Physics 12 Mr. Jean January 13th, 2012

The plan: Video clip of the day Finish BBC video Types of radioactive decays.

Friday the 13th in 2012… There are 3 Friday the 13th’s in 2012 Each Friday the 13th is exactly 13 weeks a part from each other. Spooky?

Symbols Used for Nuclear Reactions: Neutron Proton Electron Alpha particle Gamma ray Beta particle

Nuclear Reactions: Nuclear reactions are somewhat different than chemical reactions. In chemical reactions, the equation is balanced when the number of each of the different elements on the reactant side equals the number of the different elements on the product side. In nuclear reactions, the atomic number and the mass number for each element must be balanced on both sides (in addition to the number of elements). We say that the mass number and atomic number must be conserved. The effect of balancing the atomic number is to actually balance the charge of the reactant and product.

Alpha Decay: Alpha particles. The symbol for the alpha particle is .  particles are helium nuclei. Each alpha consists of 2 protons and 2 neutrons. 's have a positive charge (+2). 's are only slightly deflected by a magnetic field (because of their large mass). They are stopped easily by a sheet of paper.

Alpha Decay Fact: All nuclei with more than 83 protons spontaneously begin to decay. They are always trying to get back to a more stable substance. The most stable substance in the world is iron… there’s a million things I can say about iron, stars and spectrum analysis.. So let’s leave this for another day….. Ok.. Here’s a teaser…

Another measure of stability is binding energy, the amount of energy needed to overcome the strong nuclear force and pull apart a nucleus. The binding energy per nucleon for the most stable isotope of each naturally occurring element is illustrated in Figure 2. Binding energy reaches a maximum of 8.79 MeV/nucleon at 56Fe. As a result, there is an increase in stability when much lighter elements fuse together to yield heavier elements up to 56Fe and when much heavier elements split apart to yield lighter elements down to 56Fe, as indicated by the arrows.

Alpha Decay: Alpha decay: In alpha decay, an unstable nucleus produces a daughter nucleus and releases an  particle. The reason alpha decay occurs is because the nucleus has too many protons which cause excessive repulsion. In an attempt to reduce the repulsion, a Helium nucleus is emitted. The way it works is that the Helium nuclei are in constant collision with the walls of the nucleus and because of its energy and mass, there exists a nonzero probability of transmission. That is, an alpha particle (Helium nucleus) will tunnel out of the nucleus.

Key: The atomic mass of the original element has decrease by 4 because helium has an atomic mass of 4. The atomic number of the original element has decreased by 2 because helium has an atomic number of 2. (2 Protons)

Beta Decay: There are three types of Beta Decay: Beta particles. The symbol for the beta particle is . 's are electrons, so they have a have a negative charge (-1). They can be greatly deflected by a magnetic field (because of their small mass and negative charge). 's penetrate matter a greater distance than  particles, but they still aren’t very penetrating. They can be stopped by a layer of metal foil.

Beta Decay: Beta decay occurs when the neutron to proton ratio is too great in the nucleus and causes instability. In basic beta decay, a neutron is turned into a proton and an electron. The electron is then emitted. Here's a diagram of beta decay with hydrogen-3 going to Helium-3:

Beta decay: In  decay a neutron in the nucleus of the unstable radioactive parent decays and becomes a proton as it emits a  particle (an electron).

Example: Thorium-234 has one of its neutrons become a proton - this increases the atomic number by one, but has no effect on the mass number since a neutron and a proton are both nucleons. A beta particle is also produced. Note that the atomic number on the left is equal to the total atomic number on the right. During electron capture, the atomic number of the daughter decreases by one, there is no change to the mass number. Note that in this reaction, you could produce gold from mercury. (When you build a machine that can do this let me know, I want in…)

Positron Emission:

There is also positron emission when the neutron to proton ratio is too small. A proton turns into a neutron and a positron and the postiron is emitted. A positron is basically a positively charged electron. Here's a diagram of positron emission with carbon-11:

Electron Capture:

The final type of beta decay is known as electron capture and also occurs when the neutron to proton ratio in the nucleus is too small. The nucleus captures an electron which basically turns a proton into a neutron. Here's a diagram of electron capture with beryllium-7:

Danger from Beta Decay: Electron fired out of the reactions at very high velocity. Luckily can be blocked relatively easily. High energy particles are also releases. Gamma Ray This is because your element is going from a high particle state to a lower energy states (ie: energy is released)

Gamma Decay: Gamma rays. The symbol for gamma rays is . Very short wavelength, high frequency photons. Gamma rays have no charge. They are not deflected by magnetic fields They are the most penetrating form of radiation. Stopping ’s requires great thicknesses of heavy materials such as lead or concrete.

Gamma Decay:

Gamma decay occurs because the nucleus is at too high an energy Gamma decay occurs because the nucleus is at too high an energy. The nucleus falls down to a lower energy state and, in the process, emits a high energy photon known as a gamma particle. Here's a diagram of gamma decay with helium-3:

Finishing the BBC Video: If there is time.. There is still 10 minutes left to the video from earlier on this week.