Stable and Unstable nuclei

 

Most isotopes have the right numbers of protons and neutrons to be stable.  However some isotopes are unstable.  This results from the nucleus:

Radiation is the process by which an unstable parent nucleus becomes more stable by decay into a daughter nucleus by emitting particles and/or energy.  The basic form of radioactive decay can be summed up in this diagram:

The decay can consist of several steps.  The unstable nucleus decays to another nucleus of a different atom by a process called transmutation.  If the new nucleus is unstable it will decay again.  This is known as a decay chain.  There may be several steps, some of which last a very long time indeed, or can be very short.  Some elements have a decay time of thousands of millions of years.  In others the decay time can be microseconds.  Whichever, the process is entirely random.  If you watch an individual nucleus, the decay may occur in 10 seconds, or several million years.  There is nothing whatever you can do to speed up the process.

Elements have different isotopes.  An element and its isotope have:

If the isotope is unstable, it is radioactive and is called a radioisotope

There are three kinds of radiation:

 

These kinds of radiation can be emitted individually or in any combination, depending on the type of isotope that is emitting the radiation.  Often when an alpha particle is emitted the nucleus is excited and releases the excess energy in the form of a gamma ray or gamma photon.

When specimens of radioactive isotopes decay they do so entirely randomly.  There is no pattern whatsoever, and the rate of decay is not affected by temperature or other physical factors, or chemical reactions.  This means you cannot speed it up by hitting the material, heating it strongly, or reacting it with strong acids.

The table helps us to compare the properties of radiation

 

Radiation

Description

Penetration

Ionisation

Effect of E or B field

Alpha (a)

Helium nucleus

2p + 2n

Q = + 2 e

Few cm air

Thin paper

Intense, about 104 ion pairs per mm.

Slight deflection as a positive charge

Beta (b)

High speed electron

Q = -1 e

Few mm of aluminium

Less intense than a, about 102 ion pairs per mm.

Strong deflection in opposite direction to a.

Gamma (g)

Very short wavelength em radiation

Several cm lead, couple of m of concrete

Weak interaction about 1 ion pair per mm.

No effect.

 

We will look at the mechanisms of production of alpha and beta radiations later.

 

Radiation Protection

We need to be aware that elements with unstable nuclei can be harmful to living organisms.

People working with radioactive materials must take precautions to ensure their safety, such as:

Work with highly radioactive materials is carried out remotely behind walls that are 2 metres thick.

In a school physics lab, the sources are VERY weak, but you must always follow the rules in their handling.  Students under 16 are not allowed to handle radioactive materials.

 

Possible modes of decay for unstable nuclei

Alpha radiation (a) mostly comes from heavy nuclides with proton numbers greater than 82, but smaller nuclides with too few neutrons can also be alpha emitters.  The term Q stands for the energy.  The animation shows the idea:

 

 

The general decay equation is summarised below.

 

 

 

 

  We should note the following:

 

 
Question 1

  Is this equation balanced?  Explain your answer.

Answer

 

 

Alpha particles are intensely ionising.  They smash through air molecules, knocking off electrons as they go.  However this reduces the kinetic energy, so that in the end they stop.  Then they pick up a couple of free electrons to become helium atoms.  To collect an appreciable sample of helium from an alpha emitter would take a very long time.

 

Neutron rich nuclei tend to decay by beta minus (b-) emission.  The beta particle is a high-speed electron ejected from the nucleus, NOT the electron clouds.  It is formed by the decay of neutrons, which are slightly more energetic than a proton.  Isolated protons are stable; isolated neutrons last about 10 minutes.

 

 

 

Watch how the neutron suddenly emits an electron (blue) and the electron anti-neutrino (black), turning into a proton.

 

The neutron, having emitted an electron, is converted to a proton, and this results in the proton number of the nuclide going up by 1.  A new element is formed.  The reaction at the nucleon level is:

 


Notice that as well as the neutron (n) and the proton (p), the beta particle is represented as an electron (e).  The strange symbol ne (‘nu-bar e’) is a strange little particle called an electron antineutrino.  The general equation for b- decay is:

 


A typical decay is:

 

 

 

Notice that:

 

Beta Plus decay

There is another kind of decay, beta plus decay.  In this case, the nucleus has too many protons, and gets rid of the excess charge by turning a proton into a neutron.  This is rare in nature, but proton-rich unstable nuclei are found in reactors.

 

 

The beta plus decay spits out a positively charged particle called a positron.  The positron is an anti-particle to the electron.  It has the same mass, but the opposite charge to the electron.  The other particle emitted is an electron neutrino.

 

Question 2

What is the balanced nuclear equation for the following decays?

(a)    emission of a beta- particle from oxygen 19

(b)   emission of an alpha particle from polonium 212

(c)    emission of a beta + particle from cobalt 56

 

Proton numbers O – 8, F – 9, Fe – 26, Co – 27, Pb – 82, Po – 84

Answer

 

 

Summary

A graph of neutron number against proton number shows that there are more neutrons in larger nuclei

 

This is needed to ensure stability of the nuclei.

 

Natural decay occurs with alpha decay

 

Or beta minus decay.