Fundamental particles?
Chemistry is very complicated because there are literally billions of different molecules that can exist. The discovery of the Periodic Table simplified things because it suggested that there were roughly 92 different elements whose atoms could be arranged to make these various molecules. The idea that atoms are made up of just three types of particle (protons, neutrons and electrons) seemed to simplify things still more, and scientists were very happy with it because it seemed to provide a very simple explanation of a complex world. Protons, neutrons and electrons were thought of as fundamental particles, which could not be subdivided further.
However, in the middle decades of the 20 th century, physicists discovered many other particles that did not fit this pattern. They gave them names such as pions, kaons, muons, etc., using up most of the letters of the Greek alphabet.
These new particles were found in two ways:
by looking at cosmic rays, which are particles that arrive at the Earth from outer space
by looking at the particles produced by high-energy collisions in particle accelerators (Figure 16.9).
Figure 16.9 Particle tracks in a bubble chamber detector. A particle has entered from the left and then struck another particle just to the right of the centre. Four new particles fly out from the point of impact.
The discovery of new particles with masses different from those of protons, neutrons and electrons suggested that these were not fundamental particles. Various attempts were made to tidy up this very confusing picture. In principle, we can never know for certain whether a particle such as the electron is truly fundamental; the possibility will always remain that a physicist will discover some deeper underlying structure.
Families of particles
Today, sub-atomic particles are divided into two families:
- Hadrons such as protons and neutrons. These are all particles that are affected by the strong nuclear force.
- Leptons such as electrons. These are particles that are unaffected by the strong nuclear force.
The word ‘hadron’ comes from a Greek word meaning ‘bulky’, while ‘lepton’ means ‘light’ (in mass). It is certainly true that protons and neutrons are bulky compared to electrons.
At the Large Hadron Collider (Figure 16.10) at the CERN laboratory in Geneva, physicists are experimenting with hadrons in the hope of finding answers to some
Figure 16.10 Particle accelerators have become bigger and bigger as scientists have sought to look further and further into the fundamental nature of matter. This is one of the particle detectors of the Large Hadron Collider (LHC), as it was about to be installed. The entire collider is $27 \mathrm{~km}$ in circumference.
fundamental questions about this family of particles. In 2013, they announced the discovery of the Higgs boson, a particle which was predicted 50 years earlier and which is required to explain why matter has mass.
Inside hadrons
To sort out the complicated picture of the hadron family of particles, Murray Gell-Mann in 1964 proposed a new model. He suggested that they were made up of just a few different particles, which he called quarks.
Figure 16.11 shows icons used to represent three quarks, together with the corresponding antiquarks. These are called the up (u), down (d) and strange (s) quarks. Gell-Mann’s idea was that there are two types of hadron: baryons, made up of three quarks, and mesons, made up of two quarks. In either case, the quarks are held together by the strong nuclear force. For example:
- A proton is made up of two up quarks and a down quark; proton $=$ (uud) .
- A neutron is made up of one up quark and two down quarks; neutron $=$ (udd).
- A $pi^{+}$ meson is made up of an up quark and a down antiquark; pi+ meson $=$ (ud).
- A phi meson is made up of a strange quark and an antistrange quark; phi meson $=($ ss $)$.
Antiquarks are shown with a ‘bar’ on top of the letter for the quark. Antiquarks are needed to account for the existence of antimatter. This is matter that is made of antiparticles; when a particle meets its antiparticle, they annihilate each other, leaving only photons of energy.
Figure 16.11 Icons representing three ‘flavours’ of quark, up, down and strange, and their antiquarks.
Discovering radioactivity
The French physicist Henri Becquerel (Figure 16.12) is credited with the discovery of radioactivity in $1896 . \mathrm{He}$ had been looking at the properties of uranium compounds when he noticed that they affected photographic film – he realised that they were giving out radiation all the time and he performed several ingenious experiments to shed light on the phenomenon.
Figure 16.12 Henri Becquerel, the discoverer of radioactivity, in his laboratory. His father and grandfather had been professors of physics in Paris before him.
Radiation from radioactive substances
There are three types of radiation which are emitted by radioactive substances: alpha $(a)$, beta $(\beta)$ and gamma $(\gamma)$ radiations come from the unstable nuclei of atoms. Nuclei consist of protons and neutrons, and if the balance between these two types of particles is too far to one side, the nucleus may emit $\alpha$ – or $\beta$-radiation as a way of achieving greater stability. Gamma-radiation is usually emitted after a or $\beta$ decay, to release excess energy from the nuclei.
In fact, there are two types of $\beta$-radiation. The more familiar is beta-minus $\left(\beta^{-}\right)$radiation, which is simply an electron, with negative charge of $-e$. However, there are also many unstable nuclei that emit beta-plus ( $\left.\beta^{\prime}\right)$ radiation. This radiation is in the form of positrons, similar to electrons in terms of mass but with positive charge of $+e$. Positrons are a form of antimatter. When a positron collides with an electron, they annihilate each other. Their mass is converted into electromagnetic energy in the form of two gamma photons (Figure 16.13).
Figure 16.13 Energy is released in the annihilation of matter and antimatter.
