Edexcel IGCSE Physics - (Unit 7) Radioactivity

Units for Radioactivity (Topic 7)

becquerel, centimeter, hour, minute, second (skip as necessary)

Atomic number

The number of protons an element has.

Relative atomic mass

The mass of an element's atom relative to one twelfth of carbon-12.

Isotope

An atom with the same number of protons (same element) but a different number of neutrons.

(Types of) ionizing radiation

Alpha (α), Beta (β), Gamma (γ)

Alpha Radiation

2 emitted protons and neutrons, +2 charge, mass of 4, <5cm distance, stopped by paper, highly ionizing

Beta Radiation

Emitted when a neutron becomes a proton, -1 charge, mass of about 0, 1m distance, stopped by aluminum, low ionizing power

Gamma Radiation

A high energy electromagnetic wave, no charge, no mass, >1km distance, stopped by lead/concrete, very low ionizing power

Ion

A (positively or negatively) charged atom.

Ionization

The process where an atom becomes an ion by gaining or losing electrons.

Unstable isotopes are...

radioactive isotopes (radioisotopes)

Investigating penetration of ionizing radiation (practical)

Block a source of each type of ionizing radiation with paper, lead, and concrete; detect radiation with GM tube; wait for count rate to plummet.

Alpha Radiation's Effect on a Nucleus

2 protons and 2 neutrons are lost; mass number -4, atomic number -2

Beta Radiation's Effect on a Nucleus

1 neutron is converted to an electron and a proton; atomic number + 1 (no mass change)

Gamma Radiation's Effect on a Nucleus

Energy is lost from an atom in the form of an electromagnetic wave; mass and atomic numbers go unchanged

Geiger-Müller (GM) tube

A particle detector that measures radioactivity with a photographic film.

Sources of background radiation

Primarily radon gas (50%), cosmic rays, food and drink, buildings and ground, medical equipment, nuclear industry

Activity trend from a radioactive source

It decreases over time and is measured in Becquerels (Bq)

Half-life

The average time taken for an isotope's count rate (or nuclei/mass) in a sample to halve.

Home smoke detectors

Emits Americium-241 (a weak source of alpha radiation) in its casing with oxygen and nitrogen which are ionized and conductive. Smoke will break the circuit between the particles and sound the alarm.

Radioactive Tracers

A radioisotope is added to the body or pipe for use as a tracer. The radiation penetrates objects and can leave the body or pipe. The escaping radiation is detected with a Geiger-Müller counter.

Food/Medical Product Sterilization

Low doses of gamma rays ionize the product - this destroys and prevents regrowth of organisms.

Radiotherapy

Intense beams of gamma rays destroy cells in the body. Whilst cancer cells can't repair themselves, healthy cells can. Dosages must be correct; too little won't stop the cells spreading, and too much will damage more healthy cells.

Measuring Paper Thickness

Radiation sources emit radiation to both sides of a paper roll. Machines are calibrated to produce specific amounts of radiation - thickness is signaled by the volume of radiation penetrating (which should be consistent.)

Carbon Dating

A radioisotope that every living organism has, which is used to measure one's age as it has a large half-life of 5730 years.

Irradiation

Occurs when an object is exposed to foreign source of radiation: doesn't cause the object to become radioactive, can be blocked with shielding, stops when source is removed.

Contamination

Occurs if radioactive source is in or on the object: will be radioactive whilst the source is on or in it, radiation cannot be blocked from it, difficult to remove all the contamination.

Dangers of Ionizing Radiation

They can damage cells and tissues, cause mutations in living organisms, and contaminate our environment. Adequate shielding can stop a source from affecting the environment.

Fission

A nuclear reaction in which a large radioisotope splits into smaller nuclei and neutrons, generating large amounts of energy.

Chain Reaction

When neutrons released as a product of a fission reaction collide with other radioisotopes to create more fission reactions.

Fusion

When two small nuclei are joined (fused) to form a larger nucleus, generating large amounts of energy.

Radioactive Decay

When radioisotopes release (heat) energy in the form of radiation.

Uranium-235 Fission Reaction

A neutron is absorbed into a Uranium-235 nucleus, forming the radioisotope Uranium-236. It splits to form two smaller nuclei, three neutrons, and gamma radiation.

Role of control rods

They are inserted and retracted into the core to absorb neutrons, slowing down the rate of reaction.

Role of the moderator

A material like graphite absorbs some of the kinetic energy in neutrons which slows them down. Slower neutrons are more easily absorbed by Uranium-235.

Role of shielding around a nuclear reactor

Made of steel and 5-meter-thick concrete to prevent radiation and neutrons escaping.

Fission vs. Fusion

Fission splits larger nuclei into smaller nuclei, and Fusion collides smaller nuclei to form a larger nucleus

Mass decrease in Fusion

Hydrogen isotopes collide at a high speed, which reduces the mass from the smaller nuclei in the reaction.

Energy source for stars and the sun

Fusion is their main energy source; hydrogen undergoes fusion to make helium in our sun.

Temperature requirement for fusion

Must be high to overcome repulsive forces between positively charged nuclei from each isotope.

Pressure requirement for fusion

High levels increase the chance for fusion between nuclei.