Introduction to NMT 1002 :Introduction to NMT 1002 Nuclear Physics Tim Marshel, R.T. CNMT Keiser College, Miami, Florida


12.1 The basic properties of nucleus :12.1 The basic properties of nucleus We have frequently made use of the fact that every atom contains a massive, positively charged nucleus, much smaller than the overall dimensions of the atom but nevertheless containing most of the total mass of the atom. It is instructive to review the earliest experimental evidence for the existence of the nucleus.


Slide 3:It is well known that an atom has a nucleus in the center of the atom. The most obvious feature of the atomic nucleus is its size, of the order of 20,000 to 200,000 times smaller than the atom itself. Although the “surface” of a nucleus is not a sharp boundary, experiments determined an approximate radius for each nucleus. The radius is found to depend on the mass, which in turn depends on the total number A of neutrons and protons, usually called mass number.


The structures of nucleus :The structures of nucleus The electrical charges of nucleus: Nucleus is composed of protons and neutrons. Proton （positive charge） and neutrons （no electronic charges）. Nucleus has positive charges and it rotates. Because of this, it has angular momentum and magnetic moment. The total number of positive charges, the number of protons, is denoted by Z which is also called the atomic number.


Slide 5:Nuclear masses: Mass of an atom includes the mass of electrons and the mass of nucleus. As the mass of a proton or a neutron is very close to 1u, the atomic mass number A is equal to the total number of protons and neutrons. Therefore,


Slide 7:Nuclear forces and nuclear energy levels: The force of making the protons and neutrons together is obviously not electromagnetic force as neutrons are charge free and it is not the gravitational force either. Experiments show that such a force is a special interaction force which is called nuclear force.


Slide 8:Nuclear force properties: (1). It is a “short distance force” as its effective distance is about 10-15m and out of this range it reduces to zero sharply; (2). The force is the strongest force we have observed so far; (3) the force has a feature of saturation, the nucleus can interact only with its nearest neighbor;


Slide 9:The interaction of nuclear force does not depend on charged condition of nuclei. That is that interaction forces from n-n, n-p and p-p are almost the same. Nuclei are same as the atoms and they also have discrete energy levels (angular momentum and magnetic moment). Energy transmission can be also happened under the surrounding perturbations.


Nuclear binding energy and mass defect :Nuclear binding energy and mass defect Nucleus is composed of nucleons and its mass should be equal to the sum of the mass of all the individual nucleon composing the nucleus. If the masses of the nucleus, the proton and the neutron are denoted by mx, mp, mn respectively, we should have such an equation


Slide 11:Unfortunately, the experiments show that the nucleus mass is less than the sum on RHS of the above equation and the difference is m, called the mass defect. Relativity points out that if there is mass defect, there must have energy changes. The difference of energy and the difference of the mass has the following relation.


Slide 12:Where is the mass defect. E is called the nuclear binding energy. Such an energy was emitted when the nucleus was created. The higher the binding energy is, the more condensed and the more stable the nucleus is. This is why the nuclei are stable.


Brief Review :Brief Review what does the symbol mean? what are the nuclear force properties? , what is the mass defect? what is the nuclear binding energy?


12.2 The decay types of the atomic nucleus :12.2 The decay types of the atomic nucleus Nuclide has two big classes. One of them is radioactive nuclide and the other is stable nuclide. The radioactive nuclide can emit particle rays and be changed into another nuclide. This phenomenon is regarded as nuclear decay .


Slide 15: decay The heavy nucleus with its mass number A>209 could emit  particles. The decay process can be written as Radioactive (or parent) nucleus Daughter nucleus Decay energy Nucleus of Helium Kinetic


Slide 16:A > 209  unstable Stability is from the attractive nuclear force and the repulsive electrical force. nuclear force favors pairs of nucleons, In the absence of electrical interactions, the most stable nuclei would be those having equal numbers of neutrons and protons, N = Z.


Slide 17:The electrical repulsion shifts the balance to favor (like) greater numbers of neutrons, but a nucleus with too many neutrons is unstable because not enough of them are paired with protons. A nucleus with too many protons has too much repulsive electrical interaction to be stable.


Slide 18:2.  decay  Decay denotes the parent nucleus becomes another nucleus spontaneously with its mass number unchangeable. Of course this decay is a decay to emit an electron. Such a decay has three kinds of forms that can be described by the following equations:


Slide 20:3.  decay and inner transition Most of the daughter nuclei after α or β decay are in excited state and release their energies in  rays, and then leave their in ground state. This phenomenon is called  decay. In this process, a  photon is emitted.


Slide 21:The inner transmission is the above phenomenon but no  photon emission. The energy was transferred to the electrons outside the nucleus, the electron may become an free electron, leave the nucleus in a ground state, companied by x-rays generation


12.3 The nuclear decay law :12.3 The nuclear decay law The decay law If a radioactive sample contains N radioactive nuclei at some instant, it is found that the number of nuclei, N, that decay in a small time interval t is proportional to N: N = N t Where  is a constant called decay constant. The negative sign signifies that N decrease with time; The value of  for any isotope determines that rate for which that isotope will decay.


Slide 23:The decay rate is defined as the number of decay per second. The decay Law is


Slide 24:The half-life t = T, N = N0/2 The unit of half-life is minute, hour, day or year.


Slide 25:The radioactivity The radioactivity is defined as the nuclear number of decay in unit time. It is sometimes called radioactive intensity or decay rate or activity, denoted by I. The units of radioactivity are initially in Becquerel (Bq).


Slide 26:1Bq = 1 decay / s (Bq = Becquerel) 1 Ci = 3.7  1010 Bq (Ci = Curie) = 3.7  104 MBq (M=mega ~106) = 3.7  10 GBq (G=Giga ~ 109) = 3.7  10-2 TBq (T = Tera ~ 1012) = 103 mCi (millicurie) = 106 Ci (microcurie) It is known that they are similar to meters used.


Slide 27:Example 12-1: The activity of radium: the half-life of the radioactive nucleus 22686Ra is 1.6  103 years. If the sample contain 3.0  1016 such nuclei, determine the activity at this time. Solution: first, let us convert the half-life to seconds So the decay constant can be obtained as:


Slide 28:We can calculate the activity of the sample at t = 0 using