Measurements  of  Liquid  Scintillator  Light  Yield  for  Future  Neutrino  Experiments  

 Athena  Ierokomos  

University  of  California,  Berkeley  2013  University  of  California,  Los  Angeles  REU  Program  

 

 Abstract    Neutrinoless  double-­‐beta  decay,  though  a  rare  process,  could  be  used  to  determine  if  the  neutrino  is  a  Majorana  or  Dirac  particle.    In  searching  for  very  rare  processes,  it  is  often   useful   to   build   very   large   detectors,   which   are   frequently   filled   with   liquid  scintillator  materials.   In  order  to  decide  which   liquid  scintillator  would  be  the  most  useful,   there   are   multiple   characteristics   that   should   be   compared,   including   light  yield,  attenuation  length,  and  cost.    In  this  paper,  liquid  scintillators  for  future  use  in  these   detectors   are   compared   to   determine   which   have   the   highest   light   yield.    Diisopropylnaphthalene  has  been  shown  to  be  the  most  efficient  scintillator.    Introduction    A  Majorana   particle   is   a   fermion  which   is   its   own   antiparticle.     One   of   the   leading  possibilities   for   a   Majorana   particle   is   a   neutrino.     The   neutrino   is   a   very   weakly  interacting  particle  with  almost  zero  mass.    In  recent  years,  a  lot  of  progress  has  been  made  towards  understanding  the  characteristics  of   these  particles,  but   the  question  of   the   exact   nature   of   the   particle   remains.     There   is   only   one   major   method   of  determining  if  the  neutrino  is  a  Majorana  or  Dirac  particle:    neutrinoless  double-­‐beta  decay.    A   Dirac   neutrino  would   be   limited   to   the   two   neutrino   double-­‐beta   decay.     In   this  process,   the   antineutrino   partner   of   the   electrons   is   emitted   from   the   system,  carrying   away   some   of   the   energy   of   the   decay.     Since   neutrinos   are   not   directly  detectable,   this   results   in   a   spectrum   of   total   energy   for   the   two   electrons.     A  Majorana  neutrino  would  also  undergo   this  process,  but   is   also  allowed   to  undergo  neutrinoless   double-­‐beta   decay.     Here,   instead   of   being   emitted   away   from   the  system,  the  neutrino  undergoes  light  Majorana  neutrino  exchange.    If  the  neutrino  is  not  emitted,   the  electrons  carry   the   full   energy  of   the  decay  away   from   the  system,  resulting  in  a  peak  in  counts  at  the  end  of  the  two  neutrino  double-­‐beta  decay  energy  spectrum.    This  peak  makes  it,  in  principle,  very  easy  for  an  experimentalist  to  look  for  the  signal  arising  from  Majorana  neutrinos.    However,  the  decay  we  are  looking  for  is  very  rare,  with  a  half-­‐life   several  orders  of  magnitude   longer   than   the  age  of   the  universe.     In  this  case,  we  need  a   large  enough  detector  that  the  events  will  occur  with  sufficient  frequency   to  produce   a  measurable   signal.    One  of   the  best   techniques   for  building  large  detectors  is  using  liquid  scintillators.    These  detectors  should  have  1-­‐10  tons  of  isotope,  which  would  be  dissolved  in  the  scintillator  to  a  concentration  on  the  order  of   1   g/L.     In   this   case,   a   kiloton-­‐scale   detector   is   needed,   which   corresponds   to   a  sphere  with  a  radius  greater  than  10  m.    A  scintillator  is  a  compound  that  fluoresces  when  exposed  to  charged  particles.    Some  organic  molecules  have  their  carbon  molecules  arranged  in  rings  in  such  a  way  that  

