Physics 1202
Week 13 Discussion
MC1:U-238 decays into Pb-207 with a half life of 0.71 billion years. If all the Pb-207 in a rock comes from this decay,
how old is a rock with three times as many Pb-207 nuclei as U-238 nuclei?
A) 0.71 billion years
B) 1.07 billion years
*C) 1.42 billion years
D) 2.13 billion years
E) None of these
MC2:How many neutrons are there in
206
82 P b
?
A) 82 *B) 124 C) 126 D) 206 E) None of these
MC3:In a nuclear reaction, the total mass of the constituents decreases by 0.10 u (you have to look up what a u unit
is). How much kinetic energy is released?
A) 0.01J B) 0.01 eV C) 9.31 MeV *D) 93.1 MeV E) 931 MeV
MC4:Two isotopes have the same
*A) proton number
B) neutron number
C) nucleon number
D) mass number
E) atomic weight
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Long Problem, Week 13
Homer enters the visitors’ chambers, and his Geiger beeper sounds. He shuts off the beeper, removes the device from
his shoulder patch, and holds it near the only new object in the room—an orb that is to be presented as a gift from
the visiting Cartesians. Pushing a button marked monitor, Homer reads that the device is reading a counting rate
of 4000 counts/s above the background counting rate. After 10 min, the counting rate has dropped to 1000 counts/s
above the background rate. (a) What is the half-life of the source? (b) How high will the counting rate be counting
above the background counting rate 20 min after the monitoring device was switched on?
Alternate Long Problem, Week 13
The relative abundance of 40K (potassium 40) is 1.2 ◊ 10–4 . The isotope 40K has a molar mass of 40.0 g/mol, is
radioactive and has a half-life of 1.3 ◊ 109 y. Potassium is an essential element of every living cell. In the human
body the mass of potassium constitutes approximately 0.36 percent of the total mass. Determine the activity of this
radioactive source in a student whose mass is 60 kg.
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