Measurement of the neutron Measurement of the neutron detection efficiency of the KLOE detection efficiency of the KLOE

calorimetercalorimeter

P.GauzziP.Gauzzi

(Universita’ La Sapienza e INFN – Roma)(Universita’ La Sapienza e INFN – Roma)

for the KLONE Groupfor the KLONE Group

1010thth ICATPP Conference ICATPP Conference October 8-12, 2007 – Como October 8-12, 2007 – Como

M.Anelli, G.Battistoni, S.Bertolucci, C.Bini, P.Branchini, C.Curceanu, G.De Zorzi, A.Di Domenico, B.Di Micco, A.Ferrari, S.Fiore, P.G.,

S.Giovannella,F.Happacher, M.Iliescu, M.Martini, S.Miscetti, F.Nguyen, A.Passeri, A.Prokofiev, P.Sala, B.Sciascia, F.Sirghi

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The KLOE calorimeterThe KLOE calorimeterPb - scintillating fiber sampling calorimeter of the KLOE experiment at

DANE (LNF):

• 1 mm diameter sci.-fi. (Kuraray SCSF-81 and Pol.Hi.Tech 0046)– Core: polystyrene, =1.050 g/cm3, n=1.6, peak ~ 460 nm

• 0.5 mm groved lead foils

• Lead:Fiber:Glue volume ratio = 42:48:10

• X0 = 1.6 cm =5.3 g/cm3

• Calorimeter thickness = 23 cm

• Total scintillator thickness ~ 10 cm

1.2 mm1.35 mm1.0 mm

Lead

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The KLOE calorimeterThe KLOE calorimeter• Operated from 1999 to 2006 with good performance

and high efficiency for electron and photon

detection, and also good capability of

π//e separation

Energy resolution:

E/E=5.7%/E(GeV)

t=54 ps/E(GeV)147 ps

Time resolution

(see KLOE Collaboration, NIM A482 (2002),364)

(KSKL; KSπ+π─; KL2π0)

4

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Why neutrons at KLOE ?Why neutrons at KLOE ?• Detection of n of few to few hundreds of MeV is traditionally performed

with organic scintillators (elastic scattering of n on H atoms produces protons detected by the scintillator itself)

efficiency scales with thickness 1%/cm

• Use of high-Z material improves the neutron efficiency

(see C.Birattari et al., NIM A297 (1990) and NIM A338 (1994)

and also T.Baumann et al., NIM B192 (2002))

• Preliminary estimate with KLOE data (n produced by K interactions in the apparatus) showed a high efficiency (40%) for neutrons with

En< 20 MeV, confirmed by the KLOE MC

• n detection is relevant for the DANE-2 program at LNF; two proposals:– search for deeply bounded kaonic nuclei (AMADEUS)

– measurement of the neutron time-like form factors (DANTE)

A test has been performed with the neutron beam of the The Svedberg Laboratory of Uppsala (October 2006 and June 2007)

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The neutron beam @ TSLThe neutron beam @ TSL

EKIN (MeV)

5.31 mKLOE calorimeter moduleKLOE calorimeter moduleKLOE calorimeter moduleKLOE calorimeter module

( 2 cm)

• Neutrons produced in the reaction 7Li(p,n)7Be

• Proton beam energy from 180 MeV to 20 MeV

• Neutron energy spectrum peaked at max energy

(at 180 MeV fp=42% of n in the peak)

• Tail down to termal neutrons

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Experimental setupExperimental setup1. Old KLOE prototype:

total length 60 cm 35 cells (4.2 cm 4.2 cm)

read out at both ends by Hamamatsu/Burle PMTs

2. Beam Position Monitor: array of 7 scintillating counters 1 cm thickness

3. Reference counter : NE110; 5 cm thick; 1020 cm2 area (in June 2007 two other NE110 counters 2.5 cm thick)

All mounted on a rotating frame allowing for vertical (data taking with n beam) and horizontal (for calibration with cosmic rays) positions

(1)

(3)

(2)

