Nuclear Physics

Spring 2013

Intro:

1.What atom is this?

2.Where do you find protons?

3.Where do you find neurons?

4.Where do you find electrons?

5.How many protons does it have?

6.How many neutrons?

7.Is it a neutral atom, and how do you know?

The Atom

In the nucleus (nucleons)

Proton- (+) charged particle

Neutron- no charge

Outside the nucleus

Electron- (-) charged particle

has almost no mass

Nucleons

• Are particles occupying the nucleus

• Consist of + charged protons and neutral neutrons

• Have almost 2000 times the mass of electrons

• Where can you find the number of protons?

• It’s the atomic number (found on the periodic table)

Nuclear Notation

Atomic number = no. of protons

Atomic mass = protons + neutrons

Atomic number is the same as the number of electrons in an uncharged atom

5

B10.811

Atomic Number

Atomic Mass

1a. How many protons?

1b. How many neutrons?

1c. How many nucleons?

You may see atomic number written many ways. The smaller number is the atomic number and the larger is the atomic mass

Question 1

• has 13 protons and 14 neutrons for a total of 27 nucleons

• has 13 protons and 15 neutrons for a total of 28 nucleons

• The identity of an element depends on the number of protons

2813

Isotopes:

• Atoms of the same element with different numbers of neutrons (different masses)

Most common stable isotope of carbon

Unstable radioactive isotope of carbon

Question 2

• List the four fundamental forces from strongest to weakest

1.

2.

3.

4.

Review of Fundamental forces

Strongest to weakest1.Strong Nuclear Force

2.Electromagnetic Force

3.Weak Nuclear Force

4.Gravity

Forces Acting on Nucleons:

Forces of attraction between nucleons

Strong forces– Are independent of the charge of the nucleon– Are short range (exist only between closest

neighbors)

Electrical force (electrostatic)– Force of repulsions between positively

charged protons– Are long range

When are nuclei unstable? (naturally radioactive)

a. Large nuclei (Z > 82) – electrical forces of repulsion are greater than strong forces of attraction

b. Wrong neutron : proton ratio

When are nuclei unstable?

Bigger atoms require more neutrons per proton to keep the atom stable

A radioactive isotope:

• Has an unstable nucleus

• Spontaneously emits a particle and decays into another element (to become more stable)

Transmutation

• Changing into another element through radioactive decay

Who am I?

• I worked with my husband and discovered radium, a radioactive material

Marie and Pierre Curie

• First to discover that compounds containing uranium emitted penetrating rays.

• Discovered radioactive polonium and radium

Types of Radioactive Emission

Alpha Decay

• Radiation through the loss of 2p + 2n or (helium)

Beta Decay

• Radiation where a neutron splits, giving off an electron and becoming a proton in the new element

Gamma Decay

• A change energy state gives off a gamma particle or photon

Question 3a

Balance the nuclear equation after alpha decay

Question 3a

Balance the nuclear equation after alpha decay

Question 3b

Balance the nuclear equation after beta decay

Remember in beta decay a neutron changes into a proton by giving off an electron

Question 3b

Balance the nuclear equation after beta decay

Remember in beta decay a neutron changes into a proton by giving off an electron

Extra Question

• Which radioactive isotope completes this nuclear decay equation

6

Extra Problem

• Finish off the equation

Half Life and Half Life Calculations

• Half Life- time it takes for half of the radioactive sample to decay.– Ranges from a fraction of a

second to billions of years

• Decay constant- Probability per time that a nucleus would decay

Section 2 Intro

1. Rewrite and balance the equation above

2. What kind of decay is shown above?

3. What is the particle given off during alpha decay composed of?

4. What is the particle given off during beta decay composed of?

Section 2: Nuclear Physics Math

Half Life and Half Life Calculations

y= fraction of radioactive material left

n= number of half lives

Example 1

• How much of the original radioactive material is left after 15 half-lives?

Example 1

• How much of the original radioactive material is left after 15 half-lives?

Half Life and Half Life Calculations

T1/2 = half life

λ = decay constant

The unit for λ and T1/2 will be in the same timeframe

Example 2

Cobalt-60, used in radiation therapy, has a half-life of 5.26 y. What is the decay constant for cobalt-60?

Example 2

Cobalt-60, used in radiation therapy, has a half-life of 5.26 y. What is the decay constant for cobalt-60?

Extra Examples A

• A radioactive sample has a mass of 56 mg and a half life of 30 minutes. How much of the sample remains after 60 minutes have passed?

Extra Examples B

• An unknown radioactive material has a half life of 4000 years. How much of the sample will remain after 20,000 years?

Half Life and Half Life Calculations

N = number of radioactive atoms

No = original number of radioactive atoms

t = elapsed time

λ = decay constant

e = 2.72

N = Noe-λt

Example 3

Cobalt-60, used in radiation therapy, has a half-life of 5.26 y. A sample of cobalt-60 containing 5.00 x 1012 radioactive atoms sits in a lead case in the medical stockroom for 10.09 years. How many cobalt-60 atoms remain after this amount of time?(You already solved for the decay constant in example 2) λ= 0.132 y-1

Example 3

Cobalt-60, used in radiation therapy, has a half-life of 5.26 y. A sample of cobalt-60 containing 5.00 x 1012 radioactive atoms sits in a lead case in the medical stockroom for 10.09 years. How many cobalt-60 atoms remain after this amount of time?

Useful applications of radioactivity

• Can be detected and therefore small amounts can be used as tracers for medical diagnosis

• Larger amounts can be used as treatments for certain types of cancers (cancer cells are killed before healthy cells)

• Can be used to determine the age of rocks and fossils

Show what you know

Types of Nuclear Reactions

Natural transmutation – Uranium spontaneously decays

Artificial transmutation – bombardment of a stable isotope to force it to decay

Question 4

• Balance the reaction after the following artificial transmutation.

Types of Nuclear Reactions

Artificial transmutation •First done by Earnest Rutherford

•When the bullets are positively charged, they are repelled by the nucleus they are bombarding. To overcome the repulsions, they must be accelerated to very high speeds by particle accelerators.

Nuclear Fission

• Nuclear fission - Heavy nuclei are bombarded with neutrons and split.

plus a tremendous amount of energy

Nuclear fission

• Mass of particles produced is slightly less than the mass of the reactants. This mass is converted into energy. (E=mc2)

Nuclear fission is a chain reaction. Neutrons are needed to start and released as a product which can start more reactions.

Critical mass: minimum mass of fissionable material required for a chain reaction.

Problems with Fission• Nuclear fission produces radioactive waste that has

a large half life.

U-235 Uranium 235

–Half life of U-235 is 713 million years

• We cannot get rid of this dangerous product so we store it away from anything it can harm.–We deeply bury

• Meltdown if cooling system fails the reactor can overheat and melt releasing radioactive materials

• Nuclear fusion – combination of small nuclei into larger with release of energy.

• Mass of particles produced is much less than the mass of the reactants.

• This mass is converted into energy. (E=mc2)• Can release up to 10 times that of fission• Occurs naturally in our sun and other stars• Does not give off radioactive waste

Problems with Fusion

• Fusion requires high temperatures like those in the stars.

• We cannot sustain these temperatures without vaporizing the container of the fusion reaction.

• Today many are looking into ways of making fusion work under sustainable conditions