Difference between revisions of "Determining the Half Life of 234mPa"

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Before you plot the data subtract the background counts from each measurement and calculate the uncertainty for each point.
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[[Lab Exercise with 234Th/234Pa Radionuclide Generator|Return]]
  
#Plot your measured data on a A3-sized semi-logarithmic paper. Your data should lie on a straight line (why?). Fit the best possible line through your data. Remember to plot the uncertainty also.  
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Use a high-quality data plotting and fitting program (e.g. Origin) to analyse the data. The fitting ''must'' take the uncertainity into account (do not use Excel), otherwise you will get the wrong result.
#Select a point on your fitted line from the left side (e.g. 1000 counts), note the corresponding time, t1.
 
#Now, divide the number of counts for t1 by 2 three times (for 1000 counts you get 125) and find the time, t2, your fitted line passed through this number of counts.
 
#The difference between t1 and t2 corresponds to three half lives (why?).
 
#Estimate the uncertainty in your fit by drawing two worst-case lines through your data, one with the steepest possible slope and one with the least possible slope. Analyse these lines in the same way as the main line - the respective half lives indicate your lower and upper uncertainty limits.
 
  
[[Lab_Exercise_with_234Th/234Pa_Radionuclide_Generator|Return to main page]]
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Notice that you always shall use the 1/3 of the time into each measuremnent as the "middle time point". This is due to decay - after 1/3 of the time you will have equally many counts before and after the 1/3 point (i.e. it is the "middle point".
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#For each datapoint calculate the net count (gross count - background count), the uncertainity of the net count (based on uncertainity of both the gross count and the background count). You might want to use e.g. MS Excel or similar for doing this.
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#Enter your data in a table ("worksheet" in Origin jargong): Include measurment time (relative to start of sampling) as x-value, the net count as y-value, and the uncertainity as y-error. 
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#Plot the data - does it look ok?
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#Use the Origin data-fitting functionality to determine the measured half-life and associated uncertainity.
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'''Alternativ/Extra:''' Plot the gross counts instead of the net counts and ask Origin to fit both the background and the decay.
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[[Category:Half_life]] [[Category:Master]]

Latest revision as of 22:04, 4 October 2012

Return

Use a high-quality data plotting and fitting program (e.g. Origin) to analyse the data. The fitting must take the uncertainity into account (do not use Excel), otherwise you will get the wrong result.

Notice that you always shall use the 1/3 of the time into each measuremnent as the "middle time point". This is due to decay - after 1/3 of the time you will have equally many counts before and after the 1/3 point (i.e. it is the "middle point".

  1. For each datapoint calculate the net count (gross count - background count), the uncertainity of the net count (based on uncertainity of both the gross count and the background count). You might want to use e.g. MS Excel or similar for doing this.
  2. Enter your data in a table ("worksheet" in Origin jargong): Include measurment time (relative to start of sampling) as x-value, the net count as y-value, and the uncertainity as y-error.
  3. Plot the data - does it look ok?
  4. Use the Origin data-fitting functionality to determine the measured half-life and associated uncertainity.

Alternativ/Extra: Plot the gross counts instead of the net counts and ask Origin to fit both the background and the decay.