Session BC: Instrumentation I

ZeroDegree spectrometer at RIKEN RI Beam Factory

At RI Beam Factory (RIBF) [1] at RIKEN Nishina Center, a variety of fast rare isotope (RI) beams are produced using the BigRIPS in-flight separator [2] for studies of exotic nuclei. The beam line following BigRIPS is designed to work as a forward spectrometer named ZeroDegree, so that it can be used for reaction studies with RI beams. The ZeroDegree spectrometer consists of two dipoles and six superconducting quadrupole triplets, of which designs are essentially the same as those of BigRIPS. It analyzes and indentifies projectile reaction residues, often in coincidence with gamma rays, and can be operated in different optics modes, depending on experimental requirements. The ZeroDegree spectrometer has recently been commissioned and used for a series of full-dress RI-beam experiments. Overview and status of the ZeroDegree spectrometer will be reported.\\[4pt] [1] Y. Yano: Nucl. Instr. and Meth. \textbf{B 261} (2007) 1009. \\[0pt] [2] T. Kubo: Nucl. Instr. and Meth. \textbf{B 204} (2003) 97~and T. Ohnishi et al.: J. Phys. Soc. Japan, \textbf{77} (2008) 083201.   

Ion-optical studies of BigRIPS separator and ZeroDegree spectrometer at RIKEN RI Beam Factory

The BigRIPS in-flight separator[1] and the ZeroDegree spectrometer (ZDS) have been commissioned at RIKEN RI Beam Factory (RIBF) recently. Intense radioactive isotope (RI) beams are produced, separated and analyzed by the BigRIPS and the ZDS. Both of them are operated in several optical modes according to experimental conditions. For particle identifications of RI beams, it is essentially important to achieve high resolutions in $A/Q$ ratio because RI beams are produced in several charge states in our energy region especially for heavy RI beams. Ion optical calculation with realistic magnetic field maps is indispensable for our purpose and we use COSY INFINITY[2] for that. Measured field maps are incorporated in the COSY calculations. In 2008, the ZDS was commissioned for the first time in three different modes. Experimental results and comparison with the COSY calculations will be presented in this report. [1]T. Kubo: Nucl. Instr. Meth. {\bf B204}, 97 (2003). [2]K. Makino, M. Berz: Nucl. Instr. Meth. {\bf A558}, 346 (2006).   

Ion Optics Simulation for Fragment Separator

We have developed a Monte-Carlo simulation code for unstable-nuclear beam experiments using a fragment-separator. This code primarily aims at calculating beam traces in the fragment separator BigRIPS and ZeroDegree Spectrometer (ZDS) at RIBF(RIKEN RI-Beam Factory). This code uses externally given transfer-matrices of ion optics such as an output of COSY Infinity[1]. We have applied this code to recent campaign of experiments using $^{48}$Ca at 345MeV$/$u as primary beam. In the experiments, two modes of ion optical settings , namely ``Standard'' and ``High Brho'' modes were used. The former is an ordinary used starndard setteing, which has a limit in the maximum rigidity ($B\rho<9.2$Tm). On the other hand, the ``High Brho'' setting has been developed for a secondary beam with higher rigidity, such as for very neutron rich nuclei $^{22}$C (A/Z=3.67). In this talk, we compare properties of these two optics settings and evaluate beam traces, emittances, and transmissions.\\[4pt] [1] K. Makino, M. Berz Nucl. Instr. Meth. A 558 (2005)   

Construction of high resolution beam line for SHARAQ spectrometer at RIKEN RI Beam Factory

A high resolution beam line [1] has been constructed for the SHARAQ spectrometer [2] at RIKEN RI Beam Factory (RIBF), in order to achieve dispersion matching that allows high resolution measurement at the focal plane of the spectrometer. This beam line is formed by the existing BigRIPS separator [3] at RIBF and a newly constructed beam line that diverges from BigRIPS and leads to the target position of SHARAQ. The ion optics is so designed that it can be operated in the dispersion matching mode. The new part of the beam line consists of two 30-degree bend dipoles, three quadrupole singlets and three superconducting quadrupole triplets. Recently the beam line has been successfully commissioned together with the SHARAQ spectrometer. Overview of the beam line will be reported. [1] T. Kawabata et al.: Nucl. Instr. and Meth. B 266 (2008) 4201. [2] T. Uesaka et al.: Nucl. Instr. and Meth. B 266 (2008) 4218. [3] T. Kubo: Nucl. Instr. and Meth. B 204 (2003) 97.   

