The Nucleus

Nuclear Concepts to Master• Compare the benefits and

dangers of radioactivity.• Which isotopes are used for

treatments of which illnesses?• How is background radioactivity

different from natural radioactivity?

• How did the transuranium elements come into existence?

• Does fusion or fission produce more energy?

• What are the equations for fission and fusion?

• Which reaction has not been achieved and which one is used in nuclear power plants (fission or fusion)?

• Compare fission and fusion.• What is the shielding effect?• Write and balance equations that

involve alpha, beta, neutron, positron, or proton particles and gamma radiation.

• Which direction will alpha particles move when placed in an electric field?

• What is the band of stability?• What is the difference between and

artificial and natural transmutation?• Use table N and O to solve problems.• Solve problems involving half-life using the

equation or the “walk-the-dog” method.• Half-life is constant!• What is a Geiger counter used for?• How do scientist use uranium isotopes to

date rocks?• How do scientist use carbon isotopes to

date mummies?• Why do doctors use radioisotopes with

shorter half-lies when performing a medical procedure?

• Which radioisotope is used for sterilizing things like medical equipment?

• Which isotope is used in the irradiation of meat?

Vocab

• Absorption• Alpha particle• Artificial radioactivity• Background radioactivity• Beta particle• CPM• Daughter nucleus• Decay• Emission• Endothermic• Exothermic• Fission• Frequency• Fusion• Gamma radiation• Half-life• Ionization

• Irradiated• Isotope• Natural radioactivity • Nuclides• Penetrating distance• Positron• Radioactive isotopes• Radioactivity• Radionuclide• Shielding• Transmutation• Transuranium elements• Wavelength

Labs• Radioactive decay of Uranium• Half-Life Simulation

Electromagnetic Spectrum

Ionization is the ability of an atom to become an ion (charged particle).

If electrons are lost a cation is formed.

If electrons are gained an anion is formed.

Ionization is the ability of an electron to jump away from or to another atom.

MORE harmful to the bodyLESS harmful to the body

Ionizing Power

• The ionizing power of ionizing radiation measures how many ions are formed in a given area when the radiation passes through it.

• It indicates how densely packed incidents of ionization will be. In human tissue ionization occurring in a limited location means concentrated tissue damage and will increase the probability of cell death or mutations due to DNA damage.

Radioactivity• Unstable isotopes• To become more stable, the atom spits out

subatomic particles and rays of energy• This release of subatomic particles and rays

is radioactivity

31H = H3

1 =H3= H-3

Unstable Nucleus?In the band of stability, atoms are stable. The ratio n/p  is 1 for low nuclear masses and it increases steadily up to 1.5 for high nuclear masses.

In region A, a nucleus with too many neutrons is unstable because not enough of them are paired with protons.

In region B, a nucleus with too many protons has too much repulsive electrical interactions to be stable.

Sulfur

Isotope Atomic

#

Mass

#

# of Protons

# of Neutron

s

# of Electrons

% found in

NATURE

S-32 94.93S-33 0.78S-34 4.29

Calculate ATOMIC MASS

Map of U.S. Natural Radioactivity

The colors indicate uranium concentrations: red is high, yellow is medium, blue is low.

Natural Radioactivity

•The earth is radioactive (air, water, and soil).•Due to elements that spontaneously and uncontrollably emit radioactivity.•Radioactive elements are also called:

–Radioactive isotopes–Radionuclide–Nuclides

• A beta particle is like a fast electron.

– It’s mass is small like an electron’s.

– Since it’s small, it can penetrate your skin!

Types of Radiation - Beta

Beta Decay

• A positron  is an like an electron but with a positive charge!

– It’s small like an electron

– It has a charge of +1 (opposite to an electron!)

– When it meets with an electron it annihilates it!

– Often when nuclei that release positrons also release gamma radiation.

– Can penetrate your skin!

Types of Radiation - Positron

Positron Decay

Types of Radiation - Alpha• An alpha particle  is a particle that is made up of two protons and

two neutrons.

– It is therefore a helium nucleus with a mass number of 4 and a atomic number of 2. 4

2He

– It has a mass of 4.0 amu - massive on a nuclear scale!

– Too BIG to penetrate your skin.

• The gamma ray  is a photon of high energy.

– It has no mass.

– It has no charge.

– Can penetrate your skin!

• For gamma ray emission to occur the nucleus must still be in an unstable state after emitting an alpha, beta or positron particle. So to become more stable, it spits out more energy.

Types of Radiation - Gamma

Gamma radiation released with beta decay

Radiation in an Electric Field

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Try # 5 – 9 on your own before clicking to get the answers.

These are often referred to a SHIELDS and they Protect you from radiation. Distance is also a shield.

Artificial Radioactivity• Man-made• Stable isotopes are

bombarded with particles such as neutrons to cause a nuclear reaction.

• Elements with atomic numbers over 92 have been artificially made. They are called the transuranium elements.

• (92 is the atomic number of the heaviest naturally occurring element, Uranium.)

• Atom Smashers• Nuclear reactors

Fission Larger nucleus is split into smaller nuclei.

Mass is converted to energy!

Control rods needed in nuclear power plants! WHY? It’s a CHAIN REACTION

FusionFusion is when two small nuclei combine to form a single bigger nucleus.

