Measurement and simulation of neutron detection

1
Measurement and simulation of neutron detection
efficiency in lead-scintillating fiber
calorimeter
M. Anellia, S. Bertoluccia, C. Binib, P.
Branchinic, C. Curcenaua, G. De Zorzib, A. Di
Domenicob, B. Di Miccoc, A. Ferrarid, S. Fioreb,
P. Gauzzib, S. Giovannellaa, F. Happachera, M.
Iliescua, M. Martinia, S. Miscettia, F. Nguyenc,
A. Passeric, B. Sciasciaa, F. Sirghia a
Laboratori Nazionali di Frascati, INFN, Italy
b Universita degli Studi La Sapienza e
Sezione INFN di Roma, Italy c Universita
degli Studi Roma Tre e Sezione INFN di Roma3,
Italy dFondazione CNAO, Milano, Italy
ABSTRACT The overall detection efficiency to
neutrons of a small prototype of the KLOE
Pb-scintillating fiber calorimeter has been
measured at the neutron beam facility of The
Svedberg Laboratory, TSL, Uppsala, in the kinetic
energy range 5,175 MeV. The measurement of the
neutron detection efficiency of a NE110
scintillator provided a reference calibration. At
the lowest trigger threshold, the overall
calorimeter efficiency ranges from 30 to 50.
This value largely exceeds the estimated 8
expected if the response were proportional only
to the scintillator equivalent thickness. A
detailed simulation of the calorimeter and of the
TSL beam line has been performed with the FLUKA
Monte Carlo code. The simulated response of the
detector to neutrons is presented, as well as a
first data-Monte Carlo comparison. The reasons of
such an efficiency enhancement, in comparison
with the typical scintillator-based neutron
counters, are explained, opening the road to a
novel neutron detector.
The neutron beam line at TSL Blue Hall
The KLOE Pb-scintillating fiber calorimeter

• The KLONE (KLOe Neutron Efficiency) group has
  measured the neutron detection
• efficiency of a KLOE calorimeter prototype, at
  The Svedberg Laboratory (TSL), Uppsala,
• Oct 2006 Jun 2007, performing also the whole
  simulation of the experiment.
• Motivations
• Detection of neutrons of few to few hundreds of
  MeV is traditionally performed with
• organic scintillators (elastic neutrons
  scattering on H atoms ? production of protons
• detected by the scintillator itself) ?
  efficiency scales with thickness ? 1/cm
• Preliminary measurement at KLOE (neutron from
  K? beam pipe interactions) showed
• an efficiency of ?40 for Ekin 20 MeV. An
  efficiency of ?10 would be expected
• if the response were only due to the
  equivalent amount of scintillator in the
  calorimeter
• Enhancement of neutron detection efficiency for
  fast neutron is observed in presence
• of medium-high Z materials, particularly
  lead, as in the extended range rem counters
• for radiation protection
• The KLOE e.m. calorimeter has an excellent time
  resolution, good energy resolution,

KLOE calorimeter module

• Active material
• 1.0 mm diameter scintillating fiber (Kuraray
  SCSF-81, Pol.Hi.Tech 0046),
• emitting in the blue-green region lPeak
  460 nm
• Core polystyrene, r1.050 g/cm3, n1.6
• High sampling structure
• 200 layers of 0.5 mm grooved lead foils (95
  Pb and 5 Bi)
• Glue Bicron BC-600ML, 72 epoxy resin, 28
  hardener
• LeadFiberGlue volume ratio 424810
• Good time resolution, energy response and high
  photon efficiency
• sE/E 5.7 / vE(GeV)
  sT 54 ps / vE(GeV)

3 m

• A quasi-monoenergetic neutron beam from protons
  on
• 7Li target (7Li(p,n)7Be), 50 of neutrons
  at max energy
• Three different energies used 174, 46.5 and
  21.8 MeV
• Round collimator of 2cm Ø
• Calorimeter from 5 to 6 m from target
• Absolute neutron flux in the peak measured after
  the
• last collimator by beam intensity monitor
• Cyclotron RF period from 45 to 78 ns, depending
  on
• energy

174 MeV nutrons
EKIN (MeV)
The experimental set up and data sets
Measurement of overall neutron detection
efficiency

• Small prototype of the KLOE calorimeter 60 cm
  long, 3 x 5 cells (4.2 x 4.2 cm2), read out at
  both ends by PMTs
• Reference NE110 scintillator counter, 1020 cm2,
  5 cm thick read out at both sides with PMTs
• Rotating frame allows for detector positioning
  (data taking with n beam - calibration with
  cosmic rays)
• Low beam intensity (3-10 kHc/cm2) at collimator
  exit provides negligible contribution of double
  neutron counting per event
• Trigger built by the coincidence of the
  discriminated signals of the two sides for each
  detector. For the calorimeter the analog sum of
  the first four (out fo five) planes is used. A
  phase locking with RF signal defines a precise
  start for the event and allows time of flight
  measurement.
• Typical runs consists of 1 Mevents acquired at
  2 kHz rate, thus allowing to perform scans at
  different trigger thresholds

