Installation and commissioning of EURICA – Euroball-RIKEN Cluster Array

Nuclear Instruments and Methods in Physics Research B 317 (2013) 649–652

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Installation and commissioning of EURICA – Euroball-RIKEN Cluster Array P.-A. Söderström a,⇑, S. Nishimura a, P. Doornenbal a, G. Lorusso a, T. Sumikama b, H. Watanabe a, Z.Y. Xu c, H. Baba a, F. Browne a,d, S. Go e, G. Gey a,f, T. Isobe a, H.-S. Jung g, G.D. Kim h, Y.-K. Kim h, I. Kojouharov i, N. Kurz i, Y.K. Kwon h, Z. Li j, K. Moschner a,k, T. Nakao e, H. Nishibata l, M. Nishimura a, A. Odahara l, H. Sakurai a,c, H. Schaffner i, T. Shimoda l, J. Taprogge a,m,n, Zs. Vajta a,o, V. Werner p, J. Wu a,j, A. Yagi l, K. Yoshinaga q a

RIKEN Nishina Center, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan Department of Physics, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan c Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan d School of Computing, Engineering and Mathematics, University of Brighton, Brighton BN2 4JG, United Kingdom e Center for Nuclear Study, University of Tokyo, Hirosawa 2-1, Wako, Saitama 351-0198, Japan f LPSC, Université Joseph Fourier Grenoble 1, Institut Polytechnique de Grenoble, CNRS/IN2P3, 38026 Grenoble Cedex LPSC, France g Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA h Institure for Basic Science, Rare Isotope Science Project, Yuseong-daero 1689-gil, Yuseong-gu, 305-811 Daejeon,Republic of Korea i GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany j School of Physics and State key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China k Institut für Kernphysik, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany l Department of Physics, Osaka University, Machikaneyama-machi 1-1, Osaka, 560-0043 Toyonaka, Japan m Departamento de Fı´sica Teórica, Universidad Autónoma de Madrid, E-28049 Madrid, Spain n Instituto de Estructura de la Materia, CSIC, E-28006 Madrid, Spain o Institute for Nuclear Research, Hungarian Academy of Sciences, P.O. Box 51, Debrecen H-4001, Hungary p Wright Nuclear Structure Laboratory, Yale University, New Haven, CT 06520-8120, USA q Department of Physics, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, Japan b

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Article history: Received 8 February 2013 Available online 22 March 2013 Keywords: Fragmentation facilities c-Ray spectroscopy b-Decay Germanium detectors Silicon detectors

a b s t r a c t EURICA is a project at RIKEN Nishina Center aimed at studying a wide range of exotic nuclei through bdecay measurements and high-resolution c-ray spectroscopy. The setup is located behind the BigRIPS fragment separator and the ZeroDegree spectrometer at the RIBF. EURICA consists of the HPGe cluster detectors from the previous Euroball and RISING projects, together with double-sided silicon-strip detectors for b-decay counting and lifetime measurements. In total, this setup provides us with the possibility to study several aspects of the exotic nuclei produced at the RIBF. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The Radioactive Isotope Beam Factory (RIBF) is a state-of-theart radioactive ion beam facility, operative at RIKEN Nishina Center (RNC) since 2006. Recent discovery of 45 new isotopes using an uranium primary beam at 345-MeV/nucleon demonstrated the great potential of RIBF [1,2], which enables us to shed light on the structure of exotic nuclei to finally gain a deeper understanding of the nuclear force and of the nucleosynthesis mechanism operating in the cosmos. ⇑ Corresponding author. Tel.: +81 48 462 7946. E-mail addresses: [email protected] (P.-A. Söderström), nishimu@ribf. riken.jp (S. Nishimura). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.03.018

Spectroscopy experiments with both fast and stopped beams using BigRIPS [3] and the ZeroDegree Spectrometer (ZDS) [4] are well established at RIBF. For example, c-ray spectroscopy experiments using fast beams carried out using high intensity 48Ca and 238 U beam were successful to explore the island of inversion [5,6]. In 2009, decay spectroscopy in the 110Zr region was conducted implanting isotopes in double-sided silicon-strip detectors (DSSSD) surrounded by four Clover germanium detectors. Although the detection efficiency of this modest c-ray equipment was only about 1.5% at a c-ray energy of Ec ¼ 662 keV, the output of the experiment was remarkable. Identification of isomeric states and b-delayed c-rays provided important nuclear structure clues in this region crucial for the r-process [7–10].

