Difference: CosmicRayLab (7 vs. 8)

Revision 82014-08-26 - jrodrig

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-- JorgeRodriguez - 2013-10-29
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+ Lab Assignment: Measure the angular distribution of cosmic ray flux
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+ Lab Assignment: Cosmic Ray Lab
 In this lab you will measure the cosmic ray flux as a function of the zenith angle and determine its shape by fitting your data with a function that represent theoretical expectations. Does it conform to those expections?
 Setup and calibration 

Before you begin your flux measurements you should become familiar with the experimental apparatus. To do this we'll first perform a calibration of the Cosmic Telesope, basically determine what high voltage to use for the PMT input and then take measurments of count rates before we plunge into our flux measurements.
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+First: Connect the HV supply to the PMTs and use the oscilloscope to find the output signal from the PMT. You will likely need to adjust the time scale or sweep (horizontal axis) and the voltage scale (verticle axis) on the scope. We expect output signals from the PMT to be 10-20 ns long and the output voltage to be a few milivolts for this Cosmic Ray telescope setup. You'll also need to set up the scope to self trigger on the out pulse; that is set the trigger channel to the channel in which you've pulgged the PMT output. The scope also has a discriminator threshold (the "level" knob located on the tigger portion of the scope.). You will need to adjust that to some fraction of the PMT's average output voltage. Since our PMT output voltage is negative you'll want to set the threshold below 0 V to display any events.
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+First: Connect the HV supply to the PMTs and use the oscilloscope to find the output signal from the PMT. You will likely need to adjust the time scale or sweep (horizontal axis) and the voltage scale (verticle axis) on the scope. We expect output signals from the PMT to be 10-20 ns long and the output voltage to be a few milivolts for the Cosmic Ray telescope setup. You'll also need to set up the scope to self trigger on the output pulse; set the trigger to trigger on the channel the PMT output is on. Note that the output will only trigger the scope if it exceeds a set discriminator (disc) threshold (the "level" knob located on the tigger portion of the scope). Adjust the level knob to some fraction of the PMT's average output voltage. Since our PMT output voltage is negative you'll want to set the threshold below 0 V to display any events.
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+Once you have your scope displaying multiple PMT pulses and have acertained a reasonable value for your discriminator threshold you can take the PMT output and redirect it to the discriminator NIM module. As with the disc threshold on the scope the disc threshold on the NIM Module is used to filter out noise pulses from signal pulses. You can adjust the disc threshold on the NIM module by turning the tiny set screw hidden inside the whole labled "THR". The threshold volatage can be read with multimeter.
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+Once you have your scope displaying multiple PMT pulses and have acertained a reasonable value for your discriminator threshold you can take the PMT output and redirect it to the discriminator NIM module. As with the disc threshold on the scope the disc threshold on the NIM Module is used to filter out noise pulses from signal pulses. You can adjust the disc threshold on the NIM module by turning the tiny set screw hidden inside the whole labled "THR". The threshold volatage can be read with a multimeter with leads connected to ground and the disc reading pad.
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+Before you proceed with the experiments, PART A and B below lets determine the proper volatage to input into the PMTs given the disc threshold. We do this because you want to run your PMTs, each an individual with, even if manufactured identically, can have different optimal operating voltages which can vary over time.
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+Before you proceed with the experiments, PART A and B below lets first determine the proper input volatage to run the PMTs at, given the chosen disc threshold settings that you've selected. We do this to insure that the PMT pulses and thus count rates are stable across a sizable range of input voltages which may drift over the course of the experinent. We'll do this by measuring the count rate as a function of input voltage and look the input voltage that corresponds to start of the count platue.
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+The best way to determine the operating voltage of your setup is to basically check to see that the count rate, given disc threshold setting and PMT input voltage, varies little over a set of input voltages. This process is sometime referred to as "Platue the PMTs", although here its not just the PMT that are involved. To do this setup up your cosmic telescope by connecting the PMT output to the NIM discriminator then connect the disc output to the counter. Setup the counter so as to collect events over a time window to collect a few 100 events and then vary the input voltage and record the counts. I would vary the voltage by 100 volts starting at around 1000 volts. Do not go beyond 2000 volts please. What you should see is the number of counts varies a lot when the voltage is set too high or too low. You'll want to use the a random event source, like the Sr 90 to insure you have real triggers instead of noise. DO this for BOTH PMTs and provide in your lab report the plots you make. Unfortunately you have a single HV power supply that can only ou
 PART A
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+Begin by setuping up the cosmic telescope by connecting the PMT output to the NIM discriminator then connect the disc output to the counter. Set the counter time window to collect a few 100 events. Make sure that you are counting real signals by insuring that the disc is set sufficiently high, check with the scope. Take count reading at variuos PMT input voltage and plot the results. Please DO NOT go beyond 2000 VOLTS. What you should see is that the count rates varies a lot when the voltage is set too high or too low. You'll want to use the a random event source, like the Sr 90 to insure you have real triggers instead of noise with proper disc setting. DO this for BOTH PMTs and provide in your lab report the plots you create.
 Part A: Observation of Poisson and Gaussian distributions in radioactive decay of Sr 90.
 You will only need to use one of the scintillator/PMTs for this part of the experiment. Decide on how to best setup your cosmic telescope viz. discr setting and HV input from the calibration setup above. 

