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

課題番号 Project No.

装置責任者 Name of responsible person

2012A0075 (2009S03)

Takashi Ino

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Measurement of neutron lifetime by using pulsed neutron

BL05

beam

実施日 Date of Experiment

実験責任者名 Name of principal investigator2012/05/29-2012/06/05

Kenji Mishima
所属 Affiliation
ICEPP, the university of Tokyo
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.
We didn’t use any sample. See next paragraph.

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.

We are developing a new experiment for the neutron lifetime measurement to be going to achieve
to 0.1% accuracy. In this experiment, we determines neutron lifetime as the ratio of decay count of
the beta decay to cold neutron beam flux. We use a time projection chamber to count of electrons of
the beta decay, and 3He(n,p)3H reactions from 3He doped in the gas to estimate the neutron flux.
the neutrons are delivered as bunches of 30-40 cm length, shaped by a spin flip chopper, to TPC of
1 m length in order to define fiducial volume and reduce neutron induced background.
In the present state, we have built up an apparatus and are going to evaluate the apparatus forthe physics measurements. Based on the state, we carried out measurements to derive neutron
fluxes and evaluate them. We calculate the neutron fluxes by using counts of 3He(n,p)3H reactions
thus the counts should be by only 3He(n,p)3H reaction, also all 3He(n,p)3H should be counted in therequired efficiency.
12. 実験方法及び結果（つづき） Experimental method and results (continued)
The TPC gas composition was natHe of 85 kPa, CO2 of 15 kPa, and 3He dopant of order of hundredmPa. The pulse height spectra of TPC by neutrons are shown in Fig.1. A cutoff between electron
signals (less than 600 ch) and 3He(n,p)3H signals(more than 600 ch)

can be seen well. By

normalizing the two measurement of different
partial pressure ofelectrons and3He,

we can deduce the3He(n,p)3H

spectrum. We are

analyzing the detail.
Fig.2 is a 2D plot of time of flight for x axis andevent position of TPC of beam passing direction.
Note that the event positions were calculated bycenter of mass of charge on wires. Only 3He(n,p)3H
events were selected by taking higher pulse

Fig.1. A plot of pulse height spectra with 3He of 615
mPa (black), 130 mPa (red), and their difference(blue) are shown. The pink is difference ofspectrum normalized by 3He partial pressure toindicate beta spectrum.

heights shown in Fig.1. Five lines can be seen in 15
ms to 35 ms, which corresponds to beam bunches
chopped by the spin flip chopper at the upstream.
The lines show that neutron bunches go forward,
thus the slopes of lines indicate the velocity ofneutron. We can clearly see that the velocities weregetting slowly with time of flight. All velocities
measured by the slopes were consistent with ones
controlled by the spin flip chopper. Unexpectedsignals were detected at the end of TPC. They are

33Fig.2. A plot of He(n,p) H events with position
on beam axis and TOF of chopped beam. Thelines indicate neutron bunches.

assumed to be caused by 6Li(n,t)4He events from abeam catcher set on the end of the TPC. The beam catcher was made by compound of 6LiF-PTFE tocapture neutrons without emission of gamma ray by 6Li(n,t)4He reactions. This reaction emits nogamma rays, but ~2 MeV of alpha and triton, thus we covered the beam catcher by PTFE of 100
m thickness to stop the ions. A lack of the cover can provide events at the end of TPC.
A stability of the counts is also important for this experiment. In order to study the stability of theefficiency of 3He(n,p)3H reaction, we measured a time change of a ratio of 3He(n,p)3H to beam
monitor, which set at downstream of the spin flip chopper. As a results, the ratio increased only
0.100.06 mPa/day in term of 3He pressure in measurement of 4 days with total gas pressure of
100 kPa. This value corresponds to 0.1% change for 4days run with 3He of 500 mPa.

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