XMASS:液体キセノン中の ラドンバックグラウンドの研究

XMASS:液体キセノン中の ラドンバックグラウンドの研究
14/12/6,12/7@名古屋大学
神戸大学大学院理学研究科物理専攻 修士１年
藤田 黎
contents
•
XMASS実験紹介
•
My work : 液体キセノン中のラドンバックグラ
ウンドの研究
2
XMASS実験
•
液体キセノンを用いた多目的実験
•
•
XMASS検出器
暗黒物質
•
太陽ニュートリノ
•
double beta decay
PMT 642本
など…
暗黒物質探索
約80cm
•
液体キセノンを用いた世界最大の
検出器(832kg)
•
キセノンと暗黒物質が弾性散乱す
る際に落としたエネルギーをシン
チレーション光として捉える。
3
液体キセノン : 832kg
神岡
実験室LabC
XMASS collaboration
Kamioka Observatory, ICRR, the University of Tokyo: K. Abe, K. Hiraide, K.
Ichimura,Y. Kishimoto, K. Kobayashi, M. Kobayashi, S. Moriyama, M.
Nakahata, T. Norita, H. Ogawa, H. Sekiya, O. Takachio, A. Takeda, M.
Yamashita, B. Yang
Kavli IPMU, the University of Tokyo: K.Martens, Y. Suzuki
Kobe University: R. Fujita, K. Hosokawa, K. Miuchi, Y. Ohnishi, N. Oka, Y.
Takeuchi
Tokai University: K. Nishijima
Gifu University: S. Tasaka
Yokohama National University: S. Nakamura
Miyagi University of Education: Y. Fukuda
Insitiute of Socio-Arts and Sciences, The University of Tokushima : K.Fushimi
STEL, Nagoya University: Y. Itow, R. Kegasa, K. Kobayashi, K. Masuda, H.
Takiya
Sejong University: N. Y. Kim, Y. D. Kim
KRISS: Y. H. Kim, M. K. Lee, K. B. Lee, J. S. Lee
5
液体キセノンの利点
•
大きい原子番号 Z=54と密度 ρ=約3g/cm^3
➡ コンパクトで大質量の検出器
•
大きな発光量(46000photon/MeV)
•
発光波長:175nm
➡ 真空紫外領域で波長変換材を使わずPMTで直接観測可
能
•
液ガス両方の状態での純化が可能
6
低バックグラウンド環境への取り組み
•
暗黒物質との反応は非常に稀
第 3 章 XMASS 実験
➡バックグラウンドを減らすこと
が重要
3.3 800kg 検出器の特徴
3.3.1
液体キセノンによるガンマ線の自己遮蔽
本実験では約 800kg の液体キセノンを使用する。液体キセノンは直径 80cm の球体に収めら
にキセノンは原子番号が Z = 54 と大きいのでガンマ線と相互作用する確率が高く、遮蔽体と
•
XMASSでは次のようなことを行って
いる。
る。本実験では液体キセノンが入っている直径 80cm の球のうち、外側 20cm の領域はガンマ線の
純水タンク
よって内側 20cm の領域をガンマ線 BG が非常に少ない領域にすることが出来る。これにより暗
の領域から選ぶことができる。事象の反応点は 642 個の PMT の光量分布から再構成する。図
表面から発生させたガンマ線の遮蔽効果のシミュレーションである。青い線が発生したガンマ線
の領域が 800kg すべての液体キセノン領域を、濃いピンクが液体キセノンの有効体積を表して
ションでは液体キセノンの表面から 238 U を 106 個崩壊させて、放出されるガンマ線がどの程度
•
地下1000mで実験(宇宙線μ粒
子を10万分の1に減らせる)
積まで届くのかを示している。この自己遮蔽効果が実際に使用する PMT からのガンマ線に対し
のかを確かめたのが図 3.11 と図 3.12 に示すシミュレーション結果である。このシミュレーショ
る 238 U からのガンマ線 BG を調べ、conservative に評価するために PMT1 個あたり 1.8mBq の
3.11 は、液体キセノンの外側からそれぞれ 5cm、10cm、20cm 内側の領域を色分けしたものである
分けと対応している。図 3.12 は PMT から発生するガンマ線が液体キセノン中の作るガンマ線
こで赤色の分布が、液体キセノンの外側 20cm を自己遮蔽として使用した場合に得られるガンマ
•
純水タンクによるシールド
これよりこの PMT と液体キセノンの自己遮蔽効果を用いれば、半径 20cm の有効体積では暗黒
れると予想される 100KeV 以下のエネルギー領域において、目標 BG レベルの 10−4 event/day/k
回ることが分かる。
•
検出器に放射性物質の少ないも
のを用いる(PMT,無酸素銅等)
•
キセノンによる自己遮 (キセノ
ンは高密度、Zが大きいのでγ線
をよく遮 する) 発生したガンマ線は中心付近に入り込めない
Blue : γ tracking
Pink : whole liquid xenon
Deep pink : fiducial volume
7
現状とこれから
• 2010年10月
2012年6月 コミッショニングラン
• 予期せぬバックグラウンドが見つかる(PMT
• 2012年6月
Al シール)
2013年10月 バックグラウンド低減のための改造作
業
-
1桁以上のBG低減に成功
• 2013年11月
観測開始
-
低閾値での季節変動の探索
-
検出器の中心での原子核反跳事象の探索
• 将来計画
• XMASS-1.5
•
標的質量1トンでの暗黒物質探索
• XMASS-Ⅱ
•
標的質量10トンで様々な物理を高統計、高精度で測定
8
これまでの結果
XMASS Collaboration / Physics Letters B 724 (2013) 46–50
軽い暗黒物質探索
WIMP-原子核の弾性散乱
counts/kg/day/keVee
Physics Letters B 719 (2013) 78-82
Data
18GeV MC
12GeV MC
