Session JH: Instrumentation III

Mitigation of Beta-Gamma Summing in a Planar Germanium Double-Sided Strip Detector

Beta-decay spectroscopy experiments at fragmentation facilities are typically performed using a position-sensitive solid-state detector as a stopping medium for radioactive ion implantation. Subsequent beta decays are detected and correlated to the previously implanted ions based on position and time information. The results from these beta-decay spectroscopy experiments are pertinent to nuclear structure and astrophysics applications. To maximize the beta-decay detection efficiency a novel planar germanium double-sided strip detector (GeDSSD) has been implemented at the National Superconducting Cyclotron Laboratory. While the GeDSSD offers a beta-decay detection efficiency that will be close to 90{\%}, the detector also has a very high efficiency for low-energy gamma rays (15.7{\%} at 250 keV, for example). This leads to a large percentage of events in which the simultaneous energy deposition from the beta decay and gamma ray sum together in the GeDSSD. In order to mitigate the beta-gamma summing effects and recover the high gamma-ray detection efficiency, an algorithm has been developed in an attempt to separate the energy deposition of beta-decay electrons from gamma-rays. Results of the algorithm in both GEANT4 simulation and experimental data will be presented.   

Commissioning of a new decay-detection array/tape transport station for CARIBU

The CARIBU facility [1] at Argonne National Laboratory provides a unique opportunity for research in nuclear structure, nuclear astrophysics and applied applications. A new decay-detection array for performing $\beta $-$\gamma $ coincidence measurements is being commissioned for use with exotic stopped beams. The new array consists of the existing ``X-array,'' with five HPGe detectors for detection of $\gamma $ rays, and a plastic scintillator for $\beta $-particle detection. Two operational modes are possible: ``Mode 1'' utilizes a stand-alone scintillator chamber; ``Mode 2'' incorporates a tape transport system into a modified chamber, offering significant contamination removal that would otherwise result from the subsequent decay chain. The design of the tape station has been adopted from a prototype diagnostic system currently installed at CARIBU. Here, a general overview of the apparatus, commissioning runs and analysis of data collected whilst operating in both modes will be discussed. \\[4pt] [1] G. Savard \textit{et al Nuclear Instruments and Methods in Physics Research B} 266 (2008) 4086--4091   

Measuring $(d,p\gamma)$ Gamma Decays with Apollo at HELIOS

The role of neutron capture reactions is critical for nucleosynthesis processes far off of stability. Unfortunately, due to the radioactive nature the target isotopes of interest and the difficulty in producing a neutron target, these reactions will never be amenable to direct measurement. Further, for most astrophysical environments favored for the $r$-process, the required reaction networks are so large as to make direct experimental treatment of all of the reactions of interest beyond the range of what is feasible. Neutron transfer reactions, such as $(d,p)$, combined with intense beams of radioactive ions can help to elucidate the nuclear physics at play. The HELIOS instrument at Argonne National Laboratory has been successfully used to study a range of reactions in inverse kinematics. To complement this effort, we have designed a scintillator array APOLLO to be used in conjunction with HELIOS to measure gamma-decay properties following neutron transfer. This faced challenges related to operation under vacuum and the 3~T field at HELIOS. The first measurements with this new instrument, including efficiency, resolution, and coincidence efficiency will be discussed.   

Coupling the ORRUBA and Gammasphere Arrays

The measurement of transfer reactions in inverse kinematics using heavy beams poses a number of experimental challenges. Even for nuclei in close proximity to double shell closures, the fragmentation of single-particle strength can result in relatively complex spectra with level spacings as low as of tens of keV. Coincident measurement of de-excitation gamma rays in coincidence with the charged reaction products can aid significantly in resolving the states populated, and provide constraints on numerous other properties, such as spin-parities, branching ratios and lifetimes of levels. The ORRUBA detector array, with extended angular coverage, is being coupled to Gammasphere in order to facilitate such measurements. The motivation, details and current status of the coupled arrays will be presented. *This work is supported in part by the U.S. Department of Energy and the National Science Foundation.   

SIPT---An Ultrasensitive Mass Spectrometer for Rare Isotopes

Over the last few decades, advances in radioactive beam facilities like the Coupled Cyclotron Facility at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU) have made short-lived, rare-isotope beams available for study in various science areas, and new facilities, like the Facility for Rare Isotope Beams (FRIB) under construction at MSU, will provide even more exotic rare isotopes. The determination of the masses of these rare isotopes is of utmost importance since it provides a direct measurement of the binding energy of the nucleons in the atomic nucleus. For this purpose we are currently developing a dedicated Single-Ion Penning Trap (SIPT) mass spectrometer at NSCL to handle the specific challenges posed by rare isotopes. These challenges, which include short half-lives and extremely low production rates, are dealt with by employing the narrowband FT-ICR detection method under cryogenic conditions. Used in concert with the 9.4-T time-of-flight mass spectrometer, the 7-T SIPT system will ensure that the LEBIT mass measurement program at MSU will make optimal use of the wide range of rare isotope beams provided by the future FRIB facility.   