they  alternate  one  double  bond  and  one  single  bond.    In  this  case,  a  moving  charged  particle,   such   as   an   electron,   can   deposit   some   energy   into   the   system  by   colliding  with   it.    This  excites  the  electrons   in  the  pi  bond,  allowing  them  to  resonate.    When  these   electrons   relax   again,   they   can   emit   a   photon   within   a   certain   range   of  frequencies,  which  are  at  lower  energies  in  general  than  that  which  excites  them.    In  detector   systems,   these   photons   are   collected   by   photomultiplier   tubes   (PMTs)   to  produce  a  measurable  signal.        There   are   several   characteristics   that   must   be   taken   into   consideration   when  choosing  an  efficient   liquid  scintillator.    These   include  safety  and  cost,  but   the  most  important   characteristics   are   light   yield   and   attenuation   length.     In   particular   for   a  large  scale  detector,  the  attenuation  length  is  a  crucial  measurement.    The  attenuation  length  of  a  liquid  scintillator  is  related  to  the  transparency  of  the  liquid  to  light.    Any  molecule   has   characteristic   absorption   and   emission   spectra.     These   frequently  overlap   to   a   certain   extent.     If   a   photon   is   emitted   on   the   high   energy   end   of   the  emission  spectrum,  it  could  fall  into  the  low  energy  end  of  the  absorption  spectrum.    If   this   is   the   case,   it   would   not   get   all   the   way   across   the   detector   without   being  reabsorbed.    For  experiments  at  the  kiloton  scale,  attenuation  lengths  on  the  order  of  10  m  are  crucial  for  reliably  getting  a  signal  out  of  the  detector.    These  lengths  can  be  achieved  by  increasing  what  is  known  as  the  Stokes'  shift,  or  the  spacing  between  the  emission   and   absorption   peaks.     In   our   experiment,   this   is   achieved   using   a  compound   known   as   diphenyloxazole   (PPO),  which   serves   as   a  wavelength   shifter.    PPO  will   tend   to   shift   the   emission   spectrum   of   the   pure   solvent   lower   in   energy,  which  allows  more  light  to  traverse  the  detector  without  being  reabsorbed.        Another  important  characteristic  is  the  light  yield.    The  light  yield  of  a  scintillator  is  the  amount  of   light  emitted  by  a  molecule  per  deposited  energy.    Since  neutrinoless  double-­‐beta  decay   is   such  a   rare  process,  when   it  occurs  we  want   to  be  able   to  get  enough  light  out  of  the  scintillator  that  it  is  detectable,  and  that  the  energy  resolution  is  good  enough.    Thus  we  would  like  to  have  a  high  light  yield  for  our  scintillator.    PPO  can  also  be  helpful  for  this  purpose.    The  structure  of  PPO  is  three  rings,  two  of  which  are   aromatic.     This  makes   it   very   likely   for   PPO,   once   excited,   to   deexcite   through  emission  of  a  photon  rather  than  by  mechanical  energy  loss  or  any  other  process.    By  transferring  energy   to  PPO   from  the  solvent   it   is  dissolved   in,  PPO  can   increase   the  light  yield  of  the  solvent.    Experimental  Setup    This  summer,  the  purpose  of  my  experiment  was  to  find  the  optimal  concentration  of  PPO  for  light  yield  in  a  variety  of  scintillators,  given  in  the  table  below.      

 Table  1:    The  solvents  to  be  tested  and  their  structures.  

   For  this,  we  used  a  cuvette  (1cm  x  1  cm  x  3.5  cm)  to  hold  the  solvent.    This  cuvette  was   held   in   place   by   a   teflon   reflecting   block   to   increase   the   number   of   photons  emitted   by   the   scintillator   that  make   it   into   the   PMT.     Attached   to   the   back   of   the  block  was  the  radioactive  source  that  stimulated  scintillation.    For  all  experiments,  a  Cesium  137   source  was   used.     The   cuvette  was   coupled   to   the  PMT  using   a   grease  with   the   same   refractive   index   as   the   glass.     The   PMT  was   held   in   place  with   two  metal  plates.    It  was  placed  in  a  fixed  position  in  a  dark  box.    For  our  tests,  the  PMT  was   powered   by   a   voltage   of   1675V.    We   used   an   AlazarTech  Waveform   Digitizer  acquisition  card  on  the  computer  set  to  a  2V  scale.    

 Figure  1:    The  inside  of  the  dark  box  contains  the  sample  and  the  PMT,  along  with  other  structures  to  hold  them  in  

place  consistently  from  test  to  test.  