Fig. 7. Simulated WIMP energy spectra in the XMASS detector assuming the maximum cross section that provides a signal rate no larger than the observation in any
bin above 0.3 keVee.
[cm2 ]
select these events a time-of-flight correction is made to the timing distribution of each event assuming the event vertex is on
effect is dominant in this energy range (<100 keV), and (3)the
thesurface of the PMT closest to the charge-weighted center of
gravity
process after the axioelectric effect is exactly the same as that of the event. After this correction the timing distribution
K. Abe et al. / Physics Letters B 719
78–82
81
of (2013)
Cherenkov-like
events is found to be narrower than that for
for the photoelectric effect. In the simulation, we considered the
scintillation-like events. Events with more than 60% of their PMT
nonlinearity of the scintillation yield for gamma rays, the optical
hits occurring within the first 20 ns of the event window are reprocesses of the scintillation photons in the detector, the photomoved as Cherenkov-like. The ratio of the number of PMT hits
electron distributions and discrimination threshold of photomultiwithin the first 20 ns relative to the total number of hits in the
pliers, and the trigger conditions of the data acquisition system.
event window for all events (head-to-total ratio) is shown in Fig. 4.
The detailed description of the simulation and efficiencies Each
werestep of the data reduction is shown in Fig. 5.
The expected WIMP acceptance efficiency of these cuts was
previously reported [36,35]. After taking into account the reducestimated with the detector simulation. In the simulation WIMP
tion efficiency described in the next section, the expected energy
recoil energy spectra were generated for each WIMP mass and MC
spectra for various masses of axions are obtained.
events were distributed uniformly throughout the detector volume
using a liquid scintillation decay constant of 25 ns [16]. Fig. 6
3. The data
shows the resulting signal acceptance efficiency at energies below 1 keVee. The size of the error bars comes primarily from the
The XMASS detector is a large liquid-xenon detector locatedsystematic
ununcertainty in the xenon scintillation decay constant,
derground (3000 m water equivalent) at the Kamioka Observatory,
25 ± 1 ns, which is estimated based on the difference between the
Japan. It contains an 835-kg liquid-xenon target with a surface
of model [16] and the NEST model [17] based on [18]. A sysXMASS
tematic error on the selection efficiency is determined based on
a pentakis-dodecahedron that is tiled with inward looking photothe error resulting from a linear fit to the points in the figure. At
multiplier
tubes (PMT),
which
have
hexagonal
Fig. 6. WIMP
signal acceptance
efficiency630
after of
data
reduction
for the
analysis. and 12 have
0.3 keVee analysis threshold this error is 6.1%.
round photocathodes. The PMTs (R-10789, Hamamatsu) are the
spe-
n
48
SI
Light WIMP search
Fig. 6. WIMP signal acceptance efficiency after data reduction for the analysis.
