Page 212 - Physics Form 5 KSSM_Neat

Half-life
When a sample of radioactive material decays, the number of parent nuclei which has not
decayed decreases with time while the number of daughter nuclei increases. Let us do the
following activities to get an idea about half-life.

Activity 6.2 ISS ICS
KEMENTERIAN PENDIDIKAN MALAYSIA
SCAN ME
Aim: Watch the animated video to get an idea about the half-life SCAN ME
and discuss the decay series of a radioactive source Video of half-life
Instructions:
1. Carry out this activity in pairs.
http://bit.ly/2CPwomw
2. Scan the QR code to watch the video on half-life and browse
the Internet for more information on half-life.
3. Based on the video and the information obtained from the website, discuss:
(a) What is half-life?
(b) What is radioactive decay series?
(c) State the types of radiation emitted, the elements produced and the time taken for the
uranium-238 decay series.
(d) Explain the importance of the uranium-238 decay series in determining the age of rocks and
the age of the Earth.
4. Prepare a short multimedia presentation and present it in class.

The half-life, T 1 is the time taken for a sample of radioactive nuclei to decay to half of its
2
initial number. After one half-life, the number of nuclei that are not decayed will be half of its
initial value.
When an unstable radioactive nucleus decays, the new nucleus that is formed may also be
unstable. The new nucleus will experience a series of continuous decay until a stable nucleus
is formed. Figure 6.5 shows a complete decay series from uranium-238 to lead-206 and their
respective half-life.

α β β α α
238 234 234 234 230
Th
Pa
Th
92 U 4.5 × 10 years 90 24.1 days 91 1.8 minutes 92 U 2.5 × 10 years 90
5
9

7.5 × 10 years
4
α α α β, γ β, γ
226 222 218 214 214
Pb
Ra
Po
Ra
88 1 622 years 86 3.83 days 84 3.05 minutes 82 26.8 minutes 83 Bi

19.7 minutes
α β β α
214 Po 210 Pb 210 Bi 210 Po 206 Pb
84 1.64 × 10 –4 82 22.26 years 83 5.0 days 84 138.4 days 82 Stable
seconds
Figure 6.5 Uranium-238 decay series
202 LS 6.1.2