 The  data   acquisition   card   reads   a   voltage   to   the   computer,  which   is   converted   to   a  charge   by   integrating   under   the   voltage-­‐time   output.     The   scintillator   emits   light  according  to  a  Compton  spectrum.    Light  from  the  source  will  scatter  off  electrons  in  the  solution  with  sufficient  energy  to  ionize  the  molecules,  allowing  a  free  electron  to  propagate   into   the   system.     The   lower   the   scattering   angle,   the   higher   the   energy  imparted  to  the  electron  from  the  incident  light.    So,  there  is  a  maximum  energy  that  can  be   imparted,  which  occurs   at   a   scattering   angle   of   0  degrees.  Thus,   this   charge  spectrum   is   distributed   around   a   distinct   Compton   edge   corresponding   to   the  maximum  energy  electron  from  the  average  energy  radiation  emitted  by  the  source.    By   finding   this  maximum  charge,  we  can  determine   the   light  yield  of   the  sample  by  retracing  the  conversion  of  energy  through  the  sample  and  PMT.    The  energy  from  the  source   comes   into   the   sample,   and   stimulates   scintillation.    A   certain  percentage  of  the   scintillation   light   hits   the   PMT   photocathode,  which   converts   light   to   electrons  with  a  given  efficiency  related  to  its  quantum  and  collection  efficiencies.    The  number  of  electrons  is  then  magnified  by  the  gain  of  the  PMT,  and  the  current  is  collected  by  an  anode  at  the  end.    The  area  under  the  voltage  vs.  time  graph  gives  charge  with  a  factor  of  resistance.    So,  all  other  things  in  the  experiment  being  carefully  controlled,  we  can  simply  compare  the  charge  numbers  to  determine  which  scintillators  have  the  highest  light  yield.    

Initially,  there  were  a  few  problems  that  needed  to  be  addressed  in  the  setup.    First,  any   oxygen   in   the   system   will   reduce   the   light   yield   of   the   scintillator.     Oxygen  reduces   the   efficiency   with   which   energy   is   transferred   between   the   scintillator  solvent   and   the   PPO,   which   encourages   deexcitation   by   mechanical   modes   rather  than   scintillation.     Leaving   a   sample   of   high   purity   DIN   open   to   oxygen,   even   in   a  cuvette   with   a   small   exposed   surface   area,   for   just   15  minutes   decreases   the   light  yield  of  the  sample  by  2%,  and  for  3  hours  reduces  the  light  yield  nearly  7%.    Since  there  is  no  way  to  determine  how  much  oxygen  is  dissolved  in  the  solvent,  it  is  very  important   to   set   procedures   in   place   that  minimize   oxygen   contamination.     In   our  tests,  nitrogen   is  bubbled   through  the  sample   for   ten  minutes  after  placing   it   in   the  cuvette.    The  nitrogen  will  displace  the  oxygen  dissolved  in  the  sample.    Additionally,  all   samples   are   stored   under   a   nitrogen   blanket   to   minimize   any   new   oxygen  dissolving  into  the  solvent.    After  fixing  the  oxygen  control  procedure,  there  was  still  large  variation  in  the  results.    These  variations  were  present  even  when  the  cuvette  was  simply  removed  from  the  PMT  and  replaced,  indicating  that  there  was  some  variation  in  the  coupling  process.    Due  to  the  deposition  process  that  places  the  photocathode  on  the  glass  of  the  PMT,  the  photocathode  itself  is  not  uniformly  distributed.    Thus  the  reflective  block  holding  the  cuvette  in  the  same  place  relative  to  the  position  of  the  PMT  was  not  sufficient  to  ensure  that  the  exact  position  of  the  sample  relative  to  the  dynodes  within  the  PMT  was  the  same  every  test.    Additionally,  the  PMT  is  no  longer  in  the  same  position  with  respect   to   the  magnetic   field   in   the   room.     Since   dynodes   themselves   are   also   not  cylindrically  symmetric,  this  was  determined  to  be  the  cause  of  the  variation.    Fixing  the   orientation   of   the   PMT   between   tests   resulted   in   differences   between  measurements  being  reduced  to  around  0.5%  (RMS)  if  the  coupling  was  done  newly  after  each  measurement  but  the  cuvette  was  kept  closed.    Results    Each  solvent  was  tested  at  six  concentrations  of  PPO:    0.5,  1,  2,  5,  10,  and  50  g/L,  the  exception   being   LAB,   for  which   the   highest   tested   concentration  was   40   g/L.     The  resulting  Compton  spectra  are  represented  below  by  histograms.    The  figure  below  is  a  set  of  the  six  concentrations  for  a  single  solvent,  namely  PXE.    