7GeV MC
Fig. 2. Observed energy spectra. The horizontal axis shows the “scaled energy” calcially
developed
for this correction
low-background
The photoelecselect these
events
a time-of-flight
is madedetector.
to the tim4.
Results and
discussion
culated
by dividing the number of photoelectrons by the photoelectron yield at the
tron
yield
at
the
center
of
the
detector
is
evaluated
at
14.7
phoing distribution of each event assuming the event vertex is on
center of the detector, 14.7 p.e./keV. Error bars are statistical only. In this figure
57
toelectrons
(p.e.)/keV
internal Co source.
The positional
the surface
of the PMT
closest using
to thean
charge-weighted
center of
Fig. 8.horizontal
Spin-independent elastic WIMP-nucleon cross section as a function of WIMP
we also show the efficiencies for the Cherenkov cut (closed circles with
Fig. 7 shows
simulated WIMPs energy spectra overlaid on the
(maximum
15%) of the
the timing
photoelectron
yield caused
by
gravity ofdependence
the event. After
this correction
distribution
mass.
uncertainties except that from Leff are taken into account in
forenergy
the applicable
1 fordetector
100%) and
for the
of all All
oursystematic
cuts
Fig. 7. Simulated bars
WIMP
spectra in range;
the XMASS
assuming
thecombination
maxiobserved spectrum after the data reduction was applied. WIMPs
the XMASS
90% C.L. limit line. The effect of the Leff uncertainty on the limit is
of Cherenkov-like
events
is found
be and
narrower
than that
for
the angular
acceptance
of to
PMTs
absorption
of scintillation
mumlight
cross section
that circles).
provides Only
a signal
rate trigger
no larger
than theisobservation
any
(open
at the
threshold
the overallinefficiency
not
dominated
are assumed
to be distributed in an isothermal halo with v =
shown in the band. Limits from other experiments and favored regions
are Collaboratio
also
binacquiabove
0.3 keVee.
XMASS
scintillation-like
events.
Events with
more
thanCarlo
60% of
their PMT Data
by the Cherenkov cut efficiency. The inset shows the sameo quantities
for energies
are taken
into account
in the
Monte
simulations.
shown [4–9].
220
km
/
s,
a
galactic
escape
velocity
of
v
=
650
km
/
s,
and
an
esc
extending up to 100 keV.
hits occurring
the firstif 20
ns or
of more
the event
window
are re-than 0.2 p.e.
sitionwithin
is triggered
four
PMTs
have more
average density of 0.3 GeV/cm3 . In order to set a conservative
moved aswithin
Cherenkov-like.
The
ratio
of
the
number
of
PMT
hits
200 ns. The trigger efficiency around the trigger threshold
nuclear-recoil and electronic events, and many events remain in
upper bound on the spin-independent WIMP-nucleon cross secwithin the
first
20 ns relative
to the
totalatnumber
of hits wall.
in the
was
examined
by LEDs
placed
the detector
The observed
the analysis sample, the present result excludes part of the paramtion, the cross section is adjusted until the expected event rate
event window
for all
events
ratio)
is shown
in Fig.
