The Nucleus and Radioactivity

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

Potassium-38

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

Types of Radiation Include:

Alpha rays : positive charge

Beta rays negative charge

Gamma rays : no charge

and rays consist of streams of particles

rays consist of electromagnetic radiation

positron: an antiparticle of a β particle (their charges are opposite, but their masses are the same)

•A positron has a +1 charge and is called a “positive electron.”

positron: β+ or 0+1

e

•A positron is formed when a proton is converted to a neutron.

10

n11

p

neutronproton

+ 0+1

e

positron

particle:

contains 2 protons and 2 neutrons

identical to helium nucleus

travel only short distances

particle:

electrons produced in the nucleus, then emitted

travel greater distances than particles

Ray:

High-energy ray similar to an X ray

Travel great distances

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

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

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.

Nuclear Reactions: Alpha Emission

Alpha emission is the decay of a nucleus by emittingan a particle.

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 + 4He 98 96 2

24195

Am 42

He + 237

93

Np

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 = 84

Symbol of element 84 = Po

222Rn 218Po + 4He 86 84 2

Beta Decay

The unstable nucleus converts a neutron into a proton (emitting an electron from the nucleus)

The mass number of the new nucleus remains the same

The atomic number of the new nucleus increases by 1

1n 0e + 1H0 -1 1

Beta emission is the decay of a nucleus by emitting a β particle; 1 neutron is lost and 1 proton is gained.

Nuclear Reactions: Beta Emission

Example: Potassium - 42 is a beta emitter.

42K new nucleus + 0e19 -1

Mass number : (same) = 42

Atomic number: 19 + 1 A = 20

Symbol of element 20 = Ca

42K 42Ca + 0e19 20 -1

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

60Co 27

Learning Check

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

60Co 60Ni + 0e 27 28 1

Solution

16

Nuclear Reactions: Positron Emission

Positron emission is the decay of a nucleus by emitting a positron, β+; 1 proton is lost and 1 neutron is gained.

• 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 99Tc + 43 43

Gamma Radiation

Summary of Radiation

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

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

Note: The half-life of a radioactive isotope is a property of a given isotope and is independent of the amount of sample, temperature, and pressure.

Half-Lives Vary Dramatically Between Elements

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

1 half-life 2 half-lives

Initial40 mg

20 mg10 mg

Half-Life Calculations

Practice:

If the half-life of iodine-131 is 8.0 days, how much of a 100. mg sample remains after 32 days?

Determine how many half-lives occur in thegiven amount of time.

32 days 1 half-life8.0 days

x = 4.0 half-lives

For each half-life, multiply the initial mass by one-half to obtain the final mass:

100. mg

initial mass

x 12

x 12

x 12

x 12

The mass is halved four times.

= 6.25 mg

final mass

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

Learning Check

Half life = 13 hrs

Number of half lives = 2

Amount remaining = 64 mg x 1 x 1 = 16 mg 2 2

13 hrs 13 hrs

64 mg 32 mg 16 mg

Solution

Radiation and Health

Free Radicals: Very reactive compounds that can cause mutations, cancer; usually caused by long-term exposure to low-level radiation

Radiation Sickness: Illness and symptoms caused by short-term exposure to intense radiation

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

Uses of Radioisotopes

Common Imaging Techniques

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

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

Health/Agriculture: food irradiation

Radioactive dating: determine age of fossils

• 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)

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