Gas Ring-Imaging Cherenkov Detector Construction Introduction

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1 Gas Ring-Imaging Cherenkov Detector Construction Author: Brandon Tyler Blankenship, The University of the South Advisor: Dr. Todd Averett, College of William & Mary August 5, 2016 Introduction Scientists in Hall A at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) have an interest in scattering polarized electrons from polarized helium-3 targets to understand the neutron spin structure. In the pursuit of this experiment, Jefferson Lab has commissioned the upgrade of the BigBite (BB) Spectrometer. The upgraded BB must be capable of handling significantly higher event rates than past experiments, meaning that the associated detectors and apparatus must be able to cope with the increased event rate. One of the detectors associated with the BB project is the Gas Ring-Imaging Cherenkov (GRINCH) detector. Cherenkov radiation is electromagnetic radiation emitted when a particle travels faster than the speed of light through a medium. Cherenkov radiation is emitted in a cone, and its behavior can be thought of as analogous to that of the Mach cone that is produced when an object travels faster than the speed of light. The angle at which the radiation is emitted is governed by the following equation: where ß is the velocity of the particle divided by the speed of light and n is the refractive index of the medium through which the particle is traveling. Cherenkov radiation was first discovered by Alekseyevich Cherenkov, for which he was awarded the Nobel Prize in 1958, and Cherenkov radiation s applications for detectors in particle physics were recognized soon after. Interestingly, Cherenkov radiation had been predicted even earlier (as far back as 1888) by Oliver Heaviside. Cherenkov detectors are a very effective means of discerning between different types of particles. This is because by choosing the refractive index of the medium inside the detector, scientists can ensure that only particles above a certain velocity will produce Cherenkov radiation. The number of photons emitted via Cherenkov radiation for a given range of wavelengths can be determined from the following equation:

2 where N is the number of photons emitted per x, distance travelled, and α represents the fine-structure constance and λl and λh are minimum and maximum wavelengths, respectively. The GRINCH detector is intended to discern electrons from other particles such as pions as these particles travel through the detector. The detector takes advantage of the fact that electrons have a small rest mass when compared to other particles, therefore, the electrons will have a higher relativistic speed when they enter the detector, i.e. a larger ß value, meaning that the electrons will emit Cherenkov radiation, while other particles will not. This is the general operating principle behind all Cherenkov radiation detectors. There are several aspects that make the GRINCH detector unique. Chief among these is the GRINCH detector s ability to operate in a high background environment while simultaneously registering the high sampling rates required to be used in conjunction with the BigBite Spectrometer. Additional differences will be described in subsequent portions of this paper. My role in the GRINCH detector has involved varying aspects of the construction and testing of the detector vessel itself and the associated apparatuses such as the data acquisition system, the photomultiplier tube (PMT) array, and the mirror assembly that will eventually be housed within the detector vessel. Detector Vessel The GRINCH detector vessel itself is not particularly unique in comparison to other Cherenkov detectors. It is constructed from steel and aluminum and has four openings: a particle entry window and a particle exit window as well as a window for the installation of the photomultiplier tube array and a door for access to the sliding mirror assembly, both of which will be discussed at length later. A project that I have participated in heavily over the course of the summer is the sealing of the detector vessel. The vessel must be sealed completely in order to prevent light contamination of the interior, as well as to prevent gas leaks, as both of these issues have the potential to negatively influence the detector s performance. The actual sealing of the vessel is accomplished through the use of an epoxy that is made up of sealant and a hardening agent that are mixed in a precise stoichiometric ratio along with black dye to produce a flexible epoxy that will, in theory, flex and bend with the natural movement and manipulation of the detector without cracking and breaking the seal. This epoxy has been applied or will be applied at some point in the future to every possible point of ingress for gas or light contamination into the detector, including every bolt hole, the edges of all of the windows, and around the door. The door itself presented a particular problem to sealing as it is designed to be taken off of the detector to allow access to the interior of the vessel, meaning that we could not simply epoxy around the edges of the door to seal it to the detector vessel as we had done for other parts of the detector. Additionally, the door is mounted onto a set of iron bars that hold it in place on the side of the detector meaning that the alignment of the door is subject to change if the iron bars are not sturdily secured. This problem was thought to have been solved through the implementation of a brace system that holds the iron bars securely against the inside of the detector allowing for accurate alignment of the door as well as making

the process of taking the door on and off of the side of the detector much easier. Although this has presented a partial solution, more work is needed to completely resolve the issue.

