The atom can be thought of as a system containing a positively charged nucleus and negatively charged electrons which are in orbit around the nucleus.
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The nucleus is the central core of the atom and is composed of two types of particles: protons which are positively charged, and neutrons which have a neutral charge. Each of these particles has a mass of approximately one atomic mass unit (amu). (1 amu = 1.66E-24 g)
Electrons surround the nucleus in orbitals of various energies. (In simple terms, the farther an electron is from the nucleus, the less energy is required to free it from the atom.) Electrons are very light compared to protons and neutrons. Each electron has a mass of approximately 5.5E-4 amu.
A nuclide is an atom described by its atomic number (Z) and its mass number (A). The Z number is equal to the charge (number of protons) in the nucleus, which is a characteristic of the element. The A number is equal to the total number of protons and neutrons in the nucleus. Nuclides with the same number of protons but with different numbers of neutrons are called isotopes.
For example, deuterium (2,1H) and tritium (3,1H) are isotopes of hydrogen with mass numbers two and three, respectively. There are on the order of 200 stable nuclides and over 1100 unstable (radioactive) nuclides. Radioactive nuclides can generally be described as those which have an excess or deficiency of neutrons in the nucleus.
Radioactive nuclides (also called radionuclides or radioisotopes) can regain stability by nuclear transformation (radioactive decay) emitting radiation in the process. The radiation emitted can be particulate or electromagnetic or both. The various types of radiation and examples of decay are shown below.
Alpha particles (α)
Alpha particles have a mass and charge equal to those of helium nuclei (2 protons + 2 neutrons). Alpha particles are emitted during the decay of some very heavy nuclides (Z > 83).
226,88Ra --> 222,86Rn + 4,2a
Beta particles (β+ β-)
Beta particles are emitted from the nucleus and have a mass equal to that of electrons. Betas can have either a negative charge or a positive charge. Negatively charged betas are equivalent to electrons and are emitted during the decay of neutron rich nuclides.
14,6C --> 14,7N + 0,-1B + neutrino
Positively charged betas (positrons) are emitted during the decay of proton rich nuclides.
22,11Na --> 22,10Ne + 0,1B + g
Gamma rays (γ)
Gamma rays (also called gammas) are electromagnetic radiation (photons). Gamma rays are emitted during energy level transitions in the nucleus. They may also be emitted during other modes of decay.
99m,43Tc --> 99,43Tc + g
In certain neutron deficient nuclides, the nucleus will capture an orbital electron resulting in conversion of a proton into a neutron. This type of decay also involves gamma emission as well as x-ray emission as other electrons fall into the orbital vacated by the captured electrons.
125,53I + 0,-1e --> 125,52Te + g
For a few radionuclides, a neutron can be emitted during the decay process.
17,7N --> 17,8O* + 0,-1B (*excited state)
17,8O* --> 16,8O + 1,0n
X-rays are photons emitted during energy level transitions of orbital electrons. Bremsstrahlung x-rays (braking radiation) are emitted as energetic electrons (betas) are decelerated when passing close to a nucleus. Bremsstrahlung must be considered when using large activities of high energy beta emitters such as P-32 and S-90.
Characteristics of radioactive decay
In addition to the type of radiation emitted, the decay of a radionuclide can be described by the following characteristics.
The half-life of a radionuclide is the time required for one-half of a collection of atoms of that nuclide to decay. Decay is a random process which follows an exponential curve. The number of radioactive nuclei remaining after time (t) is given by:
A = Ao/(2N) » N = t / T*
Ao = original number of atoms
A = number remaining at time t
t = decay time
T = half-life of the radioisotope
* = same units must be maintained
The basic unit used to describe the energy of a radiation particle or photon is the electron volt (eV). An electron volt is equal to the amount of energy gained by an electron passing through a potential difference of one volt. The energy of the radiation emitted is a characteristic of the radionuclide.
The transfer of energy from the emitted particle or photon to an absorbing medium has several mechanisms. These mechanisms result in ionization and excitation of atoms or molecules in the absorber. The transferred energy is eventually dissipated as heat.
Ionization is the removal of an orbital electron from an atom or molecule, creating a positively charged ion. In order to cause an ionization, the radiation must transfer enough energy to the electron to overcome the binding force on the electron. The ejection of an electron from a molecule can cause dissociation of the molecule.
Excitation is the addition of energy to an orbital electron, thereby transferring the atom or molecule from the ground state to an excited state.
Alpha particles (α)
Interactions between the electric field of an alpha and orbital electrons in the absorber cause ionization and excitation events. Because of their double charge and low velocity (due to their large mass), alpha particles lose their energy over a relatively short range. One alpha will cause tens of thousands of ionizations per centimeter in air. The range in air of the most energetic alpha particles commonly encountered is about 10 centimeters (4 inches). In denser materials, the range is much less. Alpha particles are easily stopped by a sheet of paper or the protective (dead) layers of skin.
Beta particles (β+ β-)
Normally, a beta particle loses its energy in a large number of ionization and excitation events. Due to the smaller mass, higher velocity and single charge of the beta particle, the range of a beta is considerably greater than that of an alpha of comparable energy. Since its mass is equal to that of an electron, a large deflection can occur with each interaction, resulting in many path changes in an absorbing medium.
If a beta particle passes close to a nucleus, it decreases in velocity due to interaction with the positive charge of the nucleus, emitting x-rays (bremsstrahlung). The production of bremsstrahlung increases with the atomic number of the absorber and the energy of the beta. Therefore, low Z materials are used as beta shields.
A positron will lose its kinetic energy through ionizations and excitations in a similar fashion to a negative beta particle. However, the positron will then combine with an electron. The two particles are annihilated, producing two 511 keV photons called annihilation radiation.
Gammas and x-rays differ only in their origin. Both are electromagnetic radiation, and differ only from radio waves and visible light in having much shorter wavelengths. They have zero rest mass and travel with the speed of light. Photons have no definite maximum range. However, the total fraction of photons passing through an absorber decreases exponentially with the thickness of the absorber.
The electrons from ionizations and pair production will themselves go on to cause more ionization and excitation events in the same way as described for beta particles.