K. Morita

    
    The gas-filled recoil separator GARIS is used at the RILAC facility at the RIKEN laboratory. The setup consists of a differential pumping section, a rotating-wheel target, a magnetic recoil separator filled with helium gas, and a detection section. Ions of ⁶⁴Ni and ⁷⁰Zn are accelerated by RILAC. The beam is introduced to the target chamber into the gas-filled region after passing through the differential pumping section. Thus, no foils are needed to separate the vacuum region of the beam transport system from the gas-filled region. Reaction products of our interest recoiling out of the targets are separated by GARIS from the primary beam or unwanted particles such as target recoils or transfer reaction products. Then, the products are implanted into the semiconductor detector placed at the focus of GARIS.

    The energy is measured by two methods. One is by measuring the magnetic-rigidity Bρ value using a 90°-bending magnet. The other one is a time-of-flight method using two cylinder-shaped electrode sets in the straight section of the beam line. The difference between the energies determined by the two methods is ∼ 0.2%. The overall ambiguity in energy determination is ± 0.4 MeV.

 

 

    The targets are prepared by evaporation of ²⁰⁸Pb metal (isotopically purified up to 98.4%) and ²⁰⁹Bi metal deposited on carbon backing foils of 30 μg/cm² thickness. The thicknesses of the targets range from 190 μg/cm² to 450 μg/cm². An additional 10 μg/cm² carbon layer is evaporated on the target to protect the target from sputtering. The beam hit the target from the 30 μg/cm² foil side. Eight targets are mounted on the rotating wheel, which rotated by a speed of 3000 rpm. The beam hit the wheel at the distance of 15 cm from the pivot.

 

   Measuring elastically scattered beam particles by a PIN photodiode located 45° from the beam axis monitors the beam intensity and the target condition. The beam intensity is deduced from the counting rate of the elastically scattered particles assuming the scattering is pure Rutherford scattering.

 

   GARIS consists of four magnets in D1-Q1-Q2-D2 configuration where D and Q denote dipole and quadrupole magnets, respectively. The primary beam is stopped in a tantalum plate located inside the chamber of the D1 magnet. The role of the D2 magnet is to reduce the background from light particles otherwise detected in focal-plane detectors. The pressure of the helium gas filled in GARIS is 75 – 86 Pa. The Bρ value of GARIS is 2.04 – 2.09 Tm.

 

   The focal-plane detectors detect the reaction products transported to the focal plane of GARIS. The detection area is separated from the gas-filled area by a 1 μm Mylar foil and evacuated down to 1.3 × 10−4 Pa. The products passe two foils of timing counters, MCP1 and MCP2. Micro channel plates are used for detecting secondary electrons emitted from the foils. The effective areas of the counters are 78 mm in diameter. A distance between the foils is 29.5 cm. Then, the products are implanted into a position-sensitive silicon semiconductor detector (PSD) that has an effective area 60 × 60 mm².
 
 

   Signals from the timing counters are used for two purposes. One is to obtain information on masses of heavy products using their time-of-flight (TOF) between two foils of the timing counters, and the energy signals from the PSD. The other is to distinguish decay events in the PSD and the implantation events. The identification of the products is based on genetic correlations of mother and daughter nucleus. All successive decays start from the products after the implantation takes place at the same position and in anticoincidence with the timing detectors. The products can be identified by the time, position, and energy correlation of the decay signals.

  

   Because stopping ranges of α-particles are much larger than that of fusion products, α-particles emitted in backward hemisphere escape from the detector. Four silicon detectors (SSDs) of the same size as the PSD but not position sensitive are set in box shape in backward direction of the PSD in order to detect the α-particles escaping from the PSD.

 

    The energy resolution for decay α-particles measured only by PSD is 70 keV (FWHM), while that for those measured as sum of PSD and SSD is 140 keV (FWHM). These resolutions were improved to 35 keV and 70 keV, respectively, in the 2003 November run by cooling the detectors down to 0°C in temperature. The beam intensity typically was 5 × 10¹²/s and the singles counting rate of the PSD at this beam intensity was 3 to 10 /s. The counting rate depends strongly on the target condition. The main components of the signals are due to the target recoils and the scattered beam particles.