Radon-222, for example, has a half-life of just four days. We’re much more familiar with radioactive elements like uranium and plutonium-these are the wild teenagers of radioactive elements, constantly hurling off particles. Depending on their makeup, some will stabilize themselves by releasing subatomic particles and turning into an atom of a different element-a process called radioactive decay. The results from the XENON1T experiment, co-authored by University of Chicago scientists and published April 25 in the journal Nature, document the longest half-life in the universe-and may be able to help scientists hunt for another mysterious process that is one of particle physics’ great mysteries. Along the way, however, the detector caught another scientific unicorn: the decay of atoms of xenon-124-the rarest process ever observed in the universe. Hope this is helpful to you.Deep under an Italian mountainside, a giant detector filled with tons of liquid xenon has been looking for dark matter-particles of a mysterious substance whose effects we can see in the universe, but which no one has ever directly observed. If this is of interest to you, you might want to review a paper by Perepelitsa and Pepper that does some comparisons between the theoretical evaluation and experimental measurements for some alpha emitters. It is also possible to make theoretical estimations of some radionuclide half-lives using a quantum-mechanical approach described as the Geiger-Nuttal Law, which provides a means for estimating the decay constant associated with alpha decay. Some include allowing the short-lived progeny, 234Th and 234Pa, to grow into the separated 238U and to count some of the progeny radiations. There are other methods for half-life evaluation as well. Counting using alpha particle energy spectrometry is effective in separating the alpha particles from the two uranium isotopes. We have also not considered the complication associated with possible interference from 234U, which also occurs in natural uranium and also decays by alpha emission. Naturally, the numbers used in the example were contrived, and the uncertainty in the result would have to consider all the uncertainties involved in the measurement. The estimated uncertainty in this value is approximately 3 x 10 6 years. See the Chart of the Nuclides on the Brookhaven National Laboratory site. This would compare to the presently accepted value of 4.468 x 10 9 years. If we solve for T 1/2 we obtain T 1/2 = 2.40 x 10 15 minutes = 4.57 x 10 9 years. If the 5 mg were deposited in a thin uniform layer and counted for its alpha activity, and we obtained a count rate of 16.9 cps with an alpha detection efficiency of 0.315 counts per disintegration (Bq-s), we would then calculateĪ = 1014 cpm/0.315 c d -1 = 3219 dpm = λ N = (ln2/T 1/2)(4.41 x 10 -3 g/238.03 g/g-atomic weight)(6.022 x 10 23 atoms/g-atomic weight). The uranium decays 100 percent of the time by alpha emission. The number of atoms is determined from the measured mass of the sample, its fractional mass content of the radionuclide, and Avogadro’s number, N o = 6.022 x 10 23 atoms g -1 atomic weight.įor example, suppose we isolate 5.00 mg of pure 238UO 2, which contains 4.41 mg of 238U. One solves for λ and gets the half-life from the relationship λ = ln2/T 1/2. The half-life is then determined from the fundamental definition of activity as the product of the radionuclide decay constant, λ, and the number of radioactive atoms present, N. In the case of 238U and some other long-lived radionuclides, one approach that has been used is to separate a pure sample of the radionuclide in a known chemical form, weigh the sample, and then measure the activity, A (disintegration rate).
In such instances, one must employ alternative techniques to evaluate the half-life. As you have apparently inferred, when a radionuclide has a half-life that is long compared to the time interval over which radioactive decay observations are possible, the overall decay rate remains substantially the same and experimental measurements of the change in the activity of a given sample with time are not sufficiently precise to allow determination of the half-life.
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