| Date | May 2019 | Marks available | 2 | Reference code | 19M.3.hl.TZ1.24 |
| Level | HL | Paper | 3 | Time zone | TZ1 |
| Command term | Determine | Question number | 24 | Adapted from | N/A |
Question
Technetium-99m, a widely used radionuclide, has a half-life of 6.0 hours and undergoes gamma decay to technetium-99.
Most of the nuclear waste generated in a hospital is low-level waste (LLW).
Determine the percentage of technetium-99m remaining after 24.0 hours.
Technetium-99 decays further, emitting beta radiation. Formulate the equation for the decay of technetium-99.
Outline what is meant by low-level waste.
Outline the disposal of LLW.
Magnetic resonance imaging (MRI) is an application of NMR technology using radiowaves.
Suggest why MRI is much less dangerous than imaging techniques such as X-rays and radiotracers. Use section 3 of the data booklet.
Markscheme
Alternative 1
half-lives = « =» 4.0 [✔]
«N(t) (%) = 100(0.5)4 =» 6.3 «%» [✔]
Note: Accept “6.25 «%»”.
Alternative 2
λ =«» 0.116 hour–1 OR =e–0.116 × 24 [✔]
6.3 «%» [✔]
Note: Award [2] for correct final answer.
99Tc → 99Ru + β–
Ru [✔]
mass number of Ru AND beta product [✔]
Note: Accept “e/e–/ e” for “β–”.
small/low amounts of radiation AND for a short time [✔]
Note: Accept “weakly ionizing radiation” instead of “small amounts of radiation”.
Accept “short half-lives” instead of “for a short time”.
stored in shielded containers until radiation level drops «to a safe level» [✔]
lower frequency/longer wavelength/lower energy
OR
does not use ionizing radiation/radionuclides [✔]
Note: Do not accept “does not cause cancer”.
Examiners report
Most candidates correctly determined the % of technetium-99m remaining after 24.0 hours. Some candidates did not read the question properly and forgot to convert to a percentage.
A common incorrect answer was to give Tc as a product or to give an incorrect symbol for beta radiation. Most candidates scored a mark for the correct mass number of the product and beta radiation.
Low level nuclear waste was poorly outlined with many superficial responses. Many candidates only gave half the required answer.
Outlining the disposal of LLW was also challenging with many candidates saying that it should be put in a container without saying the container should be shielded.
Suggesting why MRI is less dangerous than using X-rays and radiotracers was mostly answered well, but some candidates were confused and linked longer wavelengths with higher energy.
