RENO ÷ m Ç] M ö; c" e Cosmic “ ¥Æ X Ø; c 8 ýA 0 t V R Ëc Ü R ú n ÞV R Ë ƒ ºT ƒ † Ž ì ŏ Œ

RENO ÷ m Ç] M ö; c" e Cosmic “ ¥Æ X Ø; c 8 ýA 0 t  V R Ëc Ü R ú n ÞV R Ë  ƒ ºT ƒ † Ž ì ŏ Œ

#

l - > ) ç  · ® £# Ü  ¢ 9

Ä

ºÅ Ò ™ èw n   ƒ  ½ ¨™ è, „  z Œ ™@ /† < Ɠ § Ó ü t o † < Æõ , F  g Å Ò 500-757

(2012¸   7 Z 4 10{ 9  ~ à Î6 £ §, 2012¸   7 Z 4 30{ 9  à º& ñ ‘ : r ~ à Î6 £ §, 2012¸   8 Z 4 30{ 9  > F  S X ‰& ñ )

RENO (Reactor Experiment for Neutrino Oscillation) z  ´+ « >“ É r % ò F  g " é ¶  § 4  µ 1 τ  ™ è_  " é ¶  – Ð\ " f

š ¸  H ×  æ$ í p  \  ¦  Ž Ø  ¦ # Œ ”  1 l x   ¨ 8 Š  © œÃ º (θ

)\  ¦ 8 £ ¤& ñ   H z  ´+ « >s  . 2011¸   8 Z 4 Ò'  X <s ' \  ¦ ~ à Î

“

¦ e ”   H X < Cosmic Á »“ : r \  _ ô  Ç X <s '   H ß ¼>  3t – Ð ì  r À Ó½ + É Ã º e ”  . Á »“ : r \  _ K  Ò q t$ í  ) a ×  æ$ í  

Ã

º™ è\  Ÿ í S \ ‰ ÷ &# Q €  • 2.2 MeV_  \  -t \  ¦ ~ ½ ÓØ  ¦   H  â Ä º, ×  æ$ í   [  t o ³ o u \  Ÿ í S \ ‰ ÷ &# Q €  • 8 MeV _

 \  -t \  ¦ ~ ½ ÓØ  ¦   H  â Ä º, Õ ªo “ ¦ stopping Á »“ : r \  _ K  Michel electron`  ¦ µ 1 ÏÒ q t   H  â Ä º e ”  .

Gaussian fitting`  ¦ : Ÿ x K  y Œ •  â Ä º\  \  -t _  ¨ î ç  H ° ú כ (mean value)`  ¦ ¶ ú ˜( R4 Ÿ § Ü ¼– Ð+ ‹  Ž Ø  ¦ l _  î ß –& ñ $ í

` 

¦ — ¸m ' a A % i  .

Ù þ

˜d ” # Q: Ä ºÅ ҂  , à º™ èŸ í S \ ‰, Gd Ÿ í S \ ‰, Michel „    Û ¼& 7 ˜à Ô! 3 

Study of Neutron Capture Events Produced by Cosmic Muons in the RENO

In Sung Yeo · Kyung Kwang Joo ^∗

Institute for the Universe & Elementary Particles,

Department of Physics, Chonnam National University, Gwangju 500-757 (Received 10 July 2012 : revised 30 July 2012 : accepted 30 August 2012)

The RENO (Reactor Experiment for Neutrino Oscillation) aims to measure the neutrino mixing angle θ

by using antineutrinos emitted from the Yonggwang nuclear power plant. RENO has taken data since August 2011. Data containing muons are classified into 3 categories because when cosmic muons cross the detector, neutrons are created. Firstly, neutrons are captured by hydrogen and release an energy of ∼2.2 MeV. Secondly, neutrons are captured by gadolinium and release an energy of ∼8 MeV. Lastly, muons are stopped inside the detector and generate Michel electrons.

By checking the mean value of the energy with a gaussian fitting, we monitor the stabilities of the detectors on a daily basis.

