Cobalt-60 is commonly used in radiation therapy for cancer treatment because it predominantly emits which type of penetrating radiation?

Introduction / Context:
Cobalt-60 is a radioactive isotope widely used in medical applications, especially in radiotherapy units for treating cancer. Different types of nuclear radiation (alpha, beta, and gamma) have different penetrating powers and biological effects. This question asks which type of radiation emitted by cobalt-60 makes it particularly suitable for targeting tumours deep inside the body, a key point in nuclear medicine and general science.

Given Data / Assumptions:
- The radioactive isotope mentioned is cobalt-60 (Co-60). - It is used in radiation therapy for cancer. - Options include alpha rays, beta rays, gamma rays, and X-rays. - We assume standard knowledge of radioactive emissions and their properties.

Concept / Approach:
Cobalt-60 decays by beta emission, producing an excited nickel nucleus that then emits high-energy gamma rays. These gamma rays have strong penetrating power and can pass through body tissues to reach tumours located below the surface. Alpha particles, by contrast, are stopped by a sheet of paper or skin, and beta particles have only moderate penetration. X-rays are electromagnetic radiation like gamma rays but are usually generated by electronic transitions or X-ray tubes. The correct approach is to identify gamma rays as the key therapeutic radiation from cobalt-60 sources.

Step-by-Step Solution:
Step 1: Recall that cobalt-60 undergoes beta decay to form nickel-60, and in the process, the daughter nucleus emits gamma rays. Step 2: These gamma rays are very penetrating and can be carefully collimated and directed toward a tumour from outside the body. Step 3: Alpha rays (helium nuclei) are heavy and carry a double positive charge; they have strong ionising power but very low penetrative ability and are stopped by skin. Step 4: Beta rays (electrons or positrons) have more penetration than alpha particles but still cannot penetrate deep tissues as effectively as gamma rays. Step 5: X-rays are also high-energy electromagnetic waves, but in the context of cobalt-60 therapy, the radiation actually used is gamma radiation from nuclear transitions. Step 6: Therefore, the correct choice is gamma rays, option c.

Verification / Alternative check:
Medical physics and radiology textbooks describe cobalt therapy units as gamma-ray sources with characteristic energies around 1 megaelectron volt. These gamma rays are used in external beam radiotherapy to deliver controlled doses to tumours. Although other modalities like linear accelerators now generate high-energy X-rays for similar purposes, the specific association with cobalt-60 in exam questions is gamma radiation. This confirms the selection of gamma rays as the correct answer.

Why Other Options Are Wrong:
Alpha rays (option a) have very low penetration and are not practical for external beam therapy; they might be considered in some internal radiotherapy approaches but not for cobalt-60 units. Beta rays (option b) emitted in cobalt-60 decay are mostly absorbed in the source and shielding and do not form the main therapeutic beam. X-rays (option d) are generated differently and, although used in radiotherapy from other machines, are not the primary radiation directly emitted from cobalt-60 nuclei. Hence, these options do not correctly describe the radiation from cobalt-60 used in therapy.

Common Pitfalls:
Students sometimes confuse X-rays and gamma rays because both are electromagnetic radiation and may be used for imaging or therapy. The key distinction is their origin: X-rays arise from electronic transitions or machine-generated beams, while gamma rays come from nuclear transitions. Another pitfall is thinking that the initial beta emission is what treats the tumour; in practice, it is the gamma radiation that has the right combination of penetration and controllability for external therapy. Keeping this distinction clear will help you answer questions accurately.

Final Answer:
Cobalt-60 is used in radiation therapy because it emits highly penetrating gamma rays suitable for treating deep-seated tumours.