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PRINCE SATTAM BIN ABDUL AZIZ UNIVERSITYCOLLEGE OF PHARMACY

Nuclear PharmacyNuclear Pharmacy((PHT 433PHT 433 ) )

Dr. Shahid JamilDr. Shahid Jamil

The stability of the atoms depends on the neutron to The stability of the atoms depends on the neutron to proton ratio in the nucleus (N/Z). Above atomic proton ratio in the nucleus (N/Z). Above atomic number 83, all elements are radioactive. number 83, all elements are radioactive. The nucleons are in a state of continual motion The nucleons are in a state of continual motion (natural isotopes). (natural isotopes). If additional or a deficiency of neutrons occurs and If additional or a deficiency of neutrons occurs and disturbs the stability of an atom, the atom attempts disturbs the stability of an atom, the atom attempts to regain its stability by giving off either a photon, to regain its stability by giving off either a photon, such as gamma ray, or a particle from the nucleus to such as gamma ray, or a particle from the nucleus to attain a more stable N/Z ratio and the nucleus is attain a more stable N/Z ratio and the nucleus is transformed into another. transformed into another. This phenomenon is known as radioactivity or This phenomenon is known as radioactivity or radioactive decay.radioactive decay.

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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)

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Radioactivity could be through an artificial Radioactivity could be through an artificial transmutation, e.g. the bombardment of nitrogen of transmutation, e.g. the bombardment of nitrogen of N=14 with a helium nucleus of mass 4 (an alpha N=14 with a helium nucleus of mass 4 (an alpha particles) to produce radioactive oxygen ofparticles) to produce radioactive oxygen of O = 17 and a proton:O = 17 and a proton:

It will be noted that nuclear equations must balance. It will be noted that nuclear equations must balance. This same reaction may also be represented byThis same reaction may also be represented by

14 1714 17 N (N (αα, p) O, p) O

14 4 1 17 N + He H + O

7 2 1 8

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A) Natural radioactivityThe 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.

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Nuclear reactions are classified as follows:

1- Charged-particle reactions:

Reactions of these types may be produced with

protons (1H1 or p), deuterons (2H1 or d), alpha

particles (4He2 or α) or occasionally by electrons or

beta particles (°e-1 or °β-1).

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In proton initiated reactions, non radioactive sodium

is bombarded with protons to form 23Mg and a

neutron (1n0).

23Na11 + 1H1 23Mg12 + 1n0

In deuteron induced reactions,

27 Al13 + 2H1 25Mg12 + 4He2

In alpha particle initiated reaction.

14 N7+ 4He2 17p8 + 1H1

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2- Photon-induced reactions:Electromagnetic radiations or photons ofhigh energy may also , induce nuclear reactions e.g.

γ + 9Be4 8Be4 + 1n0

The source of electromagnetic energy utilized in these types of reactions may be a gamma emitting radionuclide or a high voltage X-ray generator.

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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).

23N11 + 1n0 24Na11 + (very important in

activation

analysis technique)

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Radioactive DecayAs 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.

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Kinetics of Radioactive DecayDecay 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.

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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, N

NN

o

t ln

Where N0 is the number of atoms present at zero time Nt is the number of atoms present at time t

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Nt = No e – λ t

Radi

oact

ivit

yThis relation is illustrated graphically

Time

Radioactive decay curve

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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,

NAA

o

t ln At = A0 e –λtor

In At = 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).

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Log.

Rad

ioac

tivi

ty

Time

log At = log Ao – 303.2

t

Radioactive decay curve

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The half-life of a radioactive species is the time required for one half 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/λBoth 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.

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types of Nuclear Decay1. 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 + QZ Z-2 2Where: 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

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The α-particles are the heaviest and having the

greatest kinetic energy of the particles emitted from

nuclei

238 234 4 U Th + He + Q 92 90 2

e.g.05/04/23L3-L4

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2. Beta - Decaya. Negatron (β-) emission.Nuclei with excess neutrons gain stability bythe 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.,

1 1 0 n p + β + ν 0 1 -1

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A A 0 X Y + β + Q + ν (neutrino)Z Z+1 -1

generally

e.g. 32 32 0 P S + β + Q + ν 15 16 -1

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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. 1 1 0

P n + β + ν 1 0 +1

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There is no change in mass number but the atomic number is reduced by one unit, e.g.:

11 11 0 C B + β + Q 6 5 +1 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-1 +1

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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 1 P + e- n1 0

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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 νZ -1 Z-1

hν = electromagnetic radiations or photons

51 0 51 Cr + e V + h ν24 -1 23

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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 g rays (electromagnetic radiation of high energy), so that the final energy of the product nucleus is at its ground state.

4. Gamma - emission (y- emission).05/04/23L3-L4

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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 (99mTc, 110mAg, 234mPa)

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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.

11C

11B

β+

0.97 Mev100%

32P

32S

β-

1.17 Mev100%

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60Co

γ1

β-

0.314 Mev100%

1.173 Mev

1.48 Mev

0.01%

1.332 Mevγ2

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40K

γ1

β-

1.36 Mev88.5%

1.173 Mev

K electroncap.

11.5 %40Ca

40Ar

the disintegrating atoms are only 88.5% of the β particles and 11.5 % of the γ rays are counted (mixed decay scheme

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