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In this context, the EURICA (Euroball-RIKEN Cluster Array) project was proposed to the Gammapool [11] Owners Committee in July 2011 and the approval was granted. Twelve Euroball IV HPGe cluster detectors [12–14], and the RISING stopped-beam electronics and support structure [15–17] were moved to RIKEN to assemble the EURICA spectrometer. EURICA is one of the highest-efficiency c-ray detectors existing, coupled to the world’s most intense in-flight radioisotope beams to create a unique opportunity for the worldwide nuclear physics community. This paper details the progress and the current plans of the EURICA project. 2. EURICA Each EURICA cluster consists of seven tapered, hexagonal HPGe crystals, with one central crystal and the remaining six crystals placed in a surrounding ring. The 84 encapsulated HPGe crystals and twelve cryostats arrived at RIKEN in November and December 2011, for testing and assembly of EURICA. The spectroscopy tests were always carried out in the same conditions, with a 6 ls shaping time and the same gain of the amplifier. For an incoming c-ray energy of 1332.5 keV, the average EURICA resolution was 1:99  0:08 keV, while the average RISING resolution was 2:06  0:05 keV. 2.1. Infrastructure and geometry In parallel with the conditioning of the cluster detectors, the infrastructure was installed at the F11 focal point of ZDS. The clusters are arranged in three rings at 51°, 90° and 129° relative to the beam axis at a nominal distance of 22 cm from the center. The 51°, 90° rings each consist of five clusters while the 90° ring has two clusters in the left half of the support structure (in the beam direction). The three empty positions in the right half of the support structure can be used for ancillary detectors, like NaI (thallium doped sodium iodide) [18] or LaBr3(Ce) detectors [19].

implemented in the sorting. When a signal is seen in multiple neighboring crystals within a time interval of 400 ns, the energies from these were added together and treated as one c-ray if the number of neighboring crystals were less than three. If the signals in these crystals instead were outside the 400 ns time interval they were treated as individual c-rays. If a signal was seen in three or more neighboring crystals, that cluster was not included in the event. Using a 60Co source with an activity of 68 kBq, the peak-to-total ratio of the full-energy peaks at 1173 keV and 1333 keV relative to the total number of counts was 16.3%. With the add-back algorithm implemented, this value increased to 25.8%. The total energy resolution of the array at 1333 keV was 2.89 keV without the add-back algorithm and 3.17 keV when the add-back algorithm was used. The efficiency was measured using a 19 kBq 152Eu source. The results from this measurement is shown in Fig. 1. 3. WAS3ABi For implantation and b-decay, two different DSSSD arrays are available. One of these two is SIMBA (Silicon Implantation detector and Beta Absorber), that has been successfully used in the 100Sn region at GSI [23]. The other is WAS3ABi (Wide-range Active Silicon Strip Stopper Array for b and Ion detection) that has been developed in collaboration with the TU-Munich and constructed at RIKEN. WAS3ABi has been designed to consist of eight 40  60 mm2 area and 1 mm thick DSSSDs, each with 40 þ 60 strips. Four of these detectors have been provided by RIKEN and four by Institute for Basic Science, South Korea. The signals from the DSSSDs are split into a low-gain branch for measuring the energy of the implanted ion, and a high-gain branch for measuring the energy deposited by b particles, protons or a particles. 4. Data acquisition 4.1. Silicon Detector DAQ

2.2. Electronics The energy and timing information for all strips of the DSSSDs is stored by using 23 CAEN V785 ADCs and six CAEN V1190 TDCs, which are arranged in two VME crates. Data from these crates is read out in serial by a VMI/VME controller installed in the master VME crate and then sent to the event builder. A dead time of less than one millisecond is expected for an implantation event. With a proper zero-suppression for ADCs, a dead time of 500 ls can be achieved for beta decay event. For the offline analysis, data from

Efficiency (%)

The cluster’s electronics scheme corresponds to that employed for RISING at GSI [16]. The two output channels from a crystal’s pre-amplifier are sent to two individual branches for energy and timing, respectively. The energy branch is processed by digital cfinder (DGF) modules manufactured by XIA [20]. With these modules a total energy resolution of less than 3 keV at Ec ¼ 1332:5 keV for the full array was achieved for the first EURICA experimental campaigns. The analogue timing branch originates from the second preamplifier outputs of the cluster detectors. The readout circuit is composed of a timing filter amplifier (TFA), a constant fraction discriminator (CFD) and a time-to-digital converter (TDC). To increase the range for time measurements, two TDCs are used in parallel. One is a V775 CAEN TDC [21] with a 1.2 ls range and 0.31 ns digital time resolution at the full range, and the other is a V767 CAEN TDC [22] with 800 ls range and 0.73 ns digital time resolution at the full range. As the CFDs have a higher energy threshold than the DGFs, the DGF time can also be read out with a 25 ns digital time resolution.

