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Spontaneous Nature of Radioactive Decay

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Matthew Broadberry

on 10 March 2015

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Transcript of Spontaneous Nature of Radioactive Decay

Spontaneous Nature
of Radioactive Decay
Radioactive decay is completely random. You can't predict which atom will decay when.

Although you can't predict the decay of an individual atom, if you take a very large number of atoms, their overall behavior shows a pattern.
There is equal probability of any nucleus decaying, It cannot be known which particular nucleus will decay next.

It is only possible to estimate the proportion of nuclei decaying in the next time interval.
Non Dependent on time
The chance that a given atom will decay never changes, that is, it does not matter how long the atom has existed.

This chance is called the decay constant.
Any sample of a particular isotope has the same rate of decay i.e. the same proportion of atoms will decay in a given time.
Nuclear Decay
Rate of Decay
The activity of the sample (number of atoms that decay per second) is proportional to the size of the sample.
The decay constant lamda measures how quickly an isotope will decay - the bigger the value of lamda, the faster the rate of decay. Rate measured in s^-1
activity = lamda X Number of Atoms
Activity is measured in Becquerels (1 decay per second) (s^-1).
Definition of Half life.

The half life of an isotope is the average time it takes for the number of undecayed atoms to halve.
Measuring the number of un-decayed atoms is difficult. In practice, half life isn't measured by counting atoms, but by measuring the time it takes the activity to half.
The longer the half-life of an isotpe,
the longer the it stays radioactive.
The decay constant and half-life are related by the equation:
decay constant = ln(2)/(1/2)halflife
If you have 1 nucleus, you don't know
when it will decay.


If you have a range of nuclei, you dont
know which one will decay next.
Random/Spontaneous Decay
Interesting extra stuff

Exam Questions
Explain what is meant by
the random nature of
nuclear decay. (1)
Radioactivity involves the spontaneous decay of unstable nuclei leading to the emission of radiation.
Explain the meaning of the term;
Spontaneous decay. (1)

Every Isotope Decays at a Different Rate
Why decay is random
In terms of the energy needed for an alpha particle to be emmited from a nucleus, the energy is greater than all the potential energy in the nucleus! Therefore in order for it to escape it ‘Quantum Tunnels’ out.
In Practice
Linking Decay Constant and Half-Life
Typical exam Question...
Full transcript