26 - particle and nuclear physics

8/18/2019 26 - Particle and Nuclear Physics

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Modern Physics

Marline Kurishingal

Particle and Nuclear Physics


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The results of the α-particle

scattering experiment&

the existence and small size of the

nucleus

Rutherford Alpha Particle Scattering

Experiment


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Rutherford Alpha Particle Scattering Experiment

• Rutherford‘s alpha particlescattering experiment changed

the way we think of atoms.

• Before the experiment the

best model of the atom wasknown as the Thomsonor "plum pudding" model. Theatom was believed to consistof a positive material"pudding" with negative"plums" distributedthroughout.


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Rutherford Alpha Particle Scattering Experiment(continued from previous slide)

• Rutherford directed beams of alpha particles (which are the nuclei ofhelium atoms and hence positively charged) at thin gold foil to test thismodel and noted how the alpha particles scattered from the foil.

Note : Diagram

is only for your

reference, its

not in syllabus


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•

Rutherford made 3 observations:

Most of the fast, highly charged alpha particleswent whizzing straight through un-deflected. This

was the expected result for all of the particles ifthe plum pudding model was correct.

Some of the alpha particles were deflected backthrough large angles. This was not expected.

A very small number of alpha particles weredeflected backwards! This was definitely not asexpected.


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• To explain these results a new model of the atom was needed.

• In this model the positive material is concentrated in a small butmassive (lot of mass - not size) region called the nucleus. Thenegative particles (electrons) must be around the outsidepreventing the atom from trespassing on its neighbours space tocomplete this model.

• The diagram in next slide will help you to understand the results ofthe experiment.


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Particles in the Atom


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2


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Recap…


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N

Note : This

equation is an

example for

balanced

equation. Do notconsider it for

alpha emission.


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-1


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• In alpha decay 2 protons and 2 neutronsare emitted. Notice that this reducesthe nucleon number by 4 and theproton number by 2. A new element isthus formed.

• In beta decay a neutron changes into aproton (which remains in the nucleus)and an electron (which is emitted asbeta radiation). The net effect is anincrease in proton number by 1, while

the nucleon number stays the same.Again a new element is formed.

• When a nucleus has undergone alpha orbeta decay it is often left in a high-energy (excited) state. This energy can

be lost in the form of an emitted agamma ray. Because the composition ofthe nucleus is unchanged no newelement is formed.

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Recap…


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Recap….


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26 - particle and nuclear physics

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