function updateGammaConstant() {
var selector = document.getElementById("isotopeSelect");
var constantInput = document.getElementById("gammaConstant");
var selectedValue = selector.value;
if (selectedValue !== "custom") {
constantInput.value = selectedValue;
} else {
constantInput.value = "";
}
}
function calculateDoseRate() {
// 1. Get Inputs
var gamma = parseFloat(document.getElementById("gammaConstant").value);
var actAmount = parseFloat(document.getElementById("activityAmount").value);
var actUnit = document.getElementById("activityUnit").value;
var distAmount = parseFloat(document.getElementById("distanceAmount").value);
var distUnit = document.getElementById("distanceUnit").value;
// Validation
if (isNaN(gamma) || isNaN(actAmount) || isNaN(distAmount)) {
alert("Please enter valid numerical values for Gamma Constant, Activity, and Distance.");
return;
}
if (distAmount <= 0) {
alert("Distance must be greater than zero.");
return;
}
// 2. Normalize Inputs to Standard Units (Curies and Meters)
// Standard Formula used: Dose (R/hr) = (Gamma * Activity(Ci)) / (Distance(m))^2
// Assuming Gamma Constant input is in R·m² / (Ci·hr)
// Convert Activity to Curies (Ci)
var activityInCi = 0;
if (actUnit === "Ci") {
activityInCi = actAmount;
} else if (actUnit === "mCi") {
activityInCi = actAmount / 1000;
} else if (actUnit === "GBq") {
// 1 Ci = 37 GBq
activityInCi = actAmount / 37;
} else if (actUnit === "TBq") {
// 1 TBq = 1000 GBq = 1000/37 Ci
activityInCi = (actAmount * 1000) / 37;
}
// Convert Distance to Meters (m)
var distanceInMeters = 0;
if (distUnit === "m") {
distanceInMeters = distAmount;
} else if (distUnit === "cm") {
distanceInMeters = distAmount / 100;
} else if (distUnit === "ft") {
distanceInMeters = distAmount * 0.3048;
}
// 3. Calculate Dose Rate in R/hr (Roentgens per hour)
// Formula: D = (Γ * A) / d^2
var doseR = (gamma * activityInCi) / (Math.pow(distanceInMeters, 2));
// 4. Convert R/hr to other units
// Conversion assumption: 1 R (exposure in air) approx equals 0.00877 Gy (air kerma)
// For radiation protection purposes, 1 R is often approximated as 1 rem or 10 mSv (0.01 Sv) for gamma.
// More precise conversion: 1 R ≈ 0.96 rem (tissue) ≈ 9.6 mSv.
// However, standard safety estimation often uses 1 R ≈ 10 mSv (or 1 rem).
// Let's use the standard conversion: 1 R ≈ 0.01 Sv (10 mSv).
var doseSv = doseR * 0.01;
var dosemSv = doseSv * 1000;
var dosemuSv = dosemSv * 1000;
// 5. Display Results
document.getElementById("resultContainer").style.display = "block";
// Formatting numbers
document.getElementById("resR").innerHTML = doseR.toFixed(4) + " R/hr";
document.getElementById("resSv").innerHTML = doseSv.toFixed(6) + " Sv/hr";
document.getElementById("resmSv").innerHTML = dosemSv.toFixed(4) + " mSv/hr";
document.getElementById("resmuSv").innerHTML = dosemuSv.toFixed(2) + " μSv/hr";
}
Understanding Gamma Dose Rate Calculations
The Gamma Dose Rate Calculator is an essential tool for health physicists, radiographers, and radiation safety officers (RSOs). It estimates the intensity of radiation exposure at a specific distance from a point source of gamma radiation. Understanding these calculations is critical for establishing safety perimeters (barriers) and ensuring compliance with ALARA (As Low As Reasonably Achievable) principles.
The Inverse Square Law
The fundamental physics principle behind this calculator is the Inverse Square Law. This law states that the intensity of radiation is inversely proportional to the square of the distance from the source. In simple terms, if you double your distance from the source, the dose rate drops by a factor of four. Conversely, halving the distance increases the dose rate by a factor of four.
The formula used for a point source is:
D = (Γ × A) / d²
D: Dose Rate (typically in R/hr)
Γ (Gamma): The specific gamma ray constant for the isotope (R·m²/Ci·hr)
A: Activity of the source (Curies)
d: Distance from the source (Meters)
Common Gamma Constants
Different isotopes emit gamma radiation at different energy levels and probabilities. The Gamma Constant (Γ) quantifies this output. While values can vary slightly depending on specific literature or shielding assumptions, common approximate values (in R·m²/Ci·hr) include:
Cobalt-60 (Co-60): ~1.32
Cesium-137 (Cs-137): ~0.33
Iridium-192 (Ir-192): ~0.48
Radium-226 (Ra-226): ~0.825
Safety Units and Conversions
Radiation measurements can be confusing due to the mix of traditional units (Roentgens, Curies, Rems) and SI units (Sieverts, Becquerels, Grays). This calculator provides outputs in both systems to assist with reporting.
Roentgen (R): A measure of ionization in air.
Sievert (Sv): A derived unit of ionizing radiation dose that accounts for biological effects.
Conversion: For gamma radiation, 1 Roentgen is approximately equivalent to 0.01 Sieverts (10 mSv) of effective dose.
Precautionary Note
Warning: This calculator assumes a "point source" geometry and does not account for shielding (attenuation) provided by lead, concrete, or steel containers. It also does not account for air attenuation or scatter, which become significant at large distances. Always use calibrated survey meters to verify actual dose rates in the field.