Blaum, K.

Block, Michael

Droese, C.

Eliseev, S.

Herfurth, F.

Laatiaoui, M.

Lautenschläger, F.

Marx, G.

Minaya Ramirez, E.

Schweikhard, L.

Thirolf, P. G.

GSI Repository

First Extraction Measurementswith the Cryogenic Gas Stopping Cell at SHIPTRAP∗C. Droese1, K. Blaum2, M. Block3, S. Eliseev2, F. Herfurth3, M. Laatiaoui4, F. Lautenschla¨ger5, G.Marx1, E. Minaya Ramirez4, L. Schweikhard1, and P. G. Thirolf61Ernst-Moritz-Arndt-Universita¨t Greifswald; 2Max-Planck-Institut fu¨r Kernphysik Heidelberg; 3GSI Darmstadt;4Helmholtz-Institut Mainz; 5Technische Universita¨t Darmstadt; 6Ludwig-Maximilians-Universita¨t Mu¨nchenOne of the open questions in modern nuclear physicsis the existence and the exact location of the so called’Island of Stability’ of superheavy elements. The evolutionof the nuclear binding energy close to the region of thesuperheavy elements, an indicator of nuclear stability,can be obtained by high-precision mass measurements.Recently, the masses of 252−255No [1] and 255,256Lr[2] were directly measured for the first time with thePenning-trap mass spectrometer SHIPTRAP. From theresults the strength of the deformed shell closure at neutronnumber N = 152 has been determined experimentally.To extend the reach of SHIPTRAP towards superheavyelements it is crucial to increase the overall efficiencyof the setup. The bottleneck is the deceleration of thefusion-evaporation reaction products from SHIP from akinetic energy of MeV down to meV in a gas stopping cell.Thus, a second generation gas stopping cell (CryoCell)operating at cryogenic temperatures was built as describedin detail in [3]. An increase of the overall efficiency by atleast a factor of 5 [4] is anticipated.In August 2012 the first measurement of the extractionefficiency was performed with a 223Ra (half-life 11.43days) recoil ion source. Figure 1 shows the alpha spectrumobtained in vacuum to determine the initial activity of223Ra. Afterwards, the source was placed axial symmetri-Figure 1: Alpha-decay spectrum obtained in 3600 s with a223Ra recoil ion source.∗We acknowledge the support of the German BMBF (Grants05P09HGFN5, 05P12HGFN5, 06ML9148), GSI (Grants LMTHIR1012and GFMARX1012) and the Max-Planck Society.cally in front of the largest ring electrode of the RF-Funnelinside the CryoCell. From there, the recoil ions, i.e.219Rn+ (half-life 3.96 s), are extracted. The number ofextracted 219Rn+ ions was determined with a silicondetector placed behind the extraction radio-frequencyquadrupole (RFQ). The 219Rn recoils were accumulatedon an aluminium foil with a thickness of 0.8µm at -1400 Vpositioned in front of the detector. Direct implantationof the 219Rn+ ions would corrupt the result. Due to itsnoble gas configuration a major fraction of the extractedions would diffuse out of the silicon detector before thedecay occurs. The resulting alpha spectrum looks similarto that shown in Fig. 1 however without the lines ofthe 223Ra decay. The number of 219Rn+ ions detectedbehind the Extraction RFQ is compared to the number of219Rn+ recoil ions emitted by the 223Ra source that can bederived from the initial activity. Taking into account theexponential decay of the initially determined activity of223Ra and the solid angle covered by the silicon detectorthe extraction efficiency was calculated. The measurementwas performed at room temperature and at 40 K. A similarbuffer-gas flow (1 mbar·l·s−1) of ultra pure helium (purity99.9999 %) was used which corresponds to a pressureof 60 mbar at 300 K and 21 mbar at 40 K, respectively.DC-gradients of 7 V/cm at the DC-Cage, 2 V/cm at theRF-Funnel and 0.2 V/cm at the Extraction RFQ wereapplied. RF amplitudes of 160 V at the RF-Funnel fora frequency f = 1014 kHz and 240 V at the ExtractionRFQ (f = 907 kHz) were used. At room temperature amaximum extraction efficiency of 37(3) % was obtained.The extraction efficiency increased to 64(3) % as thesystem was cooled to 40 K. The operation at cryogenictemperatures, e.g. the increased buffer-gas cleanliness,result in a relative gain in transmission efficiency of 75 %compared to the operation at room temperatures. Anexperiment with a radioactive ion beam, directly deliveredby SHIP, will be performed soon to confirm this promis-ing result that will pave the way towards high-precisionmass measurements of superheavy elements at SHIPTRAP.References[1] M. Block et al., Nature 463 (2010) 785[2] E. Minaya Ramirez et al., Science 337 (2012) 1207[3] C. Droese et al., GSI Scientific Report 2012[4] S. Eliseev et al., Nucl. Instr. Meth. B 266 (2008) 4475GSI SCIENTIFIC REPORT 2012 PHN-ENNA-EXP-11141

First Extraction Measurements with the Cryogenic Gas Stopping Cell at SHIPTRAP

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First Extraction Measurements with the Cryogenic Gas Stopping Cell at SHIPTRAP

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