13.2 Effects of Radioactivity

Effects of Radioactivity

Radioactivity has real-world applications across medicine, industry, and energy production. Understanding both the benefits and the risks of radiation is essential for honors chemistry, since it connects nuclear decay concepts to practical consequences you can actually observe and measure.

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Applications of Radioactivity in Medicine

Diagnostic Imaging

Radioactive tracers are isotopes that emit detectable radiation from inside the body, letting doctors visualize organs and metabolic activity without surgery. Several imaging techniques rely on different types of nuclear decay.

Positron Emission Tomography (PET) Scans

PET scans use positron-emitting tracers (commonly $^{18}F$-fluorodeoxyglucose, or FDG). When the emitted positron meets an electron, both particles annihilate and produce two gamma rays traveling in opposite directions. Detectors surrounding the patient pick up these gamma ray pairs and reconstruct a 3D map of metabolic activity. Because cancer cells consume glucose faster than normal tissue, they light up on PET scans.

Gamma Imaging & SPECT

Gamma imaging uses a gamma camera to detect gamma rays emitted by an injected radioisotope (such as $^{99m}Tc$) and form a 2D image of an organ like the thyroid. SPECT (Single-Photon Emission Computed Tomography) builds on this by rotating the camera around the patient to produce 3D images, giving more spatial detail about blood flow or organ function.

Radiotherapy

Radiotherapy uses high-energy radiation to damage the DNA of cancer cells, preventing them from dividing. There are two primary approaches:

• External beam radiation directs focused beams from outside the body toward the tumor. The beam is shaped to match the tumor geometry, reducing exposure to surrounding tissue.
• Brachytherapy places sealed radioactive sources directly inside or next to the tumor. This delivers a high local dose while sparing more distant healthy tissue.

Both methods exploit the fact that rapidly dividing cancer cells are more vulnerable to DNA damage than most normal cells.

Sterilization

High-energy gamma rays (typically from $^{60}Co$) can kill bacteria, viruses, and other pathogens on surgical instruments and medical supplies. Because gamma radiation penetrates packaging, items can be sterilized after they're sealed, keeping them sterile until use. The process doesn't leave residual radioactivity on the equipment.

Radioactivity in Manufacturing Industries

Non-Destructive Testing (NDT)

Radiography is a common NDT method: gamma rays or X-rays pass through a material (a weld, a casting, an aircraft wing), and a detector on the other side records how much radiation was absorbed. Internal cracks, voids, or inclusions show up as differences in the image. This is critical in aerospace, construction, and pipeline inspection where cutting a sample open isn't an option.

Radiotracers

Small amounts of a radioactive isotope are added to a substance flowing through an industrial system. Detectors placed outside pipes or vessels track the tracer's movement, revealing flow rates, blockages, or leaks without shutting down the process. For example, $^{131}I$ or $^{24}Na$ tracers can map fluid flow in oil pipelines.

Food Irradiation

Exposing food to controlled doses of ionizing radiation (usually gamma rays from $^{60}Co$) kills microorganisms that cause spoilage or foodborne illness. The process extends shelf life and eliminates pests without significantly changing taste or nutritional value. The food does not become radioactive, since the radiation passes through rather than being absorbed into the nuclei of the food's atoms.

Radioactivity in Energy Production

Nuclear Reactors

Nuclear reactors use controlled fission of heavy isotopes (most commonly $^{235}U$) to generate heat. Here's the basic energy conversion chain:

1. Neutrons strike $^{235}U$ nuclei, causing them to split into smaller fragments and release additional neutrons plus a large amount of energy.
2. Those released neutrons go on to split more $^{235}U$ nuclei, sustaining a chain reaction.
3. Control rods (made of neutron-absorbing materials like boron or cadmium) are inserted or withdrawn to regulate the reaction rate.
4. The fission energy heats water into steam.
5. The steam drives turbines connected to generators, producing electricity.

A single kilogram of $^{235}U$ can release roughly the same energy as burning 2,500 metric tons of coal, which is why nuclear power is so energy-dense.

Radioisotope Thermoelectric Generators (RTGs)

RTGs convert heat from radioactive decay directly into electricity using thermocouples. They're used on deep-space missions (like the Voyager probes and the Curiosity Mars rover) where solar panels aren't practical due to distance from the Sun. $^{238}Pu$ is the most common fuel because it produces steady heat through alpha decay and has a half-life of about 87.7 years, providing reliable power for decades.

Biological & Environmental Impact of Radiation

Biological effects

• Acute exposure at high doses causes radiation sickness (also called acute radiation syndrome). Symptoms progress from nausea and weakness to hair loss, skin burns, organ failure, and potentially death. Doses above roughly 1 Sv (sievert) begin producing noticeable symptoms; doses above 6 Sv are often fatal without medical intervention.
• Chronic low-level exposure can damage DNA over time, increasing the risk of mutations and cancer. The body can repair some DNA damage, but accumulated errors may eventually lead to uncontrolled cell growth.

Environmental impact

• Nuclear accidents like Chernobyl (1986) and Fukushima (2011) released large quantities of radioactive isotopes ($^{137}Cs$, $^{131}I$, among others) into the environment. Long-term studies of these areas show measurable ecological effects on plant growth, animal reproduction, and population sizes.
• Radioactive contamination can spread through air, water, and soil. Isotopes that enter waterways accumulate in sediment and can move up the food chain through a process called bioaccumulation, affecting agriculture and ecosystems far from the original release site.

Safety Measures & Regulations

Radiation safety follows three core principles, plus protective equipment:

• Shielding: Dense materials absorb radiation before it reaches people. Alpha particles are stopped by paper or skin. Beta particles are stopped by a few millimeters of aluminum. Gamma rays require thick lead or concrete. Neutrons are best slowed by hydrogen-rich materials like water or polyethylene.
• Distance: Radiation intensity follows the inverse-square law: $I \propto \frac{1}{d^2}$. Doubling your distance from a source reduces your exposure to one-quarter. Even a few extra meters make a significant difference.
• Time: Less time near a source means less total dose. Workers rotate shifts and plan procedures carefully to minimize exposure duration.
• PPE (Personal Protective Equipment): Lead-lined aprons, gloves, and thyroid shields are standard during radiological procedures. Dosimeters (film badges or electronic monitors) track each worker's cumulative exposure.

Regulations & oversight

The International Atomic Energy Agency (IAEA) sets global standards for the safe handling, transport, and disposal of radioactive materials. National agencies (like the NRC in the United States) enforce dose limits for workers and the public, regulate nuclear facility operations, and oversee radioactive waste management. These frameworks ensure that the benefits of nuclear technology are pursued while keeping exposure as low as reasonably achievable (a principle known as ALARA).

Conclusion

Understanding the effects of radioactivity ties together nuclear decay, energy transformations, and interactions between radiation and matter. The same nuclear processes that power PET scans and electricity generation can also cause serious biological harm and environmental contamination. Honors chemistry asks you to evaluate both sides: how radioactive isotopes are chosen for specific applications (based on half-life, decay type, and energy) and how safety principles like shielding, distance, and time protect people from their hazards.