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Laboratory #22: Absorption of β and γ Rays Goal: Detect nuclear radiation with a gas-filled detector; understand statistical uncertainty. Study the absorption of β and γ rays in matter. Equipment: "The Nucleus" counting station with Geiger-Müller tube, counter and timer. Set of absorbers of varying thickness, 90 Sr and 60 Co source. Monitor-4 Geiger counter, Vernier interface, computer. (A) Counting Statistics Here, we want to convince ourselves that repeating a counting experiment N times leads to results n i ( i =1, N ) that are distributed like a Gaussian around a mean value <n>. In order to demonstrate this well, we need many measurements. Thus we enlist the help of a computer to do this for us. Place a 60 Co source in the source holder in the black wooden bench, turn on the computer and the Monitor-4 (its little red light should now start flashing with each count). Click on SW137 and then double-click on the &Exp04 Statistics± icon. Click &experiment±, then &sampling± and set the experiment length to 300s and the count interval to 1s. Click &collect±. You now should be making 1s measurements, acquiring a histogram as you go. Adjust the distance between the counter and the source until the count per s is between 25 and 30. Click &stop± and &collect± again to restart the experiment. Wait till the experiment is finished. Copy the values f n (number of times, the result n occurred) of the measured distribution into your logbook. Calculate the average <n> for the N =300 measurements from >= < n n f f n n / , ( ) where the sum extends over the bins of the histogram. Plot the measured distribution f n . On the same graph also plot the theoretically expected distribution. This is a Gaussian with a mean = <n> and a standard deviation σ 2 = <n> : > < > < = n n n o n e m
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This note was uploaded on 01/20/2012 for the course P 309 taught by Professor Urheim during the Spring '11 term at Indiana.

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