Nuclear Physics

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** Atom and Introduction to Nuclear physics **

Introduction

Alpha-particle scattering

2.1 Experimental set-up

➢ Alpha particle
o is a high-energy helium nucleus with 2 protons and 2 neutrons. o has 2 positive charges.
➢ Gold foil was chosen since it has a high atomic mass. Thin gold foil, ~10-6m was bombarded by high-energy alpha particles. ➢ Angular deflections/ scattering of alpha particles were measured by observing the scintillation when alpha particles struck on the zinc sulphide screen on the turntable microscope. No. of bright spots vs. angles was measured. ➢ The experiment was carried out in an evacuated container to avoid collision between alpha particles and air molecules.

Fig 1 : Expérimental set up

Fig 2 : Trajectory of (-particles around a nucleus
2.2 Observations & conclusions

|Observations |Explanation | |Most of the alpha particles, ~90%, passed through the foil | | |undeflected. | | | | | | | | | | | |A small number of particles, 1 in 800 particles, were | | |scattered considerably at large angles, deflected by 90o - | | |180o | | |A tiny portion, 1 in 20000 particles was deflected backwards,| | |i.e. at 180o | | | | |

Graph of no. of scintillations vs. angles

Rutherford’s simple model of atom

i. An atom consists of neutron, proton and electron.
ii. Nucleus is a small, dense, positive charge in the centre of an atom that contains protons and neutrons. iii. Neutron and proton are in the nucleus and make up most of the mass of the atom - collectively known as nucleons. Electrons are orbiting the nucleus as shown in Fig. 3.

| |Neutron |Proton |Electron | |Relative mass/ u |1 |1 |1/1850 | |u=1.66x10-27C | | | | |Charge/ e=1.6x10-19C | | | |

Notation of nuclide [pic]

| |A |Z |N |
|[pic] | | | |
|[pic] | | | |
|Electron | | | |
|Proton | | | |
|Neutron | | | |
|(-particle | | | |
|(-particle | | | |
|Helium | | |...
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