Shielding with half-value layers
Used on the ASP, CSP and CHST. The worked example below is computed by code and checked against the source.
I = I0 × e^(−μx)
μ = 0.693 ÷ HVL
I0 = unshielded intensity; x = shield thickness; HVL = thickness that halves intensity (same units as x).
Worked example
80 mR/h, lead HVL 0.5 in, 1.5 in of lead.
80 × e^(−(0.693 ÷ 0.5) × 1.5) = 10.0 mR/h
1.5 in is three HVLs: 80 → 40 → 20 → 10.
Where it appears on the exam
ASP: ASP11 Domain 1, Mathematical Calculations
Blueprint: ASP11 Domain 1, task 15: radiation exposure (shielding)
Practice: Radiation Calculations: Decay, Distance, Shielding
CSP: CSP11 Domain 6, Occupational Health and Applied Science
Blueprint: CSP11 Domain 6, task 1: occupational exposures (radiation)
Practice: Applied Science: Radiation, Ventilation, Heat and Force
CHST: CHST5 Domain 1, Hazard and Risk Identification and Control
Blueprint: CHST5 Domain 1, knowledge 19: radiation exposure
Source
The formula comes from
OSHA, Ionizing Radiation: Introduction to Health Physics handout(opens osha.gov in a new tab)“I = I o Be -µx = intensity on other side of shield ... Half-Value Layer (HVL): The shield thickness necessary to reduce intensity by half. HVL = 0.693 / µ.”
Questions
- What is the formula for shielding with half-value layers?
- I = I0 × e^(−μx); μ = 0.693 ÷ HVL. I0 = unshielded intensity; x = shield thickness; HVL = thickness that halves intensity (same units as x).
- Will this formula be given on the BCSP exam?
- BCSP says the formula is included in the question whenever one is needed, unless the question is testing whether you know the formula. You can't bring a formula sheet into the exam, so learn what each formula does and when to use it.
BCSP on formulas at the exam
BCSP, What to Expect When Sitting for a BCSP Exam (item 7)(opens bcsp.org in a new tab)“For any question requiring a mathematical formula, the formula will be included within the exam question (unless the question is specifically testing your knowledge of a formula). You will not be provided or permitted to bring a formula sheet.”
Related
Checked 2026-10-16. Spot a problem? Email hello@passcited.com.