PACS numbers: 25.40.L, 96.40.T

Keywords: Cosmic muon, Hydrogen capture event, Gd capture event, Michel electron spectrum

∗

E-mail: [email protected]

-974-

I. " e  ] Ø

RENOz  ´+ « >“ É r  Ž Ø  ¦ l ü <  Ž Ø  ¦ l î ß –\  [ þ t # Q  H Ó  o^ ‰$ 3  F 

g Ž Ø  ¦6   xÓ  o (LS, Liquid Scintillator)_  — ¸Ž  H ] j› ¸ = å Q


“

¦ 2011¸   8 Z 4 Ò'  X <s ' \  ¦ ~ à Γ ¦ e ”  . RENO  Ž Ø  ¦ l 



 H 1 l xd ” " é ¶: Ÿ x+ þ A — ¸€ ª œÜ ¼– Ð ½ ¨$ í ÷ &# Q e ”   H X <, î ß –A á ¤ Ü ¼– ÐÂ Ò '

   ¿ (target), γ-catcher, ! Q( (buffer), q ž Ð (veto)– Ð s

À Ò# Q4 R e ”  .   ¿ “ É r  ß ¼w n =– Ð ë ß –[ þ t # Qt   H X < [  t o 

³ o

u s  6   x K   ) a Ó  o^ ‰$ 3 F  g Ž Ø  ¦6   xÓ  os  G 0 >t “ ¦, γ-catcher• ¸



ß ¼w n =– Ð ë ß –[ þ t # Qt “ ¦ Ó  o^ ‰$ 3 F  g Ž Ø  ¦6   xÓ  os  G 0 >”   . ! Q (

  H Û ¼_ …“  o Û ¼ Û ¼ 9 – Ð ë ß –[ þ t # Qt “ ¦ p W 1³ 1 Ï š ¸{ 9  (min- eral oil) s  [ þ t # Q  H X < F  g7 £ x; Ÿ ¤› ' a (PMT, Photo Multiplier Tube) s  # Œl \  [ O u   ) a  . q ž Ѝ  H & ñ ] j  ) a í  H à ºô  Ç 3  7 £ x À

Óà º– Ð G î  r   [1]. % i Z …  Ô  æ õ  (IBD, Inverse Beta De- cay, ν e + p → e ⁺ + n) õ & ñ `  ¦ : Ÿ x K   Ž Ø  ¦ ÷ &  H ×  æ$ í p  

\

 ¦ & ñ S X ‰ y  ¹ 1 Ô ? /l  0 AK " f  H X <s '  î ß –\  e ”   H back- ground\  ¦ · ú ˜  ô  Ç . Background Ò  re  ¦“ É r 3 t – Ð
è  H



. Z  }“ É r \  -t \  ¦ ”   cosmic Á »“ : r s  € Œ ™$ 3 õ  Ø  æ[  t K  ×  æ

$ í

 \  ¦ Ò q t$ í r †   . s X O >  ë ß –[ þ t # Q”   ×  æ$ í    Ž Ø  ¦ l  î

ß –Ü ¼– Ð [ þ t # Qš ¸  H X <  Ž Ø  ¦ l î ß –\  [ þ t # Q“ : r ×  æ$ í   à º™ è\ 

Ÿ

í S \ ‰ ÷ &# Q €  • ∼2.2 MeV_  \  -t \  ¦ ~ ½ ÓØ  ¦ ½ + Éà º• ¸ e ” “ ¦  [

 t o ³ o u \  Ÿ í S \ ‰ ÷ &€   €  • ∼8 MeV_  \  -t \  ¦ ~ ½ ÓØ  ¦ >   ) a



. s M : [  t o ³ o u s   Ž Ø  ¦ l  ? / Ò\  t    H q ×  æ s  ± ú 



 à º™ è\  Ÿ í S \ ‰ ÷ &  H & ñ • ¸\  q K  s  $ ™à Ôà º  H & h  . ¢ ¸ô  Ç stopping Á »“ : r \  _ ô  Ç Michel electron spectrums  µ 1 ÏÒ q t ô

 Ç . s ü <° ú  s  s  : r& h Ü ¼– Ð · ú ˜“ ¦ e ”   H \  -t ü < Ÿ í S \ ‰ r  ç

ß – (capture time)`  ¦ ½ ¨K  q “ § ì  r$ 3  “ ¦ { 9 Z >  ì  r Ÿ í\  ¦ : Ÿ x K

 z  ´] j  Ž Ø  ¦ l \  [ O u   ) a F  g7 £ x; Ÿ ¤› ' a s  î ß –& ñ & h Ü ¼– Ð  Œ •1 l x

“ ¦ e ”   H t \  ¦ — ¸m ' a A   H  כ s  ×  æ כ ¹  .

