Fig. 1 shows a teacher's diagram of uranium-238 decay in a sample of rock. The first change is alpha decay. Students are asked to use conservation of mass n...
Question 1 Report
Fig. 1 shows a teacher's diagram of uranium-238 decay in a sample of rock. The first change is alpha decay. Students are asked to use conservation of mass number and atomic number to identify the new nucleus. The diagram includes the original uranium nucleus and an emitted alpha particle.
(a) Calculate the mass number of the new nucleus. [1]
(b) Calculate the atomic number of the new nucleus. [1]
(c) What element has atomic number 90? [1]
(d) Describe how alpha emission changes the charge of the original nucleus. [2]
Table 1 shows activity readings from a tracer used to find a leak in an underground water pipe. Fig. 1 shows a detector placed at points along the ground above the pipe. The tracer is chosen because it emits gamma radiation, which can pass through soil and be detected above ground.
| Distance along pipe / m | Corrected count rate / counts per minute |
|---|---|
| 0 | 18 |
| 10 | 21 |
| 20 | 24 |
| 30 | 146 |
| 40 | 27 |
(a) Which position is most likely to be above the leak? [1]
(b) Give two properties of a suitable tracer for this investigation. [2]
(c) Describe why a tracer with a long half-life is not preferred. [2]
Answer Details
Uranium alpha decay
- (a) Mass number is conserved: \[238-4=234\] The new nucleus has mass number \(234\). [1]
- (b) Atomic number is conserved: \[92-2=90\] The new nucleus has atomic number \(90\). [1]
- (c) Element 90 is thorium. [1]
- (d) Alpha emission removes two protons from the original nucleus. Its nuclear charge therefore decreases by 2, becoming less positive by two elementary charges. [2]
Pipe-leak tracer
- (a) The detector reading is greatest at 30 m, at 146 counts per minute, so 30 m is most likely above the leak. [1]
- (b) A suitable tracer emits gamma radiation so it can be detected through soil. It should have a short half-life and be chemically safe or non-toxic; it must also be able to mix with water. Any two properties gain credit. [2]
- (c) A long-half-life tracer remains radioactive for longer. It could therefore cause longer-lasting contamination and a greater radiation dose. [2]
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