4.
behavior
was
well (head-to-total
reproduced by
the Monte
Carlo
simulations.
eter space favored by other measurements [4–6] when those data
inSigXMASS does not exceed the observed one in any energy bin
Each step of the data reduction is shown in Fig. 5.
are interpreted as a signal for light mass WIMPs. Finally, we are
nals from each PMT are fed into charge ADCs and TDCs whose
above 0.3 keVee. Implementing the systematic errors discussed in
The expected WIMP acceptance efficiency of these cuts was
working on modifications to the inner surface of XMASS, especially
the text above, the resulting 90% confidence level (C.L.) limit deresolution is around 0.05 p.e. and 0.4 ns, respectively. The liquidestimated with the detector simulation. In the simulation WIMP
around the PMTs, to improve the detector performance.
rived from this procedure is shown in Fig. 8. The impact of the
xenon detector is surrounded by a water Cherenkov veto counter,
recoil energy spectra were generated for each WIMP mass and MC
uncertainty
from
L
is
large
in
this
analysis,
so
its
effect
on
the
eff
which
is 10.5 uniformly
m in height
and 10 the
m detector
in diameter.
It is equipped
events were
distributed
throughout
volume
Acknowledgements
limit is shown separately in the figure.
with 72
20-inch PMTs
signals
using a liquid
scintillation
decay whose
constant
of 25 are
ns fed
[16].into
Fig. the
6 ADCs and
After careful study of the events surviving the analysis cuts,
TDCs.
Data acquisition
is triggered
if eight
or morebe20-inch PMTs
shows the
resulting
signal acceptance
efficiency
at energies
We gratefully acknowledge the cooperation of Kamioka Mining
their origins are not completely understood. Contamination of 14 C
have
hits.
The
detector
is
described
in
detail
in
Ref.
[36].
low 1 keVee. The size of the error bars comes primarily from the
and Smelting Company. This work was supported by the Japanese
in the GORE-TEX® sheets between the PMTs and the support strucThe data set
in thescintillation
solar axion
search
experiments ture
cov-may explain a fraction of the events. Light leaks through this
systematic uncertainty
in used
the xenon
decay
constant,
Ministry of Education, Culture, Sports, Science and Technology,
21–27,based
2012.onA the
sequence
of standard
data reduction
February
25 ± 1 ns,ers
which
is estimated
difference
between the
Grant-in-Aid for Scientific Research, and partially by the National
material are also suspect. Nonetheless, the possible existence of a
XMASS model
[16] and
the NESTevents
model caused
[17] based
[18]. A sysis applied
to remove
by on
afterpulses
and electronic
Research Foundation of Korea Grant funded by the Korean GovernWIMP signal hidden under these and other backgrounds cannot
tematic error
on The
the selection
determined
based of
on cuts: (1)bethe
ment (NRF-2011-220-C00006).
excluded. Although no discrimination has been made between
ringing.
standard efficiency
reductionisconsists
of a series
the errorevent
resulting
from a linear
theliquid-xenon
points in thedetector;
figure. At(2) the time
is triggered
only fit
bytothe
the 0.3 keVee
analysis
this error
difference
to threshold
the previous
eventisis6.1%.
more than 10 ms; (3) the root
WIMP mass[GeV]
Solar axion search
Physics Letters B 724 (2013) 46-50
• 太陽中で制動放射やコンプトン散乱に
よって放出される可能性がある。
• 光電効果のような反応
• 実験的探索としては世界最高感度を達
成
counts/kg/day/keVee
energy[keVee]
mean square of the hit timing is less than 100 ns and is used to
4. Results
and discussion
reject
events caused by afterpulses of PMTs due to bright events;
and (4) the number of PMT hits in the first 20 ns divided by the
Fig. 8. Spin-independent elastic WIMP-nucleon cross section as a function of WIMP
uncertainties except that from Leff are taken into account in
limit line. The effect of the Leff uncertainty on the limit is
Fig. 7 total
shows
simulated
WIMPs
energy
on which
the the numnumber
of hits
is less
thanspectra
0.6 foroverlaid
events in
mass. All systematic
observed ber
spectrum
after the data
reduction
was applied.
WIMPs
of photoelectrons
is less
than 200.
The fourth
cut was the
applied
XMASS 90% C.L.