II. Ä ] Ø Â ] Ø

1. • ¤} º; c ƒ ºT ƒ †c Ü R ú n ÞV R Ë 

Á

»“ : r X <s ' \ " f ×  æ$ í   à º™ è\  Ÿ í S \ ‰ ÷ &# Q ∼2.2 MeV _  \  -t \  ¦ ~ ½ ÓØ  ¦   H X <s ' \  ¦ — ¸Ü ¼ 9€   # Œ Q  t

 s  $ ™à Ô ‚  Z >  › ¸| [ þ t`  ¦ & h 6   x K   ô  Ç . ×  æ$ í   µ 1 ÏÒ q t

 )

a Ê ê à º™ è\  Ÿ í S \ ‰ ÷ &  H ¨ î ç  H r ç ß –“ É r ∼180 µs – Ð · ú ˜ 94 R e ” 



. ‘ : r ƒ  ½ ¨\ " f  H €  • 800 MeVs  © œ_  Á »“ : r \  _ K  µ 1 ÏÒ q t

 )

a ×  æ$ í    Ž Ø  ¦ l ? /_  à º™ è\  Ÿ í S \ ‰ ÷ &# Q \  -t \  ¦ ~ ½ Ó Ø

 ¦ l  t   H r ç ß –\    É r \  -t  ì  r Ÿ í (Fig. 1)\  ¦  

„

½ ÓÜ ¼– Ð ×  æ$ í   s  $ ™à Ô_  µ 1 ÏÒ q tr ç ß –\  @ /ô  Ç ‚  Z >  › ¸| `  ¦

 

& ñ % i  . ¢ ¸ô  Ç, : £ ¤& ñ PMT @ / Òì  r _  ’    ñ\  ¦ ° ú   H accidental background • ¸ ] j K  Å Ò% 3  .

Fig. 1. (Color online) Comparison of photoelectron dis- tribution with different capture time.

Fig. 2. Number of photoelectrons with gaussian fitting.

Mean value of photoelectrons corresponds to ∼2.2 MeV.

Fig. 3. Neutron capture time on hydrogen.

þ

j7 á x& h Ü ¼– Ð Á »“ : r \  -t   H 800 MeV s  © œ, ×  æ$ í   s 

 $

™à Ô µ 1 ÏÒ q tr ç ß – (T )“ É r Á »“ : r s  $ ™à Ô µ 1 ÏÒ q tr ç ß –“   160 µs

< T < 350 µs, Õ ªo “ ¦ Q max ( þ j@ / „   \  ¦ ° ú   H PMT _ 

„

  ) / Q tot („  ^ ‰ „   ) < 0.03 › ¸| `  ¦ Å Ò>   ) a  .   H  

Fig. 4. (Color online) Time variation of photoelectrons for neutron events captured by hydrogen.

Fig. 5. (Color online) Time variation of capture time for neutron events captured by hydrogen.

o

  Ž Ø  ¦ l _   â Ä º 1{ 9  é ß –0 A– Ð \  -t _  ¨ î ç  H ° ú כ`  ¦ ½ ¨ 

%

i “ ¦ " é ¶  o   Ž Ø  ¦ l _   â Ä º  H 2{ 9  é ß –0 A– Ð \  -t  ¨ î ç  H

° ú

כ`  ¦ ½ ¨ % i  . 0 A_  › ¸| `  ¦ Å Ò>  ÷ &€     H  o   Ž Ø  ¦ l _ 

 â

Ä º 500ë ß – s  $ ™à Ô\ " f 5000 s  $ ™à Ԗ Ð ×  ¦ >  ÷ &“ ¦ " é ¶  o 

 Ž

Ø  ¦ l _   â Ä º 500ë ß – s  $ ™à Ô\ " f 2000 s  $ ™à Ԗ Ð ×  ¦ >   ) a



.   H  o   Ž Ø  ¦ l _  X <s ' \ " f ½ ¨ô  Ç ×  æ$ í   à º™ è\ 

Ÿ

í S \ ‰ ÷ &# Q 2.2 MeV_  \  -t \  ¦ gaussian fitting ô  Ç Õ ªa Ë >s  Fig. 2 \  e ”  . ×  æ$ í   Ÿ í S \ ‰ r ç ß –_   â Ä º Fig. 1õ  ° ú  “ É r

\

 -t  ì  r Ÿ í\ " f peaks  e ”   H ½ ¨ç ß –`  ¦ ‚  × þ ˜ “ ¦ Á »“ : r \ 

-t   H 900 MeV s  © œ, ×  æ$ í   s  $ ™à Ô µ 1 ÏÒ q tr ç ß –“ É r 0 < T

< 700 µs Å Ò% 3 “ ¦ fitting function“ É r a × exp ⁻

+b (a, b  H



© œÃ º)\  ¦  6   x % i  (Fig. 3). s \  ¦ 8 > h Z 4ç ß –   H  o   Ž Ø  ¦ l

  H 1{ 9 , " é ¶  o   Ž Ø  ¦ l   H 2{ 9  X <s '  ì  r | ¾ ÓÜ ¼– Ð \  -t  _

 ¨ î ç  H ° ú כõ  ×  æ$ í   à º™ è\  Ÿ í S \ ‰ ÷ &  H ¨ î ç  H \  -t _  { 9

Z >  ì  r Ÿ í\  ¦ Õ ª§ 4   (Fig. 4, Fig. 5).

