Radioactivity

Som e subs tance s are r ad i oac t i ve . The i r a t om s re l ease s om e o f t h i s e ne r gy a s r ad i a t i on

Radioactivity

Radiati on in the environment and is caused by the radioacti ve in

rocks, the air and our bodies, and also from cosmic rays from outer

space we called it as “Background Radiati on”.

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Radiation Detector

Gold Leaf electroscope

Geiger Muller Tube

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Radiation Detector

Diffusion Cloud Chamber

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Characteristics of α, β, and γ Radiations

• Ionizing Power

• Penetrating Power

• Deflection by Electric and Magnetic Fields

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Ionizing Power

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Penetrating Power

Deflection of α, β, and γ Rays

Type of Radiations• Alpha (α) particles

> Helium Nucleus ( 42He)

> Positive Charge> high mass> a few centimeters range in air> slight deflection in electric/magnetic fields.> strong ionization> can be stopped by thin paper> Pure source: Americium (Am-241)

Type of Radiations• Beta (β) particles

> High-energy electron (0-1e)

> Negative Charge> low mass> a few meters range in air> great deflection in electric/magnetic f.> weak ionization> can be stopped by aluminium (5mm)> Pure source: Strontium (Sr-90)

Type of Radiations• Gamma (γ) rays

> High-energy electromagnetic radiation> un-charge> zero mass> a few hundred meters range in air

> none deflection in electric/magnetic field.

> very weak ionization> can be stopped by thick lead or concrete> source: Cobalt (Co-60) covered by aluminium

Radioactive Decay• After ejecting particles, a nucleus becomes the

nucleus of a different element. It’s called radioactive decay.

• If the new element is also unstable, the process of decay will continue until there are atoms with stable nuclei.

Pu24294 U238

92 Th23490

Radioactive Decay• Alpha Decay.

XAZ X4-A

2-Z + He42

Parent nuclide

Daughternuclide

Alpha particle(helium nucleus)

Ra22688 Rn222

86 + He42

Radioactive Decay• Beta Decay.

XAZ XA

1Z + e01-

Parent nuclide

Daughternuclide

Beta particle(electron)

Sr9038 Y90

39 + e01-

Radioactive Decay• Gamma Rays are usually given off during both

Alpha decay and Beta decay.

Sr9038 Y90

39 + e01- +

Ra22688 Rn222

86 + He42 +

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Half Life

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Half Life

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Plutonium-238 (plutonium oxide). The plutonium glows in the dark as a result of nuclear fission

reactions which release enough energy to increase the metal's temperature to red-heat. The heat

produced by plutonium has been used as an energy source on spacecraft.

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Sample Problems1. A radioactive source has a half-life of 10 minutes.

What fraction is left in 1 hour?

2. Carbon-14 has a half-life of 5700 years. A 10 g sample of wood cut recently from a living tree has an activity of 160 counts/minute. A piece of charcoal taken from a prehistoric campsite also weight 10 g but has an activity of 40 counts/minute. Estimate the age of the charcoal.

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Sample Problems3. The ratio of the number of atoms of argon-40

to potassium-40 in a sample of radioactive rock is 1 : 3. Assuming that there was no potassium in the rock originally and that argon-40 decays to potassium-40 with a half life of 1500 million years, estimate the age of the rock.

4. Radioactive Uranium-238 (U, proton number 92) decays to thorium (Th), emitting an alpha particle and gamma rays. Show this decay reaction as a nuclide equation