LabIntro

From MariachiWiki

Introduction to Detecting Cosmic Rays

The goal of the Mariachi project is to develop an understanding of cosmic rays and where in the universe they come from. Cosmic rays are free particles that have a certain amount of energy associated with them. Higher energy ones are constantly entering our atmosphere and creating cosmic ray showers. You may of heard of Solar Winds before. This term refers to the low energy protons and electrons that are constantly being ejected from the sun toward Earth. They are of fairly low level energies though (~1 keV), and for this reason most of then don't make it through the atmosphere. This is also why we don't care about them. What we do care about though are the high energy particles that are capable of creating cosmic ray showers.

The tool used to detect these particles is the scintillator. They are inside the black gun cases you see around the room. If you look inside one of the cases, you'll find a scintillator which is a piece of special clear plastic that detects particles by exciting electrons and emitting light that is collected in the photomultiplier tube. This device utilizes the photoelectric effect. If you don't remember what this is from your intro physics class, it is based on electrons being ejected from a metal atom due to an interaction with a photon of light. We use these electrons to detect cosmic ray particles. This is the photo- part of the photomultiplier. The -multiplier part comes from the "amplification" of the electron. Detecting one electron is very hard to do. To solve this problem, many layers of metal are spaced a distance apart like they would be in a capacitor, except here voltages are induced across each pair to excite the electron to jump to the next plate with a greater energy than it came in with. Due to this greater energy, a greater number of electrons are emmitted from this next plate than originally "shot" at it. This happens many times thus creating a snowball effect. 1 electron goes to 3 electrons, 3 electrons goes to 9 electrons, 9 electrons goes to 27 electrons, so on and so forth. As a result, from one electron (from our original cosmic ray detection), we get a current of electrons that we can measure easily.

We then send this current of electrons to a coincidence box, which is used to process the incoming currents from multiple scintillators. This tool exists because it is very useful in detecting cosmic ray showers. One problem that we encounter from the scintillator setups is that they often fire when a cosmic ray hasn't penetrated it. To solve this dilemma, we always use at least two scintillators. Since it is unlikely that both of the scintillators will misfire at the same time, we can trust their coincidence (when both claim to detect a cosmic ray) to be a true detection within a degree of error.

Using the Equipment

You should notice on the front of the gun cases near the handles that there are two adapter ports. One of them is used to detect signal and the other is used to supply voltage to the photomultiplier tube. Let's start with the signal output. This will always be connected to the coincidence box with a signal cable. You should notice that the coincidence box has two sets of inputs. You will use the inputs on the right. They are labeled 1-5, which means you can have up to 5 scintillators connected at once. Now let's look at the voltage input on the scintillator case. This must be connected or you won't get any cosmic ray detections. This voltage can be supplied by an outside voltage source, but you will almost always use the coincidence box to supply a set voltage. If you open up the lid to the box, you will find resistor inputs on the top right. Hopefully, there is one connected already connected. You might be thinking "Why can't I use the same resistor for the scintillator I'm connecting?" The answer is that not all of the scintillators have the same internal resistance, so a different resistor must be used to compensate for this. Follow this link to learn how to choose a correct resistor. After reading this over, you should know that the coincidence box supplies a voltage of 7 volts. We want the scintillators to run at about 5.9 volts. This means you must choose a resistor to make this the case. BE CAREFUL NOT TO EVER MAKE THE VOLTAGE GREATER THAN 6.2 V. THE PHOTOMULTIPLIER TUBE MIGHT BREAK IF YOU DO. If you find the method of choosing a resistor in the link you just read confusing, you can instead use some logic to choose the correct resistor. We know that the voltage in the coincidence box is 7 volts. Since the circuit in the link you looked at above is a series circuit, then all we need to know is the voltage drop across the resistor we put in the coincidence box and we'll know the voltage in the scintillator case by subtracting 7 volts from that. If we have a voltage drop of 7 - 5.9 = 1.1 V, then we will have the 5.9 V we wanted applied to the scintillator. Note: there is a switch that must be turned on in the coincidence box for the voltage source to supply voltage to the photomultiplier tube. Also make sure that any time you want to change a resistor that the voltage source is turned off

Now that we have our voltages set and our signal cables connected, we can now open up the daq (data aquisition) computer program to process the information sent to the coincidence box by the photomultiplier tube. Go to this daq link to learn how to open and run the program. Now that you know how to use the daq program, let's take a second look to understand what is being counted. The first set is straight counts and frequencies from each of the scintillators individually. The second set, underneath the first set, measure coincidences between different counters. For example, between 1 and 2 or 1, 2, and 4. These numbers correlate to the labels on the input jacks of the coincidence box. These coincidences are more important to us than the individual counters since as mentioned before, we look at coincidences because this will give us a more accurate cosmic ray rate. Recall, each individual counter often misfires, so we must look at the coincidence of counters to get a good approximation. Run the program by pressing the arrow button near the top left of the screen. You should see the numbers in the boxes start to change.

Transferring data to Microsoft Excel

One last thing we should talk about is how to use Excel. Let's look at an example to get an understanding. Let's say we want to take several trials of the coincidence rate of scintillators 1 and 2 at a 30 second interval and graph them. We should first label columns to organize the data. In this example, we should have columns for "Trial Number" and "Rate". Next we take five measurements in the daq program at this interval each time writing the measurement in the "Rate" column. The "Trial Column" should just list 1 through 5 down the column. You should also include error whenever you measure something. To calculate error in the rate, use this formula Error = {[(rate)*(time duration)]^(1/2)]/(time duration), where time duration is how long you took each measurement for in the daq program. If you just wanted to find error in a raw count, all you need to do is take the square root of it. For example, the square root of 100 is 10, so the answer would be 100 + - 10 counts. You should create an error column in Excel so that you can include this in your graph later. A picture of what this should look like is below.

Now that we have this set of data, we can create a graph. This is done by first highlighting the data we want to graph (including titles) by holding down the mouse button and dragging over all of the columns from the top left to the bottom right. Then open the Insert tab on the top left of the page and click on Chart. You will be presented with a list of different types of graphs to work with. For this graph, we just want to see the data points, so we pick the X-Y (Scatter) plot. You'll then be presented with further choices. For this graph, we should choose the first one since, again, we only want to plot data points. Click next and an example of what your graph should look like will come up. If it's not what you wanted, you can go back and play with the options until it fits your needs. Note: you might need to use the series tab if you have several columns of data. Now, click next and label the axes and title box. Click next again to finish. A chart should come up. To add error bars, double click on a point on your graph. A screen should pop up labeled "Format Data Series". Go to the "Y Error Bars" tab and click on the both choice under display. Then click on custom on the bottom. Next click on the red and white button to the right of the "+" box. Highlight just the numbers in the error column you want to include and click the same button to get back to the prior screen. Do the same for the "-" box and click ok. Error bars should now be on your graph. An example of what your graph should look like is below.