Q#19 (Past Exam Paper – June 2016 Paper 21 Q7)
(a) Give one example of
a hadron:
a lepton:
[1]
(b) Describe, in terms of the simple quark model,
(i) a proton, [1]
(ii) a neutron. [1]
(c) Beta particles may be emitted during the decay of an unstable nucleus of an atom. The emission of a beta particle is due to the decay of a neutron.
(i) Complete the following word equation for the particles produced in this reaction.
neutron → .................................... + .................................... + .................................... [1]
(ii) State the change in quark composition of the particles during this reaction. [1]
[Total: 5]
Solution:
(a)
hadron: neutron/proton
and
lepton: electron/(electron) neutrino
(b)
(i) proton: up up down or uud
(ii) neutron: up down down or udd
(c)
(i) neutron → proton + electron + (electron) antineutrino
{Note that the decay of a neutron to produce proton is a known one and can be directly inserted to obtain the answer.
Beta particles are emitted. So, one of the blanks should be beta. Beta particles are the same as electrons. So, we can write electrons instead of beta.
Beta particles / electrons are examples of leptons. In an equation if a lepton is emitted, its associated antimatter should also be emitted so that the overall lepton number is zero.
For leptons, a matter (e.g. electron, neutrino) has a lepton number of +1 while an antimatter (e.g. anti-electron, antineutrino) has a lepton number of -1.
So along with the electron, an antimatter should be emitted – either anti-electron or antineutrino.
If an antielectron is emitted, the overall charge on the left-hand side would be 0 (charge of neutron) while that on the right-hand side would be (+1 – 1 + 1 =) +1 (due to proton, electron and antielectron respectively). The charge is not conserved. It cannot be an antielectron.
So, the other particle is an anti-neutrino.}
(ii)
up down down (quarks) change to up up down (quarks)
or
down (quark) changes to up (quark)
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