Constructing and calibrating a neutron detector using cosmic rays

1. Abstract The particles that make up the nucleus of an atom are so infinitesimally small that it takes a detector of large magnitude in order to predict where protons and neutrons are located. This certain device requires the construction and utilization of one-meter-long rectangular plastic bars called scintillators. Attached to the left and right ends of each bar are Photomultiplier Tubes (PMTs) and bases with outlets for high voltage and signal wires. In experiments, there is a thick wall of lead positioned in front of the detector which excludes nearly all charged particles and permits primarily neutrons to enter and react with the nuclei inside the bars. When charged particles do pass through the scintillators, photons are released and bounce around until they reach a light guide and are collected by the PMTs. Within these there is liberation of electrons which in turn provide an analog signal to the electronics. A data acquisition system (DAQ) comprised of ADCs (Analog-to-Digital Converters) and TDCs (Time-to-Digital Converters) then store the data into files for later replay and analysis. By doing so, we can better measure the type of particle detected, it’s trajectory, and the amount of energy that it deposits. To ensure that these complex apparatuses are working at an acceptable level, scientists manipulate the constant flux (100 particles/m2·s) of cosmic rays. Because they constantly bombard the atmosphere and collide with other particles, muons fall at a steady rate and can be easily detected by the scintillators and determine the accuracy of the devices. Once the neutron detector is fully constructed and calibrated, it will be run in future experiments such as E07-006 (Short Range Correlations for the Triple Coincidence (e, e’pn) Reaction) for detecting neutrons released in particular collisions. Constructing and calibrating a neutron detector using cosmic rays Cosmic Rays Construction Calibration • Energetic particles from space impinging on Earth’s atmosphere • 90% protons; 9% helium nuclei; 1% electrons, heavier elements, and gamma ray photons • Emitted largely from solar flares as individual particles, not rays • Can reach energies of over 1020 eV • Collide with interstellar matter and split into lighter nuclei (cosmic ray spallation) • Decay into smaller particles such as pions, neutrinos, and muons • Produce a cascade of lighter particles called an air shower • The Hall A Neutron Detector (HAND) was originally designed with 4 layers of scintillators with 17% detection efficiency • 2 new layers (HAND 2), composed of 24 scintillators, and a thinner lead wall will be used to reach an efficiency of ~30% • An extruded aluminum l frame was built around each layer to guarantee no jostling or interference during experimentation • The veto layer allows the detector to filter out unwanted electrons and protons • Voltage readings from a PMT roughly correspond to ADC channels • The purpose of calibration is to correlate those voltage readings with the energy of the detected particles and have uniform readouts for all PMTs • Each PMT base pair is unique and needs a slightly different “gain” • Gain refers to the amount of voltage output for a given particle energy input • If the gains are too low, then the voltage to the PMT is increased, and vice versa • Through the process of gain matching, an optimum high voltage setting is determined for individual PMTs Matthew Mendonca Woodside High School Mentor: Dr. Doug Higinbotham and Lawrence Selvy Electronics • After the PMT gives out a signal, it is intensified by the amplifier • If the pulse is <50mV then the discriminator will disregard the signal • The delay cables prevent the ADC from taking data until the Trigger is activated and the TDC begins counting • The trigger supervisor, TDC, and ADC send their results to the computer for storage and analysis • ~800 ns worth of data is collected for one particular pulse High Voltage Scintillator Base PMT x10 Amplifier 2-Output Split Blueprint/Diagram of HAND Updated construction with 6 planes Delay Cables Discriminator 2-Output • Ethyl and Isopropyl alcohol are squirted on the PMTs and wave guides to pristinely clean the surface for no obstruction, and Elastosil glue attaches them together • Black electric tape wrapped around white computer paper covers every inch of the plastic so that it is light tight • Light testing is done to make sure there are no holes in the cover • High voltages of 1300-2000V are inputted into the base of the PMT in order to check that they will properly detect cosmics ADC TDC Steps for Relative Calibration Fit the pedestal, located at TDC channel 0, with a Gaussian curve (above left) Use the mean of the Gaussian to zero the ADC plot Fit the ADC plot, minus the pedestal, with a Landau curve (above right) Extract the gain from the MPV of the Landau Use the gains of various voltages to plot the gain curve Logic Unit/Trigger Trigger Supervisor Fast Bus Crate Conclusion Through the process of observing cosmic rays, we can prepare a neutron detector to be used in the experimental hall. Although the flow of these particles is entirely random, we can create a relative calibration to better understand the dynamics of the Hall A Neutron Detector. DAQ Special thanks to Doug Higinbotham, David Abbott, Or Chen, David Anez, Vincent Sulkosky, Navaphon (Tai) Muangma, Eliazer Piasetcky, Elena Long, and Aidan Kelleher Cosmic ray reading on an oscilloscope Layout for light testing the scintillators Wire chamber that holds the electronics