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Date November 2021 Marks available 1 Reference code 21N.2.HL.TZ0.4
Level Higher level Paper Paper 2 Time zone 0 - no time zone
Command term Suggest Question number 4 Adapted from N/A

Question

Plutonium-238 (Pu) decays by alpha (α) decay into uranium (U).

The following data are available for binding energies per nucleon:

plutonium          7.568 MeV

uranium             7.600 MeV

alpha particle     7.074 MeV

The energy in b(i) can be transferred into electrical energy to run the instruments of a spacecraft. A spacecraft carries 33 kg of pure plutonium-238 at launch. The decay constant of plutonium is 2.50 × 10−10 s−1.

Solar radiation falls onto a metallic surface carried by the spacecraft causing the emission of photoelectrons. The radiation has passed through a filter so it is monochromatic. The spacecraft is moving away from the Sun.

State what is meant by the binding energy of a nucleus.

[1]
a.i.

Draw, on the axes, a graph to show the variation with nucleon number A of the binding energy per nucleon, BEA. Numbers are not required on the vertical axis.

[2]
a.ii.

Identify, with a cross, on the graph in (a)(ii), the region of greatest stability.

[1]
a.iii.

Some unstable nuclei have many more neutrons than protons. Suggest the likely decay for these nuclei.

[1]
a.iv.

Show that the energy released in this decay is about 6 MeV.

[3]
b.i.

The plutonium nucleus is at rest when it decays.

Calculate the ratio kinetic energy of alpha particlekinetic energy of uranium.

[2]
b.ii.

Estimate the power, in kW, that is available from the plutonium at launch.

[3]
c.i.

The spacecraft will take 7.2 years (2.3 × 108 s) to reach a planet in the solar system. Estimate the power available to the spacecraft when it gets to the planet.

[2]
c.ii.

 State and explain what happens to the kinetic energy of an emitted photoelectron.

[2]
d.i.

 State and explain what happens to the rate at which charge leaves the metallic surface.

[2]
d.ii.

Markscheme

the energy needed to «completely» separate the nucleons of a nucleus

OR

the energy released when a nucleus is assembled from its constituent nucleons ✓

 

Accept reference to protons and neutrons.

a.i.

curve rising to a maximum between 50 and 100 ✓

curve continued and decreasing ✓

 

Ignore starting point.

Ignore maximum at alpha particle.

a.ii.

At a point on the peak of their graph ✓

a.iii.

beta minus «decay» ✓

a.iv.

correct mass numbers for uranium (234) and alpha (4) ✓

234×7.600+4×7.074-238×7.568 «MeV» ✓

energy released 5.51 «MeV» ✓

 

Ignore any negative sign.

b.i.

«KEαKEU=»p22mαp22mU  OR  mUmα ✓

«2344=» 58.5 ✓

 

Award [2] marks for a bald correct answer.

Accept 1172 for MP2.

b.ii.

number of nuclei present =33×103238×6.02×1023«=8.347×1025» ✓

initial activity is λN0=2.5×10-10×8.347×1025«=2.08×1016Bq» ✓

power is 2.08×1016×5.51×106×1.6×10-1918 «kW» ✓

 

Allow a final answer of 20 kW if 6 MeV used.

Allow ECF from MP1 and MP2.

c.i.

available power after time t is P0eλt ✓

18e2.50×1010×2.3×108=17.0 «kW» ✓

 

MP1 may be implicit.

Allow ECF from (c)(i).

Allow 17.4 kW from unrounded power from (c)(i).

Allow 18.8 kW from 6 MeV.

c.ii.

stays the same ✓

as energy depends on the frequency of light ✓

 

Allow reference to wavelength for MP2.

Award MP2 only to answers stating that KE decreases due to Doppler effect.

d.i.

decreases ✓

as number of photons incident decreases ✓

d.ii.

Examiners report

[N/A]
a.i.
[N/A]
a.ii.
[N/A]
a.iii.
[N/A]
a.iv.
[N/A]
b.i.
[N/A]
b.ii.
[N/A]
c.i.
[N/A]
c.ii.
[N/A]
d.i.
[N/A]
d.ii.

Syllabus sections

Core » Topic 7: Atomic, nuclear and particle physics » 7.1 – Discrete energy and radioactivity
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