With f = 6×10^8 MHz = 6×10^14 Hz, λ = c/f ≈ 3×10^8 / 6×10^14 = 5×10^-7 m = 500 nm.

A spectrometer designed to measure beta particles' energies is called a beta-ray spectrometer; the OCR indicates option D.

A moderator reduces neutron energy via collisions without capturing them, which matches option A.

Based on the block's provided answer hint (B) and the decay described, option B is indicated as correct per OCR answer key.

Each collision keeps fraction cos²45° = 0.5 of energy, so (1/2)^n = 10⁻6 ⇒ n ≈ 20.

Baryons (such as protons and neutrons) are made of three quarks.

The Rutherford α-scattering experiment provided evidence for the nuclear atom model with a small concentrated positive nucleus.

Beta particles are less ionizing than alpha particles and therefore leave thinner and often discontinuous tracks in the chamber.

In the Bohr model, kinetic energy = -total energy (E_total is negative), so KE : E_total = 1 : -1.

Isotones are nuclides with the same number of neutrons but different proton and mass numbers; option C is correct.