Maynard Scott Dewey

Neutron Lifetime From
Beam Experiments
M. Scott Dewey
National Institute of Standards and Technology
Gaithersburg, MD, USA
Solvay Workshop 2014
Outline of Presentation
• A brief introduction to the neutron lifetime
• Status of current beam measurements
• J-PARC measurement
• BL2 at NIST
• Future beam measurements
• BL3 at NIST
• Conclusions
Vud and the CKM Matrix
Measurements of tn and b-decay angular correlation coefficients yield |Vud|:
Measurements of ft values for superallowed 0+→0+ b-decay also yield |Vud|:
How to Measure τn … () = 0
Direct Observation of Exponential Decay:
Similar in principle to Freshman
Physics Majors measuring
radionuclide half lives
-- only a lot harder.
“Bottle” Experiments:
− 

Observe the decay rate of N0
neutrons and the slope of
 N (t ) 
ln 
 is  1 t n

t


Form two identical ensembles of
neutrons and then count how
many are left after different times.
N (t1 )
 e t1 t2  t n
N (t2 )
Beam Experiments:
Decay rates within a fiducial
volume are measured for a beam
of well known fluence.
Decay Detector
N (t )
  N tn
t
Neutron Beam
Fiducial Volume
Neutron Detector
The State of the Neutron Lifetime
Beam Average
t n  888.0  2.1s
Storage Average
t n  879.6  0.8s
Note: This average contains result from Yue et al
Phys. Rev. Lett. 111, 222501 (2013)
The Present
Precise measurement of neutron
lifetime with pulsed neutron beam at JPARC
Kenji MISHIMA (KEK)
T. Yamada1#,
N. Higashi1, K. Hirota2, T. Ino3, Y. Iwashita4,
R. Katayama1, M. Kitaguch5, R. Kitahara6, K. Mishima3, H. Oide7,
H. Otono8, R. Sakakibara2, Y. Seki9, T. Shima10, H. M. Shimizu2,
T. Sugino2, N. Sumi11, H. Sumino12, K. Taketani3, G. Tanaka11,
S. Yamashita13, H. Yokoyama1, and T. Yoshioka8
Univ. of Tokyo1, Nagoya Univ.2, KEK3, ICR, Kyoto Univ.4, KMI, Nagoya Univ.5, Kyoto
Univ.6, CERN7, RCAPP, Kyushu Univ.8, RIKEN9, RCNP, Osaka Univ.10, Kyushu
Univ.11, GCRC, Univ. of Tokyo12, ICEPP, Univ. of Tokyo13
Principle of our experiment
Cold neutrons are injected into a TPC.
The neutron b-decay and the 3He(n,p)3H reaction are measured simultaneously.
Principle (Kossakowski,1989)
Count events during time of
bunch in the TPC
Neutron bunch
shorter than TPC
p
ν
3He(n,p)t
Neutron bunch
e
β-decay
τn
v
εe
: lifetime of neutron
: velocity of neutron
: detection efficiency of electron
3He(n,p)3H
εn
ρ
σ
: detection efficiency of 3He reaction
: density of 3He
: cross section of 3He reaction
σ0=cross [email protected], v0=2200[m/s]
This method is free from the uncertainties due to external flux monitor, wall
loss, depolarization, etc.
Our goal is measurement with 1 sec uncertainty.
8
Setup
Set up of our experiment in “NOP” beam line.
20 cm Iron shield
TPC in the
vacuum chamber
Inside of
Lead shielding
Spin Flip Chopper
In a Lead Sheald
Inside of
Cosmic ray Veto
TPC in
a Vacuum chamber
Gas line
DAQ
9
chronological table
2008
2009
2010
嶋TPC
1st JPARC
symposiu
m
2011
G10-TPC
Design
the G10-TPC
2012
(Low noise Amp)
Upgrade of
analysis framework
for physics run
Data taking2012
(commissioning)
SFC shielding
upgrade
Design and
development of
Large TPC
Analysis for
commissioning
data
Measurement
of Beam profile
First detection of
he first beam accept at Neutron β-decay
the “NOP” Beam line
100 kW
Design and
development of
Large SFC
Design and
development of
DAQ system
BG survey
20 kW
2015
2016
2017
LARGE PEEK-TPC
TPC Basic
properties test
Development of software
(Analysis framework, Geant4) Development of
DAQ system
Material test
(PEEK)
MLF
Power
2014
PEEK-TPC
Design the
PEEK TPC,
First detection of
3He(n,p) reaction
2013
Beam
intensity is
estimated to
be 18 times.
Commissioning
for the new system
Data Taking 2014
Data taking
for 1sec level
Today
200 Earthquake 200
kW
kW
Accident
300 kW of hadron
hall
300
kW
600 kW?
Increasing size the Spin Flip Chopper is planed at 2014/2015.
Intensity will be 18 times by a designed value.
We will start physics run to 1sec at 2016/2017
10
The NIST Beam Lifetime Experiment II (BL2)
n | ddu
tn 
W
2
F
G
4908.7(1.9) s
2
2
GV  3GA
p | duu
National Institute of Standards and
Technology Physical Measurement
Laboratory
e
e
The NIST beam lifetime experiment
a,t detector
B = 4.6 T
Precision aperture
p detector
Neutron beam
n
6LiF
deposit
Beam fluence
measurement
Neutron monitor
•
Proton trap
Decay product counting
volume
+800 V
(
)
(
)
Proton trap electrostatically traps decay protons and directs
them to detector via B field
• Neutron monitor measures incident neutron rate by counting n +
6Li reaction products (a + t)
Alpha-Gamma
Determining
tn
Proton rate measured as function of trap length
Proton detection efficiency
n + 6Li reaction product counting
Neutron flux monitor efficiency for
NIST 2005 Error Budget
Most
significant
improvement
Other major
improvements
Nico et al Phys. Rev. C 71 055502 (2005)
Using AG to calibrate the neutron monitor
HPGe detector
Totally absorbing
10B target foil
Neutron monitor
PIPS detector
with aperture
Alpha-Gamma
device
HPGe detector
Neutron monitor efficiency uncertainty budget
Projected Error Budget (BL2)
Most
significant
improvement
0.5s
0.1s
Other major
improvements
0.2s
0.6s
t n  1.0s
The Future
Nab Si detectors
• 15 cm diameter
• Full thickness: 2 mm
• Dead layer ≤ 100 nm
• 127 pixels
Conclusions
• Moving forward the goal is a reliable measurement
of the neutron lifetime at the 0.1—0.2 s level
• It is likely that there will be two efforts in the US
during the coming decade
• BL3: a beam experiment designed to achieve an
uncertainty of < 0.2 s.
• UCNtau: a magnetic bottle experiment
• Both experiments will be seeking funding in the
next 1—2 years