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How the radioactivity calculator works
From curvefit.com. Copyright 1999 by GraphPad Software, Inc.  All Rights Reserved.
This page explains how to use the radioactivity calculator, and explains how it works.
Calculator Discussion How it is done

A. Isotope decay Calculates radioactive decay during a specified number of days. Select one of the common isotopes, or enter the half-life of another isotope.
The result becomes one of the inputs to calculator B.

The decay constant (k) equals 0.693/HalfLife. The fraction remaining equals e-kt.
B. Conc. of stock Enter the original mCi/ml and Ci/mmole, which should be on the label. If you are using a molecule labeled with 125I, the specific activity equals 2200 Ci/mmole if each molecule is labeled with one iodine.
Also enter the percent of the original isotope remaining (calculated in A above). The calculations assume that the decay product is not biologically active, so the concentration of biologically active stock decreases over time.
The result is used as one of the inputs to calculator C.

Divide the original mCI/ml by Ci/mmole to get the original concentration in mmole/ml which is the same as molar. Multiply by the fraction remaining to get the current concentration.
C. Dilution of stock Enter the concentration in your stock solution, after accounting for decay (calculated in calculator B). Also enter the concentration and volume you want.
The result is the volume of stock you need to use.

Divide the desired concentration by the stock concentration to get the dilution factor. Multiply that times the volume desired to get the volume of stock needed.
D. Specific activity (cpm/fmol) Enter the specific radioactivity as Ci/mmole which should be on the label. If you are using a molecule labeled with 125I, the specific activity equals 2200 Ci/mmole if each molecule is labeled with one iodine.
Also enter the counter efficiency - the fraction of radioactive disintigrations that are detected. The efficiency depends on the isotope and instrumentation. With low energy isotopes such as tritium, the efficiency also depends on the experimental details such as the choice of scintillation fluid, the amount of water in the sample, and the presence of any colored substances in the sample.
The calculation does not account for radioactive decay. We assume that the decay products are not biologically active. As the molecules decay, the concentration goes down, but the number of cpm per fmol of active molecule does not change.
The result is used as inputs to calculators E, F and G

A Curie equals 2.2 x 1012 radioactive disintigrations per minute (dpm). Multiply Ci/mmole by that factor to get dpm per mmole. Multiply by the efficiency to get cpm per mmole, and divide by 1012 to get cpm/fmol.
E. Cpm to fmol/mg Enter the specific radioactivity as cpm/fmol, the number of cpm counted, and the protein content of the sample in mg. The result is the number of binding sites in fmol/mg protein.

Divide cpm by cpm/fmol to determine the number of fmols. Divide by mg to get fmol/mg.
F. Cpm to sites/cell Enter the specific radioactivity as cpm/fmol, the number of cpm counted, and the cell count . The result is the number of binding sites per cell. Divide cpm by cpm/fmol to get the number of fmols. Multiply by 6.02 x 10 8 to get the number of molecules. Divide by cell number.

G. Cpm to nM Enter the specific radioactivity as cpm/fmol, the number of cpm counted, and the volume counted. The result is the concentration in nM. Divide cpm by cpm/fmol to get the number of fmols. Divide by volume.