Date | May 2019 | Marks available | 2 | Reference code | 19M.2.HL.TZ1.2 |
Level | Higher level | Paper | Paper 2 | Time zone | 1 |
Command term | Estimate | Question number | 2 | Adapted from | N/A |
Question
A beam of electrons each of de Broglie wavelength 2.4 × 10–15 m is incident on a thin film of silicon-30 . The variation in the electron intensity of the beam with scattering angle is shown.
Use the graph to show that the nuclear radius of silicon-30 is about 4 fm.
Estimate, using the result from (a)(i), the nuclear radius of thorium-232 .
Suggest one reason why a beam of electrons is better for investigating the size of a nucleus than a beam of alpha particles of the same energy.
Outline why deviations from Rutherford scattering are observed when high-energy alpha particles are incident on nuclei.
Markscheme
read off between 17 and 19 «deg» ✔
correct use of d = = 7.8 × 10−15 «m» ✔
so radius = «fm» = 3.9 «fm» ✔
Award ecf for wrong angle in MP1.
Answer for MP3 must show at least 2 sf.
RTh = Rsi or substitution ✔
7.4 «fm» ✔
electron wavelength shorter than alpha particles (thus increased resolution)
OR
electron is not subject to strong nuclear force ✔
nuclear forces act ✔
nuclear recoil occurs ✔
significant penetration into nucleus / probing internal structure of individual nucleons ✔
incident particles are relativistic ✔
Examiners report
This question was left blank by many candidates and many of those who attempted it chose an angle that when used with the correct equation gave an answer close to the given answer of 4 fm. Very few selected the correct angle, calculated the correct diameter, and divided by two to get the correct radius.
This question was also left blank by many candidates. Many who did answer simply used the ratio of the of the mass numbers of the two elements and failed to take the cube root of the ratio. It should be noted that the question specifically stated that candidates were expected to use the result from 2ai, and not just simply guess at the new radius.
This question was very poorly answered with the vast majority of candidates simply listing differences between alpha particles and electrons (electrons have less mass, electrons have less charge, etc) rather than considering why high speed electrons would be better for studying the nucleus.
Candidates struggled with this question. The vast majority of responses were descriptions of Rutherford scattering with no connection made to the deviations when high-energy alpha particles are used. Many of the candidates who did appreciate that this was a different situation from the traditional experiment made vague comments about the alpha particles “hitting” the nucleus.