SHIGEHARU Kobayashi Saga University Faculty of Science and Engineering, Professor, 理工学部, 教授 (00039273)
MORI Shigeki University Tsukuba Institute of Applied Physics, Professor, 物理工学系, 教授 (60100822)
INAGAKI Yousuke Tokyo Kasei University, Professor, 短期大学部, 教授 (70266542)
MAEZAWA Hideki Institute of Marterials Structure Science, High Energy Accelerator Research Organization Professor, 物質構造科学研究所, 教授 (40150015)
MAKI Akihiro Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization, Professor, 素粒子原子核研究所, 教授 (40044755)
|Budget Amount *help
¥13,000,000 (Direct Cost : ¥13,000,000)
Fiscal Year 2000 : ¥500,000 (Direct Cost : ¥500,000)
Fiscal Year 1999 : ¥1,100,000 (Direct Cost : ¥1,100,000)
Fiscal Year 1998 : ¥11,400,000 (Direct Cost : ¥11,400,000)
Our aim of the present research is to measure the neutron lifetime in a precision of 0.1%. For this purpose, we invented a new method whose principle is explained as follows : There is a hollow cylinder-shaped detector in an atmosphere of ^3He gas of low pressure. When cold neutron beams pass through along the axis of the cylinder, both β-rays from the neutron decay and protons from the reaction ^3He(n, p)^3H are detected with the same detector. The neutron lifetime can be obtained from the formula τ_n=(N_p)/(N_β)・1/(ρσ_0υ_0)・(ε_β)/(ε_p), where N_p and N_β are proton counts and β-rays counts, respectively, ρ is a number of ^3He atoms in 1cm^3, σ_0 is the cross section of ^3He(n, p)^3H reaction for a neutron having a velocity v_0, ε_β and ε_p are the detection efficiensies for electrons and proton, respectively.
To realize the above-mentioned method, we constructed a τ_n-measuring apparatus inside which there are a elctron-proton detector made of plastic scintillator, together with an anti-counter in an atmosphere of ^3He gas of low pressure.
From 1998 fiscal year to 2000 fiscal year, we performed experiments to measure the neutron lifetime using the apparatus, set up at C2-3 beam port of JRR-3M Reactor in Tokai Laboratory, Japan Atomic Energy Research Institute.
The results obtained in the experiment so far are as follows : 1. We measured the energy of the proton peak from the reaction ^3He(n, p)^3H using the energy scale determined by cosmic ray events. It was 93 keV in terms of the electron energy. 2. Using proton counts from the reaction ^3He(n, p)^3H, p, and the cross section of the above reactions, we estimated the neutron intensity used in the experiment as 〜1×10^7 per cm^2 per second. This value seems to be consistent with the nominal intensity of neutron beams from C2-3 beam pont. 3. It turned out that the method we proposed is very feasible to make a precision measurement of the neutron lifetime.