About the radiation dose calculator
This calculator estimates the external radiation dose received from a gamma-emitting point source using the inverse-square law, the standard model used in health physics and radiation safety for photon exposure at a distance. It combines source activity, a dose-rate constant, distance, and exposure time, then applies an optional exponential shielding correction based on half-value layers.
The dose formula
The unshielded dose is D = (Γ × A × t) / d², where Γ is the dose-rate constant for the source (microsieverts per hour at 1 meter, per unit of activity), A is the source activity, t is the exposure time, and d is the distance from the source. Because distance is squared in the denominator, doubling the distance cuts the dose rate to one-quarter of its previous value - this is the inverse-square law.
Shielding attenuation
Shielding is modeled with the half-value layer (HVL): the thickness of a given material that reduces transmitted dose by half. The fraction transmitted through a thickness x is 0.5^(x / HVL). Two HVLs transmit about 25% of the dose, three HVLs about 12.5%, and so on. The HVL depends on both the shielding material (lead, concrete, water, and so on) and the photon energy of the source, so use the HVL value that matches your specific setup rather than a generic default.
Reading the result
The calculator reports the shielded dose, the unshielded dose rate at the entered distance, the percent reduction contributed by shielding, and the shielded dose expressed as a share of a commonly cited public annual reference level (about 1 mSv/year under ICRP guidance). These reference levels are shown for general context only, not as a personal limit.
Scope and assumptions
This is a simplified point-source photon model. It assumes a compact source, uniform exposure geometry, and a single effective HVL for the shielding material and energy in use. It does not account for scatter, buildup factors, non-point source geometries (lines, volumes, or extended fields), or internal (inhaled or ingested) dose. It is a physics screening estimate for radiation safety planning and education, not a personal dosimetry reading or medical advice - consult a radiation safety officer or medical physicist for monitoring, shielding design, and compliance decisions.