The high-efficiency $\mathrm{\gamma }\mathrm{-}\mathrm{ray}$ spectroscopy setup ${\mathrm{\gamma }}^3$ at $\mathrm{HI}\mathrm{\gamma }\mathrm{S}$

Author-Highlights

• We have extended the existing NRF setup at HIγS at TUNL to combine large LaBr and HPGe detectors.

• NRF experiments with the mono-energetic beam in combination with Gamma coincidences are possible.

• We describe the changes to the experimental setup and data acquisition as well as data analysis.

• The performance of the new setup was assessed by investigating the nucleus 32S.

• We present a more precisely measured value for the branching ratio for the $1+\to 2+$ transition.

Abstract

The existing Nuclear Resonance Fluorescence (NRF) setup at the $\mathrm{HI}\mathrm{\gamma }\mathrm{S}$ facility at the Triangle Universities Nuclear Laboratory at Duke University has been extended in order to perform $\mathrm{\gamma }\mathrm{–}\mathrm{\gamma }$ coincidence experiments. The new setup combines large volume ${\mathrm{LaBr}}_3:\mathrm{Ce}$ detectors and high resolution HPGe detectors in a very close geometry to offer high efficiency, high energy resolution as well as high count rate capabilities at the same time. The combination of a highly efficient $\mathrm{\gamma }\mathrm{-}\mathrm{ray}$ spectroscopy setup with the mono-energetic high-intensity photon beam of $\mathrm{HI}\mathrm{\gamma }\mathrm{S}$ provides a worldwide unique experimental facility to investigate the $\mathrm{\gamma }\mathrm{-}\mathrm{decay}$ pattern of dipole excitations in atomic nuclei. The performance of the new setup has been assessed by studying the nucleus ³²S at 8.125 MeV beam energy. The relative $\mathrm{\gamma }\mathrm{-}\mathrm{decay}$ branching ratio from the 1⁺ level at 8125.4 keV to the first excited 2⁺ state was determined to 15.7(3)%.

Introduction

Structural features of excited nuclear states are reflected in their decay pattern and the partial decay widths ${\Gamma }_i$ of transitions to low-lying excited states or the ground state are directly linked to electromagnetic transition matrix elements. Thus, the single decay channels are sensitive to different components in the wave function. This is especially true in the case of transitions to low-lying excited states. For these levels the de-excitation takes place via different components in the wave function compared to the excitation from the ground state. Therefore, the observation of these transitions and the determination of branching ratios reveals important experimental information needed to provide stringent and sensitive tests to modern model calculations. The method of in-beam $\mathrm{\gamma }\mathrm{-}\mathrm{ray}$ coincidences in the spectroscopy of the $\mathrm{\gamma }\mathrm{-}\mathrm{decay}$ of excited states in combination with charged particle induced reactions has been proven to be a powerful tool for measuring even small branching ratios to excited low-lying states. Until now $\mathrm{\gamma }\mathrm{–}\mathrm{\gamma }$ coincidence experiments have been performed mainly in combination with particle-induced reactions [1], [2]. In such reactions high-lying J=1 states (in even–even nuclei) are often not or only very weakly excited, which strongly limits the investigation of the decay behavior of excitation modes such as the nuclear scissors mode [3], [4], [5] or the Pygmy Dipole Resonance [6], [7], [8], [9]. Consequently, for these excitation modes the decay behavior was not studied in detail so far. The reaction of choice for studying these modes is nuclear resonance fluorescence (NRF), which nearly exclusively populates J=1 states [10]. NRF performed with single $\mathrm{\gamma }\mathrm{-}\mathrm{ray}$ spectroscopy is primarily sensitive to ${\Gamma }_0{\Gamma }_0/\Gamma$ with ${\Gamma }_0$ being the decay width to the ground state and $\Gamma$ the total width. The observation of weakly branching transitions is difficult, especially in experiments using bremsstrahlung [11], [12], [13]. Therefore, the combination of an intense mono-energetic photon beam, which defines the excitation energy, with $\mathrm{\gamma }\mathrm{–}\mathrm{\gamma }$ coincidence spectroscopy of the following decays offers ideal conditions to investigate in detail the decay behavior of photo-excited states. First NRF experiments using the completely polarized $\mathrm{\gamma }\mathrm{-}\mathrm{ray}$ beam from a laser Compton backscattering (LCB) facility have been carried out by Ohgaki et al. [14]. The first application of this method to investigate parities of nuclear levels was done by Pietralla et al. [15], [16].

The principle of the $\mathrm{\gamma }\mathrm{–}\mathrm{\gamma }$ coincidence method in combination with a mono-energetic $\mathrm{\gamma }\mathrm{-}\mathrm{ray}$ beam is illustrated in Fig. 1. After excitation by photo absorption the high-lying state at excitation energies E_x may de-excite either directly to the ground state (${\Gamma }_0$) or via intermediate states (${\Gamma }_i$). By detecting two emitted $\mathrm{\gamma }\mathrm{-}\mathrm{rays}$ stemming from a cascade in coincidence, even small branching ratios ${\Gamma }_i/{\Gamma }_0$ can be determined with good precision since non-resonant background produced by the photon beam in the target via atomic processes is strongly suppressed.

This article reports on the installation and commissioning of a new experimental array, the ${\mathrm{\gamma }}^3$ setup, at the High Intensity $\mathrm{\gamma }\mathrm{-}\mathrm{ray}$ Source ($\mathrm{HI}\mathrm{\gamma }\mathrm{S}$ [17]). This array provides sufficient efficiency in order to perform $\mathrm{\gamma }\mathrm{–}\mathrm{\gamma }$ coincidence decay spectroscopy in combination with a high intensity, narrow bandwidth photon beam. The design of the ${\mathrm{\gamma }}^3$ setup is based on three principles: High efficiency, high energy resolution and high count rate capability. A setup with a high full energy peak efficiency is mandatory to investigate small branching ratios with sufficient statistics within a reasonable amount of time. This condition requires maximum solid angle coverage which, using few detectors, implies a close geometry and demands high count rate capabilities. High energy resolution is needed when investigating higher lying states in energy regions, where the level density is large. A combination of High-Purity Germanium (HPGe) and LaBr detectors in a close geometry fulfills these requirements.

The newly installed experimental setup is described in Section 2. Section 3 describes the different parts of the data acquisition hardware and software which are used for the ${\mathrm{\gamma }}^3$ array. The details of the analysis and the results of a commissioning measurement on ³²S are presented in Section 4.