RT Barrier & Ug

10 mR/h stand-off for SE-75 / Ir-192, plus geometric unsharpness.

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SE-75 steel HVL = 8 mm (0.315″)
Each sheet = 3 HVL (≈ 3 mm lead for SE-75)
SE-75 · stand-off to 10 mR/h
— m
— ft
Total HVLs—
Unshielded—
SE-75 assumptions

Γ = 203 mR/h per Ci at 1 m · Lead HVL ≈ 1 mm · Steel HVL = 8 mm · Sheet = 3 HVL

d = √( Γ × Ci ÷ (10 × 2^HVLs) )

Ir-192 steel HVL ≈ 13 mm (0.51″)
Same physical 3 mm sheets · Ir-192 lead HVL ≈ 5.1 mm → ~0.59 HVL per sheet
Ir-192 · stand-off to 10 mR/h
— m
— ft
Total HVLs—
Unshielded—
Ir-192 assumptions

Γ = 480 mR/h per Ci at 1 m (0.48 R/h/Ci @ 1 m, QSA) · Lead HVL ≈ 5.1 mm · Steel HVL ≈ 13 mm

Lead sheets counted as 3 mm each (same physical sheets as your SE-75 3-HVL sheets).

d = √( Γ × Ci ÷ (10 × 2^HVLs) )

ASME uses SOD. If you measure FFD, pick that — SOD = FFD − OFD.
SW contact ≈ wall thickness. DW viewing: often OD (source-side wall to film).
ASME V T-274.2 / T-285 limits by nominal thickness
Geometric unsharpness Ug
— mm
— in
SOD used—
FFD—
ASME max—
Ug checkenter values
Ug formula

Ug = f × OFD ÷ SOD

Similar triangles (ASME V Art. 2). Not FFD in the denominator.

If you enter FFD: SOD = FFD − OFD, then same formula.

ASME max Ug (nominal thickness):
< 2″ → 0.020″ · 2–3″ → 0.030″ · 3–4″ → 0.040″ · > 4″ → 0.070″

Exposure time (H : M : S)
— : — : —
Dose rate at film—
Ci·s—
How shot time is calculated

t (min) = R × SFD_in² × 2^(steel_in ÷ HVL) ÷ (Ci × C)

Matched to iPhone X-Ray Timer (Lance Henderson).

Worked checks:
Ir-192 · 59 Ci · 15 mm · R=4 · 394 mm → 0:2:5
Ir-192 · 60 Ci · 0.75″ · R=2.5 · 16″ → 0:1:42
SE-75 · 10 Ci · 9.3 mm · R=2.5 · 136 mm → 0:2:8
SE-75 · 40 Ci · 1.124″ · R=2.5 · 42″ → 2:53:26

Ir-192: HVL=0.5″ (12.7 mm), C=17.75
SE-75: HVL=8 mm, C=7.55
Co-60: HVL=21 mm, C≈48.1 (not phone-checked)

Build v16 · Timer-matched shot

Field aid only. Survey the rope. Follow your procedure, code, and RSO.