Radiography
Radiation boundary distance
From the source activity and gamma constant, or a rate you measured, estimate the distance to the 2 mR/h and 100 mR/h lines.
For reference only — verify against your procedure and the governing code.
Blank uses the NRC training value, 0.48 (5.2 at 1 ft). Kept in the NRC's unit in both systems: roentgen measures exposure, so turning it into sieverts would not be a pure unit change.
0 for an unshielded (open) source.
Result
- 20 µSv/h line (2 mrem/h, 10 CFR 20.1301(a)(2))
- 110m
- 1000 µSv/h line (100 mrem/h, 10 CFR 20.1003)
- 15.5m
- Exposure rate at 1 m
- 24,000mR/h
An estimate for planning. Survey the boundary with a calibrated instrument before and during the exposure.
Formula
- Rate at d = A · Γ · (d_ref / d)² · transmission
- d = d_ref · √(A · Γ · transmission / target rate)
Units. Distances in m or ft, activity in GBq or Ci, a measured rate in µSv/h or mR/h. The gamma constant stays in the NRC's R/h per Ci at 1 m in both systems: roentgen measures exposure, so converting it to sieverts would not be a pure unit change. Activity in GBq is converted to Ci for that formula.
Assumptions and limits
- An ESTIMATE for planning only. The boundary is set and confirmed by survey with a calibrated instrument (10 CFR 34.51 and 34.53, posting under 20.1902).
- Gamma constants offered are the NRC training values (Ir-192 0.48, Co-60 1.3 R/h per Ci at 1 m). For Se-75 and Yb-169 enter the value from your source documentation.
- R and mR are treated as rem and mrem for gamma radiation (quality factor 1, 10 CFR 20.1004). Scatter from walls, the ground and the part is not included.
Sources
- 10 CFR Part 20 (eCFR, current): 20.1003 definition of high radiation area; 20.1004 units and quality factors; 20.1301(a)(2) dose in any unrestricted area.
- 10 CFR Part 34 (eCFR, current): 34.51 surveillance; 34.53 posting.
- 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.