====== Evaluation of the results ======

Now everything is measured, the values are stored in the computer and you need to calculate the results - weight/activity dependency (self-attenuation) and half-life of the present radionuclide.

The general formula for acquiring the half-life looks like this:
{{ :vrhub:halflife:64658d386f34d44f5d8d71c21b6bdc23cf22209e.svg?nolink |}}

First of all lets plot the values of CPM per gram, one can see the self-attenuation gradually diminishing the effect or added weight.

{{:vrhub:halflife:cpm.png?nolink|Illustration of self-attenuation CPM/g}}

The computation works as follows:

  * Compute log(activity) for all samples
  * Compute linear regression for the log(activity)/weight, remember the linear part (y-intercept) of the equation
  * Compute antilog of the y-intercept

{{:vrhub:halflife:logcpm.png?nolink|Illustration of dependence log(CPM/g)/g}}

  * Compute the lambda by dividing the antilog by number of 40-K atoms in each gram of KCl (9.69*10<sup>18</sup>)
  * Compute the half-life by dividing ln(2)/lambda, giving total time in seconds if you started with CPS, or in minutes if you started with CPM. Divide by hours, days and years to get the half-time.


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