Ion optical studies in the high resolution beam line and the SHARAQ spectrometer

The SHARAQ spectrometer is designed to achieve a resolving power of $p/ \delta p$ $\sim$ 15000 and a high angular resolution $\delta \theta$ $\sim$ 1 mrad with RI beam at RIBF. To avoid loss of energy resolution due to the momentum spread of RI beams, the dispersion matching technique is applied. In the commissioning run in March and May 2009, we have investigated ion-optical properties of the SHARAQ spectrometer and the high resolution beam line. We measured the first order matrix elements of the beam line and the SHARAQ spectrometer using the primary beams. The resolving power $D/M$ of the SHARAQ spectrometer is 14.7 m, which corresponds to the design value of the resolving power when the beam spot size is assumed to be 1 mm. Based on the first order elements, the beam line was tuned to be dispersion-matched to the spectrometer. In the tuning, we used correlations of beam trajectories at different focal planes. As the results, we have partially achieved the dispersion matching for the lateral and angular direction simultaneously. At present, the resolving power of $\sim$ 8000 is achieved. The tuning method and the obtained results in the commissioning will be presented.   

Field mapping measurement of SHARAQ dipole magnets

In high-resolution ion-optical analyses of radioactive isotope beams, accurate and precise knowledge of magnetic field distribution is of basic importance. We have measured magnetic field distributions in dipole magnets which make up the SHARAQ spectrometer. Search-coil method was adopted in the measurement. Details of the method, devices, and results of magnetic field distribution will be reported.   

The New DAQ System in RIKEN RIBF

The new DAQ system for RIKEN RI-Beam factory (RIBF) have been introduced. Several thousands of RI beams are produced by the fragmentation and fission reactions. The in-flight RI-beam separator named BigRIPS discriminates RI beams by using many beam profiling detectors placed at seven focal plains along the beam line of 77-meter-long. RI beams identified by BigRIPS are impinged on the reaction target. The reaction products are transported to spectrometers, and measured by particle and gamma detectors. The detector section of BigRIPS is used in all experiments, but the other detector sections vary according to the experimental condition. Since many experiments with a different setup are shifted one after another in several weeks, the DAQ system is required the flexibility and the scalability. Therefore, we developed the new DAQ system with the functions of hierarchical event build in online and parallel data readout from CAMAC/VME modules. It is remarkable that these functions are achieved by only using commodity computer and network equipments and standard CAMAC/VME modules with the flexibility and the scalability. In this paper, we will introduce the configuration and the performance of this new DAQ system.   

Performance test of detection system for $\beta-\gamma$ spectroscopy at RIBF

In RIBF at RIKEN, the nuclear structure in the region of heavy neutron rich nuclei is studied with high intensity U-beam. We will research decay-schemes and excited states of nuclei through the $\beta-\gamma$ spectroscopy. We install 10 Double-sided Silicon Strip Detectors, DSSDs, as active stopper at the end of the beam-line. When the secondly beam particles are implanted into the DSSDs, we measure the energy loss and the stopping position of particles in the DSSDs. Then $\beta$ decay is detected by the DSSDs. Clover-type Ge detectors are arranged around DSSD to measure an energy of $\beta$-delayed gamma-ray. To veto the $\beta$-ray, thin plastic scintillator is placed on the front of each Clover. In addition, BGO, high-density scintillator, are arranged around each Clover to decrease background from Compton-scattering. We will report the performance test of these detectors and read out electronics with standard sources.   

