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Particle Physics Questions

Q1.

(a) Describe how the strong nuclear force between two nucleons varies with the separation of the nucleons quoting suitable values for separation. (3 marks)

Here AQA want information from their syllabus.

At a spearation of less than 0.5 fm the force is repulsive.It then becomes attractive over a short range, peaking at 1.3 fm at a force of 10-4 N. and becoming negligible by about 3 fm separation.

(b) An unstable nucleus can decay by the emission of an alpha particle.

(i) State the nature of an alpha particle.  (1 mark)

By 'the nature' the board mean say what it is 'in the real world' - not to physicists. They wanted you to say that it was a helieum nucleus. If they had asked about a beta particle they would expect you to say a high energy electron.

helium nucleus OR 2 protons and 2 neutrons

If you just said helim you would not get the mark - it has no electrons therefore it is a nucleus. Best bet is to say:

'It is a helium nucleus i.e. it is composed of two neutrons and teo protons, it therefore has a positive - double the charge on a single proton.

(ii) Complete the equation below to represent the emission of an alpha particle by a uranium nucleus.

(2 marks)

(c) Uranium 238 decays in stages by emitting particles and particles, eventually forming a stable isotope of lead, called lead 206.

(i) State what is meant by 'isotope'. (2 marks)

Isotopes of an element have the same number of protons as each other, but a different number of neutrons. (This results in the same chemical properties but different physical properties).

Be careful with the wording of your answer. There is no such thing as a 'normal' version of an element. For example carbon 12 is the common isotope of carbon, not its normal version! There are radioactive isotopes and stable isotopes, common and rare isotopes.

Don't just say 'An isotope has the same proton number but a different nucleon number'... you haven't said different and same to what!

(ii) If there are eight alpha decays involved in the sequence of decays from uranium 238 to lead 206 deduce how many decays are involved. (show your working!)

Proton number decreases by two, -2 for each alpha emission and increases by one for each beta decay +1

Nucleon Number decreases by 4 for each alpha emission , but does not change for beta. We can therefore not use the nucleon number change to look at how many betas.

Proton number change: 92 - 82 = 10

8 alpha decays means proton number decreases by 16.

It only deceases by 10 so there must be 6 beta decays.

(3 marks)

(Total 11 marks)