RIKEN Accel. Prog. Rep. 48 (2015)

Ⅰ. HIGHLIGHTS OF THE YEAR

Results of ﬁrst online tests of small ion-surﬁng RF carpet gas cell atGARIS-IIP. Schury,∗1 F. Arai,∗1,∗2 H. Haba,∗1 Y. Ito,∗1 I. Katayama,∗1 D. Kaji,∗1 K. Morimoto,∗1 M. Reponen,∗1T. Sonoda,∗1 M. Wada,∗1 and H. Wollnik∗1,∗3

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Fig. 1. Combined stopping and extraction eﬃciency for205
Fr as a function of degrader thickness. The thickness was varied by rotating the degrader. Gas cell wasoperated at room temperature.
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beam On/Oﬀ

214Ac

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213Ac216Th

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215Ac

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217Pa

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Fig. 2. α-decay spectra seen on silicon detector after thegas cell. Pulsing the linac beam with a short period, astrong suppression of short-lived isotopes was observed.

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Of perhaps greater interest was the stopping andextraction eﬃciency for shorter-lived isotopes. Thiswas studied with a Tantalum target using the reactions 181 Ta(40 Ar,p4n)216 Th and 181 Ta(40 Ar,4n)217 Pa
to produce species with T1/2 of 26 ms and 3.5 ms, respectively. The α-decay spectra seen after the gas cellusing a chopped beam and measuring decays during“beam oﬀ” are shown in Fig. 2. The red spectrum
was measured by quickly pulsing the linac beam while
the green spectrum was measured with longer periodpulses. For the long period beam pulses, short-lived
isotopes extracted during “beam on” decay prior to“beam oﬀ” and are suppressed.
For these beams, having higher-Z and lower energy
than 205 Fr, the degrader was inoptimally thick. However, we were able to set lower limits on the stoppingand extraction eﬃciencies. For the short-lived 216 Th,
we were able to achieve ∼15% combined eﬃciency,
while for the extremely short-lived 217 Pa ∼12%.

As part of the SLOWRI project, we have begun development of a new low-energy experimental facility,
SlowSHE, for use with radioactive isotopes (RI), suchas Super Heavy Elements (SHE), that are created via
fusion-evaporation reactions. This new facility utilizes
a small gas cell1) placed after the GARIS-II separator
to thermalize the ions before they are transported toa multi-reﬂection time-of-ﬂight mass spectrograph2) .
A very thin mylar ﬁlm (2.4 µm thick) on a rotatable
frame is placed before the gas cell for use as an adjustable degrader to remove, in combination with twothin (1.1 µm and 2.4 µm thick) mylar windows, a large
fraction of the ions kinetic energy. The remaining
kinetic energy is removed by collisions with He gas,
which the cell is pressurized with upto 100 mbar.
In this ﬁrst experiment, we desired to verify the combined stopping and extraction eﬃciency for a varietyof chemical elements using isotopes with a wide rangeof half-lives. We ﬁrst studied stopping and extractionof the long-lived 205 Fr (T1/2 =3.8 s), produced via thereaction 169 Tm(40 Ar,4n)205 Fr.
The degrader was initially replaced with a large silicon detector array to measure the incoming rate. The
α-decay rate of 205 Fr was calibrated against the rateof elastically scattered incoming beam near the reaction target. The detector was then replaced by thedegrader and the rate of α-decay on a silicon detector
placed after the gas cell was measured as a function ofthe degrader angle. The result, shown in Fig. 1, featured a ﬂat plateau with eﬃciency peaking near 30%.

RIKEN Nishina Center
Department of Physics, University of Tsukuba
Department of Chemistry and BioChemistry, New Mexico
State University

These results were obtained with the gas cell operated at room temperature; we believe that using cryogenic temperatures should provide further improvements by reducing the possibility of charge exchangewith gas impurities and reducing the rate of diﬀusionof the ion cloud as it transits the length of the gas cell.
Even with the eﬃciencies reported herein, it should
be possible to perform precision mass measurements ofeven trans-Uranium isotopes with exceedingly low production rates. First such measurements are planned totake place within calendar year 2015.
References
1) F. Arai et al., this volume2) P. Schury et al., Nuclear Instr. and Methods in Physics
Research B 335 (2014) 39-53- 22 -

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