Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 50 (2017)

Towards two-neutron transfer reactions with a tritium target:
Characterization of a prototype TiD0.24 target
T. Koiwai,*1 T. Kumon,*1 Y. Shimizu,*1 N. Imai,*2 M. Niikura,*1 K. Wimmer,*1 and Y. Hatano*3Two-nucleon transfer reactions are well-suited to investigate specific nuclear structure properties, like shape coexistence and pairing correlations. In particular neutron-adding reactions are of great interest because they can
populate very exotic systems. The large neutron excesscombined with the rather simple structure of the tritium
nucleus makes it an ideal probe or target nucleus for
two-neutron pair transfer reactions. With radioactive ion
beams two-neutron transfer reactions have to be performed
in inverse kinematics and therefore a radioactive tritium
target is required. After pioneering work at
REX-ISOLDE,1,2) tritium targets are now also available at
ATLAS at Argonne National Laboratory and TRIUMF’s
ISAC II facility. Transfer reactions are best performed atbeam energies close to the Coulomb barrier. At RIBF, thefull potential of this spectroscopic tool will be exploitedwith the completion of the OEDO facility.3) Here, thehigh-intensity radioactive beam produced by projectile
fragmentation and in-flight fission will be slowed down in adegrader. Re-bunching and focusing devices are used torestore the beam quality. The challenge in building a radioactive tritium target for RIBF is twofold. First the size ofthe target has to be sufficiently large. At RIBF, typical beam
spot sizes are approximately 2 cm in FWHM, in stark contrast to the values of approximately 1 mm that are achieved
through post-acceleration at ISOL facilities. In order toperform two-neutron transfer reactions at RIBF a tritium
target with an area of 2.5 x 2.5 cm2 is required. Secondly,
the effective thickness of the tritium target material has tobe sufficient to achieve reasonable luminosities for experiments with radioactive beams at RIBF. The target thickness
has to result in a total activity below the limit of 10 TBq,
but it also has to be suitable for a wide variety of experiments with different physics goals as well as beam energies.
The approach used for RIBF is the loading of a titanium foilwith hydrogen. Theoretically a hydrogen to titanium ratio
H:Ti = 2:1 can be reached. However the large amount ofhydrogen critically affects the mechanical stability.4) This isa challenge for the construction of a large-area tritium target.
In order to establish a production and handling method for atritium target, prototypes containing non-radioactive deuterium instead of tritium are used.
At the Hydrogen Isotope Research Center at Toyama University a deuterated target with an intended ratio of D:Ti =0.24:1 has been produced, the thickness of which is 20 Pm
Ti before loading. In order to characterize the properties ofthe target, and in-beam experiment has been performed.
䠆1
䠆2
䠆3

Department of Physics, University of Tokyo
Center for Nuclear Study, University of Tokyo
Hydrogen Isotope Science Center, Toyama University

The goal of the experiment was to (i) independently determine the deuteron content of the target, (ii) examine thebehavior under high beam current, and (iii) investigate otherpossible types of radioactive contamination of the target due
to fusion reactions of the Ti carrier material. The experiment was performed at the E7b beam line. A 20Ne beam at
an energy of 8.2 MeV/u was irradiated on the TiD target.
Elastically scattered deuterons were detected in three pin
diodes in the forward direction. In the backward direction
an array of Si strip detectors was placed to detect protons
from (d,p) transfer reactions.

At the end of the beam line, the unreacted beam was impinged on two additional Ti foils to investigate the background that could be created by fusion reactions on the Ti
material. In order to establish a limit on the beam current
that might cause the release of hydrogen from the Ti material, the mass distribution of the residual gas was monitored
in a quadrupole mass spectrometer. The beam intensity wasvaried from 5 pnA to 200 pnA. Up to 20 pnA (1.3 x1011pps) the partial pressure of A/q =2 was stable at backgroundlevel. At higher currents the release of deuterium from thetarget was observed.
The analysis of the scattering as well as the activation datais ongoing. New prototype targets with different D:Ti loading ratios are being produced. The production of the final
tritium target is envisioned for 2018.
References1) K. Wimmer et al., Phys. Rev. Lett. 105 252501 (2010).
2) K. Nowak et al., Phys. Rev. C 93 044335 (2016).
3) S. Shimoura, Letter of Intent for OEDO project, the 14th
NPPAC for nuclear physics experiment at RIBF, June 2014.
4) I. Lewkowicz, Solid State Phenomena, 49-50 239 (1996).

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