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A Double Time of Flight Method For Measuring Proton Light Yield


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

National Laboratory.

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.

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