Passage VII

A physicist tested various sheets for their ability stop gamma rays (γ-rays) that had different energies. Any γ-ray that was not stopped by a sheet struck a detector (see Figure 1), which then registered a detection, or count.

Each sheet was made of either Material 1, Material 2, or Material 3. The thickness, x, of each sheet was uniform, but sheets of the same material had different thicknesses.

l₀ represented the number of counts per minute (cpm) with no sheet between the γ-ray source and the detector, and I represented the cpm with 1 sheet between the source and the detector.

Experiment 1

First, the physicist measured I₀ for a source of γ-ray having an energy of 4 million electron volts (Mev). Next with this same γ-ray source, she measured I for various sheets of Material 1, each having a different thickness x. Then, she plotted a graph of I versus x (in cm) for Material 1. Similarly, she produced graphs of I versus x for Material 2 and Material 3 (see Figure 2).

Experiment 2

The physicist repeated the procedures from Experiment 1 with other sources of γ-ray having different energies. For each combination of sheet material and γ-ray energy, she produced a graph of I versus x, These graphs are not shown.

From the graphs of I versus x, she found the halfvalue thickness, x_half, for each combination of sheet material and γ-ray energy. A sheet of thickness x_half stopped half of the γ-rays striking the sheet; that is, I equaled . (For example, based on Figure 2, for 4 MeV γ-rays striking sheets of Material 1, x_half equaled 1.0 cm.) Finally, the physicist plotted a graph of x_half versus γ-ray energy for each material (see Figure 3).