Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 49 (2016)

Development of 178m2 Hf isomer target
N. Kitamura,∗1 Y. Yamaguchi,∗1 S. Michimasa,∗1 N. Imai,∗1 and H. Haba∗2
Since the discovery of superdeformation in the nucleus, many superdeformed states have been observed1). As the further deformed state, hyperdeformation,in which the ratio of the long axis to the short axis is3 : 1, has been predicted as extreme quadrupole deformation2) . However, no such state had been discoveredyet. Furthermore, recent theoretical calculation predicts that even a deformed torus shape appears at ahigh-spin state3) . Such deformed states will provide us
with a stringent test of our understanding of nuclear
physics. The key to produce such states is transferring high angular momentum to the nucleus. If thehigh-spin target or beam is available, it would be moreprobable to populate such an exotic state. Thus, weare developing a target of 178m2 Hf, a) which has a highspin of 16h̄ and a long half-life of 31 y. Among several
reactions, the 176 Yb(α, 2n) reaction is known to havea relatively high cross section to produce 178m2 Hf4) .
The pioneering work to produce 3 × 1014 atoms of178m2
Hf was performed in Dubna5) . The isomeric state
was produced by the fusion reaction of (α, 2n) withan enriched 176 Yb target. At an incident energy of
35.5 MeV, the cross section was 9 mb and the isomer ratio was 0.5 %. The AVF cyclotron at RIBF can provide
40-MeV α beams of 10 eμA, which are enough to makea nanogram sample of 178m2 Hf with a beam time of afew days. Before stepping into mass production, we
performed the feasibility study for production. Thereare three issues to be checked. The ﬁrst issue is themeasurement of the excitation function of the production cross section of 178m2 Hf to optimize the incident
beam energy. The second issue is the production crosssections of 175,172 Hf, which will be the main contaminants after the irradiation. Since they have relatively
short half-lives, 70 d and 1.87 y, respectively, the radioactivities of these contaminants will be much larger
than that of 178m2 Hf if we use a natural Yb target. Theproduction cross sections of these nuclei determine thedegree of enrichment required for 176 Yb. The third
issue is the heat damage to the Yb target. It is not obvious that the target can withstand the high-intensity
α beam since the melting point of Yb is about 824 ◦C.
The experiment was performed at the C03 beam lineof the AVF cyclotron. An α beam of 40 MeV irradiated a target stack for 8 hours. The target stack wasmade of ten nat Yb foils with a thickness of 20 μm each.
The Yb foils were sandwiched by two 2-μm Ti foils
to check the energy of the beam using the monitor
reaction nat Ti(α, xn)51 Cr. The target was placed on∗1∗2a)

Center for Nuclear Study, University of Tokyo
RIKEN Nishina Center
There are two isomeric states in 178 Hf. One is 178m1 Hf witha half-life of 8.0 s, the other is 178m2 Hf.

- 176 -Fig. 1. Experimental cross sections for nat Yb(α, xn)175 Hf(circles) and 172 Hf (triangles) are compared with thecalculations by PACE4 (dashed and dotted, respectively). The horizontal errorbars stand for the loss ofenergy in the foils.

the water-cooled Ta Faraday cup to measure the total
dose of the beam. In addition, He gas was ﬂown on
the surface of the target to take the heat away. The
irradiation time was determined by the regulation for
production of 175 Hf in this facility.
When we opened the chamber and checked the target after cooling for 3 days, it turned out that the ﬁrst
ten foils, one Ti and nine Yb, were adhered. On theother hand, the last two foils, Yb and Ti, could be separated. This means that as long as the foils are attachedtightly to the water-cooled Faraday cup, they can withstand the 10-eμA α beams. γ rays were measured bya HPGe detector to evaluate the radioactivities produced. The deduced cross section of 51 Cr was foundto be consistent with the value with an incident energyof 40 MeV. The cross sections of nat Yb(α, xn)175,172 Hf
are presented in Fig. 1. The PACE4 code7) reproduced
these experimental values. For detecting the decay
from 178m2 Hf, we have to measure γ rays in triple coincidence to distinguish them from other activities. We
are now preparing Ge detectors array GRAPE8) forthe measurement.
References
1) X. L. Han and C. L. Wu, At. Data Nucl. Data Tables
63, 117 (1996).
2) J. Dudek et al., Phys. Lett. B 211, 252 (1988).
3) T. Ichikawa et al., Phys. Rev. Lett. 109, 232503 (2012).
4) S. A. Karamian, Yad. Fiz. 68, 1827 (2005).
5) Yu. Ts. Oganessian et al., J. Phys. G: Nucl. Part. Phys.
18, 393 (1992).
6) IAEA-TECDOC-924, Vienna (1997)7) http://lise.nscl.msu.edu/pace4.html
8) S. Shimoura, Nucl. Instrum. Methods A 525, 188(2004).