DP Physics (last assessment 2024)

Question 20N.2.HL.TZ0.6c(iii)

Date	November 2020	Marks available	[Maximum mark: 2]	Reference code	20N.2.HL.TZ0.6c(iii)
Level	HL	Paper	2	Time zone	TZ0
Command term	Calculate	Question number	c(iii)	Adapted from	N/A

c(iii).

[Maximum mark: 2]

20N.2.HL.TZ0.6c(iii)

A sample of waste produced by the reactor contains $1.0 kg$ of strontium-94 (Sr-94). Sr-94 is radioactive and undergoes beta-minus ( $β^{-}$ ) decay into a daughter nuclide X. The reaction for this decay is

${}_{38}^{94}{Sr} \to X + {\bar{v}}_{e} + e$ .

The graph shows the variation with time of the mass of Sr-94 remaining in the sample.

(c(iii))

Calculate the mass of Sr-94 remaining in the sample after $10$ minutes.

[2]

Markscheme

ALTERNATIVE 1

$10 \min$ $= 8 t_{1 / 2}$ ✓

mass remaining $= 1.0 \times {(\frac{1}{2})}^{8} = 3.9 \times 10^{- 3} « kg »$ ✓

ALTERNATIVE 2

decay constant $= « \frac{\ln 2}{75} = » 9.24 \times 10^{- 3} « s^{- 1} »$ ✓

mass remaining $= 1.0 \times e^{- 9.24 \times 10^{- 3} \times 600} = 3.9 \times 10^{- 3} « kg »$ ✓

Examiners report

This was answered well with most candidates (even at HL) going down the number of half-lives route rather than the exponential calculation route.

Syllabus sections

Core

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

Core » Topic 7: Atomic, nuclear and particle physics

Question 20N.2.HL.TZ0.6c(iii)

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