40
energy[keV]
mass[keV]
Fig. 3. Comparison between the observed data (points with error bars) and Fig.
expected
4. Limits on gaee . The thick solid line shows the limit obtained in this
10
main contribution to the remaining background in this energy region stems from
214 Pb. From our simulation we estimate this background alone to contribute 2
other background contributions are smaller but less certain, we do not subtract
calculating our limits. Our detector’s low background allows us to directly use
the signal region to extract our limit on the inelastic scattering cross section of
solid line:
pre-selection
nuclei.
Using Eq. 6 and taking into account the nuclear form factor and our si
solid line
; XMASS(90%
dashedderive
line: aR 90%
cut C.L. upper limit for this cross section,
which
in Fig. 4 is C.L.)
compared
dashed
line
:
DAMA/LXe
doted Refs.
line: timing
cut
[12,13]. The gray band reflects our systematic uncertainties. The system
1
30
40
50
60
70
80 90 100
energy [keV]
Fig. 2. Energy spectra of the simulated events after each reduction step. As an example we chose a WIMP mass
of 50 GeV and
the form factor in
Ref. [29]. From
top to bottom,
simulated energy
spectrum after pre-selection
, Prog.
Theor.
Exp.
Phys.
2014,
063C01
(solid line), cut (2) (dashed line), cut (3) (dotted line), and cut (4) (solid line). As we do not apply the proper
radial correction for energy, a shift in our energy scale seems to occur after our fiducial volume cut (2). As
• WIMP-原子核の非弾性散乱
we are only using events in a very limited fiducial volume and our energy scale is based on calibration at the
PTEP 2014, 063C01
counts/keV
events/keV
counts/keV
events/keV
105
104
Data
103
102
104
103
102
10
MC
10
10
20
30
40
50
60
70
10
80 90 100
energy [keV]
20
30
40
50
60
70
102
10
80 90 100
energy [keV]
1
energy[keV]
energy[keV]
102
Downloaded from http://ptep.oxfordjournals.org/ at Kokusai Hoken Keikakugaku (UNIV OF TOKYO) on June 15, 2014
1
1
H. Uchida et al.
solid line: Band cut
center of the detector, the energy scale of the surviving events is correct within 4%.
cross section [pb]
20
Asymptotic cross section (σas
I ) [pb]
10
Downloaded from http://ptep.oxfordjournals.org/ at Kokusai Hoken Keikakugaku (UNIV OF TOKYO) on June 15, 2014
WIMP-nucleus Inelastic Scattering search
103
WIMP mass [GeV]
Fig.
2. for
Energy
spectra
of days
the simulated
events
Fig. 3. Energy spectra of the observed events after each reduction
step
our 165.9
live
of data. From
topafter each reduction step. As an example we chose a WIMP mass
of
50
GeV
and
the
form
factor
in
Ref.
[29].
From
to bottom, the observed energy spectrum after pre-selection (solid line), cut (2) (dashed line), cut (3) (dotted top to bottom, simulated energy spectrum after pre-selection
4. The
black
solid
is ourthe
90%
C.L. upper limit on the asymptotic cross section
(solid
line), cut (2) (dashed line), cut (3) (dotted line), and cutFig.
(4) (solid
line).
As we
do line
not apply
proper
line), and cut (4) (solid line). The fiducial volume contains 41 kg
of LXe.
129
Cut1:
pre-selection
using
the volume
same form
factors
on afterXeour
radial correction for energy, a shift in our energy scale seemstering
to occur
fiducial
cut (2).
As as DAMA. The gray band covers its variation
we are only using events in a very limited fiducial volume andCut2:
our energy
scale
is
based
on
calibration
at
the
uncertainty.
