Page 182 - Physics 10_Float

ATOMIC AND NUCLEAR PHYSICS

to decrease from 1000 to 250 per min, hence
Time taken = 2 × 40 min. = 80 min.
(b) It takes 3 half-lives for the count rate to decrease from For your information
1000 to 125 per min, hence
Time taken = 3 × 40 min. = 120 min = 2 h
(c) Graph is shown in Fig 18.8.
count-rate
per min.
1000
800 During brain radiotherapy,
patient is carefully positioned
600
in the helmet to ensure that
400 the gamma rays converge at
the desired point in the brain.
200 A lead apron protects the body
from exposure to radiation.
0
T ½ 2T ½ 3T ½ Time/min.
Fig. 18.8: Decay of unstable element

18.6 RADIOISOTOPES AND THEIR USES
Nuclei which do not emit radiations naturally are called stable
nuclei. In general, most of the nuclei with atomic number . . . . . . . . .
1 to 82 are stable nuclei. While the elements whose atomic
number is greater than 82 are naturally unstable. They emit
different types of radiations, all the time, and hence
continuously change from one type of element to another.
The stable and non-radioactive elements can also be
changed into radioactive elements by bombarding them with
protons, neutrons or alpha particles. Such artificially
produced radioactive elements are called radioactive
isotopes or radioisotopes. Here are some examples of the
production of radioisotopes:
1 23 24
1. 0 n + 11 Na Na + gamma ( )-rays

11
neutron stable a sodium
sodium radioisotope
nuclide
4 27 30 1
2. 2 He + Al 15 P + n
0
13
alpha stable a phos-
particle aluminium phorous
nuclide radioisotope
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