A new force called the strong nuclear force keep them together.

Transcription

1 Nuclear Physics Tiny central group of particles in an atom. A hydrogen atom is the smallest atom and is on the order of about m across. A nucleus is only about m across. A nucleus is made up of the protons and neutrons. Nucleus has a net positive charge. If the protons are positively charged and the neutrons are neutral, how does the nucleus stay together?

2 Protons repel each other via the electric force. Because the protons are so close together and there mass is so small, the gravitational force is not strong enough to keep them together. A new force called the strong nuclear force keep them together. There is another force called the weak nuclear force which is involved in radioactive beta decay. These 4 forces are the fundamental forces that determine how the universe behaves.

3 Three types of glue in the universe keeps particles together. 1) Gravitational force holds objects together at the astronomical level. 2) Electric force holds solids and liquids together at the atomic level. 3) The nuclear force holds the nucleus together. The nuclear force is very strong but acts only up to a distance of about m. (As the size scale goes down, the strength of the forces go up.)

4 Nucleus has protons and neutrons. Each of these are made up of quarks. There are 6 types of quarks. Protons and neutrons are made up of Up and Down quarks. Up quarks have charge of +(2/3)e Down quarks have charge of (1/3)e Protons have 2 up quarks and 1 down quarks. Neutrons have 2 down and 1 up quarks.

5 Masses: Mass of proton is x kg Mass of neutron is x kg Neutron is a tiny bit larger. We will see later that a neutron can decay into a proton with some other junk left over. Elements are described as having atoms with the same number of protons. Example: all oxygen atoms have 8 protons. The number of protons is called the atomic number.

6 Atoms with the same number of protons but different number of neutrons are isotopes. Example: ordinary hydrogen has 1 P and 0 N. Deuterium has 1 P and 1 N. Tritium has 1 P and 2 N. All have the same atomic number = 1. Atomic weight or atomic mass or mass number is the number of protons and the number neutrons added up. Different isotopes of the same element have the same atomic number but different atomic masses. Another example is carbon C, 6C, C

7 Typically elements with small atomic numbers have only a few neutrons. (#p about equal to #n) For large atoms, such as uranium ( 92 P), there are a lot more neutrons (for uranium there are about 140 N). The large number of protons need more neutral neutrons to be in between them.

8 Radioactivity First observed by accident when a photographic plate was stored in a drawer with an uranium compound. The plate had been exposed even though it has kept in a dark drawer. Energy from the uranium exposed the plate.

9 Radioactivity results when an unstable nucleus emits energy in the form of particles or electromagnetic waves. The original material decays, or turns into two entities which when added up have the same energy as the original material. Radioactive isotopes isotopes that are radioactive. All isotopes with an atomic number > 83 is radioactive. Some smaller isotopes are also radioactive. Example: 14 C.

10 Stable nuclei don t naturally radioactively decay. They need some outside influence. Unstable nuclei decay with no outside disturbance. 3 types of radioactive decay: alpha radioactive nucleus spits out an alpha particle. Alpha particles ( ) are identical to a helium nucleus. 2 p and 2 n. Air can absorb alpha particles. They will collide with the air and pick up electrons and become normal helium atoms.

11 beta decay ( ) radioactive nucleus spits out an electron. Amazingly, a nucleus can make an electron. Most radioactive isotopes can only give off either alpha or beta particles. (one or the other) However these two are usually accompanied by a gamma-ray photon. This is the 3 rd type of decay. Gamma rays are a form of electromagnetic radiation (light with wavelength smaller than m)

12 Parent nucleus is what is present before the decay (nuclear reaction). Daughter nucleus is present after the nuclear reaction. 238 U Th uranium is the parent thorium is the daughter

13 Half life determines the rate of how fast the nuclear material decays. half life the time that is needed for half of the material to decay. see tables 15.1 for some examples and the graph below. The half life for each radioactive isotope is constant. Using this information it is possible to date old objects depending on the amount of radioactive material that is present. Carbon dating using the 14 C isotope is common.