CONTENTSWhat is Radioactivity?Radioactive Nuclei 1) Strong Nuclear Force 2) Stability of NucleusWhat are Radioisotopes? How many? How are they manufactured? Significance

What are Radioactive Decay? α Decay β Decay Gamma DecayThe Decay Law, Decay Constant

and Half-life of radioactive Elements

What are the uses of Radioisotope?What are the biological effect of

Ionization Radiation?

Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves.

There are numerous types of radioactive decay. The general idea:

An unstable nucleus releases energy to become more stable

1) Strong Nuclear Force The nuclear force (or nucleon-nucleon interaction or

residual strong force) is the force between two or more nucleons. It is responsible for binding of protons and neutrons into atomic nuclei. The energy released causes the masses of nuclei to be less than the total mass of the protons and neutrons which form them.

An important factor affecting nuclear force between particles is a characteristic of each particles called spin. When a neutron and a proton get together to form a deuteron, it is only possible if the spin of the two particles are parallel. When the spins are anti-parallel, the nuclear force between them gets weaker by the factor of 2.(stability of nucleus)

The nuclear strong force and the electromagnetic force are the strongest of the four fundamental forces

2) Stability of Nucleus Each nucleus consists of a number of protons and neutrons. This items are

called nucleon. Whether the nucleus is stable or not, depends on the ratio of the numbers of two particles. Nuclides with more neutrons or equal numbers of neutrons and protons seem to be more stable. The separation distance between nucleons is comparable to the range of the strong nuclear force.

The stable nuclides can be characterized as follows;  The lightest nuclides have almost equal numbers of protons and neutrons. The heavier nuclides require more neutrons than protons. Most nuclides have both an even of number protons and an even number

of neutrons.  The unstable nuclides, on the other hand can be characterized as follows; Disintegrations occur to produce daughter nuclei which are more stable

than the original or parent nuclei. The heavier nuclides decay such as to increase the number of protons. The

neutron to proton ratio decreases, thus shifting towards more stable nuclei. The nuclide residing below the stability line decay such as to decreases

the number of protons. The neutron to proton ratio increases, thus shifting towards more stable nuclei.

 

Atoms with a different number of neutrons than a usual atom, with an unstable nucleus that decays, emitting alpha, beta and gamma rays until the isotope reaches stability. Once it's stable, the isotope becomes another element entirely. Radioactive decay is spontaneous so it's often hard to know when it will take place or what sort of rays it will emit during decay.

How many?

There are around 3800 radioactive isotopes. Atpresent there are up to 200 radioactive isotopes used on a regular basis, and while some are found in nature, most others have to be manufactured to suit specific needs, such as for hospitals, research labs and manufacturers.

How are they manufactured?

Radioactive isotopes can be manufactured in several ways, the most common by neutron activation in a nuclear reactor which involves capturing a neutron by the nucleus of an atom which results in an excess of neutrons (neutron rich). Some radioactive isotopes are produced in a cyclotron in which protons are introduced to a nucleus resulting in a deficiency of neutrons (proton rich).

Significance

Radioactive isotopes have very useful properties. Alpha, beta and gamma radiation can permeate solid objects like an x-ray, but are progressively absorbed by them. The amount of this penetration depends on several factors including the energy of the radiation, mass of the particle, and density of the solid. These properties can lead to many uses for radioisotopes in the scientific, medical, archaeological and industrial fields. The uses of radioactive isotopes in these fields depend on what element they become after they reach stability.

Radioisotopes has unstable nuclei that does not have enough binding energy to hold the nucleus together.

Radioisotopes would like to be stable isotopes so they are constantly changing to try and stabilize.

In the process, they will release energy and matter from their nucleus and often transform into a new element. This process, called transmutation, is the change of one element into another as a result of changes within the nucleus.

The radioactive decay and transmutation process will continue until a new element is formed that has a stable nucleus and is not radioactive. Transmutation can occur naturally or by artificial means.

The nucleus has too many protons which cause excessive repulsion.

In an attempt to reduce the repulsion, a Helium nucleus is emitted. The way it works is that the Helium nuclei are in constant collision with the walls of the nucleus and because of its energy and mass, there exists a nonzero probability of transmission. That is, an alpha particle (Helium nucleus) will tunnel out of the nucleus. Here is an example of alpha emission with americium-241:

   

Alpha Decay of Americium-241 to Neptunium-237. Adapted from Alpha Decay.

Beta decay occurs when the neutron to proton ratio is too great in the nucleus and causes instability. In basic beta decay, a neutron is turned into a proton and an electron. The electron is then emitted. Here's a diagram of beta decay with hydrogen-3:

Alpha Decay of Hydrogen-3 to Helium-3. Adapted from Stability of Nuclei.

There is also positron emission when the neutron to proton ratio is too small. A proton turns into a neutron and a positron and the positron is emitted. A positron is basically a positively charged electron. Here's a diagram of positron emission with carbon-11:

Positron Decay of Carbon-11 to Boron-11. Adapted from Types of Radioactivity.

 

The final type of beta decay is known as electron capture and also occurs when the neutron to proton ratio in the nucleus is too small. The nucleus captures an electron which basically turns a proton into a neutron. Here's a diagram of electron capture with beryllium-7:

Electron Capture of Beryllium-7. It decays to Lithium-7. Adapted from Electron Capture.

Gamma decay occurs because the nucleus is at too high an energy. The nucleus falls down to a lower energy state and, in the process, emits a high energy photon known as a gamma particle. Here's a diagram of gamma decay with helium-3:

Gamma Decay of Helium-3

The Decay Law, Decay Constant and Half life of radioactive Elements

Although the decay of radionuclides are random and spontaneous, they occur according to a certain law called the Decay law. The law is described by parameters such as the decay constant and the half-life of the particular nuclide.

