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
+1e
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
241
95
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
