The Nucleus and Radioactivity

Radioactive Decay: Emission of radiation produced by unstable nuclei changing to a more stable state

Radioactivity: Spontaneous changes in the nucleus that emit energy as radiation (particles or rays) Nuclei contain protons and neutrons; some combinations of these particles are unstable Examples of Radioactive Nuclei Include: •Uranium, Plutonium •Hydrogen-3

Types of Radiation Include: •Alpha rays α: positive charge •Beta rays β: negative charge •Gamma rays γ: no charge α and β rays consist of streams of particles γ rays consist of electromagnetic radiation

•Potassium-38

α particle:

γ Ray:

•contains 2 protons and 2 neutrons

•High-energy ray similar to an X ray

•identical to helium nucleus

•Travel great distances

•travel only short distances

Daughter Nuclei: New nuclei that result from unstable nuclei undergoing radioactive decay

β particle: •electrons produced in the nucleus, then emitted

Example: Uranium-238 gives up an α particle, resulting in a daughter nucleus of a different element, Thorium (Th)

•travel greater distances than α particles

Summary of Radiation Types

Alpha Decay • When a radioactive nucleus emits an alpha particle, a new nucleus results. • The mass number of the new nucleus is 4 less than that of the initial nucleus. • The atomic number is decreased by 2.

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ƒ In a balanced nuclear equation, the sum of the mass numbers and the sum of the atomic numbers for the nuclei of the reactant and the products must be equal.

251Cf

247Cm

98

Write an equation for the alpha decay of Rn-222. 222Rn

new nucleus

4He

+

86

2

Mass number:

222 – 4 = 218

Atomic number: 86 – 2 Symbol of element 84 222Rn 86

218Po 84

=

4He 2

Beta Decay ƒ The unstable nucleus converts a neutron into a proton (emitting an electron from the nucleus) ƒ The mass number of the new nucleus remains the same

84

= Po +

+

96

ƒ The atomic number of the new nucleus increases by 1

4He

1n

2

0

0e

+ 1H

-1

1

Example: Potassium - 42 is a beta emitter. 42K

new nucleus +

19

0e -1

Mass number :

(same) = 42

Atomic number:

19 + 1 = 20

Symbol of element

20 = Ca

42K

42Ca

19

20

+

0e -1

2

Learning Check Write the nuclear equation for the beta decay of Co-60.

Solution Write the nuclear equation for the beta decay of Co-60.

60Co

60Co

60Ni

27

27

28

Gamma γ Radiation

+ 0e −1

Summary of Radiation

• Gamma radiation is energy emitted from an unstable nucleus indicated by m. • In a nuclear equation for gamma emission, the mass number and the atomic number are the same. 99mTc 43

99Tc 43

+ γ

Some radioactive isotopes are more stable than others, and therefore decay more slowly

Radioactive Decay Over Time

Half-Life: Time required for half of the unstable nuclei in a sample to decay

Example: A Potassium-38 sample weighs 100 grams. 8 minutes later, the sample is weighed again and found to weigh 50 g. The half-life of potassium-38 is 8 minutes

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Half-Lives Vary Dramatically Between Elements

Half-Life Calculations After one half-life, 40 mg of a radioisotope will decay to 20 mg. After two half-lives, 10 mg of radioisotope remain. 40 mg x 1 x 1 = 10 mg 2 2

Initial 40 mg 1 half-life

20 mg

2 half-lives

10 mg

Learning Check

Solution Half life = 13 hrs

The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 26 hours?

Number of half lives Amount remaining

= 2 = 64 mg x 1 x 1 = 16 mg 2 2

13 hrs

64 mg

13 hrs

32 mg

16 mg

Uses of Radioisotopes Radiation and Health

•Medical: diagnosing and disease (cancer, thyroid, brain scans)

Free Radicals: Very reactive compounds that can cause mutations, cancer; usually caused by long-term exposure to lowlevel radiation Radiation Sickness: Illness and symptoms caused by shortterm exposure to intense radiation

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Imaging Techniques Using Radioisotopes PET Scans (Positron Emission Tomography): gamma rays create a 3D image of organs, used to analyze blood flow, metabolic activity and brain function

•Health/Agriculture: food irradiation •Radioactive dating: determine age of fossils

CT (Computed Tomography): X-rays are used to create series of images of the brain, identifying brain damage and hemorrhaging

MRI (Magnetic Resonance Imaging): H protons in magnetic field are used to create color images of soft tissue

• Nuclear Power Plants: Alternative energy source

Units of Radiation Curie (Ci): number of disintegrations per second per gram of radium; 3.7 x 1010 disintegrations per second

Rad (Radiation Absorbed Dose): amount of material able to deliver 2.4x10-3 cal of energy to 1 kg of tissue

Rem (Radiation Equivalent in humans): amount of biological damage caused by different types of radiation

In 1934 Radioactivity was Artificially Induced for the first time!! High-energy particles (such as neutrons) can create unstable nuclei that then undergo radioactive decay (Cyclotrons and Linear Accelerators)

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Nuclear Fission: Process in which large nuclei split into smaller nuclei when bombarded with neutrons, releasing large amounts of energy Example: When a neutron bombards U-235, an unstable nucleus of U-236 forms smaller nuclei such as Kr-91 and Ba-142.

Chain Reaction: Nuclear reaction in which the products of a reaction cause that reaction to occur repeatedly

Nuclear Fusion: Process in which small nuclei combine (fuse) to form larger nuclei

Example: Hydrogen nuclei combine to form Helium nuclei

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