Matthew Mendonca
Woodside High School
Mentor: Dr. Doug Higinbotham and Lawrence Selvy
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.
Cosmic Rays
Construction
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
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
Electronics
High
Voltage
Scintillator
Blueprint/Diagram of HAND
PMT
Base
x10 Amplifier
2-Output Split
Updated construction with 6 planes
Delay Cables
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
Calibration
Discriminator
2-Output
ADC
TDC
Logic
Unit/Trigger
Trigger
Supervisor
Fast Bus
Crate
 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
 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
DAQ
Cosmic ray reading on an oscilloscope
Layout for light testing the scintillators
Wire chamber that holds the electronics
Special thanks to Doug
Higinbotham, David
Abbott, Or Chen,
David Anez, Vincent
Sulkosky, Navaphon
(Tai) Muangma, Eliazer
Piasetcky, Elena Long,
and Aidan Kelleher
Steps for Relative Calibration
1. 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 Landau
5. Use the gains of various voltages to plot the gain curve
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.