Shielding a Temporary Boundary With Limited Material
You are the radiographer on a plant expansion job. Your exposure device holds a 120 Ci iridium-192 source, and the shot is a column weld at 18 in. The nearest uncontrolled area is a public sidewalk 12 m from the source position, with no barrier in between. Before the first exposure you survey the fence line at 12 m and read 400 mR/h. The site superintendent offers either a 15 cm stack of concrete block or a 5 cm steel plate from the site's scrap. The controlling criterion is 2 mR/h at the fence line, and you may treat the survey location as the only point of concern. Ir-192 half-value layers: concrete 5.6 cm, steel 1.5 cm, lead 0.48 cm. No other shielding is available, but the shot can be moved up to 6 m farther from the fence.
1. Determine the required number of half-value layers and evaluate whether either material offered is adequate.
Read the worked answer
Step 1 - attenuation factor needed. The 2 mR/h figure is the screening value for an unrestricted area (the 2 mrem-in-any-one-hour criterion), so it is the correct target, not a round number picked for convenience.
Factor = 400 mR/h ÷ 2 mR/h = 200.
Step 2 - convert to HVL. n = log2(200) = ln(200)/ln(2) = 5.30/0.693 = 7.6, so 8 HVL (always round up; you cannot buy a fraction of a layer and stay under the limit).
Step 3 - thickness required.
- Concrete: 8 × 5.6 cm = 44.8 cm (about 18 in)
- Steel: 8 × 1.5 cm = 12 cm (about 4.7 in)
Step 4 - compare with what is on site.
- 15 cm of concrete = 15/5.6 = 2.7 HVL = factor 6.4, leaving 400/6.4 ≈ 62 mR/h. Not adequate.
- 5 cm of steel = 5/1.5 = 3.3 HVL = factor about 10, leaving 400/10 ≈ 40 mR/h. Not adequate.
- Even stacked together (2.7 + 3.3 = 6.0 HVL, factor 64) the reading would be about 6 mR/h, still three times over.
Conclusion: neither offer is sufficient, and combining them is not sufficient either. You need roughly 45 cm of concrete or 12 cm of steel (or an equivalent combination) before the first exposure, or the shot does not run.
2. The superintendent moves the shot the full 6 m farther from the fence. How much does that help, and does it change your answer?
Read the worked answer
New distance: 12 m → 18 m. Inverse square: new rate = 400 × (12/18)² = 400 × 0.444 = 178 mR/h.
Factor still needed: 178/2 = 89 → n = log2(89) = 6.5, so 7 HVL.
Thickness: concrete 7 × 5.6 = 39 cm; steel 7 × 1.5 = 10.5 cm.
Interpretation: moving 6 m farther saved only about 1.5 HVL - useful, but it does not rescue the situation. The materials on site (15 cm concrete, 5 cm steel) still fall far short, so the answer is unchanged: no exposure until adequate shielding or a larger distance is in place. The general point to carry away is that one HVL equals a factor of 2, while doubling distance equals a factor of 4 (two HVL); distance is efficient, but a heavy source close to a public area usually needs both.
3. List the additional controls you would put in place before this shot runs, and explain the reasoning behind each.
Read the worked answer
- Recalculate from measured data, not from the source label. Source activity is decay-corrected and geometry is not a clean point source at 18 in; the fence-line survey is the number that governs.
- Collimate. A collimator cuts the radiation emitted in directions that do not reach the weld, which reduces the scattered and direct component reaching the fence line and can remove a layer or two of shielding demand.
- Use existing structure as shadow shielding (concrete walls, pipe racks, equipment) and position the shot so the beam points away from the sidewalk.
- Set two boundaries: a controlled boundary (posted, barricaded, roped, attended) well outside the shielded area, and the 2 mR/h unrestricted line at the fence.
- Control occupancy and time on the public side - nothing else may bring the dose back up if someone lingers at the fence.
- Issue alarming dosimeters to the crew and require a survey at the fence line after each exposure, not just at setup, because geometry changes as the shot is repositioned.
- Document the survey readings, distances, instrument, and calibration on the daily survey log so the ALARA decision is defensible.