3 the process of taking the door on and off of the side of the detector much easier. Although this has presented a partial solution, more work is needed to completely resolve the issue. When I began working on the GRINCH detector project, the vessel was completely unsealed with none of the bolts or panels in their respective positions and properly aligned on the vessel. Currently, all of the panels are installed, aligned, and sealed, the door has been properly aligned and sealed, and the vast majority of other sealing projects have been completed. An important part of the vessel construction that has taken part this summer aside from the sealing of the detector is the construction of the particle entry and exit windows. The windows are made from thick aluminum foil and give the particles a suitably thin material to travel through on their way to the interior of the detector vessel so as to reduce as little of the particles energy as possible, while also maintaining the integrity of the light and gas seals. Initially, a third-party manufacturer was contracted to Figure 2: A front view of the detector vessel with the particle entry and exit windows in the front and back, respectively. The PMT window is visible on the left, and the handles for the door are visible on the right. cut the aluminum foil to size and make the necessary bolt hole cuts to allow the windows to be mounted into the detector. However, the windows delivered by the

Figure 3: The removable door of the detector vessel manufacturer were found to have surface defects that were deemed unacceptable for installation in the detector, thus the construction of the

4 Figure 3: The removable door of the detector vessel manufacturer were found to have surface defects that were deemed unacceptable for installation in the detector, thus the construction of the windows became a job that fell to my advisor and me. Luckily, the manufacturers of the original windows had included metal blanks in their shipment of the windows that were the appropriate size and had the correct number of bolt holes around the edges for the creation of duplicate windows. To make the windows, we simply rolled out a suitably-sized piece of the aluminum foil and pressed it between the metal blanks to create a template for cutting the foil to the desired size. We then used a scalpel to cut the edges of the foil down to size and used a sharpened three-eighths inch striker and hammer to place the necessary holes along the edges of the windows for the bolts to mount the windows into the detector. Currently, construction has been completed on both windows; however, installation of the windows will not take place until near the end of the detector project as the windows are quite fragile and also require an environment considerably cleaner than the area in which the detector vessel is currently housed. Mirrors In order to ensure that the photomultiplier tube array attached to the detector vessel sees the maximum amount of light from the Cherenkov radiation emitted by electrons in the detector, there is a mirror assembly on the interior of the detector vessel that is used to reflect light onto the PMT array. The entire mirror assembly is made of two component parts, the mirror array and a sliding rail system. The rail system holds

the mirror array in place inside the detector vessel, as well as having the ability to slide out of the detector to allow access to the interior of the detector vessel in the event Figure 4: The

5 the mirror array in place inside the detector vessel, as well as having the ability to slide out of the detector to allow access to the interior of the detector vessel in the event Figure 4: The detector vessel with the rail system slid out of the door, including the rectangular mirror frame that any maintenance is needed. When I began working on the GRINCH detector the sliding rail system was still in its component pieces in the boxes in which it had been delivered; however, over the course of the summer both the top and the bottom portions of the rail system have been assembled and installed in the detector vessel. The second part of the mirror assembly are the mirrors themselves. This array consists of four mirrors housed in a rectangular frame that are responsible for reflecting the maximum amount of light possible onto the photomultiplier tube array as the electrons emit Cherenkov radiation. The mirrors were originally intended to be made of thin sheets of aluminized metal; however, it was determined that this material did not meet the desired specifications for reflectivity. Thus, it was eventually decided that the mirrors would be made of slightly thicker pieces of aluminized polycarbonate. Eventually, it was discovered that rectangular frame employed to hold the mirrors in place was too tall to fit inside the sliding rail system. To remedy this problem, we cut 0.25 off of both of the vertical posts of the frame of the mirror array, which, rendered the frame slightly too short. This issue was eventually resolved when we created a set of shims that are each 0.15 thick that fit inside the mirror array frame giving the frame the correct total height. After we had ensured that the frame for the mirror array was the correct dimensions to fit within the detector vessel, the next task was to align the mirrors

so that they provide the maximum amount of light reflected onto the photomultiplier tube array. This is an essential part of the design of the GRINCH detector.

