Construction of a 1 MeV Electron
Accelerator for High Precision
Beta-Decay Studies
REU Student: Brenden Longfellow, University of North
Carolina at Chapel Hill
Advisor: Albert Young, North Carolina State University
Neutron Beta-Decay
n → p + e - + νe
pfnicholls.com
sprawls.org
Detector Calibration
 Beta-decay energy calibration for detectors typically
established with conversion sources (Cd-109, Ce-139, In114m, Sn-113, Sr-85, Bi-207)
 Internal Conversion: excited nucleus interacts
electromagnetically with electron in lower atomic orbital,
ejecting it
 Achieved by placing mylar foils of conversion sources into
spectrometer (next slide)
Young
Problem and Solution
 Calibration points are not evenly distributed over beta
energy spectrum and foil backing produces
perturbations in calibration spectrum
 For improvement, use external, tunable electron
beam, coupled by magnetic field to calibrate detector
Electron Accelerator
 Pelletron Charging System: particle accelerator in which
charge is induced on chain of metal pellets connected by
insulating nylon links
 Electron Gun: 104 electrons/s with energy range of 50 keV
to 1 MeV; pulsed at 10 kHz rate with few ns width
 Magnetic field in spectrometer of 1 T and guiding fields of
0.01 to 0.05 T for electron gun to create range of pitch
angles (can be determined by spread in arrival time)
Electron Accelerator
Westerfeldt
Pelletron
 Chain rotates on two wheels, driven by motor
 Charge induced on chain as it leaves grounded end by
inductor (negatively charged electrode biased by
high-voltage supply)
 As wheel rotates, contact between pellets and wheel
is broken and positive charge is trapped on the pellets
by the insulating nylon connecting links
Westerfeldt
Pelletron
 Charged pellets pass another electrode as they arrive
in terminal causing electrode to develop mirror
(negative) charge
 Conductive pickoff wheel underneath electrode picks
up charge as chain passes and applies it to inductor
on opposite side of terminal wheel
 This inductor (positive) induces negative charge on
pellets leaving terminal
Westerfeldt
Pelletron
 Charged pellets arriving in terminal contact
conductive rim of terminal pulley, transferring charge
to terminal
 Pellets leaving terminal that have been inductively
charged by positive inductor double charging
efficiency
 High voltage built up at terminal is used to accelerate
charged particles
Progress
 Attached electrodes to accelerator column, and
installed motor control system
Progress
 Developed tensioning system for motor to provide
sufficient tension to chain (first iteration failed to
provide enough tension, second iteration currently in
machine shop)
Progress
 Used COMSOL model of accelerator column
geometry to simulate electron response
 For testing, tensioned motor sufficiently by brute
force
Results of Testing
 Generated current of 7 μA through terminal
 Resistor string of 30 GΩ gives voltage across column
of 210 kV
Next Steps
 Replace current motor with smaller one and install
tensioning system
 Enclose accelerator in pressure vessel
 Create map of magnetic fields for electron
accelerator and spectrometer
 Special thanks to Dr. Chris Westerfeldt for all of his
help with this project
 Any questions?