Thr (MeV e equiv. energy)

Measurement and simulation of the neutron response and detection efficiency of a 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, performing also the whole simulation of the experiment.

Motivations:

• Detection of neutrons of few to few hundreds of MeV is 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 amount of scintillator.

• 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.

• 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 DANE-2 program @ LNF:

• AMADEUS: study of deeply bounded kaonic nuclei.• DANTE: measurement of nucleon timelike region e.m. form factors.

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 and energy response:

(E)/E = 5.7 %/√E(GeV) (t)= 54 ps/√E(GeV)

and high photon efficiency

The neutron beam line at TSL – Blue Hall

5.31 m

KLOE calorimeter module

EKIN (MeV)

• A quasi-monoenergetic neutron beam from 178.8 MeV protons on 7Li target. (7Li(p,n)7Be)• 42% of neutrons at max energy.• round collimator of 2cm Ø• Calorimeter at 5 m from target.• Absolute neutron flux in the peak measured after the last collimator by beam intensity monitor with an accuracy of 10%. • TRF = 45 ns

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 40 % to 50 %. This value largely exceeds the estimated 8-16 % expected if the response were proportional only to the scintillator equivalent thickness. A detailed simulation of the calorimeter and of the TSL beamline 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 results show an overall neutron efficiency of about 50 %, when no trigger threshold is applied. 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.

S.Bertoluccia, C. Binib, P. Branchinic, C. Curcenaua, G. De Zorzib, A. Di Domenicob, B. Di Miccoc, A. Ferrarid, 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

( 1 ) Old prototype of the KLOE calorimeter: 60 cm long, 3 x 5 cells (4.2 x 4.2 cm2), read out at both ends by Hamamatsu/Burle PMTs.

( 2 ) Beam position monitor: array of 7 scintillating counters, 1 cm thick.

( 3 ) Reference scintillator counter: NE110, 10×20 cm2, 5 cm thick read out at both sides: S1, S2.

(4) A rotating frame allows for calorimeter positioning (data taking with n beam - calibration with cosmic rays).

(1)

(3)

(2)

3 data sets collected with beam intensities:

of 1.5 kHz/cm2, 3.0 kHz/cm2 and 6.0

kHz/cm2

For each configuration scans with different trigger thresholds Typical run: 0.5-1.5 Mevents, 1.7 kHz DAQ rate Cosmic rays run (beam off) for calibrations with MIPs.

The experimental set up and data sets

The measurement of the global efficiency

αfR

Rε

liveneutron

Trigger

mE

Tofc

lTof

kin

1

Rneutron: incoming neutron rate measured with

beam flux intensity monitors in the TSL Blue Hall

Rtrigger : Detector Trigger rates:

• Scintillator : T1 trig = S1×S2 • Calorimeter : discriminated signal of the analog sum of 12 PMs/side ,

T1 trig = SA×SB

flive: live time fraction a: for preliminary measurement, assume full acceptance and no background

• Trigger signal is phase locked with RF signal (T1 free).• Vetoed by the DAQ busy.• The final trigger signal is: T2 =T1free.AND.NOT(BUSY).● flive: T2/T1free

Z(cm)

Shielding(concrete / steel)

Calorim

eter

7Li TargetY(c

m)

X(c

m)

Z(cm)

beam

Neutron fluence

X(c

m)

Z(cm)

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 ()

The scintillator efficiency

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

The KLOE Pb-scintillating fiber calorimeter

Threshold (MeV e equiv. energy)

(%

)

The calorimeter efficiency

(%

)

Full energy spectrum

n

Z(cm)

p

n1

n2

n3

n4

X(c

m)

En = 175.7 MeV

primary vertex

En (p) = 126 MeV

Energy scale set using MIP calibration of all channels, and using the MIP/MeV scale factor used in the KLOE experiment.

10% uncertainty on horizontal and vertical scales

Stability wrt very different run conditions: a factor 4 variations of live time fraction (fLIVE=0.2 0.8) and beam intensity (1.5 6.0 kHz/cm2).

Very high efficiencyVery high efficiency, about 4 times

larger than the expected if only the amount of scintillator is

taken into account: ~ 8% for 8 cm of scintillating fibers.

Very high efficiencyVery high efficiency, about 4 times

larger than the expected if only the amount of scintillator is

taken into account: ~ 8% for 8 cm of scintillating fibers.

Compare with the scintillator efficiency measurement, scaled by the scintillator ratio factor 8/5

FLUKA simulation of beam-line and calorimeter

The enhancement of the efficiency appears to be due to the huge inelastic production of neutrons on the lead planes.: - produced isotropically; - produced 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 enhancement of the efficiency appears to be due to the huge inelastic production of neutrons on the lead planes.: - produced isotropically; - produced 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%).

Predicted integrated efficiency ~50% in reasonable agreement with test beam measurements!

ε (%

)

Ekin (MeV)

FLUKA

n

The first measurement of the detection efficiency of a high sampling lead-scifi calorimeter to neutrons, in the energy range [5,178] MeV, has been performed at TSL.

The integrated ranges between 40% and 50%, at the lowest trigger threshold used.

A detailed Monte Carlo study, carried out with FLUKA, shows that that the origin of such enhancement is related both to a shower-like effect due to the inelastic processes in the Pb-scifi structure AND to the high sampling fraction of this detector.

Data/Monte Carlo comparison is in progress and preliminary results are satisfactory

New tests to neutron beams at different energies are in program together with new tests of other calorimeter prototipes with different pb-scifi volume ratio.

This work is the starting point for the study and development of a new, compact, chip, fast and efficient neutron detector This work is the starting point for the study and development of a new, compact, chip, fast and efficient neutron detector

Conclusions and plans

Response on calorimeter module

Beam line simulation

Example of a neutron interaction

Tof(ns)

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

EKIN (MeV)

EKIN (MeV)

ns

(E

KIN

)

Study of efficiency dependence on energy

FLUKA MC prediction

Preliminary measurement by Tof