Mass of the lowest $T=2$ level in $^{32}$Cl

The mass of $^{31}$S has been measured to better than 0.5 keV/ $c^2$ using the Canadian Penning Trap mass spectrometer at Argonne National Laboratory's ATLAS facility. The result changes the mass of the lowest $T=2$ level in $^{32}$Cl substantially and improves its precision by roughly a factor of three. The new $Q_{EC}$ value for the superallowed $\beta$ decay of $^{32}$Ar to this level affects constraints on scalar currents via the $\beta-\nu$ correlation and the isospin-symmetry-breaking correction ($\delta_C$) to the $ft$ value for this decay. The quadratic isobaric multiplet mass equation (IMME) is found to fail for the lowest $T=2$, $A=32$ isobaric quintet with higher confidence than for any other isobaric multiplet; the cubic fit is excellent.   

Performance of Focal-Plane Tracking Detector CRDC for SHARAQ

The high-resolution magnetic SHARAQ spectrometer has been constructed at the RI Beam Factory (RIBF) at RIKEN. For tracking of charged particles at the dispersive focal plane of SHARAQ, we have developed two 2-dimensional position-sensitive Cathode Read-out Drift Chambers (CRDCs). The CRDCs have large active areas of 550(H) x 300(V)~mm$^2$ with i-C$_4$H$_{10}$ gas at low pressure of 15 - 30~Torr. The vertical and horizontal positions of charged particles are determined by measuring the drift time of electrons in the CRDC and deduced from the distribution of induced charges on the cathode divided 512 pads, respectively. The cathode signals are read out by using GASSIPLEX chips, which is developed at CERN for multiplexed readouts. In March and May, 2009, we evaluated the performance of the CRDCs by using the primary beam of $^{14}$N at 250A~MeV and its fragments. We found that the CRDCs have 100\% efficiency not only for heavy ion beam but also for light ion beam such as $^3$H. The performance of the CRDCs evaluated from further analyses will be presented.   

Performance evaluation of Low-Pressure Multi-Wire Drift chamber for RI beam

We are developing Low-Pressure Multi-Wire Drift Chambers (LP-MWDCs) as tracking detectors of light heavy ions with $Z$ = 1--8 at 100--300$A$ MeV in RIKEN RI Beam Factory (RIBF). The thickness of the LP-MWDCs is designed to be about $10^{-4}$ of the radiation length to reduce multiple scattering. The LP-MWDCs have 3 anode layers ($X$, $U$, and $Y$). The anode $U$ is tilted at 30$^\circ$ with respect to the anode $X$. Pure isobutane gas is used at a pressure of 10~kPa. In order to discriminate signals of the ions from the ones of $\delta$-rays, data equvalent to pluse heights are also recorded as well as the standard timing data. The performances of the LP-MWDCs were evaluated for RI beams such as $^{6}\rm{He}$, $^{12}\rm{B}$, and $^{12}\rm{N}$ at 200--250$A$ MeV. We report the efficiencies and position resolutions as a function of ions and high voltages.   

Development of ionization chamber for super-heavy elements

In the field of super-heavy elements, the direct measurements of atomic number $Z$ and mass number $A$ of produced nucleus is the most challenging tasks. One of the way to identify $Z$ and $A$ is to measure the energy loss per unit length $dE/dx$, and to measure total energy by stopping incident nucleus in a detector, respectively. $A$ is derived from the combination of total energy and velocity of the nucleus. In the region of our interest ($Z>100$, $A>250$), the density of electron-hole pairs or primary electrons is too high in semiconductor detector or even in normal gas detector, because of large $dE/dx$. Too high density of electron-hole pairs provokes the recombination of electrons and holes. As a result, the precise measurement of energy loss becomes almost impossible. In this work, we operate ionization chamber with low pressure to reduce the density of primary electrons. By this means, we try to measure the energy with a high degree of accuracy. Our first priority is to identify $A$ of super-heavy elements by measuring the total energy. In addition, Identifying $Z$ by measuring $dE/dx$ is also tried. At present, we engage a operation test using $\alpha$-source, and have a plan to examine operating characteristics for heavy ions. These results will be reported.