実験報告書様式（一般利用課題・成果公開利用）

MLF Experimental Report

提出日 Date of Report

課題番号 Project No.

装置責任者 Name of responsible person

2015A0127

Yosuke Toh

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Study of double octupole states in Pb-208

BL04

実験責任者名 Name of principal investigator

実施日 Date of ExperimentToshiyuki Shizuma

20-26 May, 2016

所属 Affiliation
National Institutes for Quantum and Radiological Science andTechnology
試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.
Following samples were used:
Enriched Pb-207 sample, D=10mm, 160mg
Enriched Pb-208 sample, D=10mm, 159.7 mg

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.
The first excited state in208

If you failed to conduct experiment as planned, please describe reasons.

Pb has the spin and parity of 3- at the excitation energy of 2.614 MeV andinterpreted as one-phonon octupole vibrational character. The vibrational nature of this state leads to theexpectation of a quartet of double octupole states (DOS) with 0+, 2+, 4+, and 6+ at roughly twice the energy ofthe 3- state, i.e., 5.2 MeV as shown in Fig. 1. In addition, the DOS
could decay to the first 3- state because of its octopole vibrational
nature. In this case, the transition between the DOS and the first 3state could be observed in -ray coincidence measurements. In order
to search for the DOS in208

Pb we carried out coincidence

measurements of -rays emitted from 208Pb produced in the neutroncapture reaction on 207Pb.
The experiment has been performed using the ANNRI instrument at
the beam line BL-04. An enriched 207Pb sample was bombarded by thepulsed neutron beam. We measured emitted -rays following the1

Fig.1. Level scheme of 208Pb.

2. 実験方法及び結果（つづき） Experimental method and results (continued)
neutron capture reaction on 207Pb using the ANNRI 4 -ray spectrometer which consists of two cluster
high-purity Ge detectors surrounding by BGO Compton-suppressed shields. Each of the Ge detectors is
composed of seven closely packed hexagonal encapsulated Ge detectors and is vertically placed at angles of±90°with respect to the neutron beam axis. Coincidence events were recorded with the energy and time
information on these -rays using the high counting rate and high resolution data acquisition system based ona digital data processing technique. We also measured the time-of-flight (TOF) information of each -ray
signal relative to the neutron time signal. The measurement time was totally about 96 hours and the total -raycoincident events were 4×108. We also made a similar measurement on an enriched 208Pb sample for about 12hour measurement time in order to subtract background components originating from 208Pb in the 207Pb data.
Figure 2 shows a TOF spectrum of neutrons in coincidence with -ray signals. Since the states in 208Pb
populated in the neutron capture reaction would depend on the incident neutron energy, gating channels of theneutron energy are carefully determined. Figure 3 shows single and coincidence -ray spectra. The 7334 keVtransition which is a -ray transition from the 1- state at 7334 keV to the ground state can be clearly seen inthe left panel of Fig.3. At the lower energy region,single and double escape peaks of this transition, i.e.,
6824 and 6314 keV transitions, are observed. In theright panel of Fig.3, the 7334 keV transition is notobserved because of no coincident -ray. On theother hand, the single and double escape peaks of the7334 keV transition are observed, because thesetransitions are in coincidence with the 511 keVannihilation -ray. We will further analyze thecoincidence data whether transitions in coincidence

Channelwith the 2614 keV transition exist or not byFig.2. Neutron TOF spectrum

appropriately gating on the neutron TOF channel.

Fig.3. Single (left) and coincidence (right) -ray spectra
obtained in the neutron capture reaction on 207Pb.

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