7.2.1 Activity and Decay | Edexcel International GCSE Science Double Award 4SD0: Physics

Section 1 of 9Unstable nuclei and radioactive decay

Most atoms have stable nuclei, but some nuclei have too many protons, too many neutrons or are simply too large.

Unstable nuclei

These unstable nuclei can become more stable by emitting radiation.

Radioactive decay

The process is called radioactive decay.

After the decay the nucleus is a different nucleus (or the same nucleus with less energy), and the radiation carries the surplus energy away.

Radioactive decay is spontaneous: it is not caused by heating, pressure, chemical reactions or any other outside influence.

Random

It is also random: we can never say when one particular nucleus will decay, only how likely it is to decay in a given time.

With a very large number of nuclei, though, the overall behaviour becomes predictable,

which is what makes half-life a useful idea.
- (see the next lesson)

Section 2 of 9Types of emission

Three kinds of ionising radiation are emitted by radioactive nuclei.

Radiation	What it is	Charge	Relative ionising power	Stopped by
Alpha (α)	Helium nucleus: 2 protons + 2 neutrons	+2	Strongest	A few cm of air, or paper
Beta (β⁻)	Fast electron from the nucleus	−1	Medium	A few mm of aluminium
Gamma (γ)	Electromagnetic wave	0	Weakest	Thick lead or many metres of concrete

Alpha particles are heavy and highly charged, so they ionise strongly and lose their energy over a short range. Gamma rays are uncharged and ionise only rarely, so they travel a long way before being absorbed.

Section 3 of 9Nuclear equations

A nucleus is written as ^A_ZX, where A is the nucleon (mass) number (protons + neutrons) and Z is the proton (atomic) number. In any nuclear equation, both the top numbers and the bottom numbers must balance on the two sides.

Alpha decay

Alpha decay the nucleus loses 2 protons and 2 neutrons. A falls by 4, Z falls by 2. Example: ²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂α.

Beta-minus decay

Beta-minus decay a neutron in the nucleus changes into a proton and an electron; the electron is emitted.

A is unchanged, Z rises by 1.
Example: ¹⁴₆C → ¹⁴₇N + ⁰₋₁β.

Gamma emission

Gamma emission the nucleus is left with excess energy after an alpha or beta decay and releases it as a gamma ray. A and Z do not change at all.

Remember that the emitted beta particle did not exist inside the nucleus beforehand; it is created at the moment the neutron changes.

Section 4 of 9Measuring activity and count rate

The activity of a source is the number of nuclei that decay each second. It is measured in becquerels (Bq), where 1 Bq = 1 decay per second. A source of 5 kBq has 5000 nuclei decaying every second.

A Geiger-Müller (GM) tube connected to a counter detects radiation that enters the tube and records counts. The count rate is the number of counts per second (or per minute). Because the tube does not intercept all the radiation (some goes in other directions and some passes through unnoticed), the count rate is always less than the true activity.

Count rate (counts per second) = number of counts ÷ time (s).

Section 5 of 9Background radiation

Even with no source nearby a GM tube clicks occasionally. This background radiation comes from natural sources: radon gas seeping from rocks, building materials, food and drink, cosmic rays from space, and a small contribution from medical procedures and fallout. Levels vary from place to place, for example higher in regions with a lot of granite.

Background radiation

When measuring a source you must first measure the background count rate with the source removed (ideally over a long time and repeated),

then subtract it:

corrected count rate = measured count rate − background count rate.

Without this step the source would seem more active than it really is, and a weak source could appear to show no decay at all.

Section 6 of 9Decay and the changing activity

As nuclei decay they are used up, so fewer undecayed nuclei remain and the activity of a sample falls with time.

Decay and the changing activity

The decrease is not a straight line: the activity drops quickly at first, when there are many nuclei, and more slowly later, when few are left.

The graph of activity against time is a curve that becomes shallower and approaches zero without ever quite reaching it.

This shape is the key to the idea of half-life.

Linkthe penetrating power of the three radiations (above) decides which detector or absorber is suitable, and later lessons on uses and dangers build directly on this.

Section 7 of 9Worked examples

A GM tube records 1260 counts in 3.0 minutes when a source is placed close to it. With the source removed, it records 135 counts in 3.0 minutes. Find the corrected count rate in counts per second.
Radium-226 (²²⁶₈₈Ra) decays by emitting an alpha particle. Write the equation and identify the product nucleus.
A sample has an activity of 3.0 kBq. How many nuclei decay in 20 seconds, assuming the activity stays constant over this short time?

Section 8 of 9Key terms

Term	Meaning
Activity	Number of nuclei in a source that decay each second; measured in becquerels (Bq).
Becquerel (Bq)	The unit of activity: 1 decay per second.
Count rate	Number of counts registered by a detector each second or minute.
Background radiation	Ionising radiation always present from natural and artificial sources.
Random	Cannot be predicted for an individual nucleus; only the probability is known.
Nucleon number (A)	Total number of protons and neutrons in a nucleus.
Proton number (Z)	Number of protons in a nucleus; defines the element.

Section 9 of 9Exam technique

1

Thinking decay can be sped up or slowed down by heating or chemical reaction. It is spontaneous and unaffected by physical conditions.

2

Forgetting to subtract background radiation. The reading with a source is source plus background.

3

Saying that an alpha particle is an atom or a hydrogen nucleus. It is a helium nucleus (2 protons + 2 neutrons).

4

In beta decay, changing the nucleon number. It stays the same; only the proton number rises by 1.

5

Believing a decaying nucleus 'knows' when it will decay. For a single nucleus the timing is random; only large samples are predictable.