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Define and Explain Nuclear Physics
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Nuclear physics, as the name implies, deals with the model and mechanics of the nucleus.
Nucleus structure
Building upon the understanding of a central positive charge—called a proton—in the nucleus of
hydrogen, it seemed reasonable to assume that other atoms also had nuclei with protons.
Sir James Chadwick demonstrated the existence of a neutral particle—called aneutron—that has
essentially the same mass as the proton. The nucleus is made of protons and neutrons that,
collectively, are called nucleons. According to the modern model of the nucleus, the atomic
number ( Z) is the number of protons in the nucleus, and the atomic mass ( A) is the number of
nucleons in the nucleus. (The number of electrons is equal to the number of protons in an
electrically neutral atom, and so the number of orbiting electrons is also given by the value of Z.)
Nuclei with the same number of protons but differing number of neutrons are called isotopes.
The chemical properties of an element are determined by the outer electrons (equal to the
number of protons); therefore, isotopes are identical in chemical nature but differ in mass. The
symbol for an element ( X) is Z A X; for example, 4 9Be is beryllium with four protons and five
neutrons.
Binding energy
When the masses of the constituent particles of a nucleus are added together, the sum is less than
the nucleus itself. For example, a deuteron is an isotope of hydrogen with one proton and one
neutron in the nucleus. The following below adds these particles in atomic mass units—
abbreviated here as amu—where 1 amu is 1/12 of a carbon atom with 12 nucleons.
The observed mass of the deuteron is 2.014102 amu, which is .002388 less than the sum. Using
the mass‐equivalence equation, E = mc 2, 1 amu corresponds to approximately 931 MeV. Thus,
the mass difference is (0.002388 amu)(931 MeV/amu) = 2.224 MeV. This quantity is
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called binding energy. The binding energy is the difference between the mass energy of the
nucleus and its constituent particles. To separate the nucleus into a proton and neutron, energy
equal to the binding energy must be added to the system.
Radioactivity
Some nuclei are unstable and spontaneously emit radiation, which is calledradioactivity. The
radiation is of three types:

Alpha decay, in which the emitted particles are helium nuclei of 2 protons and 2 neutrons

Beta decay, in which the emitted particles are electrons

Gamma decay, in which high energy photons are emitted
The original nucleus is called the parent nucleus, and the nucleus remaining after the decay is
called the daughter nucleus. The process of one element changing into another through
radioactivity is called transmutation.
If a nucleus emits an alpha particle, it loses two protons and two neutrons; therefore, the daughter
nucleus has an atomic mass of 4 less and an atomic number of 2 less than the parent nucleus. An
example of alpha decay of uranium is92 238U → 90 234Th + 2 2He.
If a nucleus emits a beta particle, it loses an electron. Since the mass of the electron is so small
compared to that of a proton and a neutron, the atomic mass of the parent nucleus is the same as
the daughter nucleus. The atomic number of the daughter nucleus is one greater than that of the
parent nucleus. An example of beta decay of bismuth is 83 212Bi → 84 212P 0 + − 0 e.
Frequently the daughter nucleus is left in an excited state after either alpha or beta decay. Then
the nucleus can give up excess energy by emission of gamma radiation. The following example
shows a typical situation where gamma decay occurs: 5 12B → 6 12C* + −1 0 e; then, 6 12C*
→ 6 12C + γ, where the asterisks indicate an excited nucleus.
The rules for radioactive decay are based on conservation laws. Examination of the preceding
examples reveals that the number of nucleons and the electric charge are conserved; that is, the
total on one side of the equation equals the total on the other side of the equation. Other
conservation laws that must be observed are those of energy, momentum, and angular
momentum.
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Half-life
The decay rate ( R) or the activity of a sample of radioactive material is defined as the number of
decays per second, given by R = − λ N, where N is the number of radioactive nuclei at some
instant and λ is the decay constant.
The half‐life ( T) is defined as the time required for half of a given number of radioactive nuclei
to decay. It is different for each type of radioactive element:
The general decay curve for a radioactive sample relating the number of nuclei present at a given
time to the original number of nuclei is exponential. The expression is 0 1 n + 92 235U → 56 241Ba
+ 36 92Kr + 3 0 1 n. The total rest mass of the products is less than the original rest mass of the
original uranium by 220 MeV. This is an enormous amount of energy compared to energy
releases in chemical processes and when considering that a relatively modest piece of uranium
has so very many nuclei. Nuclear fusion occurs when light nuclei are combined to form a heavier
nucleus. The sun is powered by nuclear fusion.
The binding energy is related to stability. When the mass energy of the parent nucleus is greater
than the total mass energy of the decay products, spontaneous decay will take place. If the decay
products have a greater total mass energy than the parent nucleus, additional energy is necessary
for the reaction to occur. Energy is released when light nuclei combine (fusion) and when heavy
nuclei split (fission).
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