Date | November 2021 | Marks available | 3 | Reference code | 21N.2.HL.TZ0.4 |
Level | Higher level | Paper | Paper 2 | Time zone | 0 - no time zone |
Command term | Show that | 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.
Draw, on the axes, a graph to show the variation with nucleon number of the binding energy per nucleon, . Numbers are not required on the vertical axis.
Identify, with a cross, on the graph in (a)(ii), the region of greatest stability.
Some unstable nuclei have many more neutrons than protons. Suggest the likely decay for these nuclei.
Show that the energy released in this decay is about 6 MeV.
The plutonium nucleus is at rest when it decays.
Calculate the ratio .
Estimate the power, in kW, that is available from the plutonium at launch.
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.
State and explain what happens to the kinetic energy of an emitted photoelectron.
State and explain what happens to the rate at which charge leaves the metallic surface.
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.
curve rising to a maximum between 50 and 100 ✓
curve continued and decreasing ✓
Ignore starting point.
Ignore maximum at alpha particle.
At a point on the peak of their graph ✓
beta minus «decay» ✓
correct mass numbers for uranium (234) and alpha (4) ✓
«MeV» ✓
energy released 5.51 «MeV» ✓
Ignore any negative sign.
«» OR ✓
«» ✓
Award [2] marks for a bald correct answer.
Accept for MP2.
number of nuclei present ✓
initial activity is ✓
power is «kW» ✓
Allow a final answer of 20 kW if 6 MeV used.
Allow ECF from MP1 and MP2.
available power after time t is ✓
«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.
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.
decreases ✓
as number of photons incident decreases ✓