Using a random source, that ⁹⁰ Sr is as good a random source as any. Take 100 measurements with the count time interval set to collect on average 1 event per time interval. Histrogram the results. Your histrogram will have on the horizontal axis the number of counts per interval and on the vertical axis the number of times you record a particular number of counts. How is the data distributed? Then repeat the excercise but now use 100 events per time interval. You can adjust any of the experimental conditions you control to achive these measurements. You can change the distance between the source and the scintilator or adjust the discriminator level. What ever you do make sure you retain the randomness of the events by insuring you are not picking up PMT or other sources of noise. There is no need to fit the distributions in this part of the lab but please do feel free to comment on what you observe.
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+ Part B
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+ Part B: Cosmic Ray Flux
 Using both PMTs, both of which you should have already platued and are running in optimal configuration for your experiment, perform the flux vs. angle measurement. Here you'll need to use the coincide NIM module inline between the discriminator outputs and the counter. The coincidence counter will trigger and output a NIM pulse when it detects two NIM pulses. This particular coincidence counter uses the size of the input pulse to select the size of the window it will use to detemine if there is a coincidence, check the manual for the device if you are interested in details, something you'll have to find online. As you take data do plot the distribution. In the slac paper, linked below you'll see an estimate of the count rate. Make sure your count rate is consistent with your expectation. It typically takes 20 minutes to take one reading and you don't want take 10 of these 20 min reading to find out you did something wrong. and fit a function form to your data and discuss what you see. There is a nice write up on this part that you can access as a reference. See document from slac linked below.
 Equipment:
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+To count the number of decays of the radioactive source you will need to setup equipment that can detect the emission of electrons in this energy range. You will use the cosmic ray telescope in the lab for the dection of the electrons. Basically the telescope consists of a piece of plastic scintillator made out of material that when exposed to charged particles reacts by emitting light. The light travels through the plastic material, designed to be transparent to the emitted light, reflecting from surfaces, colliding with other electrons etc, some of the photons end up at the front of the photomultiplier tube (PMT). The PMT is an electronic device based on the photoelectric effect that first converts a small number photons, into an amplified electrical signal sufficiently large to be easily recored by standard laboratory equipment. You should provide in your writeup an a short description of how the emitted electrons are detected by the equipment you use in this experiment.
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+Our Cosmic Ray Telescoe consists of a piece of plastic (organic Bicron BC 409) scintillator made out of material that when exposed to charged particles reacts by emitting light. The light travels through the transparent plastic material, reflecting from surfaces until eventually some of the photons emerge at the front face of the PMT. The PMT is an electronic device based on the photoelectric effect that first converts a small number photons, into an amplified electrical signal sufficiently large to be easily recored by standard laboratory equipment. You should provide in your writeup an a short description, longer than this, of how this works. You should also include a paragraph or two about cosmc rays, what are they where do they come from and what does our detector actually "see".
 The list of necessary equipment is: 
 Scintillation counters each with its own PMT (Cosmic Ray Telescope: Note for this experiment you will need to use both counter)

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