Veto Scintillating Detector Design Testing

As a part of the upgrades from PRad I, the new PRad II will have the addition of an extremely low angle Veto Scintillating Detector. This detector will consist of 4 actuating panels of scintillating material.These panels will be able to be brought in to cover extremely low angles for calibration. While the beam is running, the panels will retract and allow for differentiation between e-p and e-e events at these low angles. The ability to separate at these low angles will improve statistics for the proton radius measurement in a region that was previously unable to be separated in PRad I.

Currently, there are two potential designs prepared for this detector. The difference between the two is the thickness of the scintillating panels. The panels can either be 5mm thick or 3mm thick. The preference would be to use as little material as possible to lower multiple scattering effects, and minimize energy losses in this detector. However, it is currently being checked if the signal from the 3mm panels will be good enough to use during the experiment. See details for this project below.

Scintillating Panel Thickness Testing

In order to check the responses of the two scintillator thicknesses (3mm and 5mm) there will modules with one of each thickness constructed in Youri Sharabian's lab and then tested in the black box he has in his lab. The simultaneous testing will allow for direct comparisons to be made. The scintillator modules were set up so that the 3mm and 5mm paddles are directly in line with one another vertically while the light guides and components are not above each other. This allows for a DAQ trigger to be set to save only events where both modules experience an event in coincidence with each other. This means that the events are from the same particle which means passing through each module and their responses can be used to see the performance of each module. Ideally, the thinner 3mm module is to be used in the actual construction of the Veto Scintillator due to it introducing the least amount amount of material possible into the experiment. GEANT4 simulations have shown that the 3mm thickness should be adequate for the needs of PRad II, but a real hardware test is needed for final confirmation.

Originally, Aruni, Buddhiman, and Erik were provided with scrap scintillating material to practice gluing the larger surfaces without bubbles or any other issues. This also allowed us to check the transparency of the UV-activated glue we intend to use for the test and it seems to be good enough for the test especially since any effects could be expected to be equal in each module. The testing modules have since been constructed and wrapped with reflective material. Youri manufactured two holding units for holding the PMT and light guide of the module together under a small amount of pressure from a stiff foam. This pressure is necessary due to the contact being only maintained with optical grease, which while ensuring a good optical contact, is not an adhesive. This same type of contact will be used in the final construction, so the intent is to test the module under the same conditions. These holding units allow for the test to be set up and stable. Armen Stepanyan of the Fast Electronics Group also terminated the PMT dividers to the HV and LEMO connections needed for the test. Sergey Boyarinov was asked by Youri to set up the DAQ for two scintillator thickness test.

On March 05, 2025 Youri with the aid of Aruni, Buddhiman, and Erik were able to collect various signals from the two scintillating modules with varied high voltage supplies. This was done to show that there would be an adequate response on the modules of either thickness, and to help determine the running voltage for the runs with the DAQ that Sergey set up. An oscilloscope was used for this first order check. This preliminary check agreed with simulated findings by Yuan Li where the 3mm response was adequate for the the final design of the Veto Scintillator. To see these preliminary results, please refer to the first set of slides linked below. On March 06, 2025 the data taking with the DAQ began to have an ample number of signals collected to compare the responses in coincidence with one another to evaluate the performance of the modules. The 3mm module was run at 1300V and the 5mm module was run at 1250V.

Slides for the Thickness Testing Results

• Veto Scintillator Thickness Testing Preliminary Results Slides, March 05, 2025
• PRad Collaboration Weekly Meeting Slides 03/07/2025 (by Erik Wrightson). Please see slides 1-7 for information on the Veto Scintillator Thickness Testing.

Contributors

• Youri Sharabian (Hall B, JLab)
• Aruni Nadeeshani (Mississippi State University)
• Buddhiman Tamang (Mississippi State University)
• Erik Wrightson (Mississippi State University)
• Armen Stepanyan (Fast Electronics Group, JLab)
• Sergey Boyarinov (Hall B, JLab)
• Yuan Li (Shangdong University)

Veto Scintillator Alignment Tolerance

Due to the nature of the Veto Scintillating Detector needing to reach very low angles, there is the reasonable worry of how well aligned the scintillating modules must be with HyCal and each other to ensure the detector's ability to actually discriminate the Møller (e-e) events from the proton-electron events at these low angles. Initial GEANT4 simulations undertaken by Yuan Li showed that the alignment quoted as achievable by Hall B staff of 0.5mm would be more than well within the needs if the alignment of the scintillator before an impact was seen on the ability of the detector to be used to reject Møller events. These results were preliminary and would require higher statistics to confirm. The rest of the simulation and analysis was done by Buddhiman Tamang. Doing this he was able to further confirm the findings of Yuan. These simulations were done by running e-e and e-p events through the PRad Apparatus geometry in GEANT4 with the Veto Detector perfectly aligned, rotated by 1 degree, and rotated by 2 degrees. For refernce 1 degree out of alignment translates to being 0.87mm out of alignment at the edge of the crystal.

After the initial findings confirmed that the alignment needs could be met for the critical low angle event, there was some interest in understanding the impact of alignment on the events at the upper reaches of the Veto Detector around 1 degree off of the beam line. This required that many more events be generated in the angular regions above 0.7 degrees from the beam line. While this targeted generation would make the samples not realistically representative, the final goal was to look at a relative value (the rejection efficiency) so the absolute numbers do not need to be as realistic as long as the ratio in each angular bin is. Even with the selected generation, there is the complication that at these higher angles is when there starts to be a very drastic difference in energy for the products of e-e and e-p events. This is good for the experiment as a more simple energy cut can be used to veto Møller events, but it made getting enough statistics generated that were of an energy to be ambiguous more difficult. After generating this new subset of data, Buddhiman showed that the rejection efficiency at these further off beam line angles was also not sensitive to a one or two degree misalignment. See images below:

Overall, this work has concluded that the 0.5mm precision quoted by Hall B staff at JLab will definitely be sufficient towards meeting the needs of the Veto Scintillator.

Slides for Alignment Tolerance Simulation

• PRad Collaboration Meeting Veto Scintillator Update 03/03/2025 by Buddhiman Tamang. Please see slides 3 and 7-12 for relevant information on this project.

Contributors

• Buddhiman Tamang (Mississippi State University)
• Dipangkar Dutta (Mississippi State University)
• Yuan Li (Shangdong University)