The** radioactive decay law** states that the probability per unit time that a nucleus will decay is a constant, independent of time. This constant is called the **decay constant** and is denoted by λ, “lambda.” This constant probability may vary greatly between different types of nuclei, leading to the many different observed decay rates. The radioactive decay of a certain number of atoms (mass) is exponential in time.

**Radioactive decay law: N = N.e ^{-λt}**

The rate of nuclear decay is also measured in terms of **half-lives**. The half-life is the amount of time it takes for a given isotope to lose half of its radioactivity. If a radioisotope has a half-life of 14 days, half of its atoms will have decayed within 14 days. In 14 more days, half of that remaining half will decay, and so on. Half-lives range from **millionths of a second for highly radioactive **fission products to **billions of years for long-lived materials** (such as naturally occurring uranium). **Notice that** short half-lives go with large decay constants. Radioactive material with a short half-life is much more radioactive (at the time of production) but will obviously lose its radioactivity rapidly. No matter how long or short the half-life is after seven half-lives have passed, there is less than 1 percent of the initial activity remaining.

The radioactive decay law can also be derived for activity calculations or mass of radioactive material calculations:

**(Number of nuclei) N = N.e ^{-λt} (Activity) A = A.e^{-λt} (Mass) m = m.e^{-λt}**

where N (number of particles) is the total number of particles in the sample, A (total activity) is the number of decays per unit time of a radioactive sample, and m is the mass of remaining radioactive material.

## Decay Constant and Half-Life

In radioactivity calculations, one of two parameters (**decay constant** or **half-life**), which characterize the decay rate, must be known. There is a relation between the half-life (t_{1/2}) and the decay constant λ. The relationship can be derived from the decay law by setting N = ½ N_{o}. This gives:

where *ln 2 *(the natural log of 2) equals 0.693. If the decay constant (λ) is given, it is easy to calculate the half-life and vice-versa.

## Decay Constant and Radioactivity

The relationship between **half-life** and the amount of a radionuclide required to give an activity of one curie is shown in the figure. This amount of material can be calculated using **λ**, which is the **decay constant** of certain nuclide:

The following figure illustrates the amount of material necessary for **1 curie** of radioactivity. Obviously, the longer the half-life, the greater the quantity of radionuclide needed to produce the same activity. Of course, the longer-lived substance will remain radioactive for a much longer time. As can be seen, the amount of material necessary for 1 curie of radioactivity can vary from an amount too small to be seen (0.00088 gram of cobalt-60), through 1 gram of radium-226, to almost three tons of uranium-238.

## Example – Calculation of Radioactivity

A sample of material contains 1 microgram of iodine-131. Note that iodine-131 plays a major role as a radioactive isotope present in nuclear fission products and is a major contributor to the health hazards when released into the atmosphere during an accident. Iodine-131 has a half-life of 8.02 days.

**Calculate:**

- The number of iodine-131 atoms is initially present.
- The activity of the iodine-131 in curies.
- The number of iodine-131 atoms will remain in 50 days.
- The time it will take for the activity to reach 0.1 mCi.

**Solution:**

- The number of atoms of iodine-131 can be determined using isotopic mass as below.

**N**_{I-131}** = m**_{I-131}** . N**_{A}** / M**_{I-131}

**N**_{I-131 }**= (1 μg) x (6.02×10**^{23}** nuclei/mol) / (130.91 g/mol)**

**N**_{I-131}** = 4.6 x 10**^{15}** nuclei**

- The activity of the iodine-131 in curies can be determined using its
**decay constant**:

The iodine-131 has a half-life of 8.02 days (692928 sec), and therefore its decay constant is:

Using this value for the decay constant, we can determine the activity of the sample:

3) and 4) The number of iodine-131 atoms that will remain in 50 days (N_{50d}) and the time it will take for the activity to reach 0.1 mCi can be calculated using the decay law:

As can be seen, after 50 days, the number of iodine-131 atoms and thus the activity will be about 75 times lower. After 82 days, the activity will be approximately 1200 times lower. Therefore, the time of ten half-lives (factor 2^{10} = 1024) is widely used to define residual activity.