A Double Time of Flight Method For Measuring Proton Light Yield
- Author(s): Brown, Josh Arthur
- Advisor(s): Vujic, Jasmina
- et al.
Organic scintillators have been used in conjunction with photomultiplier tubes to detect fast
neutrons since the early 1950s. The utility of these detectors is dependent on an understanding
of the characteristics of their response to incident neutrons. Since the detected light in
organic scintillators in a fast neutron radiation field comes primarily from neutron-proton
elastic scattering, the relationship between the light generated in an organic scintillator and
the energy of a recoiling proton is of paramount importance for spectroscopy and kinematic
imaging. This relationship between proton energy deposited and light production is known
as proton light yield.
Several categories of measurement methods for proton light yield exist. These include
direct methods, indirect methods, and edge characterization techniques. In general, measurements
for similar or identical materials in the literature show a large degree of variance
among the results. This thesis outlines the development of a new type of indirect method
that exploits a double neutron time of flight technique. This new method is demonstrated
using a pulsed broad spectrum neutron source at the 88-Inch Cyclotron at Lawrence Berkeley
The double time of flight method for proton light yield measurements was established
using two commercially available materials from Eljen Technology. The first is EJ-301, a
liquid scintillator with a long history of use. Equivalent materials offered by other manufacturers
include NE-213 from Nuclear Enterprise and BC-501A from Saint-Gobain Crystals.
The second material tested in this work is EJ-309, a liquid scintillator with a proprietary
formulation recently introduced by Eljen Technology with no commercial equivalents. The
proton light yield measurements were conducted in concert with several system characterization
measurements to provide a result to the community that is representative of the material
itself. Additionally, the errors on the measurement were characterized with respect to systematic
uncertainties, including an evaluation of the covariance of data points produced and
the covariance of fit parameters associated with a semi-empirical model.
This work demonstrates the viability of the double time of flight technique for continuous
measurement of proton light yield over a broad range of energies without changes to the
system configuration. The results of the light yield measurements on EJ-301 and EJ-309
suggest answers to two open questions in the literature. The first is that the size of the
scintillation detector used to measure the proton light yield should not effect the result if
the spatial distributions of Compton electrons and proton recoils are equivalent. Second, the
shape of the scintillation detector should not effect the light yield with the same constraint
on the spatial distributions.
A characterized hardware and software framework has been developed, capable of producing
proton light yield measurements on additional materials of interest. The acquisition,
post processing, error analysis, and simulation software were developed to permit characterization
of double time of flight measurements for a generic system, allowing it to be utilized to
acquire and analyze data for an array of scintillation detectors regardless of detector size or
geometric configuration. This framework establishes an extensible capability for performing
proton light yield measurements to support basic and applied scientific inquiry and advanced
neutron detection using organic scintillators.