Understanding Radioactivity Half Lives and Their Mechanism

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In summary, the exact mechanism for radioactivity that results in a half life is that radioactive nuclei decay randomly. While the decay of a single nucleus cannot be predicted, a large sample can be used to determine the average decay rate and half life. The long half life of uranium is due to its inertial properties, not because of interactions with other nuclei. Attempts to reduce or change half lives by spreading out nuclear material over a large surface area are not effective, as the number of nuclei and their decay rate remain constant. Different isotopes of an element can have a range of half lives, with some being very long-lived and others being much shorter.
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Denton
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Ive been thinking about this for a while, what is the exact mechanism for radioactivity that results in there being a half life.

Say you have a single radioactive isotope of Uranium, this particle would ultimately emit an alpha particle or some other form of radiation and thereby quickly returning to a stable element. However this does not happen, we have huge half lives for uranium which I presumed was because when densely packed enough, the radiation emmited by one would then increase another and therefore it would take a long time for it to spread to the outside.

But if this were the case, we could just spread out nuclear material over a very large surface area and reduce its half life significantly. But this is incorrect by what I've heard that you can't reduce or change half lives.

can anyone fill me in on what I am missing?
 
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The "exact" mechanism for radioactive nucleis is that they decay randomly. You can't say when a certain nuclei will decay, but if you have a large sample (of the order 10^6 and bigger), you can empirrically measure the half life, the time when half of the sample has dissapeared, and relate that to the decay constant [tex] \lambda [/tex], which is the AVERAGE probablilty that a particle decays each unit time. You can never extrapolate this to a small number of nuclei (ex. 1 nuclei), since the ultimate process is random, but on large scales we can find averages.

Like when you roll a dice, you can never predict what ONE single throw will yield. But if you roll the dice 1000times, then you can say that approx 160 will be 1, 160 will be 2 etc.
(However the thing is more complicated with a nuclei, but this example may enlighten the difference between one signle trial and a large collection of trials).

"Say you have a single radioactive isotope of Uranium, this particle would ultimately emit an alpha particle or some other form of radiation and thereby quickly returning to a stable element. However this does not happen, we have huge half lives for uranium which I presumed was because when densely packed enough, the radiation emmited by one would then increase another and therefore it would take a long time for it to spread to the outside."

The sencence "we have huge half lives for uranium which I presumed was because when densely packed enough, the radiation emmited by one would then increase another and therefore it would take a long time for it to spread to the outside."

Is wrong, the long half life of Uranium is due to its inertial protperties (size, shape, shell effects etc)

nope you can't change half lives. You can change the activity by decreasing the number of nuclei (N). ( Activity is : [itex] A = \lambda N [/tex] ). And by spreading a sample over a larger volume, the intensity (number of particles emitted per area) is decreasing, but the half life don't change (if you still have a "large" number of nucleis per unit area of course), since you have less radioactive particles om each area, and then you get less emitted particle per area too of course.
 
  • #3
Therefore, uranium is not particulary radioactive by itself. A large mass of depleted U is required to produce a significant amount of emissions but practically no alpha particles will reach a handler unless it is vaporized and breathed in.
 
  • #4
Denton said:
Ive been thinking about this for a while, what is the exact mechanism for radioactivity that results in there being a half life.

Say you have a single radioactive isotope of Uranium, this particle would ultimately emit an alpha particle or some other form of radiation and thereby quickly returning to a stable element. However this does not happen, we have huge half lives for uranium which I presumed was because when densely packed enough, the radiation emmited by one would then increase another and therefore it would take a long time for it to spread to the outside.

But if this were the case, we could just spread out nuclear material over a very large surface area and reduce its half life significantly. But this is incorrect by what I've heard that you can't reduce or change half lives.

can anyone fill me in on what I am missing?
Isotopes of any element will have a range of half-lives. Some long, some short.

Here is a nice overview of the natural radioactive decay series.
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radser.html
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radact.html

Natural U is mostly U-238 (about 99.3%), with about 0.7% U-235. There will also be traces of U-234. U-238 has a half-life of ~4.5 billion years, while U-235 has a half-life of ~700 million yrs.

Another long-lived radioisotope is Th-232, which has a half-life of ~ 14 billion yrs.

The longer half-lived isotopes will survive long enough to be found in nature. The shorter the half-life, the smaller the amount found in nature.

Here is a useful resource for radionuclides - http://www.nndc.bnl.gov/chart/
Place cursor over the chart and left click on a location of interest, then click on the 1 under the Zoom (top right) for details of a nuclide and its neighbors.
 

FAQ: Understanding Radioactivity Half Lives and Their Mechanism

What is radioactivity and how does it work?

Radioactivity is the process by which unstable atomic nuclei emit particles or energy in order to become more stable. This emission is known as radiation, and it can take the form of alpha, beta, or gamma particles. Radioactivity occurs spontaneously, and the rate at which it occurs is determined by the half-life of the element.

What is a half-life and how is it calculated?

A half-life is the amount of time it takes for half of the atoms in a radioactive substance to decay into a more stable form. It is a constant value for each element and can range from fractions of a second to billions of years. The calculation for half-life is determined by dividing the natural logarithm of 2 by the decay constant of the element.

How do scientists use half-lives to study radioactive decay?

Scientists use half-lives to study the rate of radioactive decay by measuring the amount of radioactive material present at different time intervals. By plotting this data on a graph, they can determine the half-life and the decay constant of the element. This information can then be used to make predictions about future decay and to understand the behavior of radioactive materials.

What factors can affect the half-life of a radioactive element?

The half-life of a radioactive element can be affected by several factors, including temperature, pressure, and the chemical environment. Additionally, the presence of other elements or isotopes can also impact the rate of decay. However, the half-life of an element remains constant under normal conditions.

What are some real-world applications of understanding radioactivity half-lives?

Understanding radioactivity half-lives has many practical applications, including in medical imaging, nuclear energy, and environmental monitoring. For example, doctors use radioactive isotopes with short half-lives to diagnose and treat diseases, while nuclear power plants use elements with longer half-lives to generate electricity. Additionally, scientists can use the half-life of certain elements to determine the age of fossils and archaeological artifacts.

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