HALF-LIFE

It’s impossible to know exactly when an unstable atom will decay.

We can however predict how many will decay in a period of time.

A half-life is the time taken for half a group of unstable nuclei to decay.

Half-lives vary according to the isotope that is decaying – these can range from microseconds, to thousands of millions of years.

HALF-LIFE

HALF-LIFE

1st Half life –the time it takes for 50% of the nuclei to

decay

2nd Half life –the time it takes for

50% of the remaining nuclei to

decay

3rd Half life –the time it takes for

50% of the remaining nuclei to

decay4th Half life –

the time it takes for 50% of the

remaining nuclei to decay

The Half-life of an atom can be represented on a graph, known as a decay curve.

HALF-LIFE

The Half-life of an atom can be represented on a graph, known as a decay curve.

X

The y-axis shows the number of Californium-

252 atoms as a percentage

~2.645 yrs

To find the half-life, find 50% on the y-axis, ruling a line to the plot and match this up to the corresponding value

on the x-axis

HALF-LIFE

The Half-life of an atom can be represented on a graph, known as a decay curve.X

This tells us that the half-life of Californium-

252 is approx. 2.65 years

2.65

The second half-life (when only 25% remain un-

decayed – ie. Half of the remaining 50%) in this case, occurs in another

2.65 years, at approximately 5.3 years.

X

5.33

HALF-LIFE

X

2.65

The third half-life (when only 12.5% remain un-

decayed – ie. Half of the remaining 25%) in this case, occurs in another 2.65 years, at approximately

7.95 years.

X

5.33

7.95

X

The fourth half-life (when only 6.25% remain un-

decayed – ie. Half of the remaining 12.5%)

in this case, occurs in another 2.65 years, at approximately

10.6 years.

When does the fourth half-life occur?

X10.6

HALF-LIFE

HALF-LIFE

Use the decay curve to find:a) The Half-life of Uranium-235

b)The Second Half-life of Uranium-235

c) What fraction of the isotope will remain after 2840 million years?

d) What fraction of the isotope will remain after 4260 million years?

HALF-LIFE

Use the decay curve to find:a) The Half-life of Uranium-235

b)The Second Half-life of Uranium-235

c) What fraction of the isotope will remain after 2840 million years?

d) What fraction of the isotope will remain after 4260 million years?

710 million years

1420 million years

6.25%

1.5625%

NOW DO

HALF-LIFE ~ SIMULATION TASK

MEASURING DECAY

We can measure the ionising radiation of a

radioactive source using a Geiger counter.

• A Geiger counter detects Alpha, Beta and Gamma radiation.

• The common unit for measuring radioactive decay is Becquerel (Bq).

• Bq = number of decay’s per second.

http://atomic.lindahall.org/what-is-a-geiger-counter.html

MEASURING DECAY

Refer to the graph below, showing the decay curve of Thorium-234.

At the beginning when the decay is at large, the Geiger counter would of course be the most active, recording a

high count rate

Gradually decreasing over

time

So, if we measured the decay of a radioactive source as graphed it, it would be the same as

the decay curve

MEASURING DECAY

eg. A radioactive material is measured to have 600,000 decays per

second.

a) What is this equivalent to in Bq?

b) After 3 half-lives, what will the activity be in Bq?

600,000 Bq

One Half-life

Bq

Two Half-lifes

Bq

Three Half-lifes

Bq

CHAPTER ONE - Q 17; 23-29

NOW TRY