Radiation dosimetry is the quantitative description of absorbed radiation and its effects on tissue.

In typical radioactive decay equations of this form, Y represents an alpha particle, i.e., a helium nucleus.

A reduction to one-eighth corresponds to three half-lives, so half-life = 420 days / 3 = 140 days.

After 10 years (two half-lives), the remaining amount is 100 × (1/2)^2 = 25 g, so option A is correct.

Alpha bombardment of beryllium yields a neutron (e.g., 9Be + α → 12C + n), so option D is correct.

An alpha particle is the nucleus of a helium atom (two protons and two neutrons) and is positively charged, so option C is correct.

Reduction from 368 to 32 is ≈ a factor of 11.5, about 3.52 half-lives ×10 min ≈ 35.2 min, closest option is 40 minutes.

Kα X-rays occur when an electron falls from the L shell to fill a vacancy in the K shell (L → K transition).

Typical U-235 fission yields two or three neutrons per event, so either two or three (option D) is consistent with observed fission multiplicities.

OCR marks option A; the Lyman series of hydrogen lies in the ultraviolet region, so A is correct.