2/17/2016 L3-L4 1 PRINCE SATTAM BIN ABDUL AZIZ UNIVERSITY COLLEGE OF PHARMACY Nuclear Pharmacy (PHT 433 ) Dr. Shahid Jamil
The stability of the atoms depends on the neutron to proton ratio in the nucleus (N/Z). Above atomic number 83, all elements are radioactive. The nucleons are in a state of continual motion (natural isotopes). If additional or a deficiency of neutrons occurs and disturbs the stability of an atom, the atom attempts to regain its stability by giving off either a photon, such as gamma ray, or a particle from the nucleus to attain a more stable N/Z ratio and the nucleus is transformed into another. This phenomenon is known as radioactivity or radioactive decay. Nuclear stability 2/17/2016L3-L4 2
Radioactivity is the processes by which atomic nuclei spontaneously decay or disintegrate by one or more distinct energy levels or transitions until finally a stable state is reached. It is divided into two different types: A) Natural radioactivity B) Artificial radioactivity (man made radioactivity) Radioactivity 2/17/2016L3-L4 3
Radioactivity could be through an artificial transmutation, e.g. the bombardment of nitrogen of N=14 with a helium nucleus of mass 4 (an alpha particles) to produce radioactive oxygen of O = 17 and a proton: O = 17 and a proton: It will be noted that nuclear equations must balance. This same reaction may also be represented by N (α, p) O N (α, p) O N + He H + O /17/2016L3-L4 4
A) Natural radioactivity The radioactive decay processes take place in a material without the addition of energy. There are more than 50 naturally occurring radio-nuclides e.g. uranium 238, thorium 232, radium 226, lead 210 and potassium 40. B) Artificial radioactivity (man made radioactivity): Is the radioactivity of synthetic nuclides produced by particle bombardment or electromagnetic irradiation. It may be produced by different ways several devices depending on what bombarding particles or rays is utilized. 2/17/2016L3-L4 5
Nuclear reactions are classified as follows: 1- Charged-particle reactions: Reactions of these types may be produced with protons ( 1 H 1 or p), deuterons ( 2 H 1 or d), alpha particles ( 4 He 2 or α ) or occasionally by electrons or beta particles (°e -1 or °β -1 ). 2/17/2016L3-L4 6
In proton initiated reactions, non radioactive sodium is bombarded with protons to form 23 Mg and a neutron ( 1 n 0 ). 23 Na H 1 23 Mg n 0 In deuteron induced reactions, 27 Al H 1 25 Mg He 2 In alpha particle initiated reaction. 14 N He 2 17 p H 1 2/17/2016L3-L4 7
2- Photon-induced reactions: Electromagnetic radiations or photons of high energy may also, induce nuclear reactions e.g. γ + 9 Be 4 8 Be n 0 The source of electromagnetic energy utilized in these types of reactions may be a gamma emitting radionuclide or a high voltage X-ray generator. 2/17/2016L3-L4 8
3- Neutron induced reactions: It is one of the most important methods of producing artificial radioactive nuclides (bombardment with a source of thermal neutrons), e.g. (neutron capture). 23 N n 0 24 Na 11 + (very important in activation analysis technique) 2/17/2016L3-L4 9
Radioactive Decay As the radioactive species exist in a highly excited, unstable state characterized by an energy excess. These nuclides ultimately achieve stability through the process of radioactive decay and the release of large amounts of energy either kinetic or electromagnetic or both. 2/17/2016L3-L4 10
Kinetics of Radioactive Decay Decay rate is the time rate at which atoms undergo radioactive disintegration. It is expressed by - dN/dt where - dN is the change in the number of atoms N dt is the change in the time t The negative sign indicates that the number of atoms is decreasing in time. 2/17/2016L3-L4 11
The rate of decay (- dN/dt) is proportional to (indicated by) the number of atoms N, present at any time t Therefore: - dN/dt = λ N Where λ is a proportionality constant usually called the decay constant. The decay of radioactive atoms is therefore a first order reaction. Thus, Where N 0 is the number of atoms present at zero time N t is the number of atoms present at time t 2/17/2016L3-L4 12
N t = No e – λ t Radioactivity This relation is illustrated graphically Time Radioactive decay curve 2/17/2016L3-L4 13
A = λ N The rate of decay, - dN/dt, is called the activity (A). The activity A is proportional to the number of atoms N, Thus, A t = A 0 e –λt or In A t = In A 0 – λ t The absolute activity is usually expressed as disintegrations per second (d/s or dps) or disintegrations per minute (d/m or dpm). 2/17/2016L3-L4 14
