Nuclear Physics, Unit 4 1

UNIT 4 – NUCLEAR PHYSICS:

Atomic model: The Atomic model or Bohr´s atomic orbital model is the currently the accepted model of an atom. In this model, the atom consists of a central nucleus surrounded by orbiting electrons. In nucleus there are protons and neutrons. The sum of protons and neutrons in a nucleus is called nucleon number. The electrons exist in atomic orbits, moving around the nucleus. These orbits are set of energy states for the negatively charged electrons trapped in the electrical field generated by the positively charged nucleus.

Approximate mass

Size Charge Position in an atom

Proton 1 amumu 10-15m Positive (+e) In the nucleus

Electron 1/2000th of amu Very small Negative (-e) In the nucleus

Neutron 1u 10-15 m Neutral In orbits

1 amu= 1.66 × 10-27 kg, 1 e= 1.6 × 10-19Coulumbs,

Scattering of alpha particle: Ernest Rutherford (1871-1937) used α-particles to make it fall on the thin gold foil. He found that most of the α-particles were un-deflected, some were scattered by appreciable angle and a few about 1 in 8000 surprisingly bounced back. To explain these results, in 1911 Rutherford proposed a ‘nuclear model’ of atom:

‘Most of the space in an atom is empty. All positive charge and most of the mass of an atom formed a dense core called nucleus of very small, size compare to the size of atom. The electrons surrounding the nucleus some distance away from the nucleus.’

A length of a football pitch and diameter of a pea are in the same ratio as an atom and a nucleus.

The atomic or proton number Z of an atom is the number of proton in the nucleus. It is also the number of electron in the atom when atom is neutral.

N is the number of neutrons in an atom. Nucleon is the total number of neutrons and protons. Nuclide is one type of nucleus which has particular number of protons and neutrons. In general A=Z+N where A is the nucleon number or mass number of an atom General symbol for representing atom is

푋 for example 퐶, 푂, 푁

AS Physics 9702 – Notes, prepared by Faisal Jaffer, revised on 7-Feb-10 2

Radioactivity: The process of breaking down or decaying of

unstable nuclei to more stable form of nuclei, by emitting radiations is called radioactivity. It is random and spontaneous process.

The random process means that one cannot predict which atom is going to emit the radiation at any particular time. On decay curve the random process can be detected by the fluctuation of the points.

The spontaneous process means it is not affected by any external factor such as temperature and pressure etc.

The material that emits radiation is called radioactive material, for example uranium, radium, polonium etc.

Three common types of emissions are alpha particles, beta particles and gamma rays.

Properties of radiations: There are three types of radiation that any radioactive substance can emit. They are alpha particles (α-particles), beta particles (β-particles) and gamma rays (γ-rays).

alpha particles Beta particles Gamma radiations

Symbol α β γ Nature helium ion high energy electrons electromagnetic radiation Charge positive 2e negative 1e no charge Mass 4 times the amu* 1/2000th of amu Waves/photons Speed after emission 0.1 of speed of light 0.6 of speed of light Same as speed of light Energy of emission in the range 106 ev in the range of 106 ev 0.1×105 ev to 10×106 ev Ionization effect in gases

intense ionization effect due to frequent collision with gas molecules

much less ionization effect than alpha rays

least ionization effect of all three

Penetration effect stopped by the thick sheet of paper and have range of few millimeter in air

range of few millimeters in aluminum and several meters in air

very high penetration effect. Can only be stopped by thick sheet of lead.

Deflection deflected by electric and magnetic field

deflected by electric and magnetic field

not deflected by electric or magnetic field

amu means atomic mass unit

Nuclear Physics, Unit 4 3

Alpha Radiation (α):

In alpha-radiation the unstable parent nuclei decay into stable nuclei by emitting alpha particle which is helium nuclei and which means two protons and two neutrons. The parent X nuclei changes into Y nuclei.

푋 → 푌 + 퐻푒 + 푒푛푒푟푔푦

(A is the mass number and Z is the proton number)

Example: when the radium (Ra) of mass number 226 and proton number 88 emits α-particles, decay to radon (Rn) of mass number of 222 and proton number of 86.

푅푎 → 푅푛 + 퐻푒

The value of A and Z must be balance on both sides of the equation.

Beta Radiation (β):

In beta radiation, which involves the emission of electrons, a neutron in the parent nucleus changes to proton and electron. The electron escapes in the form of beta-particle.

푋 → 푌 + 푒 + 푒푛푒푟푔푦

Example: Radioactive carbon -14 (C), decays by emitting β-particle and changes into nitrogen-14 (N).

퐶 → 푁 + 푒

Gamma Radiation (γ):

The gamma radiation represents simple loss of energy from the nuclei.

