A graph of nucleon number A against proton number Z is shown in Fig. 7.1. The graph shows a cross (labelled P) that represents a nucleus P.

Q#15 (Past Exam Paper – November 2018 Paper 23 Q7)

A graph of nucleon number A against proton number Z is shown in Fig. 7.1.

Fig. 7.1

The graph shows a cross (labelled P) that represents a nucleus P.

Nucleus P decays by emitting an α particle to form a nucleus Q.

Nucleus Q then decays by emitting a β^– particle to form a nucleus R.

(a) On Fig. 7.1, use a cross to represent

(i) nucleus Q (label this cross Q), [1]

(ii) nucleus R (label this cross R). [1]

(b) State the name of the class (group) of particles that includes the β^– particle. [1]

(c) The quark composition of one nucleon in Q is changed during the emission of the β^– particle.

Describe this change to the quark composition. [1]

[Total: 4]

Solution:

(a)

(i) Q plotted at (82, 210)

{²¹⁴₈₄P             - - - >   ^A_ZQ      +          ⁴₂α

Proton number and nucleon number should be conserved.

214 = A + 4

A = 214 – 4 = 210

84 = Z + 2

Z = 84 – 2 = 82

The emission of an α-particle from a nucleus will cause the proton number to decrease by two and the nucleon number to decrease by four.}

(ii) R plotted at (83, 210)

{²¹⁰₈₂Q - - ->                ^A_ZR      +          ⁰_-1β

210 = A + 0

A = 210

82 = Z – 1

Z = 82 + 1 = 83}

(b) lepton(s)

{The β^– particle is a lepton. It is not made up of quarks, so it is not a hadron.}

(c)

up down down changes to up up down

or udd → uud

or down changes to up

or d → u         

{The nucleus Q is ²¹⁰₈₂Q.

Number of proton in Q = 82

Number of neutron in Q = 210 – 82 = 128

Nucleus R is ²¹⁰₈₃R.

Number of proton in R = 83

Number of neutron in Q = 210 – 83 = 127

So during the decay of Q into R, a neutron is changed into a proton.

Quark composition of neutron = udd

Quark composition of proton = uud

A d-quark has changed into a u-quark.}

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