Calibration of Scintillator Panels
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Calibration of Scintillator Panels.pdf (The pdf version)
1. – Introduction
At Brookhaven National Laboratory, several high school, college, graduate students and teachers are working on an experiment known as the MARIACHI project.The purpose of the MARIACHI project is to study various components of cosmic rays.In order to study these phenomenal particles, we created cosmic ray detectors which consist of photo multiplier tubes (PMTs), scintillator panels, gun cases, and other various tools which help us to read signals given off from the PMTs.There are many assorted ways to construct these detectors and calibrate their components.While working as an intern at BNL, I tested a new way to determine the threshold voltage that each of three PMTs and three scintillator panels operate at.This paper will compare two means of calibrating these scintillator panels.
2.1- The “New Way”
Determining the optimal threshold through this design involves using an oscilloscope, two power supplies of which you can control the voltage output, and two small scintillator panels with smaller PMTs connected to them, aside from the cosmic ray detectors.The smaller scintillator panels are to be connected to the smaller PMTs using clear, smooth plastic glue and wrapped extremely carefully in light tight wrapping (first a layer of tyvek wrapping and then thick black plastic and electrical tape).These two scintillators will be your trigger panels.Use a ring stand and two clamps strong enough to hold up the two smaller photo multiplier tubes and scintillator panels.Make sure that the spacing in between the two scintillator panels is large enough so that they are above and below the gun case.Having about an inch between the gun case and each scintillator panel on the top and the bottom is a good idea so as to ensure that you will not break the panels’ connections to the PMTs when moving the ring stand and gun case.The smaller PMTs are hooked up to a power supply that should be operating at approximately 5.0 volts and are hooked in the oscilloscope in channels A and C.Set the oscilloscope up so that it only depicts a pulse when both of the small scintillators detect a particle (trigger channels A & C).Next, take the cosmic ray detector and hook it up to the second power supply and up to the oscilloscope in channel B.Make sure that you have the oscilloscope set to infinite persistence (each pulse does not fade from the screen) and that the pulse is displayed with a negative slope.Apply a low voltage of 4.0 volts to the detector.Wait approximately 2.0 minutes for the tube to acclimate to the voltage.Let the pulses accumulate for approximately 2.0 minutes.After the pulses have accumulated, place your cursor in the location where the amplitude for the bulk of the pulses is the lowest.If there is a pulse that seems unexpectedly low, ignore it, as it is most likely an errant pulse.Record the threshold value.Increase the applied low voltage by 0.5 volts and wait 2.0 minutes for the tube to acclimate.Repeat the process of measuring the minimum threshold voltage until you have increased the applied low voltage to approximately 7.5 volts, increasing the voltage by 0.5 volt increments.As you approach 7.5 volts, you will see that the threshold voltage seems to be getting larger, but when you reach 7.5 volts, you will see that the threshold drop off to nearly zero, zero, or above zero because electrons are being rapidly knocked off and the PMT sees those events as light and sends a signal to the oscilloscope, causing false events to register.By measuring the threshold this way, we can determine the exact threshold voltage that each specific applied voltage operates the best at.This method seems to be easier and more time efficient compared to the previous way of measuring the threshold.
2.2 – Efficiency measurements:
After you have calibrated each of the scintillator panels inside of the cosmic ray detectors, you must conduct efficiency tests on all of them to make sure that the events that are detected by the trigger panels are also being registered by the detector.To do this, you will need a discriminator board, a counter, a logic board, and a coincidence board.Connect both trigger panels into the discriminator board in separate channels and plug them into the coincidence board at first in separate channels, then connect them together in another channel using a separate cable.Connect that channel to the input of one of the channels of the logic board, and then connect that logic board channel into the first counter channel.Connect the detector into its own channels in both the discriminator and coincidence boards.Next, use two more cables to connect the trigger panels and the detector into the same channel.Connect that channel into the input of another channel in the logic board, and connect that channel into the second channel in the counter (see the diagram at the end of section 3).Set the counter to count coincidences for both channels for 5.0 minutes.For each low voltage of 5.0 to 7.5, going again in 0.5 volt increments, record the number of events seen by the two trigger panels and the number of events seen by the trigger panels and the detector.Divide the number of events seen by all three panels by the number of events seen by the two trigger panels and then multiply that number by 100% (for each low voltage).Your product is the efficiency.Repeat this for all of your detectors.