The
dotted
line
is
the
limit
obtained by the DAMA group [12,13]. It was
R cut
The cut values for the three cuts that are applied after
our ofalmost
standard
pre-selection
center
the detector,
the energy
scale of were
the surviving eventscally
is correct
within
4%.
subtracting background. Our low background allows us to derive this limit witho
Bosonic super-WIMP search
Counts/keVee/kg/day
MC
pseudo
scalar
7/11
Signal (MC simulation)
10-1
cut1-3
10-2
cut1-4
gaee
cut1-2
-11
XENON100 EDW-II
-11.5
-3
10
-4
10
150
50
100
-12
150
10-1
-12.5
(a)
XMASS
Ge (solar)
(b)
-19
10
-20
10 20 30 40 50 60 70 80 90 100
-21
10-4
10-4
energy [keV]
HB stars
-22
50
100
150
50
100
150
2
Fig. 3. Energy spectra of the observed events after each reduction step for our 165.9 live days of data.-23
From top
Ωh =0.1
10-1
10-1 (solid line), cut (2) (dashed line), cut (3) (dotted
mb =after
120keV
to bottom, the observed energy
spectrum
pre-selection
-24
line), and cut (4) (solid line).
41 kg of LXe.
10-2 The fiducial volume contains
10-2
-25
Diffuse γ
-3
-3
-26
XMASS
10
10
The cut values for the
three
cuts
that
are
applied
after
our
almost
standard
pre-selection
were
-27
10-4
10-4
20
40
60
80 100 120 140
optimized for a WIMP mass of 50 GeV. Except for the radius cut, our cut values were determined
vector boson/pseudoscalar mass (keV)
50
100
150
50
100
150
-2
10
-3
Counts/keV ee/kg/day
6/11
events/keV
Counts/keVee/kg/day
counts/keVee/kg/day
Physics Review Letters 113,121301 (2014)
• warm dark matterの候補
• pseudo-scalar,vector bosonを探索
• super-WIMPは光電効果と似た反応
• 実験的に初めて制限を与えた
Cut3: timing cut
subtraction.
Cut4: Band cut
log(α’/α)
optimized for a WIMP mass of 50 GeV. Except for the radius cut, our cut values were determined
by optimizing the ratio of simulated signal events surviving the cuts in a tentative signal range from
105
30 to 80 keV over the sum of background events found in the data in two side bands ranging
fromevents
10
Observed
Data
10-1
mb = 40keV
to 30 and from 80 to 100 keV. For the radius cut, this procedure results in an extremely4 low fiducial
10
-2
10
volume, leading us to halt this optimization at 15 cm. For the remaining cuts, the values resulting
10-3 band
from our optimization were 12.91 ns for the timing cut and a ratio of 0.248 for the
103 cut. Events
with parameter values smaller than these cut values enter into the final sample. 10-4
102 50
100
Figures 2 and 3 show the impact of our cuts on the expected signal from our 50 GeV
WIMP simulation and the observed data spectrum, respectively. The signal window is defined10-1so that
it contains
mb = 80keV
10
-2
90% of the simulated 50 GeV WIMP signal with equal 5% tails to either side,10which
results in a
36–48 keV window. While the underlying simulation shown in Fig. 2 is based on
the1form factors
10-3
WIMP mass[GeV]
energy
(keVeesurviving
)
(keVee)
by optimizing the ratio of simulated signal
events
the cutsenergy
in a tentative
signal range from
energy[keV]
30 to 80 keV over the sum of background events found in the data in two side bands ranging from 10
vector boson
mass [GeV]
My work :
液体キセノン中ラドンバックグラウンドの研究
•
Motivation
214Bi-214Po coincidence 解析
214Bi-214Po coincidence 解析 : 結果
•
まとめ
•
•
Motivation
• ラドン
• XMASS実験のバックグラウンドの1つ
•
ウラン系列の希ガス 有効体積内に存在(検出器部材から放出
されている)
•
娘核の214Pb(β崩壊)の低エネルギー領域が暗黒物質探索の
BGとなる。
➡自己遮
することが出来ない
•
目標値：全体のバックグラウンドの寄与の1/10になるように
<1.2uBq/kgと定めた(小川 2011/9/16 物理学会発表)
•
ラドンは検出器に一様に分布していると考えられるのでcontrol
sampleとして事象再構成のパフォーマンスの評価に用いることが
できる。
•
今回はXMASSの改修後のデータ114.42時間をもちいて液体キセ
ノン中のラドン222の量を見積もる
12
214Bi-214Po coincidence 解析
•
222Rnを娘核である214Biから見積
もる。
‣ 214Bi β崩壊
(3.272MeV,19.9min)
‣ 214Po α崩壊
(7.687MeV,164us)
•
214Poの短い半減期を利用しBi-Poの
ペアを選別する。
13
ウラン系列(222ラドン以降)
214Bi-214Po coincidence 解析(2)
FADCの時間窓10us
sampling rate 1GHz
Bi 候補
(2)100us<dT<1000us
(β+γ崩壊)
③
①
②
dT_pre>100us
Po候補
(α崩壊)
④
cut条件 :
• preselection
preselectionで除いたイベントはペアを組まない
検出器内の事象
PMTのヒット数>=4
チェレンコフ事象カット(PMTの窓で起こった早い事象のカット）
ヒットタイミングによるノイズカット
• BiPoペアを選ぶためのカット
Bi候補と前の事象の時間差のカット(アフターパルスの除去) dT_pre>100us
2事象目の光量 > 25000p.e.