Table 16.4 shows the basic characteristics of the different types of radiation. The masses are given relative to the mass of a proton; charge is measured in units of $e$, the elementary charge.
Table 16.4 The basic characteristics of ionising radiations.
Note the following points:
Discovering neutrinos
There is a further type of particle which we need to consider. These are the neutrinos. When $\beta$ decay was first studied, it was realised that $\beta$-particles were electrons coming from the nucleus of an atom. There are no electrons in the nucleus (they ‘orbit’ outside the nucleus), so the process was pictured as the decay of a neutron to give a proton and an electron.
It was noticed that $\beta$-particles were emitted with a range of speeds – some travelled more slowly than others. It was deduced that some other particle must be carrying off some of the energy and momentum released in the decay. This particle is now known as the antineutrino (or, more correctly, the electron antineutrino), with symbol $\overline{\text { v}}$. The decay equation for $\beta^{-}$decay is written as:
Neutrinos are bizarre particles. They have very little mass (much less than an electron) and no electric charge, which makes them very difficult to detect. The Austrian physicist Wolfgang Pauli predicted their existence in 1930, long before they were first detected in 1956.
In $\beta^{+}$decay, a proton decays to become a neutron and an electron neutrino (symbol $v$ ) is released:
beta-plus $\left(\beta^{+}\right)$decay: $\quad{ }_1^1 \mathrm{p} \rightarrow{ }_0^1 \mathrm{n}+{ }_{+1}^0 \mathrm{e}+v$
The two equations highlighted above show two important features of radioactive decay. Firstly, nucleon number $A$ is conserved; that is, there are as many nucleons after the decay as there were before. In $\beta^{-}$decay, a neutron has become a proton so that the total number of nucleons is unchanged. In $\beta^{+}$decay, a proton becomes a neutron, so again $A$ is conserved.
Secondly, proton number $Z$ is also conserved. In $\beta$ decay, we start with a neutron $(Z=0)$. After the decay, we have a proton $(Z=+1)$ and a $\beta^{-}$particle $(Z=-1)$. Together these have $Z=1-1=0$. Since $Z$ tells us about the charge of each particle, we would be surprised if we had a different amount of charge after the decay than before the decay. A similar analysis shows that $Z$ is conserved in $\beta^{+}$decay.
Do these conservation laws apply to $\alpha$ decay? Here is an equation that represents a typical $a$ decay:
$
{ }_{86}^{222} \mathrm{Rn} \rightarrow{ }_{84}^{218} \mathrm{Po}+{ }_2^4 \mathrm{He}
$
In $\alpha$ decay, an alpha particle (two protons and two neutrons) is emitted by a nucleus. Although these nucleons are now outside the nucleus, the equation shows that there is the same number of nucleons after the decay $(218+4)$ as before the decay (222). So nucleon number $\mathrm{A}$ is conserved. Similarly, proton number $Z$ is conserved $(84+2=86)$.
The conservation of nucleon number and proton number are important laws in nuclear physics. They apply to all nuclear changes, not just to $\alpha$ and $\beta$ decay.
There is a third quantity that is conserved. You might expect mass to be conserved, but this is not so. For example, in the a decay equation given above, the combined mass of the polonium nucleus and the alpha particle is slightly less than that of the original radon nucleus. The ‘lost’ mass has become energy – this is where the fast-moving alpha particle gets its kinetic energy. The relationship between mass $m$ and energy $E$ is given by Einstein’s equation $E=m c^2$, where $c$ is the speed of light in free space. So, instead of saying that mass is conserved in nuclear processes, we have to say that mass-energy is conserved.
Fundamental families
Electrons and neutrinos both belong to the family of fundamental particles called leptons. These are particles that do not feel the strong nuclear force. Recall that particles that experience the strong force are hadrons, and that these are made up of fundamental particles called quarks.
So we have two families of fundamental particles, quarks and leptons. How can we understand $\beta$ decay in terms of these particles?
Consider first $\beta^{-}$decay, in which a neutron decays. $A$ neutron consists of three quarks (up, down, down or u $\mathrm{d}$ d). It decays to become a proton (u u d). Comparing these shows that one of the down quarks has become an up quark. In the process, it emits a $\beta$-particle and an antineutrino:
$
\mathrm{d} \rightarrow \mathrm{u}+{ }_{-1}^{\mathrm{O}} \mathrm{e}+\overline{\mathrm{v}}
$
In $\beta^{+}$decay, a proton decays to become a neutron. In this case, an up quark becomes a down quark:
$
\mathrm{u} \rightarrow \mathrm{d}+{ }_{+1}^{\mathrm{e}} \mathrm{e}+\mathrm{v}
$
Fundamental forces
The nucleus is held together by the strong nuclear force, acting against the repulsive electrostatic or Coulomb force between protons. This force explains a decay, when a positively charged $\alpha$-particle flies out of the nucleus, leaving it with less positive charge.
However, the strong force cannot explain $\beta$ decay. Instead, we have to take account of a further force within the nucleus, the weak interaction, also known as the weak nuclear force. This is a force that acts on both quarks and leptons. The weak interaction is responsible for $\beta$ decay.