CERN Accelerating science


High  precision  neutron cross-‐section  data  are  of  major  importance  for  a  wide  variety  of  research   fields   in  basic  and  applied  nuclear   physics   [1].   In  particular,  neutron  data   on  neutron-‐nucleus reactions  are  essential  in  Nuclear   Astrophysics   for  understanding   the   production  rate  of  heavy   elements in the   Universe, which occurs   mainly   through slow and rapid  neutron capture   processes during  the  various  phases of  stellar  evolution   [2,3].     In   the   field   of nuclear technology  new  studies aimed  at    developing  future    generation  nuclear  system    which  would  address  major  safety, proliferation   and   waste   concerns   are requiring   more accurate   data in   neutron-‐induced  fission and capture  cross-‐section on radioactive  isotopes.



The  idea  of a  new neutron  time-‐of-‐flight  facility at  CERN was  proposed  by  C.  Rubbia  in 1998  [4], as   a  follow up of the  TARC experiment, also conducted  at  CERN.


The   concept  of  the  n_TOF neutron   beam  makes   use  of  both  the  specifically  high   flux  of neutrons attainable  using the  spallation process  of 20  GeV/c  protons on a  massive  lead  target (see  Fig.1), able   to contain practically   the whole spallation shower   as well as   the remarkable instantaneous beam intensity of the  CERN Proton Synchrotron (PS).  After the initial  proposal  in 1998,  the  facility  was  then accepted  for  construction  by  CERN  in  1999.  The  CERN   n_TOF   facility  has  been  set  in  operation  and commissioned  in  2001  with  performances   matching  the  expectations.  The   PS  machine  of  CERN  can   generate  high  intensities  up  to   8.5×1012 ppp  (protons   per  pulse) -‐ high   enough  to   produce  the   vast   number of 4×101neutrons per pulse -‐ in  the  form  of short (6  ns  width) pulses  with a repetition time varying  from  1.2   s  to  16.7  s  and  a  prompt   “flash”  considerably   smaller   compared  to  electron machines. The  high neutron flux, the   low  repetition rates   and the   excellent   energy resolution of 5.5x10-4  (1 keV)  have  opened  new  possibilities   for   high  precision  cross  section measurements  in  the energy  range   from  thermal  to  GeV,   for  stable   and,  in  particular -‐   taking  advantage   of   the  high instantaneous  neutron  flux -‐ for  radioactive targets. 

After  three   years long   stop   due   to   cooling water activation   by spallation   products diluted   in   the water,  a  new  lead target  was  constructed, and  new  monitoring systems  developed in  order to  meet   safety   requirements   for  the   restart   of  the  facility.  The  new  target   assembly   consists   of  a  separated cooling and moderator circuit, which  enables  the use of different  moderator materials,  thus allowing a  greater flexibility on the  characteristics of  the  neutron beam.

Another  upgrade  performed   during   the   2010   run   has  been   the   transformation   of  the  n_TOF experimental  area   into  a   Work  Sector   Type  A,   which  has  allowed  the  possibility  to  perform measurements  of  capture  and  fission  cross-‐section  of  “unsealed”  samples   of  highly   radioactive isotopes,  such  as  actinides  like 241Am, 243Am   and  242Pu,  taking full  advantage  of  the  facility’s  high instantaneous  neutron  flux.   This   has  required  a  complete revision   of the  experimental   area and  of the related  technical services.  The  milestones of n_TOF   facility since  its   inception is summed-‐up  in   Fig.2




During  the  first   years  of  operation  2001-‐2004   the    n_TOF    collaboration  has  attained  a   rich experimental program  measuring  in  total 36  isotopes and  producing  numerous  scientific  papers  and proceedings.

One example of the  n_TOF  measurement  capabilities and  of the vast experimental campaign  is given by the measurement of the capture   cross-‐section of 186Os and 187Os, which has resulted also in a series   of featured articles   on Phys. Rev. C [5].   The precise determination of   the   neutron capture cross  sections   of  these  two  isotopes  is  important  to  define  the   s-‐process  abundance   of 187Os  at   th formation  of  the  solar   system. This   quantity  can  be  used  to evaluate the  radiogenic component   of   the  abundance of  187Os  due  to the decay of the  unstable 187Re  (t1/2  =  41.2  Gyr)  and from this  to  infer the time duration  of  the  nucleosynthesis   in our   galaxy   (the   so-‐called  “Re/Os   cosmochronometer”). The neutron capture cross sections of  186Os,  187Os, and188Os  have been measured at  the CERN  n TOF   facility  from  1  eV   to  1  MeV,   covering  the  entire  energy   range   of   astrophysical  interest.  The corresponding stellar  cross  sections  have been used to separate the  radiogenic  part  of the 187Os abundance   from its   s-‐process   component and   to   define the mother/daughter ratio  187Re/187Os.By   means of a schematic  model which assumes  an exponentially  decreasing production rate  for 187Re,  it   was  shown  that  the  new   data  limit   the  nuclear  physics  uncertainties  for  the  rhenium-‐osmium  clock   to  less   than 1  Gyr,  allowing  a more  accurate  estimate   for  the  age of  our  galaxy:  the  age  of the   universe by means of the  Re/Os clock is claimed  to  be 15.3±0.8±2  Gyr.

