RadioactivityThe Geiger Counter

bliss of productions welcome back to it physics and a series of experiments involving radioactivity the first experiment that we will do is sort of a standard procedure for setting up the instrumentation we're going to determine the proper operating voltage for a Geiger counter so this involves making a measurement of the number of counts of radioactive decay particles in a certain time interval and we'll choose a minute as the time interval we'll plot the number of counts as a function of the voltage applied to the tube and use this information to determine what the best operating voltage will be so just a couple of reminders before we begin with the instrumentation the counting procedure involves Poisson statistics rather than Gaussian statistics which we've dealt with in the past and that simply means that the uncertainty on the measured count is the square root of the number of counts so when you plot your data you'll plot the number of counts and an error bar representing the square root of the number of counts so things to be aware of you don't need to sit there and count a bunch of zeros just turn up the operating voltage until you begin to count something then we'll reset the instrument to zero time and begin to do a count at the voltage that first begins to give some indication of counts then we'll continue to increase the voltage and you'll see a sort of flat or perhaps a slowly linearly increasing number of counts as a function of voltage then you'll hit a voltage that will cause the tube to spontaneously ionize and that's not particularly good for the instrumentation so you can measure a few points into this so-called breakdown region but don't go very far into it that's really not good for for the counter so what we're looking for is the midpoint of this plateau region the part that is slowly increasing and we'll choose that as the operating voltage in order to do the plateau curve measurement all you need is the Garga counter apparatus and source of radioactive material so the instrument turns on by the power switch in the back you just press the red button to turn it on then what you'll need to do is to set the counting interval and the high voltage so you choose the display select button and that moves the cursor on the right side of the window to these various functions so let's start by setting the time and we'll just press the up button to increase the time this goes in increments of one second at first up to 10 seconds and then it goes in increments of 10 seconds and let's set this at 60 seconds so we'll do a number of counts per minute so set the time interval for 60 seconds then we press the display select button again and turn on the high voltage so I happen to know that the operating voltage for this particular tube is something in the neighborhood of 800 volts so I'm going to start there just to illustrate how the instrument works so we go pressing the UP button changes the voltage in increments of 20 volts I'll just turn this up to 800 and once you've set the voltage then you press the display select button again actually press it two more times to get it back to the count function and we start a count so you press the count button to begin and what we're doing now essentially is counting basically background radiation the source is far from the detector but once we put the source in the chamber then the counts begin to accumulate at a much faster rate so what you want to do is to adjust the voltage applied to the tube until you just begin to get some counts and then increase in increments of 20 volts until you reach the breakdown region so let me just stop this and go back to the display select function down to the high voltage and I'm going to reduce this down to 600 volts there 600 maybe below the threshold so let's find out if that's the case we'll go back to the count function and reset this to zero and press the count and it's just sitting there not counting anything so 600 volts is probably below the threshold so you might want to start around 600 and if you're not seeing anything obviously just stop this and declare that to be 0 set the high voltage again let's go up to maybe 660 and we'll see if that's enough to get the counter going so pressing the display select button again back to the count function and we'll start the count again and they're so 660 volts is enough but that may not be the threshold for your particular to of each of the tubes is slightly different so you want to change the high voltage until you find what the threshold is then count for a minute at each voltage in increments of 20 volts up until you discover the beginning of the breakdown region and then I recommend plotting the data as you go along so you can discover where the breakdown region begins and then the mid point of the plateau is the proper operating voltage for this tube so you'll set that same operating voltage each time you come in to do the next pieces of the experiment so as always I'd like to remind you to keep your exposure to radiation as low as reasonably achievable and the three common-sense ways of doing that are to maintain proper distance don't be any closer to the source then you absolutely need to be in order to do the work work efficiently to minimize the time of exposure so do your work plan what you're doing ahead of time do it quickly and efficiently and return the source to your instructor as soon as you are finished and finally if you need to be near the source for a prolonged period of time make sure that there is some sort of shielding between your body and the source of radiation so that the shielding material absorbs the radiation rather than you so keep your exposure to radiation as low as reasonably achievable when doing all of these experiments

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