Radioactivity Chapter 21  Natural occurring phenomena.  In the nucleus of an atom there are protons and neutrons. Protons are positively charged so they naturally want to repel each other. Protons are positively charged so they naturally want to repel each other. A strong nuclear force keeps them inside. A strong nuclear force keeps them inside. Neutrons have no charge – so they are neutral and help cushion the repulsive force the protons feel. Neutrons have no charge – so they are neutral and help cushion the repulsive force the protons feel. “the glue that holds the nucleus together” “the glue that holds the nucleus together”

Instability  Once the nucleus has 20 protons it needs more neutrons than protons to keep the nucleus together.  And once you get to a certain number of protons there is no number of neutrons sufficient to keep the nucleus stable.  Once the nucleus becomes unstable it wants to break down or decay.

Band of Stability  The ratio between neutrons and protons determine the stability of a nucleus.  If the number is too large or too small the nucleus is unstable.  No atoms that have an atomic numbers larger than 83 and a mass number larger than 209 are stable.

Chemical Symbol  U = Uranium  Mass number = protons and neutrons  Atomic number = number of protons

3 Main Types of Radiation

Nuclear Equations

Alpha Decay 1. Alpha emission(greek symbol alpha - α ) emission of a nucleus with a mass number of 4 and atomic number of 2. emission of a nucleus with a mass number of 4 and atomic number of 2. can be stopped by a few sheets of paper. can be stopped by a few sheets of paper.  Example:

Beta Decay 2. Beta emission (greek symbol β )  emission of a high speed electron.  can be stopped by wood or heavy protecting covering.  Example:

Gamma Decay 3. Gamma emission(greek symbol γ ) emits a high energy non-particle radiation. emits a high energy non-particle radiation. need several inches of lead to block Gamma. need several inches of lead to block Gamma.  Example:

Other Types of Particles…  Particles in nuclear equations: Proton Proton Positron Positron Neutron Neutron

Nuclear Bombardment  When atoms are bombarded by other particles.  Usually take place in a particle accelerator.

Overview of Radioactive Emission

Radiation Exposure  Radiation is measure by the non SI unit R (roentgen equivalent for man).  Average dose received annually=120 R  70% of radiation comes from natural resources such as minerals and cosmic rays. About 30% come from medical procedures like x-rays.

Fission vs. Fusion  Fission reactions - a very heavy nucleus, spontaneously or after absorbing additional light particles (usually neutrons), splits into two or sometimes three pieces. (α decay is not usually called fission.) Fission Occurs only with a critical mass. Occurs only with a critical mass.  Fusion reactions - two light nuclei join to form a heavier one, with additional particles (usually protons or neutrons) thrown off to conserve momentum. Fusion 2 H2 H + 3 H → 4 He + n 3 H 4 Hen

Fission

Fusion

Uses of Radiation  Nuclear Powerplants  Age Dating  Radioactive Tracers  Cancer Treatment  Sterilizing  Smoke Detectors  Genetic Studies

Radioactive Dating  Animals and plants have a known proportion of Carbon-14 (a radioisotope of Carbon) in their tissues.  When they die they stop taking Carbon in, then the amount of Carbon-14 goes down at a known rate (Carbon-14 has a half-life of 5700 years).  The age of the ancient organic materials can be found by measuring the amount of Carbon-14 that is left.

Radioactive Tracers  The most common tracer is called Technetium-99 and is very safe because it only emits gamma rays and doesn't cause much ionisation.  Radioisotopes can be used for medical purposes, such as checking for a blocked kidney. To do this a small amount of Iodine-123 is injected into the patient, after 5 minutes 2 Geiger counters are placed over the kidneys.  Also radioisotopes are used in industry, to detect leaking pipes. To do this, a small amount is injected into the pipe. It is then detected with a GM counter above ground.

Cancer Treatment  Because Gamma rays can kill living cells, they are used to kill cancer cells without having to resort to difficult surgery. This is called "Radiotherapy", and works because cancer cells can't repair themselves when damaged by gamma rays, as healthy cells can.  It's vital to get the dose correct - too much and you'll damage too many healthy cells, too little and you won't stop the cancer from spreading in time.  Some cancers are easier to treat with radiotherapy than others - it's not too difficult to aim gamma rays at a breast tumor, but for lung cancer it's much harder to avoid damaging healthy cells. Also, lungs are more easily damaged by gamma rays, therefore other treatments may be used.

Sterilizing  Even after it has been packaged, gamma rays can be used to kill bacteria, mould and insects in food. This process prolongs the shelf-life of the food, but sometimes changes the taste.  Gamma rays are also used to sterilize hospital equipment, especially plastic syringes that would be damaged if heated.

Smoke Detectors  Smoke alarms contain a weak source made of Americium-241. Alpha particles are emitted from here, which ionize the air, so that the air conducts electricity and a small current flows. If smoke enters the alarm, this absorbs the a particles, the current reduces, and the alarm sounds. Am-241 has a half-life of 460 years.

Half-life  The half-life of a substance is the time it takes for a radioactive sample to decay into half.