Ionising Radiation

Since half of the gamma photons are absorbed by 7.35 mm Pb, then that is the “half-value layer”. A second half-value layer placed after the first one would again absorb half of the photons that emerge from the first slab of lead. This would leave half of a half, that is, one quarter emerging from the second half-value layer. Hence ¾ have been absorbed.

Decreasing the width of the “window” means restricting the range of grey scale displayed on the monitor screen (0–255), to a more limited range of the available Hounsfield numbers (−1000 for air to 3000 for dense bone). A lung window may be −1250 to +250; a soft tissue window may be −160 to +240; a bone window may be −650 to +1350.

The other three choices, while also being forms of electromagnetic radiation, do not utilise energies that are sufficient to remove electrons from their atoms.

“X” is still technetium as the atomic number (43) has not changed. Consequently, the chemical behaviour of the two nuclides is the same. A daughter nucleus may or may not be radioactive (in this case it is).

X and gamma rays are high-energy radiation, which means high frequency, which means short wavelength and which means that many of the photons will pass through the body without interacting with any atoms. The other three choices do not distinguish between different types of radiation.

Alpha particles are He⁺⁺ nuclei, while beta particles are electrons (negative charge) or positrons (positive charge).

If you triple your distance, your exposure would be one third of a third of your exposure while closer. (1/3)² = 1/9

The production of ions (i.e. removal of electrons from an atom) is the sign of ionising radiation. Such a change in the medium through which radiation passes is significant.

X-rays are not involved in radioactivity. Electrons are beta particles.

Particulate radiation (alpha and beta rays) is not penetrating enough to pass out of the body, so it cannot be used in radiopharmaceuticals.