Radioactivity_ Unstable Atoms and Nuclear Changes

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NUCLEAR CHANGES

A Lesson on Unstable Nuclei and Radiation

RADIOACTIVITY

ATOMS

Radioactivity is the spontaneous breakdown of unstable atomic nuclei. Unstable nuclei emit particles and energy to become stable. This natural process was discovered by Henri Becquerel in 1896 using uranium.

Marie and Pierre Curie later isolated radioactive elements radium and polonium. Only isotopes with an unbalanced proton-neutron ratio are radioactive.

α Properties

1.

2.

3.

An alpha particle is a helium nucleus containing 2 protons and 2 neutrons. It has a +2 charge, is heavy, and has very low penetrating power, being stopped by paper or skin.

A beta particle is a high-speed electron emitted when a neutron changes into a proton. It has moderate penetration, being stopped by a sheet of aluminum or plastic.

Gamma radiation consists of high-energy photons. It has no mass or charge, possesses very high penetration, and requires thick lead or concrete for shielding.

Nuclear change: In alpha decay, the parent atom loses 2 protons and 2 neutrons. The atomic number decreases by 2 and the mass number decreases by 4, forming a new element.

Class activity: Practice writing the nuclear equation for Polonium-210 decay. Discuss why alpha particles are a major internal hazard but not an external one.

Beta & Gamma Radiation

β⁻

γ

Beta particle: a high-energy electron (₋₁⁰e) from a neutron changing to a proton. It has medium penetration, stopped by aluminum or plastic.

Gamma radiation consists of pure energy waves. It is low ionizing but highly penetrating, requiring dense materials like lead for shielding.

α

This slide compares beta and gamma radiation, highlighting their different properties, origins, and required shielding materials.

Shield

Quick question: Paper blocks alpha, aluminum blocks beta, and lead blocks gamma. Which type has the greatest ability to pass through matter?

RADIOACTIVE DECAY

Half-life (t½) is the time for half the radioactive atoms in a sample to decay. It is constant for each isotope, unaffected by temperature or amount. Examples include Carbon-14 (5,730 years for dating), Iodine-131 (8 days for medicine), and Radon-222 (3.8 days). The decay follows an exponential graph.

A 200 g sample with a half-life of 5 days will have 25 g remaining after 15 days. This predictable decay rate is crucial for applications like carbon dating fossils.

Radiation in Our World

Radioactivity comes from both natural and artificial sources. Natural background radiation makes up 50–80% of our total exposure, including radon gas from rocks, cosmic rays, and potassium-40 in our food and bodies. Artificial sources include medical procedures like X-rays, nuclear power, smoke detectors, and past nuclear tests.

The average annual radiation dose for a person is about 2–3 mSv. While some radiation is natural and always present, it must be managed carefully to minimize health risks.

Radioactivity in Action

Medicine: Radiotherapy targets cancer cells; thyroid scans use I-131; PET imaging aids diagnosis. Radiotherapy kills cancer but can damage healthy tissue.

How Smoke Detectors Work: Americium-241 emits alpha particles, which ionize air. Smoke disrupts this current, triggering the alarm for early fire detection.

Medicine, Industry, Energy, and Science

Power and Dating: Nuclear fission in power plants provides clean energy. Carbon-14's 5,730-year half-life allows scientists to date ancient organic materials.

Time

Shorten exposure duration to decrease absorbed radiation.

Dangers

Dist.

Increase distance; dose drops with the square of the distance.

ALARA

Block

Use proper barriers: paper for α, lead for γ radiation.

Safety

Radiation poses risks: cell damage, sickness, and cancer. Alpha is most harmful inside the body; gamma penetrates deeply. The ALARA principle guides safety: minimize Time, maximize Distance, and use appropriate Shielding.

Safety & ALARA

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