Half-life t1/2 = ln 2 / λ, so it is inversely proportional to the decay constant λ.

Alpha decay emits an alpha particle, which is a helium nucleus (helium core), so option C is correct.

Electrons in atoms are commonly modeled as arranged in concentric shells around the nucleus.

X-rays are well-suited to probing bulk crystal structure, symmetry and atomic positions but are less useful for surface-specific studies which require surface-sensitive techniques.

Stopping potential depends on photon energy so remains 0.6 V; intensity falls as 1/r^2, so current drops by (0.2/0.6)^2 = 1/9, giving 18.0 mA/9 = 2.0 mA.

After 3 half-lives the remaining amount is 400 × (1/2)^3 = 400/8 = 50 g.

Electron-positron annihilation produces two gamma (γ or 'y' in OCR) photons.

The Schrödinger equation is the fundamental equation governing quantum systems, analogous to Maxwell's role in classical electromagnetism.

Solar energy comes from nuclear fusion of protons forming heavier nuclei (synthesis of heavier elements) in the Sun's core.

Thermonuclear fusion typically requires very high temperatures on the order of 10^7 K to initiate, so option B is correct.