6 so that they provide the maximum amount of light reflected onto the photomultiplier tube array. This is an essential part of the design of the GRINCH detector. Because it is designed to work in a high background environment, it is essential that as much light as possible be collected by the photomultiplier tubes in order to ensure that the maximum amount of good data is collected. The more data that is collected, the easier it is to Figure 5: Illustration of the mirrors and the frame that mounts into the sliding rail system. Each mirror is cylindrical with a radius of curvature of 130 centimeters. The mounts on the left side of the mirror frame in the image are adjustable, allowing for proper alignment of the mirrors. differentiate these good events from the background. Therefore, the mirror alignment has been a crucial part of the project over the summer. The mirrors have multiple degrees of freedom of movement when attached to their frame, so the alignment of each mirror to a precise location is a methodical and time consuming process. Each mirror must reflect all the light it collects onto a target area that is approximately eight inches wide by ninety-six inches tall. In addition to this requirement, there is a particular pattern of light reflection that each mirror is expected to produce on the target area. To test the alignment of each mirror, we assembled the mirror frame in a clean room at William & Mary along with a linear translation system with a green laser attached. The translation system is then placed at different heights on a frame placed the same distance from the mirror assembly as the particle entry window will be from the mirror

array when the mirror array is eventually placed in the detector vessel. This way we can ensure realistic results of how the mirrors will behave and at what angles they will reflect light.

7 array when the mirror array is eventually placed in the detector vessel. This way we can ensure realistic results of how the mirrors will behave and at what angles they will reflect light. The translation system and laser allow us to make horizontal sweeps across the laser and the frame for the translation system allow us to test different vertical heights of the mirrors, therefore, we can test essentially any point on each mirror to determine how it behaves in different alignment configurations. Once this system was in place, we began the process of adjusting each mirror until it reflected all of the light it collected into the target area and created a reasonable pattern of reflected light. Having a specific pattern of collected light is necessary because, unlike with other Cherenkov radiation detectors, we want the light reflected from the mirror array to hit multiple photomultiplier tubes, thereby giving us a cluster of data. The importance of these clusters will be described in a separate section of this report, but having the mirrors aligned so as to create this pattern within the constraints of the target area is necessary for the success of the detector. Figure 6: A front-on view of the laser translation system and the mirror array.

8 When I began working on the GRINCH detector project, the mirrors had already been aligned. However, that was before it was determined that a different material was to be used for the mirrors, and also before it was determined that the frame for the mirror assembly was too tall. Additionally, the configuration of the mirrors in relation to the target area had not been the same in the original alignment as the mirrors will be once they are installed in the detector. Thus, it was necessary to redo the mirror alignment in the modified configuration with the shorter frame and the polycarbonate mirrors. Currently, all of the mirrors are correctly aligned in their frame; however, the final mirrors will not be installed in the detector vessel until much later in the construction process as once they are installed, it will be become considerably more difficult to work on the interior of the detector vessel. Figure 7: An illustration of the desired pattern that reflected light makes on the target area. Ideally, this elliptical pattern would be as flat as possible and as wide as possible while staying within the bounds of the target area. This image was collected by the W&M group from the original aluminum mirrors.

9 Photomultiplier Tubes (PMTs) Photomultiplier tubes are common in experiments used in the detection of light. Essentially, a photon is absorbed by the PMT, causing an electron to be emitted inside of the PMT. This electron is amplified through the use of multiple dynodes before it is deposited onto an anode creating an electrical signal. Photomultiplier tube arrays are an essential part of all Cherenkov detectors. However, the typical gas Cherenkov detector makes use of a handful of fairly large diameter PMTs, whereas the GRINCH detector makes use of 510 PMTs that are all 31 millimeters in diameter. Figure 8: An illustration of the box that houses the PMTs, giving an idea of the scale of the number of PMTs used and their configuration. The box is 13 wide by 76 tall by 10 deep. The PMT array is housed within a box that will eventually be mounted onto the side of the detector in the PMT window that was mentioned earlier. A key facet of the design of the PMT array is that the rows of tubes are separated by bars made of iron and a material known as mu-metal. This is because the GRINCH detector will be operating in an environment with a relatively strong magnetic field. Magnetic fields can affect the performance of photomultiplier tubes as they can seriously affect the trajectory