푋 → 푋 + 훾 + 푒푛푒푟푔푦

Background Radiation: It is the low intensity radiation present in the earth’s surface and in the atmosphere. A radioactive material exists in nearly all the rocks of the earth’s crusts, which gives off radiations. The cosmic radiation that reaches the earth from the sky also contributes to the background radiation. When detecting and measuring the radiation from radioactive substance it is important to consider the background radiations in our calculation. The radiation emission from the radioactive source is obtained by subtracting the background radiation from the actual reading on radiation counter.

Uses of radioactivity: 1. Gamma rays are used to kill bacteria specially in sterilizing medical equipment and

in preserving food. 2. Alpha particles are used in smoke alarm. 3. Beta particles are used to monitor the thickness of the paper or metal in

manufacturing factory. 4. Carbon-14 is used to find the age of living organism or plants. This method is called

radioactive carbon dating. 5. Uranium-238 which eventually decays into lead is used to find the age of igneous

rock. 6. In radiotherapy the high doses of gamma radiation are used to kill the cancer cells

AS Physics 9702 – Notes, prepared by Faisal Jaffer, revised on 7-Feb-10 4

Safety precautions in using radioactive substance: 1. Always use the radioactive symbol where there is a radioactive

substance stored; 2. always be stored in a lead-lined container; 3. be handled only with tongs; 4. never be pointed at anyone; 5. never be put in pockets; 6. should only be checked by looking at them in a mirror.

Nuclear reactions: Chemical reactions involve atomic electrons and nuclear reactions involve atomic nucleus. In chemical reaction the number of atoms conserve before and after the reaction. In nuclear reaction the charge and number of nucleons conserve and in this reaction a lot more energy is released compare to the chemical reaction. The difference of masses before and after reaction converts to energy which can be calculated by formula

∆퐸 = ∆푚푐

There are two types of nuclear reactions, fission reaction and fusion reaction.

A fission nuclear reaction is the splitting of a heavy nucleus into two or more lighter nuclei for example U-235 is bombarded with slow neutrons to produce barium (Ba-139), krypton (Kr-94) and 2 or 3 fast moving neutrons

푈 + 푛 ⟹ 퐵푎 + 퐾푟 + (2 표푟3) 푛 + 푒푛푒푟푔푦

The energy released during this reaction is about 3.20×10-11J per atom but in 1 kilogram of uranium this energy release is about 8.19×1013J!

The other 2 or 3 neutrons are produce during each reaction can further react with more uranium atoms and emit more energy. This type of reaction is called chain reaction.

In nuclear fusion reaction, two smaller nuclei join together to form a heavier nucleus. For example when two deuterium nuclei (the isotopes of hydrogen) are combined together they form one helium (He-3) nucleus.

퐻 + 퐻 → 퐻푒 + 푛 + 푒푛푒푟푔푦 If we calculate the masses before and after the reaction we will find that there is a difference of 0.004 amu or 6.68×10-30kg (1 amu = 1.6606 x 10-27 kg). If we convert this mass into energy then we get 3.27×106ev or 3.04×10-15J or energy per atom (1 eV = 1.6 x 10-19 J).

There are 6.023×1023 atoms in one kilogram and therefore total energy released in one kilogram of deuterium is 9.12×1013J which is a lot!!!

Nuclear Physics, Unit 4 5

Isotopes: Isotopes of an element are atoms which have

the same number of protons but different number of neutrons.

Isotopes have identical chemical properties since they have same number of electrons and occupy the same place in periodic table. In Greek the ‘isos’ means same and ‘topos’ means place.

Hydrogen has three isotopes 퐻 with one proton, dutreium 퐷 one proton and one neutron, and 푇 tritium has one proton and two neutrons.

Each form of element is called nuclide. Nuclides with same Z but different A are the isotopes. Radioactive isotopes are termed radioisotopes or radio nuclides, their nuclei are

unstable.

Detection of Electron beam in magnetic and electric field:

By magnetic field: An electron beam (red arrow towards right) entering the magnetic field (B) a right angles to the field experience force due to the magnetic field around it. The direction of the deflection is given by the ‘left hand rule’ method. (left hand method is for the beam of positive charges and right hand method is for negative charges).

In figure the dots represent the direction of magnetic field coming outside the paper and positive charges moving towards right perpendicular to the direction of field. The charges experience force F perpendicular to the field and the direction of motion of the electron.

By electric field: An electric field is a region where an electric charge experiences force. In the figure the two metal plates behave like a capacitor that has been charged by connecting to the voltage supply. A uniform electric field is created between them and is represented by equally spaced lines going from positive plate to negative plate. When the beam of negatively charge electrons enters the electric field perpendicular to it, it attracts towards the positively charged plate and follows a curved path.