A Neutron Counter for the CLAS12 Central Detector Detector Layout Geant4 simulations Background Studies Summary and work plan European collaboration: INFN Frascati, INFN Genova, IPN Orsay, LPSC Grenoble

Detector Layout Detector located between the central time of flight and the superconducting solenoid Available space: cylinder with R= 39 cm r = 27 cm l = 66 cm Strong constraints on readout system from limited space and high magnetic field  Silicon PMs

Plastic scintillator detector Plastic scintillator paddles with trapezoidal shape 4 layers in the radial direction with 30 paddles in j Use of central time of flight as additional layer Light readout on the two ends of each paddle to allow determination of z coordinate from time difference Neutron/photon separation through time of flight measurement y l R x r z 4 layers along r 30 paddles in j

Simulation with Geant 4 Full simulation of neutron detector with Geant 4 Material: standard plastic scintillator (BC408) density =1.032 g/cm3 ratio H:C atoms = 1.104 bulk att. length = 380 cm light output = 64% Anthracene Elastic scattering on proton is the dominant process for neutron detection Effects of light propagation, attenuation and detection included using effective parameters (l,veff, st, ...) Light quenching effect taken into account by reducing Edep for protons by a factor 5

Neutron efficiency Efficiency increases decreasing the threshold Generated neutrons with pn=0.1-1.0 GeV/c, q=90°, j=13.5° (center of 6th j slice) Efficiency: Nrec/Ngen Nrec= number of events in 6th j slice having Edep>Ethreshold q=90° q=65° Can we live with 15% efficiency? CLAS12 will have 10 times higher luminosity than CLAS (1035 cm-2 s-1) → roughly the same statistics as what was obtained for proton DVCS (assuming equal cross sections) Efficiency increases decreasing the threshold Eff ≈ 15% for thr. = 5 MeV and pn=500 MeV/c In agreement with « thumb rule »: 1% efficiency for 1 cm of scintillator

Particle Identification Neutron / gamma separation by measuring the particle time of flight Expected time resolution: s0~50 ps for MIPs for CTOF layer with pmt readout s0100 ps for MIPs for CND layers with SiPMs (to be measured, see Michel’s talk) Time resolution rescaled at actual deposited energy as s=s0√EMIPs(MeV)/√Edep(MeV) n/g separation possible up to p<1 GeV/c

Alternative Option: spaghetti calorimeter 1.2 mm 1.35 mm 1.0 mm Lead Lead-fibers calorimeter developed for the KLOE experiment Active material: 1.0 mm diameter scintillating fiber (Kuraray SCSF-81 and Pol.Hi.Tech 0046) High sampling structure: 99 layers of 0.5 mm grooved lead foils Lead:Fiber:Glue volume ratio = 42:48:10 M. Anelli et al., NIM A581 (2007) 368 Calorimeter Thickness ~18 cm Conceived as an electromagnetic calorimeter, it turned out to be very efficient for neutrons: 50% more efficient than equal volume of scintillator, measured with neutron beam (Uppsala) and reproduced by simulation (FLUKA)

First simulations of detector response Detailed simulation of the calorimeter structure with FLUKA Code Evaluation of energy deposited in each fiber Light propagation to the end of the fiber taking into account attenuation effects. Central_th=0 Central_th=10 MeV Simulation results for the neutron efficiency are consistent with the efficiency measured for the KLOE prototype on neutron beam

Neutron / Gamma separation Good timing resolution for photons: t=54 ps/E(GeV)147 ps measured at KLOE No measurement available for neutrons Hit multiplicity could be an alternative method for n/g identification Hit multiplicity for photons E= 500 MeV # fibers g n < 80 0.3% 78 % < 100 0.5 % 93 % Hit multiplicity for neutron E= 500 MeV

Pro & Con 9% increase of neutron detection efficiency at 500 MeV with Spaghetti Calorimeter based on existing measurement and confirmed by simulation What are the ToF performances of the spaghetti option? Can the multiplicity be an alternative method? Detection efficiency for neutron and gamma are comparable for plastic scintillator: 13% vs. 17% (th=2 MeV) High efficiency for gamma of spaghetti calorimeter (e=100%) may result in higher contamination of neutron sample

