Data sorting modes of phoswich detector array

TL;DR

This work addresses high-energy γ spectroscopy in the $4-10$ MeV range using the PARIS phoswich array and systematically evaluates six data-sorting modes that combine Pulse Shape Analysis, external NaI vetoing, and intra-/inter-cluster add-back. It characterizes time resolution, energy resolution, and detection efficiencies, using GEANT4 simulations and clover HPGe data as references, for a setup of three PARIS clusters (27 phoswich detectors) in a β-decay study of $^{80g+m}$Ga. The results show that the outer NaI(Tl) veto effectively suppresses escape and pileup, while add-back within clusters can boost full-energy peaks; multiplicity gating further suppresses background but reduces peak statistics. These findings inform mode selection for high-purity γ spectroscopy with phoswich arrays and have implications for nuclear structure studies and astrophysical applications.

Abstract

The different data sorting modes of the phoswich detector array PARIS used for detecting high-energy (4$-$10 MeV) $γ$ rays are investigated. The characteristics including time resolution, energy resolution and detection efficiency under various modes are studied. The present study shows that PARIS has capabilities of rejecting escape and pileup events. Compared with the 2"$\times$2"$\times$2" LaBr$_3$(Ce) detector \cite{CIEMALA200976}, even in individual mode, PARIS provides significant suppression of single- and double-escape peaks and reduces background via vetoing function of the outer-volume NaI(Tl) crystals. In contrast to the common approach of adding back the energies in LaBr$_3$(Ce) and NaI(Tl) to increase the detection efficiency of the full-energy peak, using NaI(Tl) as a veto shield provides a superior trade-off for applications where spectral purity is essential. Employing add-back analysis within each cluster of nine phoswiches or between all phoswiches could enhance full-energy peak efficiency and further suppress escape peaks and background. Applying a multiplicity condition provides a further suppression but simultaneously lowers the statistics of full-energy peaks. Notably, the methods presented in this work refer specifically to the $β$-decay experiment of $^{80g+m}$Ga conducted with three PARIS clusters comprising 27 phoswich detectors, rather than to a general report on the PARIS array or its overall performance.

Figures (11)

• Figure 1: Schematic drawing of hybrid $\gamma$-ray spectrometer. Three PARIS clusters are shown, each comprising 9 phoswiches with the two-inch cubes of LaBr$_3$(Ce) backed by the six-inch length of NaI(Tl) and then the photomultiplier tube. The distances between source collection point and phoswich detectors of PARIS, clover HPGe and coaxial HPGe are 120, 74, and 50 mm, respectively.
• Figure 2: Ratio of output signals between Q$_{long}$ and Q$_{short}$, Q$_{long}$/Q$_{short}$, from one phoswich digitized by FASTER QDC. The inset shows the same spectrum on a logarithmic scale and a schematic drawing of one phoswich detector.
• Figure 3: $\beta$-gated $\gamma$-ray spectrum of $^{80}$Ge from PARIS, where energy signals from LaBr$_3$(Ce) and NaI(Tl) components of each phoswich detector are added together (internal add-back).
• Figure 4: Time-signal distributions of NaI(Tl) (the peaks with the lowest counts) and LaBr$_3$(Ce) (the peaks with the highest counts) channels relative to $\beta$-particle signal measured by a cylindrical plastic scintillator surrounding the source collection point with a time resolution of ps magnitude, arbitrarily aligned to the 80 ns position. 1, 2 and 3 are three separate phoswiches. Note that $\gamma$ rays of the cascade of isomer should be avoided when performing time alignment. The typical time resolution observed for LaBr$_3$(Ce) and NaI(Tl) is listed in Tab. \ref{['tab2']}.
• Figure 5: Time spectrum of the 467 keV $\gamma$-line (8$_1^+$$\rightarrow$ 6$_1^+$ transition in $^{80}$Ge) and its fitting. The black curve is the time spectrum of background $\gamma$ rays adjacent to the 467 keV $\gamma$-line, which is plotted as a reference. In these spectra, the time resolution of LaBr$_3$(Ce) crystal in PARIS and the delayed-tail of 467 keV $\gamma$-line are clearly distinguishable. The extracted half-life of the 3445.3(6) keV isomeric state, 8$_1^+$, of $^{80}$Ge in the present work is 3.08(6) ns.