Radiation sources: cobalt-60 is widely used in medical therapy and industrial radiography as a sealed source emitting which dominant type of penetrating ionizing radiation?

Introduction:
Cobalt-60 (Co-60) is a man-made radioisotope produced by neutron activation of cobalt-59. Its decay scheme makes it extremely useful as a high-energy, high-intensity radiation source for teletherapy units and for non-destructive testing (NDT) in industry. Identifying its principal emission is fundamental to shielding design and safety protocols.

Given Data / Assumptions:

• Co-60 decays by beta emission to excited states of nickel-60.
• The excited daughter de-excites by emitting characteristic photons.
• Applications require deeply penetrating radiation.

Concept / Approach:
While Co-60 undergoes beta decay, the practically important radiation for therapy and radiography is the cascade of two high-energy gamma photons (~1.17 MeV and ~1.33 MeV). These γ-rays are highly penetrating, enabling treatment of tumors at depth and radiographic inspection of thick components. X-rays are produced by electron transitions or Bremsstrahlung in X-ray tubes, not by Co-60 decay directly.

Step-by-Step Solution:
Recognize Co-60 decay: β- emission to excited Ni-60.Identify emitted photons: two γ-rays near 1.17 and 1.33 MeV.Select γ-rays as the dominant useful radiation from sealed Co-60 sources.

Verification / Alternative check:
Databases and shielding charts list Co-60 as a gamma emitter with average energies around 1.25 MeV, which dictate lead or concrete thicknesses for safe operation.

Why Other Options Are Wrong:

• X-rays: Generated by tubes or secondary processes, not the primary emission of Co-60.
• α-rays: Not part of Co-60 decay.
• β-rays: Emitted internally but are fully absorbed within sealed capsules and are not the principal useful radiation.
• Neutrons: Not produced by Co-60 decay.

Common Pitfalls:
Equating the decay mode (beta) with the useful external radiation (gamma). In sealed sources, betas are shielded by the capsule; gammas escape and are used.

Final Answer:
γ-rays