Radioactivity and the disintegration theory.

The activity of a radioactive nucleus such as decays or disintegration into other nucleus is a spontaneous and random process. It means that, the process;

Cannot be controlled Cannot be predicted Is independent and not effected by any chemical

combination or physical conditions like temperature or presure.

Radioactivity only involves the nucleus of the atom and not any extra nuclear electrons (unlike chemical changes). It is a way for unstable nuclei to attain stability.

The disintegration process proceeds at a definite rate through a certain number of stages until it reaches a stable product.

The process releases energies depending on the type of particles emitted and the products of the disintegration.

Decay Law The laws for the radioactive decay physics given by the famous scientists Rutherford and

Fredrick Soddy. They both studied the radioactive decay experimentally. The laws for the radioactive decay are as follows:

  Radioactive decay phenomenon is a spontaneous process. Radioactive decay process

does not depend on the external factors like temperature, pressure etc. It is impossible to guess that which on the particular atom will decay in the particular interval of time.

In the process of radioactive decay of an atom, either an alpha particle or a beta particle is emitted. No any two particles emitted simultaneously . Even no two alpha or beta particles simultaneously. At one time only one particle is emitted.

The emission of an alpha particle from an atom causes the decrement of two in atomic number and of four in mass number in the parent atom.

ZXA                           Z–2 Y A – 4 + 2 He 4 (alpha particle)  The emission of a beta particle from an atom causes the increment in atomic number by one and

the mass number remains same. 

ZXA                          Z+1 Y A + -1e 0 (beta particle)  The number of atoms decayed per second at any instant is directly proportional to the number

of  atoms present in the sample at that instant. This law is also known as radioactive decay law. Thus, if in the sample the number of atoms is more, then the rate of decay is more and vice

versa.

Half-life of radioactive Elements There are a large range of half-lives seen in

nature Half-lives are unaffected by the nuclei’s

surroundings and only depend on what goes on inside the nucleus

238U has a half-life of 4.5 billion years To summarize, one-half of the sample will

decay in one half-fife One-half of that one-half will decay in the

next half life One-half of that one-fourth will decay in the

next half life

What are the uses of Radioisotope? Very useful in many fields regardless of

whether there are found naturally or produced artificially.

Artificially produced radioisotopes are short life (short half-life) but have greater activity comparatively. It is easy to produced and one can choose the suitable range and types of energy required for specific purposes.

Some of radioisotope uses employ the fact that they are capable of producing radiation which can be absorbed when they pass through matter.

In nuclear medicine, radioisotopes are used as tracer to identify location and concentration of certain effected cells by measuring the radition they emit.

Some of this uses are; Detecting underground pipe-line leakages such as

oil pipe-lines. Radioactive tracer in madicines, agriculture and

biological resear. In radiotherapty,gamma rays are used instead of the

expencive X-rays in the treatment of cancer. Gamma radiation used as sterilizing agents for

medical instumentation and bandages after packaging.

Dating archeological findings.

Radioisotope tracers

The use of radioisotopes are tracers depends on the ability of the radioisotopes to take part in the same prosess as its non-radioactive isptopes.

At the beginning, a selected radioisotope is injected into the patient’s body. After some times duration, the patient will be subjected to the body scan or any selected part of the body scans of the body.

As examples, gamma ray scans the body such as the brain-scan can map the concentration of the radioisotope in the patients.

The information on the activity of different part of the brain can be observed whereby tumor cells can be differentiated and identified.

Radiotracer such Iodine-131 as be used as tracer for tyroid diseases. Radioisotope phosphorus used as agricultural traers as provided

information on the suitability of the types phosphate fertilizers for particular crops and soil.

Carbon dating

Carbon dating is a variety of radioactive dating which is applicable only to matter which was once living and presumed to be in equilibrium with the atmosphere, taking in carbon dioxide from the air for photosynthesis.

Cosmic ray protons blast nuclei in the upper atmosphere, producing neutrons which in turn bombard nitrogen, the major constituent of the atmosphere . This neutron bombardment produces the radioactive isotope carbon-14. The radioactive carbon-14 combines with oxygen to form carbon dioxide and is incorporated into the cycle of living things.

The carbon-14 forms at a rate which appears to be constant, so that by measuring the radioactive emissions from once-living matter and comparing its activity with the equilibrium level of living things, a measurement of the time elapsed can be made.

What are the biological effect of Ionization Radiation?

Characteristics One characteristic of ionizing radiation on human

body is that the energy absorbed is low but the biological effects are serious. For example after receiving a lethal dose of 10 Gy, the body temperature will only increase by 0.02 oC but the dose may lead to death of all the exposed entities.

The second characteristic is the latent biological effects of radiation. Acute biological effects can occur within several hours to several days while the long term effects usually appear several years after the exposure.

Type of effects

Generally speaking, the biological effects of ionizing radiation can be classified according to the characteristics of effects, occurring times and the object that shows the effects.

Characteristic of effects Occurring time Object Effects on organs

Deterministic Effects

Acute Effects

Somatic Effects

Skin damageDamage of reproductive system

Damage of blood forming systemDamage of digestive system

Damage of central nervous system

Latent Effects

CataractDamage of immunization system

Stochastic Effects

Cancer

Genetic Effects Heredity effects

The effects of critical organs Different organs have different sensitivity to

ionizing radiation. For example, gonad and bone marrows are more sensitive organs, but the bones are less sensitive.