Precise Neutron Lifetime Measurement Using Pulsed Neutron Beams at - - PowerPoint PPT Presentation

Precise Neutron Lifetime Measurement 
 Using Pulsed Neutron Beams at J-PARC

International Workshop on Particle Physics at Neutron Sources 2018

1

Naoki Nagakura1,

Katsuya Hirota2, Sei Ieki1, Takashi Ino3, Yoshihisa Iwashita4, Masaaki Kitaguchi5, Ryunosuke Kitahara6, Jun Koga7, Kenji Mishima3, Aya Morishita7, Yusuke Nakano2, Hideyuki Oide8, Hiroki Okano2, Hidetoshi Otono9, Yoshichika Seki10, Daiichiro Sekiba11, Tatsushi Shima12, Hirohiko M. Shimizu2, Naoyuki Sumi7, Hirochika Sumino13, Kaoru Taketani3, Tatsuhiko Tomita7, Hideaki Uehara7, Takahito Yamada1, Satoru Yamashita14, Mami Yokohashi4, Tamaki Yoshioka9

The Univ. of Tokyo1, Nagoya Univ.2, KEK3, ICR, Kyoto Univ.4, KMI, Nagoya Univ.5, Kyoto Univ.6, Kyushu Univ.7, CERN8, RCAPP, Kyushu Univ.9, J-PARC Center, Japan Atomic Energy Agency10, The

• Univ. of Tsukuba11, RCNP, Osaka Univ.12, GCRC, The Univ. of Tokyo13, ICEPP, The Univ. of Tokyo14

Motivation

2

8.4 sec (3.8σ) deviation between the result of two previous methods

we plan to measure τn using different method with precision of 1 sec

in-flight method

inject neutrons into detector and detect protons from β decay

UCN storage method

store ultra-cold neutrons and count the remaining neutrons 8.4 sec (3.8σ) 888.0 ± 2.2 sec 879.6 ± 0.6 sec

Particle Data Group 2017

neutron lifetime (τn) is a fundamental parameter in the weak interaction

• 1. input parameter to Big Bang Nucleosynthesis (BBN) theory,

which predicts light element synthesis in the early universe

• 2. determine Vud element in CKM matrix

J-PARC / BL05

spallation pulsed neutron beam in MLF (Materials and Life science

experimental Facility) at J-PARC

polarized beam branch of BL05

polarization ：~ 95% neutron flux：3.9×107 /sec•cm2 (@1 MW) energy ：~ 10 meV（cold neutron）

3

facilities beamline

MLF

BL05 polarized beam branch

neutron

J-PARC bird’s eye view

Setup

Time Projection Chamber

He + CO2 gas detect e- from β decay

Spin Flip Chopper

form neutron bunches

polarized neutron magnetic mirror flipper

4 drift direction

MWPC 1 m 40 cm

30 cm 3 c m

・pulsed neutron beam from accelerator ・detect electrons from βdecay

neutron shutter

Measurement principle

TPC detects both 3He(n, p)3Hとβ decay at the same time total neutron flux can be evaluated using 3He(n, p)3H

3He

p

n

+764 keV

3H

n

+782 keV

p e-

ν

β decay

3He(n, p)3H

σ : 3He(n, p)3H cross section v : neutron velocity ε : selection efficiency N : number of events ρ : 3He number density

5

σ(v) v = σ(v0) v0 = 5333(7) barn × 2200 m/s

6

Spin Flip Chopper

Spin Flip Chopper can form neutron bunches with arbitrary length 
 by controlling neutron polarization

• perate in 5 bunch mode

bunch length is about half of TPC sensitive length

polarized neutron

filpper OFF

dump

flipper ON

flipper coil magnetic mirror

to detector

flipper off 5 bunch mode flipper on

Time Projection Chamber (TPC)

7

• count neutron β decay and 3He(n, p)3H at the same time
• 4He (85 kPa) + CO2 (15 kPa) + 3He (100 mPa)
• low background material（PEEK：PolyEthel Ethel ketone）
• inside walls covered by 6LiF to absorb scattered neutrons
• no magnetic field is applied

drift direction neutron anode (1720 V) ：24 ch field (0V) ：24 ch cathode (0V) ：40 ch×2 MWPC 6 mm

MWPC structure

1 m 30 cm 3 c m

Acquired data

8

gas No. month

3He pressure

[mPa] beam data time [hour] MLF power [kW]

I 2014/5 101 35.3 300 Ⅱ 2015/4 86 15.8 500 Ⅲ 2016/4 97 17.5 200 Ⅳ 2016/4 173 72.7 200 Ⅴ 2016/5 50 69.4 200 Ⅵ 2016/6 114 71.1 200

combined statistical uncertainty for τn ~ 10 sec

we took physics data for 6 different gas cinditions in 2014-2016

Background subtraction

9

βdecay like event at TPC

shutter open (beam in data) shutter close (beam dump data) subtracted

neutron bunch is completely inside TPC β decay events is counted only in the fiducial time fiducial time

Monte Carlo simulation

10

detector setup in beamline is constructed at Geant4 system evaluate selection efficiency and background amount

simulation processes ・β decay ・3He(n, p)3H ・55Fe X-ray（energy calibration) ・commic rays（drift velocity calibration） ・neutron scattering ・γ-ray from inside wall

electrton track (view from TPC top)

11

Separation of βdecay and 3He(n, p)3H

two kinds of signal events (βdecay and 3H(n, p)3H) in the TPC can be separated by maximum energy deposit among all wires

linear scale log scale

25 keV

12

Gas scattering background

scattered neutron interacts with a wall of the TPC, and prompt γ becomes background DC : Distance from beam Center

βdecay ~ small DC gas scattering bkg ~ large DC

DC=4 wires (48 cm from TPC center)

linear scale log scale

signal region DC

γ beam axis

βdecay

n

DC gas scattering background βdecay gas scattering background

Results

13

uncertainty values and their uncertainties for gas Ⅵ (~11 days data taking)

% Preliminary

Combined results

14

combined results of 6 gas data

Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ Ⅵ 2014 2015 2016

Preliminary

Results

15

• ur result

1.1σ deviation from PDG2017 value（880.2±1.0 sec）

Preliminary

Data taking status

16

we continue taking physics data

year gas set number MLF power [kW] total incident neutron [×1011]

2014A 1 300 0.23 2015A 1 500

0.21

2016A 4 200

1.2

2017A 9 150

0.78

2017B 9 300

3.7

2018A 6 300~500

~ 4

all combined statistical uncertainty ~ 4 sec (0.5%) new

upgrade plan1: SFC upgrade

17

polarized neutron magnetic mirror flipper

MWPC

neutron shutter

increase in size of flippers and magnetic mirros in SFC total neutron flux is expected to become 5 times 100 days for 1 sec statistic error at 1 MW operation

current setup (1 MW)

after upgrade (1 MW)

600 days 100 days

upgrade plan2: low-pressure operation

18

DC

γ beam axis

βdecay

n

DC gas scattering bkg

gas scattering event is a main 
 background for βdecay (～4%) at current TPC gas condition (He+CO2 = 100 kPa) This background can be reduced by half at low-pressure（He+CO2 = 50 kPa）operation For low-pressure operation, high gain and low power-consumption amplifier is required new amplifier (power comsumption ~ 1/50)

βdecay gas scattering bkg

19

Conclusion

• neutron lifetime is an important parameter in the weak interaction


input parameter to the BBN theory and Vud determination


• significant deviation between two previous types of measurement

• we use different method at J-PARC to measure the neutron lifetime, 


and our first result is 


• upgrade projects are undergoing to achieve our goal precision of 1 sec

Preliminary