EXPERIMENTS WITH GABRIELA DETECTOR SYSTEM

Abstract

For several years, more dozen experiments was carried out on SHELS (Separator for Heavy ELements Spectroscopy), aimed to investigation of characteristics of heavy elements and discover new isotopes. Perfect data acquisition system GABRIELA allows to investigate single particle states behavior, as well as the structure of little known elements in the Z = 102-105 and N = 148-162 region. Complex of DSSSD detects 70% alpha particles and 90% spontaneous fissions; a system of 5 coaxial Ge-detectors in 4π geometry has efficiency of gamma-quanta registration of 34-14% by scale ~ 100-1000 keV.

Keywords

• SHELS/VASSILISSA
• heavy elements
• spectroscopy

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1. Separator for Heavy Elements Spectroscopy (SHELS)

Flerov Laboratory of Nuclear Reactions (FLNR) at JINR is well-known in researches of heavy and superheavy elements (SHE). There are synthesized new elements with Z = 113–118 and a lot of isotopes of SHE with Z = 104–118. The last chemical element in Mendeleev table was named Oganesson (118, Og) in honor of academician Yu. Ts. Oganessian.

Studies of reaction products and their levels structure are produced via VASSILISSA [ 1 ] setup. Due to upgrades of VASSILISSA at 2013 years, the kinematic separator was remade in velocity filter SHELS [ 2 ], thereby a yield of facility is increased in times.

SHELS consists of the following elements (fig. 1): ● Target wheel: radius is 12 cm. The metal wheel divided by segments. In each segment was implanted target’s substance on 1.5-2 μm Ti foil. ● Two triplets of quadrupole lenses: maximum field gradient is 13 T/m, effective length of 38 cm and aperture radius of 10 cm. ● Two high voltage electrostatic deflectors: effective length of 65.7 cm, distance between plats is 10–20 cm, maximum field gradient of 40 kV/cm and rated deflection angle is 8°. ● Three dipole magnets: effective length of 59.7 cm, dipole aperture of 13.5 cm, maximum field gradient is 0.8 T. Two magnets have rated deflection angles of 21.8°, and one of 8°. ● Time of Flight system (ToF). ● Detector system GABRIELA.

The accelerated beam provides by the U-400 cyclotron, its intensity 0.5–1.5 μpA in depends of study reactions. There are use projectiles from 22Ne to 54Cr. Efficiency of the transmission of recoils nuclei (εtr) through the facility reaches 45%. This value is different for various asymmetric reaction, as an example, the reaction of 22Ne + 197Au has εtr = 6.5%. If compared to εtr from old facility, its value was 3% (tab. 1). For other reactions εtr were notably changed. 2. Detection system

The GABRIELA [ 4 ] detects the evaporation residues with their subsequent α decay or fission, as well as γ-rays, X-rays and conversion electrons. At 2013-2015 for GABRIELA was upgraded, and now it has another view (fig. 3). New content includes of the one box of DSSD and 5 Ge-detectors, which demonstrate more efficient work, than one Si-box and 7 Ge-detectors (in old version).

The box of detectors consists of large 10×10cm2 double-sided silicon strip detector (DSSSD) (fig. 4), 128×128 strips (16384 pixels) and thickness is 500 μm. This is focal plane detector, which surrounded of 8 plats 50×60mm2 DSSD (thickness of 700 μm), 32×32 strips (4096 pixels). Alpha particles registration efficiency εα = 70% and the resolution energy is 15-20 keV for α in the range of 5–7 MeV.

The Ge array consists of a large Clover installed just behind the focal plane DSSSD and 4 coaxial Ge detectors forming a cross around the DSSD. All Ge detectors are equipped with BGO shields. This configuration is more efficient! The calibration of the Ge detectors was performed using standard sources such as 152Eu and 133Ba. Registration efficiency is 38–14% by gamma-quanta energy scale from 100 keV to 1000 keV. 3. Conclusions

As for recoils registration efficiency (εreg), then achieved good results. In hot fusion reaction 40Ar + 180Hf formed compound nuclei 220Th*, by energy beam 220 Mev (fig. 5). There was integral flux of 6.2·1017 particles. Value of registration efficiency εreg was calculated by peak of 215Ac, which created through p4n-channel. Nuclide of 215Ac has short half-life time 0.17 s, it was convenient to used for calculate. Ratio of number of Rec-α correlations by number of α-particles from 215Ac peak it is εreg = 87.4%.

In experiments was studied heavy nuclei with Z=102–106 (tab. 2). System of detectors demonstrated good performance.

Z 102 104 105 106

Reaction 22Ne+238U 48Ca+204,206,208Pb 50Ti+206,208Pb [ 3 ] 50Ti+209Bi [ 5 ] 54Cr+208Pb

Isotopes

256No 250No, 252No, 254No 254Rf, 255Rf,256Rf,

257Rf 256Db, 257Db, 258Db 262-xnSg

The plans are another upgrade of the SHELS. Instead quadrupole lenses will be install lenses with more apertures of 300 mm. All Ge-detectors will be replaced on Clovers, and small DSSD on 10×10cm2 DSSSD. Also diameter of target wheel will increase before 24 cm.

References

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