If time allows, you may proceed to make attenuation measurements by taking your cosmic ray detector and your smaller scintillator panels and simply follow the same procedure for measuring the efficiency except with a few alterations.Firstly, operate the cosmic ray detector only at the most efficient low voltage.Secondly, instead of keeping the trigger panels in one place on the detector, you will be taking data from five separate places.You will need to know where the scintillator panel inside the gun case ends and begins.After you have taken measurements regarding the location of the scintillator of the detector, determine five places where you will be able to take sufficient measurements and mark them on the gun case.You need to make sure that when you are moving the trigger panels that you are not moving them to a place where there is no scintillator panel inside of the gun case.This is why you need to know where the scintillator panel is inside of the gun case.After you have made your marks on the gun case then you can begin taking your data.If your detector is near equally efficient in all spots on the scintillator panel, then you know that your detector is reliable and is calibrated.
3. – The “Old Way”
These means of measuring the threshold involves the use of a discriminator board, a logic board, and a counter, along with the small trigger scintillator panels. Connect the scintillator to the discriminator board and to the counter.Set the counter to stop counting after a period of time (5 minutes is enough).Next set the threshold of the discriminator board to approximately 15.0mV.Also set the power supply that is going into the detector to 5.0V.Set the counter and let it count the number of events that occur during the set time period and then record that number.Increase the low voltage by 0.5mV and repeat the process until you have increased the low voltage to about 7.5V, increasing the low voltage by increments of 0.5V.By doing this, you are determining the count rate at each threshold.Next, increase the threshold by 0.5mV and repeat the process.Repeat the process until the threshold reaches 55.0mV.After graphing the data, your curve should closely resemble an exponential decay curve where, eventually, the curve will level out and the count rate for all thresholds after that will be the same.This is your optimum threshold [1].The efficiency can be determined the same way as I explained above.
*The following is a diagram of the correct setup of the logic board, coincidence board, and discriminator board.
Image:Image004.gif4. - Data: *All of the following data was taken based upon the same cosmic ray detector.
The data used to construct this graph was collected using the old technique of measuring the threshold.This graph displays the efficiency of the detector when the threshold of the discriminator is set at 30.0mV.It shows that the PMT begins to plateau and is most efficient at 6.5V.This means that the optimal voltage for a threshold of 30.0mV for this PMT is 6.5V.The plateau ends at approximately 7.0V.When graphing our fit line we did not include the last data point because it would have made the fit line look odd and the last point is not important because at 7.5V the PMT “shuts off.”The data is organized using the fit of a sigmoid function.This graph is consistent with the data that we attained when plateauing the PMT our newer way.
Image:Image008.gifThe data from this graph was also acquired using the old technique of plateauing the PMT.This graph displays the efficiency of the detector when the threshold of the discriminator is set to 50.0 mV.You can see that the PMT begins to plateau and is most efficient at approximately 7.0V.This means that the optimal voltage for a 50.0mV threshold for this PMT is 7.0V.The data points are organized using the fit of a sigmoid function.This data is also concurrent with our data that was obtained using the new method.
This graph displays the data that was acquired using our new method.As you can see, at 7.0V the threshold of the PMT is at approximately 50.0mV.At 6.4V the threshold of the PMT is at approximately 30.0mV.The data is organized in an exponential curve.Our data that was attained using our method is completely concurrent to the data that was acquired using the traditional method.
5. – Why the “New Way” is More Efficient Than the “Old Way”
Using the new method of determining the optimum threshold and optimum voltage seems to be more time efficient than the old method.By using the new approach you don’t need to graph your data to determine the threshold because you are given that value directly.Instead of setting the parameters for the threshold as in the old method, you are searching for the exact threshold and therefore your data is more accurate.The new process is better because a certain scintillator might have a lower or higher optimum threshold and if so, important events will not be detected because you only used a certain set of thresholds.The setup for the new technique is simple and does not require the usage of complicated tools.Overall, the new method is simpler and more efficient than the older style.
1. Mountjoy, Ashlin and Rohlf, Scott.“COSMOS/ Quarknet Summer 2004 Internship Paper: Scintillators.”