FADCの波形解析によるカット(αイベントを選ぶ)
Bi候補とPo候補の時間差 100us<dT<1000us
14
214Bi-214Po coincidence 解析 : 結果
Entries
criteria
Bi-Po
pair
Bi_after_pe_slope
Bi_after_pecut
Bi_all_cut
Bi
Entries
2803
4246
2059
2759
Mean 2.173e+04
2.159e+04
1.748e+04
2.201e+04
RMS 1.141e+04
9976
9742
9805
2
10
preselection + 2nd
totalNPE>20+
cut1
+dT_pre>100us
+dT<1000us
10
4246
x10^3
1
3
cut3
cut1+2nd event
PE>25000
cut2+2nd decay
slope<35ns
20
40
60
80
100
#of PEs
2803
2759
Po_after_pe_slope
Po_after_pecut
Po
Entries
2803
4246
2759
Mean 8.364e+04
8.287e+04
5.748e+04
RMS 1.654e+04
1.764e+04
3.836e+04
Entries
cut2
0
2
10
cut4
cut3+dT>100us
×10
120
# of PE
2059
10
live time:114.42 hours
x10^3
3
0
20
40
60
80
100
×10
120
# of PE
#of PEs
各カットのスペクトルと残ったイベント数
最終的に2059イベントが残った。
dT_after_pe_cut
Entries
2059
Mean
317.1
時間差分布 RMS
250
191
250
214Bi
200
250
150
200
50
100
0
500
20
250
β+γ崩壊
200
100
150
p1
3
60
80
p0
Entries
Mean
100
200
×10
120
p2
2059
23.17317.1
/ 24
RMS
191
4.339e-10
± 1.066e+00
χ2 / ndf
x10^3
40
χ 2 / ndf
Entries
Entries
Entries
334.6
光量分布
260.2
69.23 / 69
15.6 ± 1.9
66.1 ± 10.0
174 ± 13.9
Entries
214Bi-214Po
coincidence
解析
:
結果
4246
23.17 / 24
453.8 ± 18.9
p0
3.533e-10 ± 1.066e+00
p1
453.8 ± 18.9
p2
162.4
162.4 ±±4.34.3
# of PE
×10 PEs
#of
120
Entries
Entries
3
0
0
160
20
40
80
120
60
100
80
100
150
# of PE
150
140
120
160
100
140
60
214Po
40
80
20
60
α崩壊
100
50
0
40
0
20
20
40
0
0
20
40
100
3
60
80
100
60
80
100
×10
120
# of PE
x10^3
3
×10
120
# of PE
#of PEs
0
0
100 200 300 400 500 600 700 800 900 1000
dT[us]
fitting function : p0+p1*(1/2)^(x/p2)
低光量の領域は銅表面で起こった事象
50
(エネルギー再構成を行っていない)
214Poの半減期：164us
フィットの結果Poの半減期とエラーの範囲でコンシステント
0
00 1000
0 100 200 300 400 500 600 700 800 900 1000
dT[us] →正しくBiPoペアを選び出している。
dT[us]
16
アクシデンタルイベントの見積もり(1)
Biのためのカット条件
• preselection + dT_pre>100us + dT>1000us
Bi_all_cut
accidental
of Bi
10
アクシデンタルのスペクトルは
Entries
2059
Entries 783327
黄色のcut criteriaでcutした後のスペクトルの面積を
Mean
Mean 2.201e+04
9460
9742
RMS
RMS
9332
アクシデンタルのイベント数になるようにスケールした
3
Entries
Entries
10
10^3
102
10^2
4
103
10 10
1
red: Bi candidate(2059 events)
blue: accidental for Bi (16.6 events)
