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Suggested Answers for 09-10 F5 Test 8
1. (a) X = 56, Y = 144
(1A+1A)
(b) Reaction 1
1A
(c) (i) Reaction 2
1A
(ii) In the centre of the Sun, the temperature is extremely high so that the nuclei have sufficient energy
to overcome the repulsion between the nuclei.
2.
(a)
(b)
3.
(a)
(b)
The  particles ionize the air molecules between the metal plates, producing a
lot of ion pairs.
Owing to the electric field between the plates,
the ions move to the metal plates,
this completes the circuit and the movement of ions forms a current.
The ammeter reading decreases and finally drops to zero when the source
moves a few centimetres away from the plates.
Since the range of  particles in the air is about several centimetres,
no ion is formed between the plates when the plates are outside the range of 
particles, and hence no current is formed.
(For effective communication)
1A
1A
1A
1A
1A
1A
1C
40 counts per min
1A
1A+1A
 and 
The count rate drops significantly after a piece of paper is inserted.
1A
This indicates that the sample emits  as alpha particles are stopped by a piece
of paper.
The count rate drops significantly when an aluminium plate is inserted.
1A
Therefore, the sample also emits  since beta particles are stopped by a 5 mm
alumninium plate.
1-5 B B A A B
6-10 C D B A C
11-15 B A D D A
16-20 D B C C B
21 A
2. 8000  4000  2000  1000  500
20 h
20 h
20 h
20 h
So required count rate is 2000 Bq.
3. No. of alpha particles emitted = (238 – 218) / 4 = 5
Because 5 alpha particles are emitted, so the atomic number should decrease by 5 x 2 = 10, but now the
atomic number only decreases by 92-84 = 8, so 2 beta particles are emitted (each beta particle will increase
atomic number by 1).
4. GM tube only detects the count rate, but cannot distinguish the types of radiation detected. So does
photographic film.
5. Alpha tracks in cloud chamber are thick and straight while beta tracks are curved and thin. Gamma rays
nearly cannot produce any visible tracks.
6. Range of beta particles is about 5 m. So a small amount of beta contributes to the reading.
7. Overdose of nuclear radiation can cause genetic changes.
8. For (1),tobacco contains radioactive isotopes that emit nuclear radiation, so (1) is true.
For (2), except plasma TV, ordinary TVs would produce x-rays because of the retardation of electron beam.
Nuclear radiation refers only to alpha, beta and gamma radiation.
For (3), we are exposing, from time to time, to various background radiation mainly from cosmic rays,
rocks and soil.
9. Trivial.
10. 4000  2000  1000  500, so 3 half-life = 18 hours
1 half-life = 6 hours.
11. After sufficiently long time, the count rate approaches the background count rate which can be determined
by looking at the horizontal part after 15 hours.
12. 1200  1100  550  650
To find the half-life, refer to the time interval between 1100 and 650.
13. Beta radiation can penetrate metal foil and a slight change in thickness causes a measurable difference in
count rate. The half-life is preferably several years, because (1) the count rate would not change considerably,
and (2) the source can be used for a longer time and need not be changed very frequently.
14. Radioisotopes are used to sterilize food and preserve the food longer.
15. Only neutrons can be used to initiate fission. Protons cannot be used because they are repelled by the
uranium nuclei.
16. A moderator is used to slow down neutrons but not absorb neutrons so that the neutrons can stay longer in
the reactor core. Boron is used to absorb neutrons, but not slow down neutrons. In pressurized water reactor
(PWR), pressurized water is used as a moderator.
18. 1 mg  0.5 mg  0.25mg  0.125 mg  0.0625
So after 4 half-lives (12 days), 0.9375 mg of the substance has decayed.
20. The energy released in a fission reaction is determined by mass difference.