実験報告書様式（一般利用課題・成果公開利用）（ ※本報告書は英語で記述してください。ただし、産業利用課題として採択されている方は日本語で記述していただいても結構です。 ）MLF Experimental Report

提出日 Date of Report
2017/6/26

課題番号 Project No.

装置責任者 Name of responsible person

2015A0305

Atsushi Kimura

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

6

Measurement of Li(n,) cross section up to 10 keV

ANNRI / BL-04

実験責任者名 Name of principal investigator

実施日 Date of Experiment

Hiroyuki Makii

2017/2/23-2017/3/3

所属 Affiliation
Japan Atomic Energy Agency
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.
In this experiment, we used a following sample.
Name of sample chemical formula from shape

Quantity

Li-6 Carbonate

Li2CO3

solid

0.5 g

Li-7 Carbonate

Li2CO3

solid

0.5 g

We also used B, Au, Si, and melamine samples to estimate the neutron flux at the sample position and to makeenergy calibration of the NaI(Tl) detectors.

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.

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

The measurement has been carried out with use of the ANNRI installed at beam line BL-04. The NaI(Tl)detectors placed at the flight length position of 27.9 m are used to detect the -rays from the samples. TwoNaI(Tl) detectors with different size are placed at 90o (330 mm in diameter and 203 mm in length) and 125o(203 mm in diameter and 203 mm in length) with respect to the neutron beam direction. An enriched 6Li2CO3
sample with a diameter of 10 mm and a weight of about 0.5 g are located in a beam duct made of carbon fiber.
The purpose of this study was to deduce the excitation function of the 6Li(n,)7Li reaction and the -ray
branching ratio of transitions from a neutron capture state to low-lying states in 7Li in incident neutron energy
range up to 10 keV using time-of-flight (TOF) technique. Because of small cross section was expected for the6Li(n,)7Li reaction, particular attention should be given to identify the background events due to the neutrons

scattered by the sample. For this purpose, an enriched 7Li2CO3 sample was irradiated in the samearrangement. In addition, we also measure the -ray spectrum without placing any sample at the sample
position to identify background events due to the neutrons scattered from the various instruments of chopper,filter, and collimator placed upstream the sample position. Neutron flux at the sample position was obtained by1

2. 実験方法及び結果（つづき） Experimental method and results (continued)
measurements of 10B(n,)7Li and 197Au(n,)198Au reactions using B and Au samples since the cross sections ofthe reactions are well known. Energy calibration of the NaI(Tl) detectors was made using the -rays from the28Si(n,)29Si and 14N(n,)15N reactions with Si and melamine samples. Measuring time and total dose of protons

for each sample are summarized in Table 1. Proton beam power in this study was about 150 kW.
Table 1: Measurement time and total dose of proton beam.
sampleMeasuring time [hours]

Proton dose (1019 protons)

6Li

67.1

6.75

7Li

48.3

4.82Empty

52.2

5.30

B

5.8

0.60

Au

4.7

0.48

Si

1.2

0.13

melamine

3.7

0.60
767

Due to the simple structure of the nuclear levels of Li, the Li(n,) Lireaction cross section can be obtained by summing the partial crosssections. Here the6Li(n,)7Li reaction proceeds through direct

radiative transitions to the ground (J = 3/2-) and to first excited (J =1/2-) states in 7Li as shown in Fig. 1. The partial cross sections andthe -ray branching ratio can be deduced from the intensities of the6.77 MeV and 7.25 MeV -rays in 7Li. Figure 2 shows -ray spectrum
for the 6Li sample putting the gate on TOF of neutrons corresponding
to the incident neutron energy (En) range at 2 ≤ En ≤ 34 keV. We see
clearly the peak considered to be the -ray from the 6Li(n,)7Li

Figure 1. Level scheme of 7Li.

reaction around 6.8 MeV. Here the background events contained in the spectra of the 7Li and empty samples
are already subtracted. The analysis to determine the absolute partial cross sections and the -ray branchingratio is now in progress.

Figure 2. -ray spectrum obtained by NaI(Tl) detector for the 6Li sample putting the gate on TOF
corresponding to the neutron energy (En) at 2 ≤ En ≤ 34 keV.
2