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To evaluate the performance of the full EURICA array, data was collected using several sources with a clock trigger of 1 kHz and 100 ls wide gate for the DGFs to accept c rays. An add-back algorithm for suppression or reconstruction of events where c rays are Compton scattered between neighboring crystals has been

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Fig. 1. Efficiency of the EURICA array as a function of c-ray energy without (dashed line, squares) and with (solid line, circles) the add-back algorithm.

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silicon data aquisition (DAQ) can be merged with BigRIPS and EURICA data files based on the time stamp information.

1500 T1/2=5.306±0.028 μs

Counts

4.2. EURICA DAQ The slow control of the DGF cards is carried out by a LabVIEW program where it is possible to adjust parameters like gain, offset, shaping time constant, and thresholds. For DGF real-time-control and readout the Multi-Branch System (MBS) [24,25] developed at GSI is used. The VME electronics are controlled by a RIO3-based real-time computer system [26]. The RIO3 computers and the event builder can be controlled by a local workstation running a LynxOS operating system. The events are handled by the remote event server that distributes the events to online monitoring as well as to the disk for final storage. For online sorting and monitoring the Go4 code [27] is used. In this code it is possible to not only monitor the EURICA HPGe detectors, but also to include data from BigRIPS as well as ancillary EURICA detectors for fast online-evaluation of the quality of the experimental data.

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In-beam commissioning of the EURICA spectrometer and ancillary detectors was performed in March and April 2012. In total, four days of beam time was devoted to the commissioning, using 18 O as a primary beam at an energy of 230 MeV/nucleon. The commissioning of the cluster detectors was primarily carried out in one day out of the two-day beam-time starting in March. The main purpose of this commissioning was to verify that the cluster detectors were working for energy and lifetime measurements of isomeric states and that it was possible to measure energies of c rays emitted after a b decay in the DSSSDs. For this part BigRIPS and the ZDS were tuned for the 16N and 15C nuclei following the fragmentation the primary beam. For bc coincidences the 3 ! 0þ transition in 16O with an energy of 6128.63 keV, and the 5=2þ ! 1=2 transition in 15N with an energy of 5269.161 keV, were selected. The spectrum from these decays, requiring a silicon trigger, is shown in Fig. 2. The clusters were calibrated using a 152Eu source, giving calibration points up to an energy of 1408 keV. Despite the absence of high-energy calibration points, the c rays at energies in the range 5–6 MeV were seen as well resolved peaks at their correct energies showing the high linearity of the setup. A transition of interest in this experiment was the 0 ! 2 transition in 16N, with an energy of 120.42 keV and a previously measured lifetime of 5:25ð6Þ ls [28] for the isomeric decay commissioning. A decay-curve was measured using a gate on

Fig. 3. Decay curve of the 0 ! 2 transition in commissioning experiment.

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N measured in the first EURICA

120.42 keV in the HPGe array and plotting the time-difference between the HPGe and the final BigRIPS scintillator, as shown in Fig. 3. We derive a lifetime for the 0 isomer in 16N isomer of 5:306ð28Þ ls, in agreement with literature data. 6. Summary The EURICA array has been successfully installed at the RIBF in Riken Nishina Center, placed behind the BigRIPS fragment separator and the ZeroDegree Spectrometer. After commissioning, several physics campaigns have been carried out using this setup. Besides the germanium detectors, two different stacks of silicon-strip detectors are available for implantation of exotic nuclei, as well as several other complementary detectors are either available or awaiting delivery and assembly. Acknowledgments This work was carried out at the RIBF operated by RIKEN Nishina Center, RIKEN and CNS, University of Tokyo. We acknowledge the EUROBALL Owners Committee for the loan of germanium

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Fig. 2. Spectrum of 15N and 16O c-rays from the EURICA commissioning after a selection of silicon triggered signals in the time spectrum. The spectrum shows the full energy, single escape (S.E.), double escape (D.E.) and positron annihilation (511 keV) peaks. The 15N and 16O full-energy peaks have a resolution of 6.13 and 6.63 keV full width at half maximum, respectively.

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detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors. Part of the WAS3Bi was supported by the Rare Isotope Science Project which is funded by the Ministry of Education, Science and Technology (MEST) and National Research Foundation (NRF) of Korea. This work was partially supported by US DOE Grant No. DE-FG02-91ER-40609. P.-A. Söderström was financed by the Japan Society for the Promotion of Science (JSPS) Kakenhi Grant No. 2301752. References [1] [2] [3] [4] [5] [6] [7]

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