Nuclides Basic information:
A nuclide (called nuclear species) is an atom that is characterized by the following specific constitution of its nucleus:
1- Proton (atomic) number ( Z) 2- Neutrons number (N) 3- The energy of the nuclear state
Both proton and neutrons are called nucleons, and thus, nuclides are composed of nucleons. The basic components of nucleons are elementary particles called quarks. Therefore, nuclides can be considered to be composed of quarks
Let A to be the number of nucleons in a nuclide, A is called also mass number, i.e N = A – Z.
The notation for a nuclide with mass number A and atomic number Z is represented by a symbol of its element X, as
According to A, Z, and the nuclear energy state of a nuclide, we can characterized the following categories of nuclides:
Isotopes: nuclides of the same element (same Z) but different A Example: (_6^12)C,(_6^13)C and (_6^14)C and (_6^14)C are isotopes of Carbon element.
Isobars : nuclides with the same A (equal in weight) but different in Z (different elements) Example: (_6^14)C and (_7^14)N are isobars.
Isotones: Nuclides with the same N,
Example: (_6^13)C and (_7^14)N are isotones.
Isomers: nuclides with the same Z and A, (i.e same isotopes) but different in their nuclear energy state (excitation state).
Example: Technetium nuclides: (_43^99)Tc and (_43^99m)Tc
Mirror Nuclides: neutron number (N) and proton number (Z) is exchanged. Example:(_1^3)He and (_2^3)He.
Mass defect, Binding energy and Stability of nuclides:
Mass defect: This is the difference in the masses of the nucleons of a nuclide and the mass of nuclide. Let mp , mn and mj to be the proton, neutron and nuclide mass respectively. The mass of the nuclide mj is lower than the mass of its constituent protons and neutrons, and Mass defect (m) of the nuclide is, then, given by m = (Z.mp + N.mn) – mj Binding energy: There is very strong attraction between the nucleons in nuclides, and the component nucleons of the nuclide are help together by the so called nuclear binding energy (E). The binding energy is , related to the mass defect of the nuclide, given by : E= mC2 The stability of nuclide is measured by the binding energy per each of its nucleons. Figure 1 shows the graphical representation of the binding energy per nucleon (E/A) versus the number of nucleons (A).
According to the stability, the nuclides are classified as stable and unstable nuclides • Stable nuclides: are stable over time, due to their high binding energy are capable of maintaining their composition unchanged for an indefinite period of time, and thus these nuclide exist for an indefinite period of time. Unstable nuclides: are not stable over time , due to their binding energy is not enough to keep their composition unchanged over time, thus they emit subatomic particles (alpha, beta, gamma, or proton) , they are called radioactive nuclides, and they eventually convert to stable nuclides by undergoing radioactive decay. - The heavy radioactive nuclides (right part of Fig. 1) can be converted to stable one by the so called nuclear fission and produced high quantity of energy. However, the light radioactive isotopes (left part of Fig. 1) can be converted to stable nuclides by fusion process and energy is released.
Radionuclides in medicine: Due to these nuclides capable of radioactive decay, they have been used in wide range of application in both diagnostic and therapeutic methods
Examples of some diagnostic methods in which radioactive nuclides are used: - Tracing method: radioactive nuclides (iodine-125, iron-59) are employed. - Scintigraphy is used for examination of thyroid gland and kidneys (iodine-131) - Single photon emission computed tomography (SPECT): (technetium-99m) are used. - Positron emission tomography (PET): fluorine-18 used The therapeutic application of the radioactive nuclides is known as Radiotherapy: Examples: - Therapy of thyroid gland tumours by radioactive iodine I-131 - Radioactive nuclides Cobalt (Co-60) and caesium (Cs-137) are used for tumour treatment by different way of applications