Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

First α-γ spectroscopic study using a Si-Ge detector arrayinstalled at the focal plane of GARIS
D. Kaji,∗1 K. Morimoto,∗1 Y. Wakabayashi,∗1 M. Takeyama,∗1,∗2 and M. Asai∗1,∗3A basic study on α-γ spectroscopy was performed
using a Si-Ge detector array installed at a focal planeof a gas-ﬁlled recoil ion separator GARIS.
The GARIS has been applied for studying production and decay properties of superheavy element(SHE) nuclides produced via Pb/Bi-based fusion reactions (cold fusion); in particular, these studies focused on the search for 113th element on the basis of209
Bi(70 Zn,n)278 113 reaction from 2003 to 20121–3) . In
the most of these experiments, the number of identiﬁed atoms was limited because of the low production
cross section of the SHE nuclides. The nuclide identiﬁcation method was based on the genetic relation between mother and daughter. It involved measuring
the α-decay and spontaneous ﬁssion SF . Therefore, adetailed decay scheme including γ-decay could not beobtained. In 2013, we newly installed a Si-Ge detector
array, as shown in Fig. 1, at the focal plane of GARIS
for studying the production and decay properties of reaction products for 248 Cm+48 Ca4) . The Si-Ge array isuseful as a probe for detecting prompt γ-ray coinciding
with SF 5) . Before the experiment, we calibrated theSi-Ge array by using a 207 Pb(48 Ca,2n)253 No reaction.
Projectiles of 48 Ca with a charge state of 11+ wereextracted from the 18-GHz ECR ion source and accelerated up to 218.5 MeV using the RILAC. The intensity of a typical beam incident on a target was
5.2×1012 s−1 (0.86 pµA). The metallic 207 Pb target
was prepared by vacuum evaporation on a 60 µg/cm2carbon foil. Target thickness was 371 µg/cm2 for207
Pb (enrichment of 99.59%). Sixteen frames of thesector targets were mounted on a φ30 cm rotating
wheel, which was rotated at 3300 rpm. The reaction products were separated in-ﬂight from projectiles and other by-products by GARIS, and they wereguided into the focal plane detection system after theypassed through the time-of-ﬂight detector. The separator was ﬁlled with helium gas at a pressure of 73
Pa. The magnetic rigidity Bρ was set to 2.064 T·m for
253
No. Gamma rays emitted in prompt coincidencewith α-particles registered by a conventional positionsensitive Si detector (PSD box)1–3) were measured using a planar-type Ge detector for counting low-energy
photons (CANBERRA BE6530; active volume: φ91.5mm×31.6 mmt ). The distance between PSD and Ge
detector was c.a. 6 mm (3 mm between PSD and
1 mmt Al window + 2 mm between Al window andGe detector). The peak eﬃciency for 122-keV pho∗1∗2∗3tons was 22.8% at the middle position of the PSD.
Figure 2 (a) shows a two-dimensional energy plot of
α- and γ-rays observed in prompt coincidence. Theenergy spectrum of γ-rays observed in prompt coincidence with α-decays is given in Fig. 2 (b). Thesedata were obtained under low-background condition,
despite the high beam intensity of 0.86 pµA. Observed
α- and γ-transitions due to 253 No agree well with previously reported values6) .
The Si-Ge array will be applicable to study α-γ(X)spectroscopy of SHE nuclides.

Fig. 1. New focal plane detector including Si-Ge array.

Fig. 2. (a) Two-dimensional plot of α-γ coincidence. (b)Projection of the events onto the γ-energy axis.

References
1) K. Morita et al.: J. Phys. Soc. of Jpn. 73, p.2593 (2004).
2) K. Morita et al.: J. Phys. Soc. of Jpn. 77, p.045001(2007).
3) K. Morita et al.: J. Phys. Soc. of Jpn. 81, p.103201(2012).
4) K. Morita et al.: In this report.
5) M. Takeyama et al.: In this report.
6) F. P. Heßberger et al.: Eur. Phys. J. A 48, p.75 (2012).

RIKEN Nishina Center
Department of Physics, Yamagata University
Advanced Science Research Center, JAEA

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