What is the approximate effective Z of air in the photoelectric region?

The effective atomic number (Z) of air in the photoelectric region is approximately 7. This value is derived from the composition of air, which consists mainly of nitrogen and oxygen, the atomic numbers of which are 7 and 8, respectively. When considering the average composition and the interaction of photons in the photoelectric effect, the effective Z reflects how air interacts with electromagnetic radiation at lower energy levels, typically below 100 keV.

In the photoelectric region, materials with lower atomic numbers play a significant role in photon absorption. The effective Z value of air being around 7 indicates that air interacts with X-rays and gamma rays primarily through the photoelectric effect and provides a useful reference for dosimetry calculations. This understanding is crucial in radiological physics and medical dosimetry, as it informs treatment planning and shielding considerations.