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Nuclear and Atomic Physics

The nature of the atom

e.g. carbon-12: 6 protons, 6 neutrons, 12C

m = atomic mass unit = 1/12 mass of C-12

 

"A chemical element is a substance that cannot be broken down into anything simpler"

"An atom is the smallest identifiable part of a chemical element"

The nature of radiation

Detecting Radiation

1.       Scintillation screen

Emits a pulse of light when ionising radiation falls on it- electronic from called scintillation counter is very accurate

2.       Cloud chamber

Ionisation leaves a trail of droplets in supersaturated ethanol vapour

3.       Ionisation chamber

Ionised air molecules form a current between the outer casing and the central electrode (needs amplifying)

4.       Geiger-Müller tube

As above but contains a gas in which the avalanche effect occurs

5.       Bubble chamber

Superheated liquid boils to form bubbles along the path of high-energy subatomic particles

Activity

The activity of a source is the rate of disintegration of it with time. One disintegration per second is one Bequerel (Bq). The energy absorbed is the dose in grays (Gy). The dose equivalent in sieverts (Sv) represents the potential damage to living tissues by radiation.

Background radiation must always be taken into account for experiments into radioactivity.

Exponential Decay

dN = -lN            where l is the decay constant

dT

The solution is:

N = N0e-lT

 

If t = t½, N = N0/2

N  = 1 = e-lt

N0    2

Since ln(½) = -ln2

t½ = ln2

        l

Physical Changes

Nuclear emission carries energy away from an atom, making it more stable. The change from one element to another is called transmutation. A second emission, especially gamma, often follows the first, producing a decay series.

The binding energy of a nucleus is the energy that would be released if the nucleus were brought together from infinity. The binding energy is carried away by a reduction in mass called the mass defect.

D E = D mc2

The nature of the electron

½mv2max = hf-f

l = h/p       where h is Plank's constant and p the momentum of the electron.