 Figure  2:    A  sample  of  six  PXE  spectra,  one  at  each  concentration.    The  x-­‐axis  is  an  adjusted  charge  of  the  event,  with  800  corresponding  to  a  charge  of  250  pC.    The  y-­‐axis  is  counts  of  events  with  that  charge.  

 The   maximum   charge   was   taken   to   be   at   the   midpoint   of   the   falling   edge.     The  maximum  charge  is  plotted  in  Figure  3  below.    The  maximum  charge  clearly  increases  with  PPO  concentration  until  around  5  g/L  as  the  rate  of  energy  transfer  rate  between  the   solvent   and   PPO   increases.     However,   at   high   concentrations   of   PPO,   the  molecules  of  PPO  interact  between  themselves,  increasing  the  likelihood  that  energy  will  be  lost  through  processes  other  than  fluorescence.    So,  at  concentrations  above  5  g/L,   the   light  yield  drops  off  slowly.    This  pattern   is  repeated  for  all  of   the  solvents,  with  the  slight  exception  of  PC,  which  peaks  around  10  g/L  and  drops  off  quickly  after  that.        

 Figure  3:    A  plot  of  the  maximum  charge  of  PXE  at  each  of  the  six  concentrations.    The  x-­‐axis  is  PPO  concentration  on  a  logarithmic  scale,  and  the  y-­‐axis  is  again  an  adjusted  charge,  with  800  corresponding  to  250  pC.      

 The   charge   numbers   for   the   various   solvents   at   all   concentrations   are   listed   in   the  table  below.      

  Charge  (10-­11  C)  

Conc.  (g/L)  

PC   PCH   DIN  (high  purity)  

DIN  (low  purity)  

Toluene   PXE   LAB  

0.5   4.19   13.06   15.19   17.94   12.78   14.59   9.81  1   9.19   15.94   22.59   20.13   17.59   18.22   13.59  2   13.41   20.00   23.16   22.84   19.69   20.78   16.97  

5   16.78   20.03   22.72   22.44   20.13   22.03   17.34  

10   20.00   17.81   22.28   21.97   19.44   21.09   16.38  

50   15.81   14.19   19.16   18.16   14.31   16.72   12.72*    

Table  2:    The  maximum  charge  of  each  solvent  at  each  concentration.      

*Measured  charge  at  a  concentration  of  39.4  g/L,  not  50  g/L.    

     

PPO Concentration (g/L)1 10

Char

ge

450

500

550

600

650

700

PXE: Charge vs Concentration

Conclusion    Examining   table   2,   it   is   easy   to   compare   the   light   yield   efficiency   of   the   different  solvents.    In  general,  it  is  clear  that  the  two  DIN  purities  have  the  highest  light  yield.    Additionally,   they  have  very  similar  numbers  at  almost  every  concentration  of  PPO,  except   for   the   lowest.     This  may   be   a   result   of   PPO   having   a   greater   affect   on   the  scintillation  rate  in  a  higher  purity  substance  than  in  a  lower  purity  substance.    More  tests  may  be  done  to  verify  these  numbers.    PC  clearly  has  the  lowest  overall  light  yield  at  the  lowest  concentrations.      However,  at  its  peak  number  at  10  g/L  PPO  and  above,  PC  actually  has  a  higher   light  yield   than  LAB,  which  is  in  general  the  second  lowest.    This  is  because  PC  peaks  at  a  higher  PPO  concentration  than  any  of  the  other  solvents.    Most  solvents  peak  between  2  g/L  and  5  g/L,  but  PC  peaks  around  10  g/L,  so  it  would  make  sense  that  at  this  point  it  could  have  a  higher  light  yield  than  LAB.    Acknowledgements    I  would  like  to  thank  Professor  Lindley  Winslow  for  taking  me  into  her  lab  and  being  a  wonderful  mentor  all  summer.    Also,  I  would  like  to  think  Dr.  Christoph  Aberle  and  Timothée  De  Guillebon  for  helping  me  get  started  and  providing  guidance  throughout  the  project.    Finally,  this  was  only  possible  because  of  Francoise  Quéval  organizing  the  REU  program  and  the  NSF  providing  funding.