Evaluating deuterated-xylene for use as a fusion neutron spectrometer

The spectrum of neutrons emitted by thermonuclear plasmas encodes information about the fuel ion distribution function. Measuring these fast neutron spectra with sufficient resolution allows for the measurement of plasma properties such as the ion temperature and strength and energy of fast ion populations. Liquid organic scintillators are a commonly used fast neutron detection technology because of their high detection efficiency and ability to discriminate between neutrons and gammas. However, performing detailed spectroscopy with these detectors is difficult because of the isotropic nature of neutron scattering on protons, the dominant mechanism of interaction. Deuterium-based scintillators have shown promise as a superior spectrometer technology because of the anisotropic nature of neutron scattering on deuterium, which significantly improves the condition number of the detector response matrix [Lawrence et al., Nucl. Instrum. Methods Phys. Res., Sect. A 729, 924 (2013)]. Deuterated-xylene, now available commercially, has advantages in light output and safety over benzene-based deuterated scintillators [Becchetti et al., Nucl. Instrum. Methods Phys. Res., Sect. A 820, 112 (2016)]. We present experimental spectrum unfoldings made by 2 in. right cylindrical protiated-xylene and deuterated-xylene detectors with response matrices generated with Geant4 and additional data from the literature. We compare their performance by measuring the neutron spectrum produced by an AmBe source and deuterium-tritium (DT) neutron generators. We find that the deuterated scintillator outperforms the protiated one for AmBe and DT spectra, suggesting deuterated-xylene should be considered for future fusion neutron spectrometry applications.