transmutations the changing of one element to another is called transmutation this occurs whenever...

Transmutations

• The changing of one element to another is called transmutation

• This occurs whenever there is an alpha decay or a beta decay

• Remember, that for a gamma decay, the nucleus just changes internal energy levels, but doesn’t change the identity of nucleons

Transmutations

• Consider an alpha decay

• 238U -> 234Th + alpha (4He)

• Have to count the number of protons and neutrons

• Protons [U-92, Th-90, He-2] Balance!!

• Neutrons [U-146, Th-144, He-2] Balance!!

• Mass No. [U-238, Th-234, He-4] Balance!!

Transmutations

• Consider a beta decay

• 234Th -> 234Pa + beta

• This means a neutron changed into a proton

• So mass number stays the same

• Proton number increases by one and the thorium changes into protactinium

• Actually an anti-neutrino appears also

Transmutations

Artificial Transmutation

• First example was Rutherford bombarding nitrogen with alpha particles

• 4He + 14N -> 17O + 1H

• He viewed the reaction with a cloud chamber

• The particles left a vapor trail in the mist

H H H

http://education.jlab.org/glossary/isotope.html

Isotopes of Carbon

Emission of alpha particles :

• helium nuclei • two protons and two neutrons • charge +2e  • can travel a few inches through air• can be stopped by a sheet of paper,

clothing.

Alpha Decay

Alpha Decay

Uranium Thorium

Beta Decay

Beta Decay

Thorium Protactinium

Types of Radioactive Decay Positron Emission

Loss of a positron (a particle that has the same mass as but opposite charge than an electron)

e01

C116

B115 + e0

1

Types of Radioactive Decay Electron Capture (K-Capture)

Addition of an electron to a proton in the nucleus– As a result, a proton is transformed into a neutron.

p11 + e0

−1 n1

0

• Gamma radiation electromagnetic energy that is released. 

• Gamma rays are electromagnetic waves.

• They have no mass.• Gamma radiation has no charge.

– Most Penetrating, can be stopped by 1m thick concrete or a several cm thick sheet of lead.

Gamma Decay

Which is more penetrating? Why?

Nuclear Stability

• Depends on the neutron to proton ratio.

Neutron-Proton Ratios• Any element with more than

one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus.

• A strong nuclear force helps keep the nucleus from flying apart.

• Neutrons play a key role stabilizing the nucleus.

• Therefore, the ratio of neutrons to protons is an important factor.

Neutron-Proton Ratios

For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.

Neutron-Proton Ratios

For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.

Neutron-Proton Ratios

For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.

Stable Nuclei

• Nuclei below the belt have too many protons.

• They tend to become more stable by positron emission or electron capture.

Some Trends

Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to

be more stable than nuclides with a different number of nucleons.

Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.

Measuring Radioactivity

• One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample.

• The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

Objectives

• Explain the relationship between number of nucleons and stability of nuclei.

• Describe the different types of radioactive decay and their effects o the nucleus.

• Define nuclear fission, chain reaction, and nuclear fusion, and distinguish between them.

• Discuss the possible benefits and the current difficulty of controlling fusion reations.

Energy in Nuclear Reactions• There is a tremendous amount of energy

stored in nuclei.

• Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy.

• In chemical reactions the amount of mass converted to energy is minimal.

• However, these energies are many thousands of times greater in nuclear reactions.

Einstein• "The intuitive mind

is a sacred gift and the rational mind is a faithful servant. We have created a society that honors the servant and has forgotten the gift." 

Man-Made Radioactive Decay on Earth

• Fission

• Fusion– Occurs naturally in space– Powers the sun– Supernovas allow atoms to fuse into heavier

elements, this is how the other elements came into existence

• "Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction."

Fission

• Nuclear fission occurs when scientists bombard a large isotope with a neutron.

• This collision causes the larger isotope to break apart into two or more elements.

• These reactions release a lot of energy.

• You can calculate the amount of energy produced during a nuclear reaction using an equation developed by Einstein: E=mc2

Nuclear Fission

1n + 235U -> 91Kr + 142Ba + 31n

Nuclear Fission• How does one tap all that energy?• Nuclear fission is the type of reaction carried out

in nuclear reactors.

Chain Reaction

Nuclear Fission

• Bombardment of the radioactive nuclide with a neutron starts the process.

• Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

• This process continues in what we call a nuclear chain reaction.

Nuclear Fission

• If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.

• Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.

Critical Mass

Atomic Bombs• Because of the tremendous amount of energy released in

a fission chain reaction, the military implications of nuclear reactions were immediately realized.– The first atomic bomb was dropped on Hiroshima, Japan, on

August 6, 1945.

• In an atomic bomb, two pieces of a fissionable isotope are kept apart. Each piece by itself is subcritical.

• When it’s time for the bomb to explode, conventional explosives force the two pieces together to cause a critical mass.

• The chain reaction is uncontrolled, releasing a tremendous amount of energy almost instantaneously.

Mushroom Cloud

Atom Bomb

Nuclear Power Plants• If the neutrons can be controlled, then the

energy can be released in a controlled way. Nuclear power plants produce heat through controlled nuclear fission chain reactions.

• The fissionable isotope is contained in fuel rods in the reactor core. All the fuel rods together comprise the critical mass.

• Control rods, commonly made of boron and cadmium, are in the core, and they act like neutron sponges to control the rate of radioactive decay.

Nuclear Power Plants (cont)• In the U.S., there are approximately 100 nuclear

reactors, producing a little more than 20% of the country’s electricity.

• Advantages– No fossil fuels are burned.

– No combustion products (CO2, SO2, etc) to pollute the air and water.

• Disadvantages– Cost - expensive to build and operate.

– Limited supply of fissionable Uranium-235.

– Accidents (Three Mile Island & Chernobyl)

– Disposal of nuclear wastes

Nuclear ReactorsIn nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

Nuclear Reactors

• The reaction is kept in check by the use of control rods.

• These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

Three Mile Island

Chernobyl

Nuclear Fusion

• Fusion is when lighter nuclei are fused into a heavier nucleus.

• Fusion powers the sun. Four isotopes of hydrogen-1 are fused into a helium-4 with the release of a tremendous amount of energy.

• On Earth, H-2 (deuterium) & H-3 (tritium) are used.

Fusion Reactions

Nuclear Fusion

• Fusion would be a superior

method of generating power.– The good news is that the

products of the reaction are

not radioactive.

– The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.

– Tokamak apparati like the one shown at the right show promise for carrying out these reactions.

– They use magnetic fields to heat the material.

Positron emission tomography, also called PET imaging or a PET scan, is a diagnostic examination that involves the acquisition of physiologic images based on the detection of radiation from the emission of positrons. Positrons are tiny particles emitted from a radioactive substance administered to the patient.

Isotopic Dating

• Cosmic rays (mostly high energy protons) strike upper atmosphere and cause transmutations that result in many protons and neutrons being sprayed out

• Protons tend to grab electrons from other atoms and become simple hydrogen

• Neutrons keep going and smash into other atoms

Isotopic Dating

• When a neutron strikes nitrogen

• 1n + 14N -> 14C + 1H

• In the atmosphere, 14C is about 1 part in 1011

• Reacts with plants just like 12C

• 14C decays via beta emission

• 14C -> 14N + beta half life of 5730 years

Half-Life

Half-Life

• There are a large range of half-lives seen in nature

• Half-lives are unaffected by the nuclei’s surroundings and only depend on what goes on inside the nucleus

• 238U has a half-life of 4.5 billion years

Half-Life

• To summarize, one-half of the sample will decay in one half-fife

• One-half of that one-half will decay in the next half life

• One-half of that one-fourth will decay in the next half life

Carbon Dating

• When a plant dies, it stops the intake of carbon

• Since the 14C decays, after 5730 years, half of it will be gone

• We can just weigh a piece of dead wood, calculate how much 14C it originally had and measure to how much it has now to get the age

Uranium Dating

• We know the half -life of 238U and 235U• They have series that end in 206Pb and 207Pb• Compare how much U vs. special lead and

calculate the age of the rock!!• Carbon dating only good for about 50,000

years• Uranium rock dating good for millions of

years

ExampleYou have 100 g of radioactive C-14. The half-

life of C-14 is 5730 years.

• How many grams are left after one half-life? Answer:50 g

• How many grams are left after two half-lives?

Problem

A sample of 3x107 Radon atoms are trapped

in a basement that is sealed. The half-life of

Radon is 3.83 days. How many radon atoms

are left after 31 days?

answer:1.2x105 atoms