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Trigger & DAQTrigger & DAQTrigger:• No beam extraction signal available• Scintillator trigger: Side 1 – Side 2 overlap coincidence • Calorimeter trigger: analog sum of the signals of the

first 12 cells (4 planes out of 5) A•B overlap coincidence • Trigger signal is phase locked with the RF signal (45 - 54 ns)DAQ: • Simplified version of the KLOE experiment DAQ system (VME

standard)• Max DAQ rate : 1.7 kHz - Typical run: 106 events • For each configuration/energy: scans with different trigger thresholds• Three data-sets:

– Epeak = 180 MeV -- October 2006 - two weeks– Epeak = 46.5 MeV -- June 2007– Epeak = 21.8 MeV -- “

4 days

n

Y XZ

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Efficiency measurementEfficiency measurement• Global efficiency measurement: integrated over all the energy

spectrum

flive = fraction of DAQ live time

= acceptance

(assuming the beam fully contained in the

calorimeter surface 1)

• Sizeable beam halo at low peak energy

Rate(trigger) must be corrected

αf)Rate(

er)Rate(triggε

live

nEn = 180 MeV

flive

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Neutron rate Neutron rate

• Absolute flux of neutrons measured after the collimator;

2 monitors of beam intensity (see A.Prokofiev et al., PoS (FNDA2006) 016):– Ionization Chamber Monitor (7 cm ): online monitor, not position sensitive

– Thin-Film Breakdown Counter (1 cm ): offline monitor; used to calibrate the ICM by measuring the neutron flux at the collimator exit

• Rate(n) = Rate(ICM) K πr2 / fp

r = collimator radius (1 cm)

K = calibration factor (TFBC to ICM)

fp = fraction of neutrons in the peak

accuracy: 10% at higher peak energy (180 MeV)

20% at lower peak energy (20 – 50 MeV)

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Scintillator calibrationScintillator calibration• Trigger threshold calibration in MeV eq.el.en.:

ADC counts

source to set the energy scale in MeV:

90Sr ─ endpoint = 0.56 MeV

90Y ─ endpoint = 2.28 MeV

25 keV/ADC count

ADC counts

Thr. [mV]ADC counts

6 counts/mV

Eve

nts

Eve

nts

Thr. [mV] 20 100Thr. [MeV] 2.5 15

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(%)

Epeak = 180 MeV

Scintillator efficiencyScintillator efficiency

• Agrees with the “thumb rule” (1%/cm) at thresholds above 2.5 MeV el.eq.en.

• Check of the method and of the beam monitor accuracy

En

(%)/ cm of scintillator

• Agrees with previous measurements in the same energy range after rescaling for the thickness

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Calorimeter calibrationCalorimeter calibration• Cell response equalized: MIP peak at 550 ADC counts

• Trigger threshold calibration

ADC counts

ADC counts

Thr. [mV]

• Energy scale calibration with the MIP/MeV conversion factor from KLOE ( 1 MIP in one calorimeter cell 35 MeV eq. en.)(see KLOE Collaboration, NIM A354 (1995),352)

6 counts/mV

Eve

nts

Thr. [mV] 15 75Thr. [MeV] 1.5 23

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(%)

Comparison with our scintillatornormalized to the same active material thickness

)th.(calor.

)th.(scint.

ε(scint.)

ε(calor.)

Calorimeter efficiencyCalorimeter efficiency

• Stable for different run conditions

• Very high efficiency w.r.t. the naive expectation ( ~ 10% @ 2 MeV thr.)

• Epeak = 180 MeV

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Energy spectrum from TOFEnergy spectrum from TOF

n

• Rephasing is needed, since the trigger is phase locked with the RF (45 ns period)• From TOF spectrum of the neutrons• Assuming the neutron mass kinetic energy spectrum

• Energy spectrum can be reconstructed from TOF

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Efficiency vs energy Efficiency vs energy • Fast MC to test the sensitivity of the time distribution to the shape of the efficiency curve• Better agreement with an efficiency decreasing with energy• Some discrepancy in the low energy part of the spectrum

Tcell(ns)

MC● Data

Ekin(MeV)

ε(%

)

Tcell(ns)

MC● Data

Ekin(MeV)

ε(%

)

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(%)/ cm of scintillator(%) - scint.

