Radiography
Exposure time for a new distance or activity
Start from an exposure that worked, change the source-to-film distance or the source strength, and get the new time.
For reference only — verify against your procedure and the governing code.
Use the source decay calculator for today's activity.
Result
- New exposure time
- 4.5min
- Distance factor (d₂/d₁)²
- 2.25
- Activity factor A₁/A₂
- 1
Formula
- t₂ = t₁ · (d₂ / d₁)² · (A₁ / A₂)
- Inverse square law: I₁ · d₁² = I₂ · d₂²
Units. Distances in mm or in., activity in GBq or Ci. Both enter as ratios, so the time comes out the same in either system.
Assumptions and limits
- Everything else stays the same: film or detector, screens, energy, material, thickness and density target.
- Treats the source as a point; very short distances relative to the source size break the inverse square law.
Sources
- U.S. NRC Training Center, "Industrial Radiography Health Physics Review" (NRC ADAMS ML12146A259) — gamma constants (slide 23), inverse square law (slide 24), half-value-layer rules of thumb (slide 14), 1 Ci = 37 GBq (slide 17).
More radiography calculators
For reference only — verify against your procedure and the governing code.
Standards and documents are cited by their publishers' names to identify them. TRACE is not affiliated with, endorsed by or sponsored by ASTM International, ASME, the U.S. Nuclear Regulatory Commission, NIST or the Laboratoire National Henri Becquerel. Formulas are paraphrased and cited by clause; get the documents themselves from their publishers.