5 m? (b) How many HVLs of lead reduce it to 2.5 mR/h at 30 cm, and what thickness (HVL_Pb ≈ 0.65 cm for Cs-137)?
3. Background is 25 cpm counted 20 min (500 counts). A 20-min sample gives 560 counts. Detected at 95% confidence? Compute L_C.
4. A worker breathes air at 0.5 DAC of Sr-90 (Class D) for 800 h in a year. What fraction of an ALI is the intake?
5. Explain why LNT is used for regulation even though the exam expects you to know its limitations. (Short essay — this is a Part II-style judgment question.)
6. A 2 Ci Ir-192 source (Γ ≈ 4.8 R·cm²/mCi·h) is stored in a shielded container. Unshielded dose rate at 1 m? If the container must limit this to 2 mR/h at 1 m, how many TVLs are needed (TVL_Pb for Ir-192 ≈ 1.9 cm)?

Answers

1. 24/8.02 = 2.99 half-lives ≈ 3. A = 250/2^2.99 = 250/7.95 ≈ 31.4 mCi. ×37 MBq/mCi ≈ 1,160 MBq.
2. (a) 40 × (0.3/1.5)² = 40 × 0.04 = 1.6 mR/h. (b) Need 40 → 2.5 = 16× reduction = 4 HVLs (2^4=16). 4 × 0.65 = 2.6 cm Pb (narrow-beam; state buildup caveat).
3. σ_B = √500 ≈ 22.36. L_C = 2.33 × 22.36 ≈ 52.1 counts. Net = 60 > 52.1 → detected at 95% confidence. (Proper σ_net = √(560+500) ≈ 32.6; using L_C = 2.33σ_B is the standard screening form — note the distinction.)
4. 0.5 DAC × 800 h = 400 DAC-h; 400/2000 = 0.20 → 20% of an ALI.
5. Model answer shape: LNT is a prudent regulatory assumption enabling a single risk coefficient and ALARA optimization; limitations — no demonstrated excess risk below ~100 mSv in epidemiology, possible hormesis/adaptive response, dose-rate effects (DDREF), and confounding at low doses. A CHP must apply LNT for compliance while understanding it is policy, not proven biology.
6. Unshielded at 1 m: Ḋ = 4.8 × 2000 / 100² = 9600/10000 = 0.96 R/h = 960 mR/h. Need 960 → 2 = 480× = log10(480) ≈ 2.68 TVLs → 2.68 × 1.9 ≈ 5.1 cm Pb (narrow-beam).

Original ETS reference writing, verified against public sources on 2026-09-30. Thin sourcing is flagged inline. Not legal advice — confirm current editions with the issuing body.