Lab Assignment: Measure the angular distribution of cosmic ray flux

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.

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.

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.

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.

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

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.

Part B

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:

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.

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)
• High Voltage Supply (to power the PMT)
• NIM module Discriminator (to decide whether the signal is a true PMT pulse and not noise)
• NIM module Linear Fan In Fan Out (this allows you to duplicate RF signals from the PMT and NIM moudules
• NIM module Counter/timer (to count the number of NIM output signals)
• NIM module Quad Coincide (produces a NIM output when two signal arrive within an interval of time determined by module's internal logic)
• Oscilloscope (to examine the output of the PMT and help set discriminator levels etc.)
• ⁹⁰Sr radioactive source

Include in your introduction a breif description of the important components of the apparatus you used to conduct this experiment:

1. The Scintillator: Look up references on how organic or plastic scintillators work. The scintillator you are using is I belive a Bicron BC 400 series plastic scintillator
2. The Photomultiplier: Describe how a PMT works. You should provide as much detail as you need to demonstrate that you understand how this device functions and why it is used.
3. The High Voltage supply, Just noting that one is used is probably sufficient.
4. The data aquisition equipment: The various NIM modules you used in your experiment. You should be familiar with the operation of each and report on their use and function in your writeup.
5. The Oscilloscope: No description required but you may want to explain who you used it.

Analysis

Histogram your flux measurments, number of counts vs. angle w.r.t zenith and fit the distribution with an appropriate probability distribution funciton. Discuss the results and the goodness of fit.

GradingRubric

• slac-tn-95-001.pdf: SLAC Cosmic Ray Telescope paper