Log. Radioactivity Time log A t = log Ao – Radioactive decay curve 2/17/2016L3-L4 15
The half-life of a radioactive species is the time required for onehalf of a given number of atoms to decay. The half-life t ½ is related to the disintegration constant, λ by the equation: The half-life of a radioactive species t ½ =0.693/ λ Both λ and t ½ are constants which characterize the rate of decay, but t ½ is the more convenient for general use. The determination of these constants is important in the planning of experiments and the maintenance of stocks. 2/17/2016L3-L4 16
types of Nuclear Decay 1. Alpha - Decay ( α - emission) This occurs with isotopes of heavy elements with nuclides of atomic number greater than 82. It involves the emission of α particle (helium nucleus He), i.e., A A-4 4 X Y + He + Q Z Z-2 2 Where: X is the parent radionuclide, Y is the Daughter resulting from decay Q is the energy required to make the reaction go or energy release as a results of the reaction 2/17/2016L3-L4 17
The α -particles are the heaviest and having the greatest kinetic energy of the particles emitted from nuclei U Th + He + Q e.g. 2/17/2016L3-L4 18
2. Beta - Decay a. Negatron (β - ) emission. Nuclei with excess neutrons gain stability by the conversion of a neutron into a proton plus a β - particle. A negative particle is emitted when a nuclear neutron is spontaneously converted to a proton, just before the emission occurs. Since the (β - particle) has a very low mass there is no change in the mass number, but the atomic number increases by one unit, i.e., n p + β + ν /17/2016L3-L4 19
A A 0 X Y + β + Q + ν (neutrino) Z Z+1 -1 generally e.g P S + β + Q + ν /17/2016L3-L4 20
b- Positron (β + ) emission This process is the converse of β - emission and occurs with neutron deficient nuclei The deficiency being resolved by the conversion of a proton into a neutron plus a positron. As with β - emission, positrons are emitted with continuous energy spectrum and energy conservation is achieved by the emission of an anti-neutrino ( ν ), i.e P n + β + ν /17/2016L3-L4 21
There is no change in mass number but the atomic number is reduced by one unit, e.g.: C B + β + Q The emission of a neutrino and anti-neutrino in both negative and positive beta decay was suggested to explain the energy change in the reaction. A A 0 X Y + β + Q + ν (antineutrino) Z Z /17/2016L3-L4 22
This is an alternative for of decay to β + emission with neutron deficient nuclei and involves the conversion of a proton into a neutron by capture of of an orbital electron (negatron) 1 P + e - n Orbital Electron capture (EC, K- capture) 2/17/2016L3-L4 23
The most frequent electron capture reactions involve the K shell. This capture causes a deficiency in the K shell, which is made by the migration of a negatron from one of the outer shells. Since the migrant negatron loses energy in the process, the surplus energy is emitted in the form of X-ray of an energy characteristic of the product atom, for example, A 0 A X + e Y + h ν Z -1 Z-1 h ν = electromagnetic radiations or photons Cr + e V + h ν /17/2016L3-L4 24
Gamma-emission is not usually a form of nuclear transformation but may follow other forms of decay. If the product nucleus is left in excited state the surplus energy is lost by the emission of one or more grays (electromagnetic radiation of high energy), so that the final energy of the product nucleus is at its ground state. 4. Gamma - emission (y- emission). 2/17/2016L3-L4 25
5. Isomeric-transition: In some cases radioactive nuclei, in addition to their instability, exist in an excited or metastable state. The excited nucleus attains a more stable configuration by gamma radiation emission (y- rays) without the emission of particles. Such nuclides are distinguished by the addition of the letter 'm' to the mass number ( 99m Tc, 110m Ag, 234m Pa) 2/17/2016L3-L4 26
Mode of Radioactive Decay "decay scheme" It give an idea about the energy levels of the radionuclides, the energy of radiation produced. the number of β + or β - particles is equal to the number of disintegrating atoms. 11 C 11 B β Mev 100% 32 P 32 S β Mev 100% 2/17/2016L3-L4 27
60 Co γ1γ1 β Mev 100% Mev 1.48 Mev 0.01% Mev γ2γ2 2/17/2016L3-L4 28
40 K γ1γ1 β Mev 88.5% Mev K electron cap % 40 Ca 40 Ar the disintegrating atoms are only 88.5% of the β particles and 11.5 % of the γ rays are counted (mixed decay scheme 2/17/2016L3-L4 29