Testing and Characterization of the JENSA Gas Jet Target

Next generation radioactive ion beam facilities are being planned and built across the globe, and with them an incredible new array of exotic isotopes will be available for study. To keep pace with the state of nuclear physics research, both new detector systems and new target systems are needed. The Jet Experiments in Nuclear Structure and Astrophysics (JENSA) gas jet target is one of these new target systems, designed to provide a target of light gas that is localized, dense, and pure. The JENSA system involves nearly two dozen pumps, a custom-built industrial compressor, and vacuum chambers designed to incorporate large arrays of both charged-particle and gamma-ray detectors. The JENSA gas jet target was originally constructed at Oak Ridge National Laboratory for testing and characterization, and will move to the ReA3 reaccelerated beam hall at the National Superconducting Cyclotron Laboratory (NSCL) for further characterization, optimization, and use. JENSA will form the main target for the proposed SEparator for CApture Reactions (SECAR), and together the two comprise the focus of the low energy experimental nuclear astrophysics community in the United States. Data on gas flow and jet characteristics of the current JENSA target system will be presented.   

Developments in precison mass measurements of short-lived r-process nuclei with CARIBU

The confluence of new radioactive beam facilities and modern precision mass spectrometry techniques now make it possible to measure masses of many neutron-rich nuclei important to nuclear structure and astrophysics. A recent mass sensitivity study (S. Brett, \emph{et al.} Eur. Phys. J., A 48, 184 (2012)) identified the nuclear masses that are the most influential to the final rapid-neutron capture process abundance distributions under various astrophysical scenarios. This work motivated a campaign of precision mass measurements using the Canadian Penning Trap (CPT) installed at the Californium Rare Isotope Breeder Upgrade (CARIBU) facility at Argonne National Laboratory. In order to measure the weakest and most short-lived (t$_{\frac{1}{2}}<$ 150 ms) of these influential nuclei, a series of upgrades to the CARIBU and CPT systems have been developed. The implementation of these upgrades, the $r$-process mass measurements, and the status of CARIBU facilty will be discussed.   

Optimizing VANDLE for Decay Spectroscopy

Understanding the decay properties of neutron rich isotopes has well established importance to the path of the r-process [1] and to the total decay heat for reactor physics [2]. Specifically, the half-life, branching ratio and spectra for $\beta$-n decay is of particular interest. With that in mind, we have continued attempts to improve upon the Versatile Array of Neutron Detectors at Low Energy (VANDLE) in terms of efficiency and TOF resolution through the use of new and larger scintillators. Details of the new implementation, design and characterization of the array will be shown and compared to previous results. \\[4pt] [1] M. Madurga et.al., Phys Rev. Lett. {\bf 109},112501 (2012)\\[0pt] [2] Rykaczewski, K. P., Physics {\bf 3}, 94 (2010)   

Experimental Results from Oak Ridge Isomer Spectrometer and Separator (ORISS)

ORISS is a linear multi reflection time-of-flight mass analyzer developed by the University Radioactive Ion Beam Consortium. It will be used to separate any isobar and many isomers for decay spectroscopy experiments. The entire system's operation was demonstrated with a less than ideal multi-isotopic ion source and achieved a mass resolving power as high as 430,000. To better characterize the system we have installed a monoisotopic 133Cs ion source. The radiofrequency quadrupole ion cooler and buncher, which serves as the ion injector into ORISS, was tested in stand-alone mode and achieved a longitudinal emittance of 22 $\pi $ eV $\times$ ns and transmission \textgreater 40{\%}. These very good results confirm our expectation that ORISS can achieve the design goals. Using the improved ion source, we expect, very soon, to demonstrate the complete system's design goals of 400,000 mass resolving power and 50{\%} transmission.   

Status of the Beam Thermalization Area at the NSCL

Beam thermalization is a necessary process for the production of low-energy ion beams at projectile fragmentation facilities. Present beam thermalization techniques rely on passing high-energy ion beams through solid degraders followed by a gas cell where the remaining kinetic energy is dissipated through collisions with buffer gas atoms. Recently, the National Superconducting Cyclotron Laboratory (NSCL) upgraded its thermalization area with the implementation of new large acceptance beam lines and~a large RF-gas catcher constructed by Argonne National Lab (ANL). Two high-energy beam lines were commissioned~along with the installation and commissioning of this new device in late~2012. Low-energy radioactive ion beams have been successfully delivered to the Electron Beam Ion Trap (EBIT) charge breeder for the ReA3 reaccelerator, the SuN detector, the Low Energy Beam Ion Trap (LEBIT) penning trap, and~the Beam Cooler and Laser Spectroscopy (BeCoLa) collinear laser beamline. Construction of a gas-filled reverse cyclotron dubbed the CycStopper is also underway. The status of the beam thermalization area will be presented and the overall efficiency of the system will be~discussed.