20.
(a) We solve Q
eff
from P = RQ
eff
:
Q
eff
=
P
R
=
P
N λ
=
mP T
1/2
M ln 2
=
(90.0 u)(1.66
× 10
−27
kg/u)(0.93 W)(29 y)(3.15
× 10
7
s/y)
(1.00
× 10
−3
kg)(ln 2)(1.60
× 10
−13
J/MeV)
=
1.2 MeV .
(b) The amount of
90
Sr needed is
M =
150 W
(0.050)(0.93 W/g)
= 3.2 kg .