livetime : 411264sec
1
102
10^-1
10-1
10^-2
10-2
x10^3
0
20
40
60
80
100
× 10
120
# of PE
10
3
0
hPE_pe_slope
Po_all_cut
103
低エネルギー側では少し純度が落ちる
Entries
2059
9662
Mean 6.183e+04
8.364e+04
1.572e+04
1.652e+04
RMS
102
Entries
Entries
# of PE
104
17
103
20
1
アクシデンタルイベントの見積もり(2)
-1
10
Poアクシデンタルのためのカット条件
• preselection
-2 + totalPE>25000 + decay slope<35ns +
10 0
20
40
60
dT>1000us
3
80
100
hPE_pe_slope
Po_all_cut
3
10
10^3
Entries
×10
120
# of PE
Entries
9659
2054
Mean 8.363e+04
6.182e+04
RMS 1.652e+04
1.571e+04
2
10
10^2
10
10
11
-1
10^-1
10
x10^3
3
-2
10 0
20
40
60
80
100
×10
120
# of PE
# of PE
red: Po candidate : 2059 events
blue: accidental for Po : 16.6 events
livetime : 411264 sec
18
アクシデンタルイベントの見積もり
アクシデンタルレート[Hz]
= １事象目のレート(p17 blue spectrum)[Hz]
x ２事象目のレート(p18 blue spectrum)[Hz] x 時間窓(900us)
= 1.90[count/sec] x 0.023[count/sec] x 900[us]
= 4.03e-5[count/sec]
アクシデンタルイベント
= アクシデンタルレート[Hz] x live time[s]
= 4.03e-5[count/sec] x 411264[sec]= 16.6[events]
アクシデンタルイベント/残ったイベント
集めたサンプルに含まれる
=16.6/2059＝0.8%
アクシデンタルの量
19
222Rn濃度 見積もり
• live
•#
time : 411264 sec
of BiPo pair : 2059
• accidental
• cut
events : 16.6
efficiency(100<dT<1000us) : 0.65
• 今の時点ではdTのみ(PE,α事象カットの見積もりはまだ)
• 液体キセノン：832kg
• activity
:(2059-16.6)/(411264x0.65x832)
= 9.17 0.20uBq/kg(stat only)
c.f.
XENON100 20uBq/kg, LUX 10uBq/kg, EXO 4uBq/kg, SK ~a few uBq/
kg(純水) , NEWAGE 25mBq/kg(CF4,14g)
参考:
XENON100 collaboration, E. Aprile et al., Study of the electromagnetic background in the XENON100 experiment, Phys. Rev. D 83 (2011)
082001
EXO-200 collaboration, J.B. Albert et al., An improved measurement of the 2νββ half-life of 136Xe with EXO-200, arXiv:1306.6106
LUX collaboration, D.S. Akerib et al., First results from the LUX dark matter experiment at the Sanford Underground Research Facility, arXiv:
1310.8214
http://www-sk.icrr.u-tokyo.ac.jp/sk/pub/master/Nakano master thesis.pdf
NEWAGE 中村D論
20
まとめ
• XMASSの改修後のデータ(114.42時間)を用いてラドン濃度を見
積もった。
• 純度よくBiPoペアを選び出した。
• 液体キセノン中に含まれるラドン222は9.17
0.20uBq/kgだと
わかった。
今後…
•
選び出した事象について事象再構成を行い再構成のパフォー
マンスを評価する。
•
時間的にラドン量が安定しているかの確認
21