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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 .

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