10 of the electrons emitted inside the tubes. Once in use, the GRINCH detector s PMT array will be responsible for collecting light reflected onto them by the mirror array as the light is produced from Cherenkov radiation. This task, like any in the detector construction process, comes with its own set of of challenges and issues that had to be addressed over the course of the summer. First of all, the box for the PMT array, like the detector vessel itself, must be completely sealed to prevent light contamination and gas leaks. This process of sealing is accomplished in the same way as that for the detector vessel; we mix a batch of epoxy, put this epoxy in thirty milliliter syringes and apply it to all of the seams and joints or any place in the PMT box where light or gas could possibly escape. Over the course of the summer the entire PMT box was sealed with epoxy, except for the seam where the box will connect to the exterior of the detector as this will be left for later in the construction project. The next challenge is that it is essential that the photomultiplier tubes see as much light as possible. There are multiple steps that have been taken to ensure that the tubes absorb the maximum amount of light, thereby giving us the maximum amount of good data. The first step taken to ensure that the PMTs absorb the maximum number of photons is to fit the area Figure 9: A set of the light catchers that will be fitted around each PMT. The light catchers are molded into strips so that there is one for each row of PMTs. In the foreground are the light catchers, while in the background is the PMT array box and the magnetic shielding bars around each photomultiplier tube with reflective material so that any light that is reflected into the target area by the mirror array, even if said light is reflected into an area where a PMT does not directly, will be directed onto the face of a tube, increasing

11 the chance that we will see good data. These reflective implements take two forms: first, there will be reflective strips on the top and bottom of each bar on which the PMTs will rest, and second, is a system of conically-molded reflective material that will be fitted around each PMT. These molded reflective pieces are known as light catchers. Initially, we had multiple options for the material to be used for the reflectivity around the PMT array, so we began the task of doing reflectivity testing on the materials that had been chosen as acceptable options. Eventually, after extensive reflectivity testing with multiple wavelengths of light and varied angles of reflectivity, one material was shown to be a more efficient reflector of light at all possible angles and all tested wavelengths of light. Presumably this is the material that will eventually be used for the reflective materials around the PMT array. The second part of ensuring that the PMTs absorb the maximum amount of light involves the mechanics of how the photomultiplier tubes themselves operate. Photomultiplier tubes come in a variety of sizes and capabilities, and, in particular, different PMTs have different wavelengths of light that they are more efficient at absorbing than other wavelengths. What percentage of a given wavelength of light a particular photomultiplier tube absorbs is called the quantum efficiency, and this quantum efficiency and the improvement of this value has been a part of the GRINCH detector project thus far. The PMTs that are eventually going to be used for the GRINCH detector have a maximum quantum efficiency of approximately 28% meaning that the best case scenario is that only 28% of the photons that hit the face of a given PMT will result in the emission of an electron within the PMT and the output of an electrical signal. With this in mind it is easy to see why it is important that we get the maximum amount of light onto the face of each PMT. The PMTs that will be used for the GRINCH detector are most efficient at absorbing light in the visible spectrum, which is unfortunate because the emission of light in the form of Cherenkov radiation intensity behaves as a function of one divided by the wavelength of the light squared. Essentially, this means that the vast majority of photons emitted in the form of Cherenkov radiation are in the ultraviolet spectrum of light, which is an area on the spectrum where the PMTs have a low quantum efficiency. This is an ongoing area of concern in the process of the detector construction; however, some previous work done by the William & Mary group has suggested that we could significantly increase the efficiency of our PMTs in the ultraviolet spectrum with the application of wavelength-shifting (WLS) paint to the face of the photomultiplier tubes. Essentially, this paint absorbs photons and then reemits them onto the face of the PMT at a larger wavelength, where the PMT is more effective at absorbing light. Currently, paint has not been applied to the PMT faces and a final decision on the type of paint to be used has not been made; however, some type of WLS paint will certainly be applied to the PMTs before the detector starts taking experimental data.