Mass is converted to energy!

Compare

• Fission– Produces hazardous

waste since nuclear fuel rods last about 3 years.

– The used rods are stored underground

– As of 2000, the amount stored is 40000 metric tons

• Fusion– Little to no hazardous

waste (He released and no reactor core)

– Inexhaustible energy – 1 gallon of sea water contains enough deuterium to support a family of 4 for 1 year.

– Why? It’s a CHAIN REACTION

Radiation•Effect of distance•Effect of shield

•thickness•type

Shielding• Shielding protects you from radiation exposure.

• Absorption (Always Artificial)

• Emission (natural DECAY)

• Artificial transmutation

• Natural Transmutation (DECAY)

• Fission

• Fusion

Determining the type of rxn

Half – Life of Radioisotopes• The time taken for a given sample of a radioisotope to decay so that its

radioactivity is one half the initial amount is called the half-life, t½, of that isotope.

• The half-life is a constant for the isotope, and shows enormous variations from one isotope to another.

• For example, 17F (fluorine-17) has a half-life of 70 seconds, while 238U has a half-life of 4.51 x 109 years. 14C (carbon-14) has a half-life of 5668 years.

• The long-lived nature of some waste products of nuclear reactors pose grave ecological problems, as some of these radioisotopes remain dangerously active for tens of thousands of years.

• Whereas the short-lived nature of some radioisotopes allow their use in medicine.

Rate of Decay – Half Life

Number of Half-lives

Timeyears

% Strontium-90 remaining

% Strontium-90 that has decayed

0 0 100 0

1 28 50 50

2 56 25 75

3 84 12.5 87.5

4 112 6.25 93.75

5 140 3.125 96.875

6 168 1.5625 98.4375

Rate of Decay – Half Life

What’s the half-life of Sr-90?

What type of curve is the above?

What would the graph look like if theThe y-axis was % being transformed?

Calculating Half LifeHow much 42K will be left in a 320 g sample

after 62 h?

Step 1: Look up the half life in Table N, the table of Selected Radioisotopes

12.4 h

Step 2: Set up a table showing the mass, time elapsed, the fraction remaining, and number of half lives.

Step 3: Fill in the table with the starting condition(s) and ending when the full time has elapsed. Time will always start with 0. Fraction will always start as 1, and Half-lives will always start at 0.

Step 4: For each half life elapsed:a) cut the mass in halfb) increase the time by an amount equal to the half lifec) cut the fraction in halfd) add one to the number of half lives

Mass Time Fraction Half Lives

320 g 0 h 1 0

320/2 = 160 g

12.4 h 1/2 1

160/2 = 80 g

12.4+12.4= 24.8 h

1/4 2

80/2 = 40 g

24.8 +12.4=

37.2 h

1/8 3

40/2 = 20 g

37.2+12.4=49.6 h

1/16 4

20/2 = 10 g

49.6+12.4=62 h

1/32 5

Calculating Half Life

Determining Half – Life from a graph

The table below shows how the activity (in counts per second) of a sample of radioactive material varies in time. Plot a graph of activity against time to determine the half-life of the material.

Time (days) 0 1 2 3 4 5 6 7 8

Activity (c/s) 800 655 536 439 359 294 241 197 162

Radioactivity

• Extremely Beneficial– Nuclear power plants– Nuclear medicine

• Cardiovascular imaging • Bone scanning• Detecting cancers

• Extremely Dangerous– Nuclear power plants

released radioactive substances into the atmosphere during nuclear accidents.

• Three Mile Island • Chernobyl

– Nuclear Bomb

Radiological dating

• We can use the natural radioactivity in some objects to date them.

• The following slide shows a rock containing naturally occurring radioactive uranium.

• As time goes by, the uranium decays to stable lead.

• By comparing the proportions of uranium and lead we can determine the approximate age of the rock.

Isotopes in Medicine

• Nuclear medicine imaging techniques give doctors another way to look inside the human body. The techniques combine the use of computers, detectors, and radioactive substances.

• It is very important to consider the half-life of a substance that is to be used for medical purposes.

• Co-60– Beta and gamma emitter– Sterilization of medical equipment– Treatment of malignant tumors

• I-131– Beta and gamma emitter– Iodine-131 is one of the radioactive isotopes of iodine

that can be used to test how well the thyroid gland is functioning.

Isotopes in Medicine

http://www.uic.com.au/nip26.htm

Co-60 and Meat• food is irradiated in an area that is

surrounded by concrete walls at least 6-feet thick which keep any rays from escaping.

• The radiation source, usually Cobalt 60, is held in a resting position in a pool of water.

• A conveyor system transports the meat or poultry product to the area.

• The radiation source is then raised out of the water and the food is exposed for a defined period of time.

• When the source is raised, lights and alarms are sounded to make people aware that the product is being irradiated.

• Once the food is irradiated, the source automatically returns to the resting position and the food leaves the area for further processing.

Detecting Radioactivity

• Geiger Counter– Measures radiation as CPM– Counts per minute– the amount of radioactive

material (counts) that is released in one minute

• Film Badge – Radioactivity will expose film.– The amount of film exposed

indicates the level of exposure to radioactivity.