Nuclear Instability Elliott

Factors Affecting Stability The stability of the nucleus depends on a combination of the proton number and the neutron number. 

For stable nuclides, we notice the following: The lightest nuclides have almost equal numbers of protons and neutrons. The heavier nuclides require up to 50 % more neutrons than protons.  The greater number of neutrons is needed to stop the nucleus flying apart, in effect diluting the repulsive force of the positively charged protons. Most nuclides have both an even number of protons and an even number of neutrons. 

For unstable nuclides, we see: Disintegrations tend to produce new nuclides that are nearer the stability line, and carry on until the stability line is reached. Nuclides above the line decay so that the proton number increases by 1, i.e. a beta emission. Nuclides below the line decay to reduce the proton number and the proton to neutron ratio increases.  This is achieved by alpha decay.

Alpha Decay Alpha radiation mostly comes from heavy nuclides with proton numbers greater than 82, but smaller nuclides deficient in neutrons can also be alpha emitters.   It is believed that the alpha particle is formed some time before its emission, and it gains its energy from the mass defect in the nucleus

Points of Note The alpha particle is a helium nucleus (NOT atom) Energy is released in the decay.  The energy is kinetic, with the majority going to the alpha particle and a little going to the decayed nucleus. The velocity of the alpha particle is much greater than that of the nucleus. The nucleon number goes down by 4, the proton number by 2.

Check Your Progress Is this equation balanced?  Explain your answer.

Answer Yes.     The number of nucleons goes down by four     The number of protons goes down by two 

Beta Decay Neutron rich nuclei tend to decay by beta minus (b-) emission. The beta particle is a high-speed electron ejected from the nucleus. 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.

Points of Note The nucleon number remains the same. The proton number goes up by 1. The beta particle is created at the instant of the decay. The antineutrino is very highly penetrating and has a tiny mass.  It is very hard to detect, as it rarely interacts with matter. A precise amount of energy is released, according to the nuclide. That energy is shared among the nucleus, the electron and the antineutrino.

Check Your Progress 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

Beta Plus Decay The positron is the anti-particle to the electron.  It has the same size, but opposite charge.   Beta-plus (b+) decay involves the emission of a positron.  It rarely occurs naturally, and is generally found in nuclear physics experiments in reactors. 

Examples  If we bombard fluorine atoms with alpha particles, we get a radioisotope of sodium, which decays by positron emission. Here we see a positively charged electron, the positron being emitted with an electron neutrino (ne).  At the nucleon level we see:

Is the charge conserved? Check Your Progress Is the charge conserved?

Answer Yes The charge on the LHS = Charge on RHS Q: +1 --> 0 + +1 + 0