An   example  of  fission   cross   section   measured   at  n_TOF  is   the  245Cm(n,f)  [6].   Transuranium elements (TRU) such as Np, Pu,  Am,  and Cm are  built up as  a  result  of  multiple  neutron captures and radioactive   decays  in  all  presently   operating nuclear  reactors   based  on   the  U/Pu  nuclear   fuel cycle. Some   highly  radioactive   isotopes  of  these  elements   constitute  the   most  important   hazard  for nuclear  waste   management.   Several  proposals  have  been   made  in   recent  years   to   reduce   the radiotoxicity  of  nuclear   waste  containing  TRU.  Clearly,  any  kind  of  system  designed  to  burn  nuclear  waste,  critical   or subcritical, thermal   or fast, will   need to  be loaded  with  fuel   containing   a large fraction of   TRU. The  response  of   these   systems   (e.g., with   respect   to criticality)  in  the presence  of   TRU  is  directly  linked to  the  fission  cross  sections  of  these  isotopes.  These  fission   cross   sections  are   therefore  key   elements  in assessing the  strategy  to  be  followed  in  detailed feasibility studies  and  in the  engineering  design  of  nuclear  waste  transmutation  systems,  i.e.,   Generation  IV  fast  reactors   or   advanced  nuclear   waste  burners.  The   245Cm(n,f)  neutron-‐induced  fission  cross-‐section  has   been measured at n_TOF   – in a single measurement -‐  from   30 meV   to 1 MeV   neutron energy   with an overall  systematic uncertainties close to   5%; significant   discrepancies with   previous measurement   and  with  evaluated   databases  –  in   selected  energy  regions   -‐  have  been   observed   and   new evaluations will take  into account  these  new data  to extract  a  coherent  and up-‐to-‐date cross-‐section description.


The  overall  efficiency  of  the  experimental  program and the  range  of  possible measurements  could be significantly  improved  with  the  construction  of  a  2ndExperimental  Area  (EAR-‐2), vertically located 20  m  on top of the  n_TOF  spallation target [7].The   realization  of  the  2ndExperimental   Area,  with  its  short   flight  path,  will  contribute  to  a substantial  improvement  in  experimental  sensitivities   and  will  open  a  new  window  to  stellar nucleosynthesis,   nuclear  technology  (such   as   transmutation   devices   or   improvement   of  safety  margins  of present   and future  nuclear energy  systems) and basic nuclear  physics  by  allowing to  measure  neutron-‐induced reactions  which are  not  or  difficult to  access  so  far  at  any  other presentlyoperating installation.

The  main advantages of the  2nd Experimental Area  are the following:

    Neutron-‐induced reaction measurements   can be  performed  on very  small   mass   samples.     This   feature   is  crucial to  reduce the   activity   of unstable  samples  and in cases where the   available  sample  material  is  limited.(ex.  238Pu, 242Pu 243Cm, 244Cm, 245Cm, 242Am,231Pa,  233Pa)

    Measurements can be   performed on  isotopes   with very small  cross   sections   for  which the optimization  of  the  signal/background  ratio  is an  essential  prerequisite  (ex. 86Kr, 138Ba,  140Ce, 208Pb )

    Measurements can be performed on much  shorter time  scales;  repeated runs  with modified conditions  are essential to check  corrections  and to reduce  systematic  uncertainties.

    Measurements of  neutron-‐induced cross   sections   at   high   energies   (En>10-‐100  MeV),  which are  difficult   to perform  in  the   existing EAR-‐1,   will  benefit. This  will  be  particularly  important  for measurements  of  (n,  charge particle) reactions at  high energies.


The  CERN  n_TOF facility has  proven to be a unique facility in the  world for  its performance. Since   2001,  with its rich scientific program,  the   n_TOF   experiment  is   contributing to  the   world efforts   aimed   at  collecting   high  quality  data,  mostly  on  capture   and  fission  neutron-‐induced reactions. The  realization of the  Experimental  Area  2  (EAR-‐2)  with  its  enhanced capabilities will  be  of  utmost importance  for the   neutron physics   community   and will   also  be complementary   to   other  future installations;   due  to   the  unique  neutron  beam  characteristics   the  installation   will  open   new opportunities  for  measurements   of  neutron-‐induced  reactions  with  unprecedented  accuracy  for various   important  fields   of  physics,   among   which  nuclear  technology,   nuclear  astrophysics   and stellar evolution,  basic  research,  medical  applications,  dosimetry  and  radiation  damage  studies.



[1]  E.   Chiaveri  et  al.  “Past,  Present   and  future  of  the   n_TOF   facility”,  Journal  of  the   Korean Physical

Society, Vol. 59, No.  2, August 2011,  pp. 1620_1623

[2]  F.  Kaeppeler, Progr. Particles Nucl. Phys. 43  (1999) 439

[3]  G.  Wallestein et  al.  Rev. Mod. Phys. 69 (1997) 995

[4]  C.  Rubbia et  al., “A  high Resolution Spallation  Driven Facility  at  the  CERN-‐PS  to measure  Neutron

Cross Sections in  the Interval from 1  eV  to  250  MeV”, CERN/LHC/98-‐02(EET)+Add1

[5]  n_TOF  Collaboration,‐for/10.1103/PhysRevC.82.015802

[6]     M.     Calviani    et     al.     (n_TOF     Collaboration),    Phys.     ReV.    C     85,    034616    (2012)

[7]      E.      Chiaveri    et    al.      “Proposal    for    n_TOF      Experimental      Area    2    (EAR-‐2)”,

( )