Lineshape response of plastic scintillator to pair production of 4.44 MeV gamma's

TL;DR

This work addresses the energy-response characterization of a plastic scintillator for beta spectroscopy by using $4.44\, \mathrm{MeV}$ gamma rays from an AmBe source and tagging the resultant $511\,\mathrm{keV}$ annihilation photons in high-Z GAGG detectors to isolate the double-escape peak at $E_{DE}\approx 3.42\,\mathrm{MeV}$. The authors measure the centroid and width of this peak and examine the lineshape tail, finding that the observed tail is significantly larger than GEANT4 predictions, primarily due to neutron-induced backgrounds, and that calibration nonlinearity exists when including an IC electron point; they model the main resolution as photon-statistics plus a constant dark current. The results provide empirical benchmarks for the energy response of plastic scintillators in beta spectroscopy and highlight neutron-background limitations, guiding improvements such as using PMTs or faster timing and exploring neutron-discriminating materials. The findings have practical implications for designing and interpreting beta-spectroscopy setups and for improving MC simulations of scintillator response in neutron-rich environments.

Abstract

We measure the distribution of energy deposited in a 40x88 mm plastic scintillator by e+ e- pair production of 4.44 MeV gamma-rays. We observe the double-escape peak of 3.42 MeV from pair production by tagging 511 keV annihilation radiation in two high-Z scintillators. The source is a standard commercial neutron source using alpha-emitting 241Am encapsulated with 9Be, which has a reaction branch feeding the first Ipi=2+ state of 12C making the 4.44 MeV gamma-rays. We demonstrate the extraction of the double-escape peak from the large neutron-produced backgound, and explore some of the features and difficulties of this technique with our apparatus.

Figures (5)

• Figure 1: GEANT4 scale model of the experimental geometry, showing 40x88 mm plastic scintillator, two 5x5cm GAGG scintillators, AmBe source location, and borated polyethylene shielding. A pair production event is shown where both annihilation $\gamma$'s miss the GAGG's.
• Figure 2: Representative timing resolution of GAGG detectors with respect to the plastic scintillator (from 16 hr of data). Timing cut is indicated.
• Figure 3: Energy spectra of the GAGG detectors in timing coincidence with the plastic scintillator (note energy calibration and resolution are different). Cuts of $\pm 2 \sigma$ are shown around the annihilation peaks, along with background regions extending the next 5 $\sigma$ above and below the signal.
• Figure 4: Top: The main technique demonstration: energy spectrum of all plastic scintillator events in timing coincidence with both GAGG's, and requiring annihilation radiation photopeaks in both GAGG's. Singles spectrum, maximum likelihood fits, and GEANT4 tail simulation are discussed in text. Energy scale is determined in the Energy Calibration subsubsection. Bottom: The energy spectrum for background events in the plastic scintillator for events meeting timing cuts with 511 keV in one GAGG but more or less energy in the other GAGG.
• Figure 5: Top: Deviations from linearity of the energy calibration. The $^{207}$Bi IC electron point excluded is the one at 995 keV. Middle: Residuals of top plot, i.e. data divided by the fit with $^{207}$Bi excluded. Bottom: Energy resolution fit to photon statistics added in quadrature with dark current.