Download Physics PHYS 276 Experimental Physics Laboratory Statistics in Counting Experiments

Physics PHYS 276
Experimental Physics Laboratory
Statistics in Counting Experiments
I. Introduction
The purpose of the first lab is to explore the role of statistical uncertainty in particle counting measurements. To
do this, a large number of identical measurements will be made using a GM detector. The distribution of the
number of detected events will be compared with the Poisson distribution discussed in class.
II. Setup: Same as last experiment.
Ortec 903
GM Tube
Ortec 906
Pulse Invert
Mechtronics 715
Counter/Timer
Source
Mechtronics 257
HV Supply
III. Procedure
A. Set up the electronics.
B. Plateau the GM tube if you have not already done so. Determine the operating voltage of the GM
tube.
C. Insert a source at such a distance as to yield at least 50 counts in the time interval you intend to
count.
D. Measure the number of counts for at least 100 trials.
E. Make a histogram of the number of times N counts were obtained as a function of N. Determine the
mean and standard distribution. Plot the predicted Poisson distribution.
IV. Logbook
In addition to the standard items, you should have the following in your logbook.
A. A plot containing a histogram of the measured number of counts and a theory curve for the Poisson
distribution described in class.
B. A calculation of the mean number of counts, the standard deviation, and the fraction of trials within
one standard deviation of the mean.
V. Questions to ponder
A. How would the histogram look different with 5000 counts per trial rather than 50? With 5?
B. Does the radiation source matter?
C. Does the distance from the source to the detector matter?
D. Does the counting time interval make a difference, other than the obvious difference of changing the
total number of counts?
E. Does a Poisson distribution seem to describe your histogram?
F. Why, in this case, do we say that the uncertainty in the number of counts is the standard deviation,
rather than the deviation of the mean, which is normally what we have done? What do we mean by
deviation of the mean?