Question SPM.2.SL.TZ0.4
| Date | May Specimen-2023 | Marks available | [Maximum mark: 6] | Reference code | SPM.2.SL.TZ0.4 |
| Level | SL | Paper | 2 | Time zone | TZ0 |
| Command term | Calculate, Complete, Outline | Question number | 4 | Adapted from | N/A |
A smoke detector uses the radioactive nuclide americium-241.
The americium is contained in a chamber that is open to the air. There are two electrodes in the chamber that are connected to a power supply and a current sensor.
Americium-241 emits alpha particles that ionize the air in the chamber. Each ionization forms one positive ion and one electron; these are called an ion pair. The electrons and the positive ions move towards the electrodes and the sensor detects a current in the air.
When smoke enters the chamber, fewer ion pairs are formed and the current in the sensor decreases, sounding an alarm.
The chamber is 0.10 m in each dimension.
A nucleus of americium-241 has 146 neutrons. This nuclide decays to neptunium through alpha emission.
Complete the nuclear equation for this decay.
[2]
✓
✓
Outline why the radioactive source is safe for use in a house.
[1]
Alpha particles only travel a few cm in air / penetration of alpha particles is poor (and will not escape the chamber) ✓
OWTTE
The initial activity of the source is 42 kBq. 33% of the alpha particles emitted by this source enter the chamber and form an ion pair.
Each alpha particle has an initial kinetic energy of 5.5 MeV.
The energy required to form one ion pair is 15 eV.
Calculate the maximum current in the chamber due to the electrons when there is no smoke in the chamber.
[3]
Each alpha gives rise to ion pairs ✓
So ion pairs per second ✓
current «A» ✓
