Nuclear Reactions and Carbon Dating M.Banu Children’s club lecture

 Remember that the nucleus is comprised of the two nucleons, protons and neutrons.  The number of protons is the atomic number.  The number of protons and neutrons together is effectively the mass of the atom. The Nucleus

 Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms.  There are three naturally occurring isotopes of uranium: Uranium-234 Uranium-235 Uranium-238 Isotopes

It is not uncommon for some nuclides of an element to be unstable, or radioactive. We refer to these as radionuclides There are several ways radionuclides can decay into a different nuclide. Radioactivity

Types of Radioactive Decay Alpha Decay Loss of an  -particle (a helium nucleus) He 4242 U  Th He

Types of Radioactive Decay Beta Decay Loss of a  -particle (a high energy electron)  0−10−1 e 0−10−1 or I Xe  + e 0−10−1

Types of Radioactive Decay Positron Emission Loss of a positron (a particle that has the same mass as but opposite charge than an electron) e 0101 C 11 6  B e 0101

Types of Radioactive Decay Gamma Emission Loss of a  -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)  0000

Penetrating Power

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 As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so- called belt of stability.

Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles.

Stable Nuclei Nuclei below the belt have too many protons. They tend to become more stable by positron emission or electron capture.

There are no stable nuclei with an atomic number greater than 83. These nuclei tend to decay by alpha emission. Stable Nuclei

Radioactive Series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

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.

Energy in Nuclear Reactions There is a tremendous amount of energy stored in nuclei. Einstein’s famous equation, E = mc 2, 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.

When atoms are bombarded with neutrons, their nuclei splits into 2 parts which are roughly equal in size. Nuclear fission in the process whereby a nucleus, with a high mass number, splits into 2 nuclei which have roughly equal smaller mass numbers. During nuclear fission, neutrons are released. Nuclear Fission

There are 2 types of fission that exist: 1. Spontaneous Fission 2. Induced Fission

U n 1 0 The Fission Process A neutron travels at high speed towards a uranium-235 nucleus.

U n 1 0

U n 1 0

U n 1 0 The neutron strikes the nucleus which then captures the neutron.

U The nucleus changes from being uranium-235 to uranium-236 as it has captured a neutron.

 The uranium-236 nucleus formed is very unstable.  It transforms into an elongated shape for a short time.

It then splits into 2 fission fragments and releases neutrons Ba Kr n 1 0 n 1 0 n 1 0

Ba Kr n 1 0 n 1 0 n 1 0

Ba Kr n 1 0 n 1 0 n 1 0

Ba Kr n 1 0 n 1 0 n 1 0

Nuclear Fission Examples U Ba n n Kr U Cs n n Rb 96 37

Energy from Fission  Both the fission fragments and neutrons travel at high speed.  The kinetic energy of the products of fission are far greater than that of the bombarding neutron and target atom. E K before fission << E K after fission  Energy is being released as a result of the fission reaction.

U Cs n n Rb ElementAtomic Mass (kg) U x Cs x Rb x n10n x

Calculate the total mass before and after fission takes place. The total mass before fission (LHS of the equation) : x x = x kg The total mass after fission (RHS of the equation): x x (2 x x ) = x kg

The total mass before fission = The total mass after fission = x kg x kg total mass before fission > total mass after fission

mass difference, m = total mass before fission – total mass after fission m = x – x m = 2.65 x kg This reduction in mass results in the release of energy.

Energy Released The energy released can be calculated using the equation: E = mc 2 Where: E = energy released (J) m = mass difference (kg) c = speed of light in a vacuum (3 x 10 8 ms -1 ) E m c2c2

E = mc 2 E = 2.65 x x (3 x 10 8 ) 2 E = x J U Cs n n Rb Calculate the energy released from the following fission reaction: m = 2.65 x kg c = 3 x 10 8 ms -1 E = E

Nuclear Fusion In nuclear fusion, two nuclei with low mass numbers combine to produce a single nucleus with a higher mass number. H He n 1 0 H Energy

H 2 1 H 3 1

H 2 1 H 3 1

H 2 1 H 3 1

H 2 1 H 3 1

He 4 2 n 1 0 ENERGY

He 4 2 n 1 0 ENERGY

He 4 2 n 1 0 ENERGY

Energy from Fusion ElementAtomic Mass (kg) 21H21H3.345 x H31H5.008 x He6.647 x n10n x H He n 1 0 H Energy

The total mass before fusion (LHS of the equation): x x = x kg The total mass after fission (RHS of the equation): x x = x kg H He n 1 0 H Energy

m = total mass before fission – total mass after fission m = x – x m = 3.1 x kg

E = mc 2 E = 3.1 x x (3 x 10 8 ) 2 E = 2.79 x J m = 3.1 x kg c = 3 x 10 8 ms -1 E = E H He n 1 0 H Energy The energy released per fusion is 2.79 x J.

Kinetics of Radioactive Decay Nuclear decay is a first-order process. The kinetics of such a process obey this equation: = -kt NtN0NtN0 ln The half-life of such a process is: = t 1/ k Comparing the amount of a radioactive nuclide present at a given point in time with the amount normally present, one can find the age of an object.

Cosmic Rays (radiation) Collision with atmosphere (N14) Forms C-14 C-14 combines with oxygen to form carbon dioxide (CO 2 )

We can take a sample of air, count how many 12C atoms there are for every 14C atom, and calculate the 14C/12C ratio. Because 14C is so well mixed up with 12C, we expect to find that this ratio is the same if we sample a leaf from a tree, or a part of your body.

Once a plant or animal dies the clock starts. The plant or animal no longer takes in C-14. The C-14 present in the plant or animal begins to decay. No more C-14 intake C-14 continues to decay

Used only on organic material Cannot be used to date rocks Maximum age limit about 60,000 years

Radioisotopes in Medicine 24 Na, t ½ = 14.8 hr, b emitter, blood-flow tracer 131 I, t ½ = 14.8 hr, b emitter, thyroid gland activity 123 I, t ½ = 13.3 hr, g-ray emitter, brain imaging 18 F, t ½ = 1.8 hr, b + emitter, positron emission tomography 99m Tc, t ½ = 6 hr, g-ray emitter, imaging agent

Brain images with 123 I-labeled compound

Chemistry In Action: Food Irradiation DosageEffect Up to 100 kilorad Inhibits sprouting of potatoes, onions, garlics. Inactivates trichinae in pork. Kills or prevents insects from reproducing in grains, fruits, and vegetables. 100 – 1000 kilorads Delays spoilage of meat poultry and fish. Reduces salmonella. Extends shelf life of some fruit to 10,000 kilorads Sterilizes meat, poultry and fish. Kills insects and microorganisms in spices and seasoning.

Questions