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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?