For each beam energy, the overall efficiency is
defined as the average over the full neutron
energy spectrum

• Rtrigger Detector trigger rates from scalers
• FH fraction of halo neutron events
  H/(HS)
• a detector acceptance 1, from MC

Neutron flux known with an accuracy of ?10
(174 MeV) , 20 (lower peak energy)
Rneutron Rate(ICM) ? K ? p r2 / fpeak ICM
Ionization Chamber Monitor ? online rate
determination TFBC Thin Film Breakdown Counter
? absolute flux calibration of peak neutrons (K)
FLUKA simulation of beam-line and calorimeter
Beam line simulation
Response on calorimeter module
Example of a neutron interaction
Neutron fluence
Proton fluence
n
175.7 MeV
beam
High probability to have interactions in lead
An efficiency enhancement w.r.t. bare
scintillator counters is related to the huge
inelastic production of neutrons on the lead
planes - produced isotropically and with a non
negligible fraction of e.m. energy and protons
which are detected in the nearby fibers - lower
energy secondaries( E 19.6 MeV) ? larger
probability of interaction in the calorimeter
with further n/p/? production (62/7/27)
Beam halo evaluation
Scintillator efficiency
n
Three evidences of a sizeable beam halo
contribution 1. Single cell clusters show
enhanced rate on lateral/central cells w.r.t.
MC 2. Special runs _at_ 22 MeV with calorimeter
out-of-beam 3. Horizontal scan with TFBC close to
the collimator exit at low energy

• The measurement of the scintillator efficiency
  gives a
• cross calibration of the measurement method
  and of the
• beam monitor accuracy, with small
  corrections due to
• the live time fraction
• The energy scale is calibrated with a 90Sr b
  source.
• 10 accuracy for horizontal scale (threshold)
  and the
• vertical one (e)
• Results agree with thumb rule (1/cm)
• 5 for 5 cm thick scintillator (at a
  threshold of ?2.5 MeV)
• Agreement, within errors, with previous
• published measurements in the same energy
• range, after rescaling them to the used
• thickness
• Similar agreement also for low energy
• measurements

Central cell Lateral cell
21.8 MeV Signal from MC Halo
shape from out-of-beam runs
174 MeV Signal from MC Halo
shape from lateral cells
Single cell clusters
Multiple cell clusters
Calorimeter efficiency

• Energy scale set using MIP calibration of all
  channels, and using the MIP/MeV scale factor of
  the KLOE experiment
• Energy cut-off introduced by the trigger
  evaluated by fitting with a Fermi-Dirac function
  the ratio of total/cluster energy
• at different thresholds
• Systematic errors on vertical scale dominated
  by halo subtraction and absolute neutron flux
• Systematics on horizontal scale conservatively
  assigned by the difference between cut-off
  determined with an
• independent method (cosmics and neutron data
  triggered with an .OR. Between scintillators and
  calorimeter)
• Stability w.r.t. very different run conditions
  a factor 4 variations of live time fraction
  (fLIVE0.2 ? 0.8) and beam
• intensity ( 3 ? 10 kHz/cm2 )

174 MeV neutrons
Very high efficiency, about 4 times larger than
what expected if only the amount of scintillator
is taken into account ( 8 for 8 cm of
scintillating fibers)
Scintillator efficiency measurement, scaled by
the scintillator ratio factor 8/5
Scintillator efficiency measurement, scaled by
the scintillator ratio factor 8/5
Conclusions and plans

• A full simulation with FLUKA is in progress.
  First data-MC comparisons are encouraging and
  allowed to disentangle a
• neutron halo component in the beam
• Further test are planned in two weeks at TSL _at_
  174 MeV with additional detectors a KLOE
  prototype with high readout
• granularity, a calorimeter with higher
  lead-scintillating fiber ratio and a beam
  position monitor

• The measurement of the detection efficiency of a
  high sampling lead-scifi calorimeter to neutrons,
  in the energy range
• 5,174 MeV, has been performed at TSL
• The efficiency ranges between 30 and 50,
  depending on the energy, at the lowest trigger
  threshold used, resulting
• four times larger than what expected for an
  equivalent scintillator thickness