Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] thorium-228



ludwik kowalski wrote:
... confirm ...
...Th-228 (half-life 1.913 years) is dissolved in water, at the concentration of 0.03 ppb. What is the activity of that isotope? (ignore activity of daughters).
Answer: 3.57*10^-4 Ci.
...
Ludwik
One curie is that quantity of material of which 3.7E10 atoms disintegrate/sec
given that this benchmark quantity is provided by 1 gm of radium.
Radium mass: 226 (let us say) Half-life 1600 yr
Thorium mass: 228 (let us say) Half-life 1.91 yr
Curie equivalent mass of Thorium: 1 / 842 gm = 1.19E-3 gm
Water mass: 16 + 2 = 18 (let us say)

Let us suppose the specified dilution represents the proportions
by mass. For unit mass of water, the proportion of Thorium
would be 3E-11 The rate at which unit mass of Thorium decays
is 1600/1.91 as fast as Radium so Thorium is 842 times as active
as that same mass of Radium
The suggested 3.57E-4 Ci of Thorium activity is provided by
3.57E-4 / 842 gm X 228/226 Thorium = 4.28E-7 gm

At a dilution of 1 : 3E-11, this would involve a mass of 14.3 kg of mixture.


Reviewing the working of this result, it is clear to me that either the definition
of the Curie given here is faulty, or Ludwik's definition of the question as an activity without a mass, is incomplete. Considering that he has some background in radioactivity, it is more likely I have misused the definition of activity

Brian W