In radiation therapy for cancer, cobalt-60 is used as a radioactive source because it emits which type of highly penetrating rays?

Introduction / Context:
Radiation therapy is an important method for treating many types of cancer. One commonly used radioactive isotope in external beam radiotherapy has been cobalt-60. Understanding why this isotope is useful requires some basic knowledge of different types of radiation and their penetrating power. This question asks which type of rays emitted by cobalt-60 make it suitable for cancer treatment.

Given Data / Assumptions:

• The isotope in question is cobalt-60, often written as Co 60.
• We are asked which radiation type it emits that is useful in cancer therapy.
• The options mention alpha, beta, gamma, X rays and neutron rays.

Concept / Approach:
Cobalt-60 is a radioactive isotope that undergoes beta decay and, importantly for therapy, emits high energy gamma rays. Gamma rays are electromagnetic waves with very short wavelength and high energy. They can penetrate deep into body tissues, allowing oncologists to target tumours located below the surface. Alpha particles have very low penetration and are stopped by a sheet of paper or the outer layers of skin. Beta particles are more penetrating than alpha but still more limited than gamma rays and are used mainly in specialised applications. X rays are similar to gamma rays in nature but are usually produced by X ray machines, not directly by cobalt-60. Neutrons are used in some advanced therapies but are not the main emission from cobalt-60 in standard radiotherapy units.

Step-by-Step Solution:
Step 1: Recall that cobalt-60 sources used in teletherapy units are known to emit strong gamma radiation. Step 2: Understand that gamma rays are capable of penetrating deeply into human tissue, which is necessary to reach internal tumours. Step 3: Compare this with alpha particles, which travel only a very short distance and are easily blocked. Step 4: Note that beta particles have moderate penetration but are not usually the primary radiation used for deep seated tumours in cobalt-60 therapy. Step 5: Recognise that X rays are usually produced by X ray tubes rather than by radioactive isotopes like cobalt-60. Step 6: Select gamma rays as the type of radiation whose penetrating power makes cobalt-60 useful in treating cancer.

Verification / Alternative check:
Descriptions of cobalt therapy or cobalt units in medical physics texts identify them as sources of gamma radiation with energies around 1.17 and 1.33 MeV. These gamma rays are used to irradiate tumours while trying to spare surrounding healthy tissue as much as possible. Imaging and radiation safety training materials also emphasise shielding against gamma radiation when working with cobalt-60 sources. This consistent emphasis on gamma rays confirms that they are the relevant radiation type in this context.

Why Other Options Are Wrong:
Alpha rays have very low penetration and are not suitable for external beam treatment of deep tumours. Beta rays from cobalt-60 exist but are not the main therapeutic component in standard teletherapy machines, which rely on gamma rays. X rays are similar to gamma rays but are typically generated electrically, not emitted directly by cobalt-60 as its principal therapeutic radiation. Neutron rays are used only in specialised neutron therapy facilities and are not the primary emission from cobalt-60 sources used in routine cancer treatment.

Common Pitfalls:
Students sometimes mix up beta and gamma radiation or assume that any high energy radiation from a source like cobalt-60 must be X rays. To avoid confusion, remember that radioactive nuclei such as cobalt-60 emit gamma rays when they transition from an excited state to a lower energy state, while X rays are generally produced when electrons decelerate in an X ray tube. Knowing that cobalt therapy units are gamma ray sources will help you answer questions about their medical role accurately.

Final Answer:
Cobalt-60 is used in cancer treatment because it emits Gamma rays with high energy and strong penetrating power.