CONSTRUCTING AND CALIBRATING A NEUTRON DETECTOR USING COSMIC RAYS

Matthew MendoncaWoodside High School

Mentor: Dr. Doug Higinbotham and Lawrence Selvy

AbstractThe 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.

ConstructionThe Hall A Neutron Detector (HAND)

was originally designed with 4 layers of scintillators with 17% detection

efficiency2 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

experimentationThe veto layer allows the detector to

filter out unwanted electrons and protons

o 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 o Black electric tape wrapped around white

computer paper covers every inch of the plastic so that it is light tight

o Light testing is done to make sure there are no holes in the cover

o High voltages of 1300-2000V are inputted into the base of the PMT in order to check that they will

properly detect cosmics

Scintillator Base

PMT

High Voltage

x10 Amplifier2-Output Split

Discriminator

2-Output

Delay Cables

ADCTDC

Logic Unit/Trigger

Trigger Supervisor

Fast BusCrate

DAQ

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

Special thanks to Doug Higinbotham,

David Abbott, Or Chen, David Anez, Vincent Sulkosky, Navaphon (Tai)

Muangma, Eliazer Piasetcky, Elena Long, and Aidan

Kelleher

Cosmic RaysEnergetic particles from space impinging on

Earth’s atmosphere90% protons; 9% helium nuclei; 1% electrons, heavier

elements, and gamma ray photonsEmitted largely from solar flares as individual particles,

not raysCan 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

Calibration

Blueprint/Diagram of HAND Updated construction with 6 planes

Cosmic ray reading on an oscilloscope

Layout for light testing the scintillators

Wire chamber that holds the electronics

ConclusionThrough 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.

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

Steps for Relative Calibration1. Fit the pedestal, located at TDC channel 0, with a Gaussian

curve (above left)2. Use the mean of the Gaussian to zero the ADC plot

3. Fit the ADC plot, minus the pedestal, with a Landau curve (above right)

4. Extract the gain from the MPV of the Landau5. Use the gains of various voltages to plot the gain curve