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Measurement and simulation of neutron detection
efficiency in lead-scintillating fiber calorimeter
Active material:
1.0 mm diameter scintillating fiber (Kuraray SCSF-81, Pol.Hi.Tech 0046),
emitting in the blue-green region: Peak ~ 460 nm
Core: polystyrene, =1.050 g/cm3, n=1.6
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 Lead:Fiber:Glue volume ratio = 42:48:10
Good time resolution, energy response and high photon efficiency
E/E = 5.7 % / √E(GeV) T= 54 ps / √E(GeV)
The neutron beam line at TSL – Blue Hall
3 m
KLOE calorimeter module
EKIN (MeV)
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
ABSTRACTThe 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.
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
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, 10×20 cm2, 5 cm thick read out at both sides with PMT’s
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
The experimental set up and data sets
Measurement of overall neutron detection efficiency
αR
)1(R
αR
Rε
neutron
trigger
neutron
signal
HF
Rneutron: = Rate(ICM) K r2 / fpeak
ICM: Ionization Chamber Monitor → online rate determinationTFBC: Thin Film Breakdown Counter → absolute flux calibration of peak neutrons (K)
Rtrigger : Detector trigger rates from scalers
FH: fraction of halo neutron events [
H/(H+S) ]: detector acceptance =1, from MC
Neutron fluence Proton fluence
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 source. 10% accuracy for horizontal scale (threshold) and the vertical one ()
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
Scintillator efficiency
The KLOE Pb-scintillating fiber calorimeter
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 (fLIVE=0.2 0.8) and beam intensity ( 3 10 kHz/cm2 )
Very high efficiencyVery 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)
FLUKA simulation of beam-line and calorimeter
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/γ production (62/7/27%)
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
Conclusions and plans
Response on calorimeter module
Beam line simulation
Example of a neutron interaction
1.2 mm1.35 mm
1.0 mm
Target Pel(%)
Pinel(%)
Pb 32.6 31.4
Fibers 10.4 7.0
Glue 2.3 2.2
High probability to have
interactions in lead
174 MeV nutrons
For each beam energy, the overall efficiency is defined as the average over the full neutron energy spectrum
Neutron flux known with an accuracy of 10% (174 MeV) , 20% (lower peak energy)
Beam halo evaluation
Three evidences of a sizeable beam halo contribution:1. Single cell clusters show enhanced rate on lateral/central cells w.r.t. MC2. Special runs @ 22 MeV with calorimeter out-of-beam3. Horizontal scan with TFBC close to the collimator exit at low energy
■ = Central cell■ = Lateral cell
174 MeV: Signal from MC Halo shape from lateral cells
21.8 MeV: Signal from MC Halo shape from out-of-beam runs
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, and high efficiency for photons. If a high neutron detection efficiency were observed, this could also be the first of a novel kind of neutron detectors
Neutron detection is important for the DAFNE-2 program @ LNF:
AMADEUS: study of deeply bounded kaonic nuclei
DANTE: measurement of nucleon timelike region e.m. form factors
Shielding(concrete / steel)
Calorim
eter
7Li Target
Y (
cm)
Z (cm)
Z (cm)
X (
cm)
beam
n
X (
cm)
Z (cm)
175.7 MeVp
n1
n2
n3
n4primary vertex
En (p) = 126 MeV
Z (cm)
X (
cm)
n
Single cellclusters
Multiple cellclusters
174 MeV neutrons174 MeV neutrons
Scintillator efficiency measurement, scaled by the scintillator ratio factor 8/5Scintillator efficiency measurement, scaled by the scintillator ratio factor 8/5
Threshold (MeV equiv. e energy)
174 MeV neutrons174 MeV neutrons
(%
)
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 @ 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