• In June 2007 we took data at lower peak energies: 21.8 and 46.5 MeV

Large errors:

• big uncertainty in the beam halo evaluation

• worse accuracy of the beam monitors

Correction factor for beam halo 0.9 0.1

Low energy dataLow energy data

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)th.(calor.

)th.(scint.

ε(scint.)

ε(calor.)

The ratio is almost independent from the halo (Ratio normalized to the same active material thickness)

(%) - calorimeter

Calorimeter efficiencyCalorimeter efficiency

Very high efficiency at low threshold

Agreement with the high energy measurements

Correct. factor for beam halo 0.8 0.1

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MC simulationMC simulation

• A detailed simulation of the calorimeter

structure and of the beamline (source,

collimator and concrete shielding) has

been carried out with the FLUKA Code

Neutron fluence along the beamline (n/cm2/29000n)

collimator

concrete shielding

KLOECAL7Li target

GLUE FIBERS

base module

replicas

200 layers

LEAD

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Preliminary results of MCPreliminary results of MC

n

Some discrepancy in the lowenergy part of the spectrum

• No cut in released energy• No trigger simulation

Upper limit on

(%)

En (MeV)

• Data - Low thresh.

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ConclusionsConclusions

• The first measurement of the detection efficiency for neutrons of 20 - 180 MeV of a high sampling Pb-sci.fi. calorimeter has been performed at the The Svedberg Laboratory in Uppsala.

• Measurement of the n efficiency of a NE110 scintillator

agrees with published results in the same energy range.

• The calorimeter efficiency, integrated over the whole neutron energy spectrum, ranges between 30-50 % at the lowest trigger threshold.

• Study of the efficiency as a function of n energy is in progress.

• Full simulation with FLUKA is in progress.

• Further test foreseen for beginning 2008 at Louvain-la-Neuve

(En = 10 – 70 MeV; larger interbunch time)

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SparesSpares

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Time structureTime structure

4.2 ms

2.4 ms

40 ns41 ns

5 ns FWHM

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Preliminary results of MCPreliminary results of MC• Simulated neutron beam: Ekin = 180 MeV

• Each primary neutron has a high

probability to have elastic/inelastic

scattering in Pb

• In average 5.4 secondaries per

primary neutron are generated,counting only neutrons above

19.6 MeV.

target Pel(%)

Pinel(%)

Pb 32.6 31.4

fibers 10.4 7.0

glue 2.3 2.2 neutrons

above 19.6 MeV

62.2%

photons 26.9%

protons 6.8%

He-4 3.2%

deuteron 0.4%

triton 0.2%

He-3 0.2%

neutrons 94.2%

protons 4.7%

photons 1.1%

Secondaries created in interactions of low energy

neutrons (below 19.6 MeV) are - in average -

97.7 particles per primary neutron.

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Preliminary results of MCPreliminary results of MC

• The enhancement of the efficiency appears to be due to the large inelastic production of neutrons in Pb. These secondary neutrons:

- are produced isotropically; - are associated with a non negligible fraction of e.m. energy and of protons,

which can be detected in the nearby fibers; - have low energy and then have a large probability to do new interactions in the calorimeter with neutron/proton/γ production.

n

Z(cm)

p

n1

n2

n3

n4

X(c

m)

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Beam haloBeam halo

• TSL beam experts measured a sizeable beam halo at low peak energy (21.8 and 46.5 MeV):– TFBC scan of the area near the collimator

integrated flux over the ICM area 5% of the “core” flux

(with large uncertainty)

halo shape also measured

• Confirmed by our background counters

• Our calorimeter is larger than the projection of ICM area

• By integrating over the calorimeter we obtain an estimate of the halo contribution to the trigger rate of (20 10)%

• Only 10% on the reference scintillator due to the smaller area

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(%)

KLOE vs othersKLOE vs others

• Comparison with other

calorimeters:

– KLOE: 23 cm thick

– Crystall Ball: NaI 40.7 cm thick

(NIMA462(2001),463)

– GRAAL: BGO 24 cm thick

(NIMA562(2006),85)