Quanta to Quarks. Focus Area 3. The Nucleus. T ypically the nucleus is less than one ten-thousandth the size of the atom, the nucleus contains more that 99.9% of the mass of the atom.
Focus Area 3
The number of protons in the nucleus, Z, is called the atomic number. This determines what chemical element the atom is.
A = Z + N
In 1920, Rutherford guessed that there had to be a new kind of particle in the nucleus, about as heavy as a proton but with no electric charge, which he called the neutron.
Since large numbers of very energetic protons were emitted from the paraffin when it absorbed whatever was coming out of the nucleus, they assumed that whatever was being ejected from the beryllium nuclei must be an extremely energetic form of gamma radiation. Chadwick, however, disagreed.
Using the velocity of the ejected protons and the laws of conservation of energy and momentum, Chadwick calculated the mass of the unknown particle. It was just a little heavier than the proton. He had no doubt that this was Rutherford's neutron.
When a nucleus ejects a positron ( ) as a result of beta decay, its atomic number will decrease by 1 but its mass number will remain unchanged. In this case, a neutrino (ν) will also be ejected. A neutrino also has no charge and little mass.
During decay it was found that beta particles could have a range of kinetic energies rather than one specific energy as in alpha decay.
The simultaneous emission of a beta particle and a neutrino in beta decay allowed for energy and momentum to be conserved.
This theory was accepted for almost a quarter of a century without any direct evidence to support it. In 1956, an experiment was performed in a nuclear reactor that could only occur if the neutrino actually existed, thus confirming its existence.
Large nuclei are unstable because protons at the surface of the nucleus are repelled by a force proportional to the total number of protons in the nucleus, but attracted towards the interior by a force proportional to the number of nucleons in its immediate vicinity (which is constant for light or heavy nuclei).
Fission or transmutation may be artificially triggered by bombardment with neutrons.
This is called a fission reaction because the nuclei split into two fragments. When the nucleus decays, more neutrons are ejected. If these neutrons cause further fissions to occur a chain reaction occurs.
At each step of this process, mass is converted into energy which is released as the kinetic energy of the particles in the system. Controlled chain reactions produce enormous amounts of energy this way and they take place in nuclear reactors, which use a fissionable material such as uranium-235.
• It must be much stronger than the proton proton electrostatic repulsion. (it’s over 100X stronger)
• It must be independent of charge and attract all nucleons... protons & neutrons.
• It must be extremely short-ranged, operating only across the tiny distances of the nucleus. (Otherwise it might cause neighbouringatomic nuclei to fuse together, and eventually pull all matter into one lump!)
The “atomic mass unit” (u) is a measure of mass devised for convenience in Chemistry. Roughly speaking, both a proton and a neutron have a mass of 1 u, although in the calculations following, you need to be much more precise.
1 u = 1.661x10-27 kg
The “electron-volt” (eV) is an energy unit that is convenient because the energy of sub-atomic particles has traditionally been measured by their behaviourwithin electric fields. 1 eV is the energy gained by an electron accelerating in an electric field with a potential difference of 1 volt. 1 eV is an extremely small amount of energy:
1 eV = 1.602 x 10-19 joules of energy
1 MeV = 1x106 (one million) eV
ΔE = Δmc2 = 0.03040 x 931.5 = 28.3 MeV
Fermi in 1939, escaped to the United States where he continued his research into fission. Delayed by economic and security constraints, he secretly build Chicago Pile 1, the world’s first nuclear fission reactor in 1942.