2. Gd ; c ƒ ºT ƒ †c Ü R ú n ÞV R Ë 

Fig. 6. Number of photoelectrons with gaussian fitting.

Mean value of photolelectron corresponds to ∼8 MeV.

Fig. 7. Neutron capture time on Gd.

[  t o ³ o u s  à º™ è\  q K   Ž Ø  ¦ l \ " f t    H q Ö  ¦ s

 10%p ë ß –Ü ¼– Ð [  t o ³ o u \  Ÿ í S \ ‰ ÷ &  H ×  æ$ í   s  $ ™à Ô



© œ@ /& h Ü ¼– Ð & h # Q   H  o   Ž Ø  ¦ l   H 3{ 9 , " é ¶  o   H 6{ 9 _  X

<s ' \  ¦ — ¸  \  -t  ì  r Ÿ í\  @ /K  gaussian fitting % i 



 (Fig. 6). ×  æ$ í   [  t o ³ o u \  Ÿ í S \ ‰ ÷ &# Q ∼8 MeV _

 \  -t \  ¦ ~ ½ ÓØ  ¦   H s  $ ™à Ô_  ‚  Z > › ¸| “ É r  6 £ § õ  ° ú  



. Á »“ : r \  -t   H à º™ è_   â Ä ºü < 1 l x{ 9  >  800 MeV s 



© œ, ×  æ$ í   s  $ ™à Ô µ 1 ÏÒ q tr ç ß –“ É r 130 µs < T < 350 µs s 

“

¦ Q max /Q _tot < 0.03 › ¸| `  ¦ Å Ò% 3   (Fig. 6). ×  æ$ í  

Ÿ

í S \ ‰ r ç ß –_   â Ä º Á »“ : r \  -t   H 900 MeV s  © œ, ×  æ$ í   µ

1 ÏÒ q tr ç ß –“ É r 0 < T < 150 µs Å Ò% 3 “ ¦ fitting function“ É r a × exp ⁻

+b (a, b  H  © œÃ º)\  ¦  6   x % i  . s  : r& h Ü ¼– Ð

· ú

˜“ ¦ e ”   H ×  æ$ í   [  t o ³ o u \  Ÿ í S \ ‰| ¨ c M : Ô  æ õ  r ç ß –“ É r

∼30 µs“  X < z  ´] j– Ð  Ž Ø  ¦ l \  PMT [ O u   ) a  Òì  r s  ! Q (

\  e ” l  M :ë  H \  # QÖ ¼ & ñ • ¸ delay ÷ &  H  כ Ü ¼– Ð ˜ Г    (Fig. 7). ×  æ$ í   [  t o ³ o u \  Ÿ í S \ ‰ ÷ &  H ¨ î ç  H \  -t \  ¦

½

¨K  8> h Z 4ç ß – { 9 Z > ì  r Ÿ í\  ¦ Õ ª 9˜ Ѐ Œ ¤  (Fig. 8).

Fig. 8. (Color online) Time variation of photoelectrons for neutron events captured by Gd.

Fig. 9. (Color online) Mean of photoelectrons of Michel electron.

3. Michel Electron

Cosmic Á »“ : r s  # Q‹ "  Ó ü t| 9 `  ¦ : Ÿ x õ  €  " f î  r1 l x \  -t 

\

 ¦ & h & h  { 9 # Q! Qo €   # QÖ ¼ í  H ç ß – " 3 ð  r  . Õ ª Q€   Á »“ : r s  µ ⁻ → e ⁻ + ν e + ν µ ü < ° ú  s  Ô  æ õ \  ¦ >  ÷ &  H X < s M :

š ¸  H „   \  ¦ Michel electron s  “ ¦ ô  Ç . Á »“ : r _  | 9 | ¾ Ó s

 105.7 MeV“  X < Ô  æ õ ô  Ç Ê ê { 9  [ þ t _  | 9 | ¾ Ós  ∼1 MeV

\

 ¦  Å t  3 l w ô  Ç . { 9   [ j> h Ò q t$ í   H X < € 9 כ ¹ô  Ç \  - t

  H  _  \ O “ ¦ @ / Òì  r î  r1 l x \  -t – Ð >   ) a  . s  [ j

>

h_  { 9   î  r1 l x | ¾ Ó ˜ Д > r \  _ K " f " f– Ð   É r ~ ½ ӆ ¾ ÓÜ ¼– Ð

”

