Rayleigh scattering measurement for Super-Kamiokande Gd project
Ryosuke AKUTSU, Institute for Cosmic Ray Research, Univ. of Tokyo / NNN2015
Ryosuke
1. Super-Kamioknade detector & Gd project
Physics target :
1. Discovery supernova relic neutrinos
● Super Kamiokande(Super-K) is an underground water Cherenkov detector
and has operated since April 1996.
● The Gd project adds 0.2% Gd2(SO4)3 to the current Super-Kamiokande detector
with ultra pure water. This will give a high neutron detectability by the large
cross section of Gd for thermal neutron(about 1.5 billion times that of Hydrogen),
so that we can distinguish between neutrinos and anti-neutrinos by a delayed
coincidence technique.
Clock
(200Hz/30Hz)
DAQ
(ATM)
2. Improve pointing accuracy for galactic supernova.
3. Precursor of nearby supernova by Si burning neutrinos.
4. Reduce proton decay background.
5. Neutrino/anti-neutrino discrimination (Long-baseline and atmospheric)
6.Reactor neutrinos
LD(375,405nm)
Gas laser(337nm)
Optical fiber
5. Set up
Z
wire
Scattered photons
Cables of PMTs
measurement
337nm :
USHO KEN1020 (600ps)
2. EGADS detector
- Evaluating Gadolinium’s Action on Detector Systems -
375nm :
SCIENTEX OPG-NIM (1-5ns)
● EGADS is a Gd2(SO4)3 loaded water Cherenkov detector for studying
the effect of Gd2(SO4)3 on the water transparency, all the materials
and the environmental neutron background.
200 m3 tank with 240 PMTs
5.2m
405 :
SCIENTEX OPG-1000-NIM
(70-80ps)
・Observe light originating from
the laser by PMTs.
Laser intensity
measurement
・The laser intensity was
adjusted to be about 1%
occupancy of PMTs.
4.7m
・The laser injecting device was
put at the center of EGADS.
・Data was taken by periodic
trigger with 200 or 30 Hz.
Y
・The scattered photon and
laser intensity measurements
were performed by turns.
Gd loaded water
circulation system
Water transparency
measurement (UDEAL)
・The observed laser intensity
was stable with about 3%.
3. Motivation
Corrected hits/event
𝐼 𝐿 = 𝐼0 exp(−α𝐿),
where L is the travel length of a light, I(L) is the intensity of a light with
an initial intensity I0 after traveling L. α can be express as the sum of
scattering coefficient β and absorption coefficient γ of the medium.
● Loading EGADS with 0.2% Gd2(SO4)3 concentration caused about 8%
decrease in the transparency from the pure water, which was
measured by UDEAL. The performance of Super-K could be affected
by the change of transparency for a given adding Gd2(SO4)3
concentration, depending on the variation of β and γ.
Barrel
Bottom
● Only the hits in the barrel part have been used for evaluating scattering coefficient to avoid a background
contamination which is thought as the halo component of the laser. But, the effect of the contamination
is not estimated yet.
Assumed negligible
background contamination
of incident photon
Ψ
Incident direction
4. Measurement principle
● Since only the scattered photons are needed for the measurement,
a device which can generate linearly polarized laser is used to
observe redirected photons of the laser due to scattering as hits of
PMTs inside EGADS.
β0.2% / β0.1%
∝ 𝑁0 × β λ × 1 − sin 2 Ψ × cos 2Φ ,
Φ
Top
cosθ
Φ [rad]
● The observed hits patterns in the barrel of the tank were consistent with a expected curves which are
derived by the differential cross section of Rayleigh scattering.
● To know whether the content of the about 8% variation is scattering or
absorption, a scattering measurement in EGADS has been conducted for
0%(pure), 0.1% and 0.2% Gd2(SO4)3 concentrations water, expecting that
the dominant process of scattering in EGADS is Rayleigh scattering
Scattered
which has the differential cross section
Polarization vector
direction
where N0 is the number of incident photons and
λ is the wave length of the incident photons.
∝β
Emit the linearly polarized laser
6. Angular distributions
● Corrections : distance between each
PMT and laser, PMTs acceptance, relative quantum
efficiency of PMTs, dark hits subtracting
● The transparency of a medium is determined by it’s total attenuation
coefficient α[m-1] defined as follows.
𝑑σ
𝑑Ω
Absorb or reflect the laser
Corrected Hits/event/PMT
15 m3 tank
to dissolve Gd2(SO4)3
About 1m
travel length
Assumed 100% sct.
Assumed 20% sct. & 80% abs.
Assumed 10% sct. & 90% abs.
7. Relative variation of β
● The measured change of β from 0.1% to
0.2% Gd2(SO4)3 concentration has been
compared with a expected value which is
calculated a spectrometer measurements
and UDEAL with an absolute value of
attenuation coefficient as the model
used in Super-K.
Model in Super-K
Wave length [nm]
Total , Absorption
Rayleigh sct. , Mie sct.
8. Summary & future plan
● A Rayleigh scattering measurement in EGADS was conducted.
● Data were compared between 0.1% and 0.2% Gd2(SO4)3 concentrations.
● Assuming that the contamination of the laser halo is negligible, the resulting
relative change of scattering coefficient, while still consistent with zero,
is at most 10%.
● Using data and an reliable Monte Carlo simulation(MC), obtain the relative
change of scattering coefficient from the pure water to the % Gd2(SO4)3
loaded water.
● Evaluation of the contamination for the halo component by MC and data.