They are surrounded by negatively charged ions known as electrons.
The number of protons is equal to the number of electrons.
Both protons and neutrons have the same mass.
The weight of an electron is relatively small compared to neutrons and protons.
The number of protons in an atom is referred to as the proton number (atomic number) and denoted by the symbol Z.
The number of neutrons isdenoted by the symbol N.
Protons and neutrons are called nucleons since they form the nucleus of an atom. The sum of both the protons and neutrons is called the mass number A or nucleon number. Therefore; A = Z + N and N = A – Z.
The masses of atoms are conveniently given in terms of atomic mass units (ᴜ) where (ᴜ) is 1/12th the mass of one atom of carbon-12 and has a value of 1.660 × 10-27 kg.
Hence the mass of one proton is equal to 1.67 × 10-27 and is equal to 1ᴜ.
The rate of collapse of the leaf depends on the nature and intensity of radiation.
The radioactive source ionizes the air around the electroscope.
Beta particles discharges a positively charged electroscope with the negative charge neutralizing the charge of the electroscope.
Alpha particles would similarly discharge a negatively charged electroscope.
To detect both alpha and beta particles a charged electroscope may not be suitable because their ionization in air may not be sufficiently intense making the leaf not to fall noticeably.
The spark counter
the detector is shown below
This detector is suitable for alpha sources due to the inadequacy of the ionization by both beta and gamma radiations.
By putting the source away from the gauze or placing a sheet of paper between the two one can determine the range and penetration of the alpha particles.
Geiger Muller (GM) tube
it is illustrated as below
The mica window allows passage of alpha, beta and gamma radiations.
The radiations ionize the gas inside the tube.
The electrons move to the anode while the positive ions move to the cathode.
As the ions are produced there are collisions which produce small currents which are in turn amplified and passed to the scale.
The scale counts the pulses and shows the total on a display screen.
After each pulse the gas returns to normal ready for the next particle to enter.
A small presence of halogen gas in the tube helps in absorbing the positive ions to reduce further ionization and hence a quick return to normal. This is called quenching the tube.
The solid state detector
This detector can be used to detect alpha, beta and gamma radiations where the incoming radiation hits a reverse biased p-n junction diode momentarily conducting the radiation and the pulse of the current is detected using a scaler.
The diffusion cloud chamber
This chamber is simplified as shown below
The bottom of the chamber is cooled by solid carbon (V) oxide to around -80o C and the alcohol vapour from the felt ring spreads downwards.
It is cooled below its normal condensing temperature.
As a particle enters the chamber it ionizes the air in its path and alcohol condenses around the path to form millions of tiny alcohol droplets leaving a trail visible because it reflects light from the source.
Alpha particles leave a thick, short straight tracks . Beta particles leave thin irregular tracks. Gamma particles do not produce tracks and since they eject electrons from atoms the tracks are similar to those of beta particles.
The activity of a sample of radioactive element is the rate at which its constituent nuclei decay or disintegrate.
It is measured in disintegrations per second or Curie (Ci) units , where 1 Ci = 3.7 × 1010 disintegrations per second 1 micro Curie (μC) = 3.7 × 104 disintegrations per second.
The law of radioactive decay states that “the activity of a sample is number of undecayed nuclei present in the sample ”
The half-life of a radioactive element is the time required for its one-half of the sample to decay.
It is important to note that although the activity approaches zero, it never goes to zero.
The half-life of a sample of a radioactive substance is 98 minutes. How long does it take for the activity of the sample to reduce to 1/16 th of the original value?
Solution Time (minutes) Activity 0 1 98 ½ 196 ¼ 294 1/8 392 1/16 → time taken = 392 minutes.
An isotope has a half-life of 576 hours. Complete the following table and show how mass varies with time from an initial mass of 1280 g?
1152 320 g ------ 1728 160 g ------ 2304 80 g ------
The initial number of atoms in a sample is 5.12 × 10 20 . If the half-life of the sample is 3.0 seconds, determine the number of atoms that will have decayed after six seconds.
Solution After the first half-life, then ½(5.12 × 1020) = 2.56 × 1020 will have decayed. The second half-life, then ½(2.56 × 1020) = 1.28 × 1020 will have decayed. The total number of decayed atoms = (2.56 + 1.28) × 1020 = 3.84 × 1020 atoms.
A radioactive element has an initial count rate of 2,400 counts per minute on a scaler. The count rate falls to 300 units per minute in 30 hours,
Calculate the half-life of the element
If the initial number of atoms in another sample of the same element is 6.0 × 1020, how many atoms will have decayed in 50 hours?
Solution a) 2,400 × ½ × ½ × ½ = 300 Three half-lives have a total of 30 hours, thus half-life = 30/3 = 10 hours b) Since half-life = 10 hrs half-lives in 50 hrs = 50/10 = 5 hrs. So the remaining undecayed atoms are ½ × ½ × ½ × ½ × ½ × 6.0 × 1020 = 0.1875 × 1020, thus The number of atoms which have decayed = (6.0 – 0.1875) × 1020 = 5.812 × 1020
Nuclear fission is a process in which a nucleus splits into two or more lighter nuclei.
This process generates large amounts of energy together with neutron emission. Nearly 80% of the energy produced appears as kinetic energy of the fission fragments.
For example Uranium-235 undergoes nuclear fission when bombarded with slow neutrons releasing 2-3 neutrons per Uranium molecule and every neutron released brings about the fission of another Uranium-235 nuclei.
Another substance which undergoes the same process is Plutonium-239 .
Substances which undergo fission directly with slow neutrons are known as fissile substances or isotopes.
Due to the ionizing radiation emitted by radiation materials, they affect living cells leading to serious illnesses. Symptoms of radiation exposures are immature births, deformations, retardedness, etc.
Their exposure to the environment through leaks may lead to environmental pollution leading to poor crop growth and destruction of marine life.
Through the identification of carbon-14 and carbon-12 absorbed by dead plants and animals.
Scientists can be able to estimate the age of a dead organism.
Since carbon is a radioactive element with a half-life of 5,600 years archeologists can be able to estimate the ages of early life through carbon dating.
Radiation is used in the treatment of cancer, by using a radioactive cobalt-60 to kill the malignant tissue.
Radiations are used in taking x-ray photographs using cobalt-60. Radiations are used to sterilize surgical instruments in hospitals.
Radioactive elements can also be used as tracers in medicine where they determine the efficiency of organisms such as kidneys and thyroid glands.
Biology and agriculture
Radioactive sources are used to generate different species of plants with new characteristics that can withstand diseases and drought.
Insects are sterilized through radiation to prevent the spread of pests and diseases.
Potatoes exposed to radiation can be stored for a long time without perishing.
Thickness of metal sheets is measured accurately using radiation from radioactive sources.
Recently the manufacture of industrial diamonds is undertaken through transmutation
In N. America, Europe and Russia nuclear reactors are used to generate electricity. The amount of fuel used is quite small hence an economical way of generating electricity energy as compared to H.E.P generation.