 ' Ÿ † < ÊÜ ¼– Ð y Œ •• ¸\    " f 4 R° ú ˜ î  r1 l x \  -t  " f– Ð



Ø Ô .   " f ô  Ç { 9  _  î  r1 l x \  -t \  ¦ Õ ª 9˜ Ð>  ÷ &€   broad > 
š ¸>   ) a  . Michel electron _   â Ä º @ /| Ä Ì 10

∼ 50 MeV & ñ • ¸\  -t \  ¦ t “ ¦
š ¸>  ÷ &“ ¦ peak ° ú כ“ É r

∼ 20 MeV& ñ • ¸
š ¸>  ÷ &“ ¦ Ô  æ õ  r ç ß –“ É r €  • ∼2.2 µs s 



. (Fig. 9, Fig. 10)

Fig. 10. Decay time of Michel electron.

Fig. 11. Comparison of photoelectron variation between H capture event (2.2 MeV) and Gd capture event (8MeV) in near detector (ND).

Fig. 12. Comparison of photoelectron variation between H capture event (2.2 MeV) and Gd capture event (8MeV) in far detector (FD).

III. + s Ç Â ] Ø

Cosmic Á »“ : r \  _ K  µ 1 ÏÒ q t÷ &  H ×  æ$ í   à º™ èü < [  t o

³ o u \  Ÿ í S \ ‰ ÷ &# Q ~ ½ ÓØ  ¦ ô  Ç ∼2.2 MeVü < ∼8 MeV_  \  - t

 ° ú כõ  Ÿ í S \ ‰ r ç ß –`  ¦ X <s ' \ " f ½ ¨ % i  . s \  ¦ normal-

ize # Œ   H  o   Ž Ø  ¦ l ü < " é ¶  o   Ž Ø  ¦ l  y Œ •y Œ •\ " f q “ § K

 ˜ Ѐ Œ ¤  (Fig. 11, Fig. 12). H captureü < Gd capture\  ¦ q

“ §Ù þ ¡`  ¦ M : normalizeô  Ç " é ¶,  H  o _  r ç ß – ì  r Ÿ í q 5 p w ô

 Ç  ⠆ ¾ Ó`  ¦ ˜ Ðs   H  כ `  ¦ S X ‰ “  ½ + Éà º e ” % 3  . 8> h Z 4ç ß – r ç ß – _

   É r H capture ü < Gd capture_  ¨ î ç  H \  -t \  ¦ 4 Ÿ § Ü ¼

–

Ð+ ‹ { 9 & ñ >  \  -t \  ¦ ~ ½ ÓØ  ¦   H  כ õ  " é ¶,  H  o  q  5

p

w ô  Ç  ⠆ ¾ Ó`  ¦ ˜ Ðe ” Ü ¼– Ð RENO  Ž Ø  ¦ l \  [ O u   ) a PMT _ 

$ í

0 p x s  î ß –& ñ & h “    כ `  ¦ S X ‰ “  ½ + É Ã º e ”  .

P

c p 8 ý ò k >

s

  7 Hë  H“ É r 2012¸  • ¸ & ñ  Ò(“ §¹ ¢ ¤ õ † < Æl Õ ü t  Ò)_  F " é ¶ Ü ¼

–

Ð ô  Dz D Gƒ  ½ ¨F é ß –_  l œ íƒ  ½ ¨ \ O  t " é ¶`  ¦ ~ à Î  à º' Ÿ  ) a

 כ

e ”  (2012-0002863, 20120001176).

Y

c p w Š à U Ø ”  ô

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[2] J. S. Park et al., Sae Mulli 58, 62 (2009).

[3] I. S. Yeo et al., New Physics: Sae Mulli 61, 739 (2011).

[4] W. Pauli, Cambridge Monogr. part.phys. Nucl. Cos- mol. 14, 1 (2000).

[5] “Observation of Reactor Electron Anti-neutrino Dis- apperance in the RENO Experiment” arxiv:1204.

0626 (2012).

[6] J. K. Ahn et al., “Disappearance in the RENO Ex- periment”, Phy. Rev. Lett. 108, 191802 (2012).

[7] “Construction & Properties of Acrylic Vessels in the

RENO Detector”, NIM-A, 686, 91 (2012).