12 Quantum Efficiency Quantum Efficiency vs λ (nm) (275nm ~ 635nm) χ 2 / ndf / 5 Prob 1 p ± p ± p ± p e-05 ± e-06 p e-08 ± e-08 p e-11 ± e-12 p e-15 ± e Photon wavelength λ (nm) Figure 10: Quantum efficiency versus wavelength of the PMTs to be used for the GRINCH. As can be seen, the PMTs absorb almost nothing below 275 nm. Data Acquisition System (DAQ) The data acquisition system for the GRINCH detector is another way in which it differs from many traditional Cherenkov radiation detectors. The DAQ system for the GRINCH detector operates with time-to-digital converters (TDCs), meaning that the meaningful data that is collected is in the form of when specific events, such as PMT electron emission started and when it ended. These events can be set to trigger the data acquisition system so that when an event takes places, all of the data from that particular moment in time is recorded and stored. Even though the GRINCH detector itself is not fully assembled, we have a working prototype of the detector in order to test the effectiveness of our data acquisition system. Initially, the GRINCH was going to use a FastBus TDC system, which is well-established technology and is the standard used for experiments that require a TDC system that can handle large data rates. However, scientists at Jefferson Lab have recently developed a newer TDC system known as the VETROC. We have been testing the VETROC to determine if it would be more advantageous to use this system over the FastBus system as the VETROC is capable of a much higher sampling rate. Testing of the VETROC system has been slowed by numerous technical difficulties as it is a relatively new technology that isn't completely understood at the present moment. We have recently obtained promising results from the VETROC system that would seem to indicate that it would be at least as effective as the FastBus TDC.

13 Aside from the time-to-digital converter system, another crucial part of the DAQ is the NINO cards. The NINO cards are front-end discriminator and amplifier cards that are used to generate digital signals from the small analog output of the PMTs. This is done because our signal from the PMT has to travel through several hundred feet of cable before it reaches the DAQ system, and digital signals travel far better over distance than do analog signals, particularly analog signals of the small scale seen from the PMTs. The photomultiplier tubes give an analog signal that is then fed into the NINO discriminator cards. These NINO cards have a threshold setting that we can manipulate within a certain range, and if the analog signal sent to the NINO card is below this predetermined threshold, there will be no output from the NINO card. However, if the analog input to the card is higher than the threshold setting then the NINO card will produce a digital pulse. This pulse is then used as an input to the TDC system which measures both the width and timing of these pulses to create a histogram of the triggers that are seen by the photomultiplier tubes as a function of time. Figure 11: The NINO cards are seen in the center of the image attached to the prototype detector prototype, with the VETROC and computer systems housed in the blue rack on the right side of the image.

Conclusions and Future Work The GRINCH detector is very much still a work in progress and will continue to be under construction and in testing for the immediate future.

14 Conclusions and Future Work The GRINCH detector is very much still a work in progress and will continue to be under construction and in testing for the immediate future. However, the progress made in construction and testing over the past ten weeks, in my opinion, cannot be overstated. When we started work at the beginning of the summer the detector was essentially just a shell and a collection of parts in delivery boxes still to be assembled. Over the course of the summer, we managed to complete the vast majority of the sealing of both the detector vessel and the PMT box, a task that while not necessarily technically difficult, required a significant amount of time. We also managed to extensively test and align the mirror array system, as well as install the sliding rail system for the mirror assembly. We also fabricated both the particle entry and exit windows, pieces of the detector that will not necessarily be installed in the near future, but are still very important for the overall construction of the GRINCH detector. Perhaps most importantly, we managed to extensively test the VETROC data acquisition system and were able to see meaningful data and results as an end product of this laborious testing process. There are obviously aspects that we would have liked to have completed over the summer that never came to fruition, particularly the testing and application of the wavelength shifting paint. However, overall we have made great strides towards full functionality over the past ten weeks. Vessel PMT array Electrons Figure 12: An illustration of the completed GRINCH detector with the particle entry window and the PMT array box visible Acknowledgement and thanks to Dr. Todd Averett, Dr. Carlos Ayerbe-Gayoso, and Scott Barcus. This work was supported in part by the National Science Foundation under Grant Nos. PHY , and , as well as the U.S. Department of Energy.

15 References Averett, Todd D. Update on the Gas Ring-Imaging Cherenkov GRINCH Detector. SBS Collaborators Meeting. 22 July Lecture. 31 July Barcus, Scott K. GRINCH Mirror Technical Report 09/10/2014. Tech. 1 August Barcus, Scott K. Technical Report on Wavelength Shifting Paint for GRINCH PMTs 7/20/15. Tech. 1 August Barcus, Scott K. VETROC Tests Report. Tech. 1 August Blankenship, Brandon T. Gas Ring-Imaging Cherenkov (GRINCH) Detector Construction. 6 July Presentation. 31 July Leo, William R. Techniques for Nuclear and Particle Physics Experiments: A How-to Approach. Berlin: Springer-Verlag, Print.

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