Background studies Precise understanding of the background sources and rates is needed to finalize the detector layout Electromagnetic background due to the interaction of the electron beam with the target may results in high rates and could affect the reconstruction of the neutron hit position and multiplicity Physics background (photons produced in hadronic reaction) may result in significant contamination Full simulations of neutron detector response in the actual configuration within the central detector are needed gemc is the best tool for this purpose

Electromagnetic background Electromagnetic background rates and spectra have been studied with geant3 by A. Vlassov Photon rate in the central detector of ~ 2 GHz at full luminosity integrating over all energies Rate of photons reaching the neutron detector is being estimated with gemc in actual configuration within the central detector L=1032cm-2 s-1 Photon Energy L=1032cm-2 s-1 Photon Angle

Physics background First estimate of hadronic background based on clasDIS event generator (pythia) Background events that could mimic a dvcs event defined as: one energetic photon (>1 GeV) in forward direction + one photon in the central detector) Estimated rate at full luminosity (1035 cm-2 s-1)  2 KHz All event rate Photon rate from ‘fake’ DVCS events

Full Simulations of Neutron Detector with gemc Neutron detector added to gemc thanks to Maurizio

Summary and Work Plan Design of neutron counter for the CLAS12 central detector is in progress Limited space and operation in high magnetic field result in strong constraints on the detector layout and readout Different detector configurations (plastic scintillator detector and spaghetti calorimeter) are being studied GEANT4 simulations of the detector response indicate that 15-20% neutron efficiency can be achieved To Do: complete evaluation of relevant parameters (efficiency, background rejection factors, angle and momentum resolution) for both detector options under study to choose best layout perform full simulations of neutron counter in realistic configuration complete study of background rates and compare with DVCS rate optimize reconstruction algorithm determine final efficiency and resolution for detected neutrons

Neutron interactions in the calorimeter Simulated neutron beam: Ekin = 180 MeV target Pel(%) Pinel(%) Pb 32.6 31.4 fibers 10.4 7.0 glue 2.3 2.2 Each primary neutron has a high probability to have elastic/inelastic scattering in Pb In average, secondaries generated in inelastic interactions are 5.4 per primary neutron, counting only neutrons above 19.6 MeV. neutrons above 19.MeV 62.2% photons 26.9% protons 6.8% He-4 3.2% deuteron 0.4% triton 0.2% He-3 Typical reactions on lead: n Pb  x n + y  + Pb n Pb  x n + y  + p + residual nucleus n Pb  x n + y  + 2p + residual nucleus neutrons 94.2% protons 4.7% photons 1.1% In addition, secondaries created in interactions of low energy neutrons (below 19.6 MeV) are - in average – 97.7 particles per primary neutron.

A typical inelastic process Z(cm) p n1 n2 n3 n4 X(cm) primary vertex En = 175.7 MeV En (p) = 126 MeV The enhancement of the efficiency appears to be due to the huge inelastic production of neutrons on the lead planes. These secondary neutrons: - are produced isotropically; - are produced with a non negligible fraction of e.m. energy and of protons, which can be detected in the nearby fibers; - have a lower energy and then a larger probability to do new interactions in the calorimeter with neutron/proton/γ production.

The measurement of the neutron efficiency @ TSL KLOE calorimeter module ( 2 cm) Neutron beam EKIN (MeV) A quasi-monoenergetic neutron beam is 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 neutrons in the peak) Tail down to termal neutrons

Experimental setup (3) (2) (1) ( 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 (1) (3) (2) ( 2 ) Beam position monitor array of 7 scintillating counters, 1 cm thick. ( 3 ) Reference counter NE110, 10×20 cm2, 5 cm thick A rotating frame allows for: - vertical positions (data taking with n beam) - horizontal positions (calibration with cosmic rays)