Neutron detector for the central part of CLAS12 European collaboration: INFN Frascati, INFN Genova, IPN Orsay, LPSC Grenoble The Central Detector Physics case: nDVCS Requirements Technical constraints Possible solutions Work plan GDR Instrumentation, 8 Avril 2008 S. Niccolai, IPN Orsay

CLAS12: the current design Forward Electromagnetic Calorimeter Forward Time-of-Flight Low threshold Cerenkov Counter Drift Chambers High threshold Cerenkov Counter Preshower Calorimeter Central detector Full detector: Central (CD), 40o<q<135o Forward (FD), 5o<q<40o Inner Electromagnetic Calorimeter CD + exploded view of one sector of FD 8/4/08 - S. Niccolai – IPNO

and/or neutron detector Central Detector Main coil Central tracker CTOF (3 cm thick) Space for calorimeter and/or neutron detector (12 cm radial thickness) Cryostat vacuum jacket Magnetic field in the center = 5 T 8/4/08 - S. Niccolai – IPNO

Neutron detector Main goal: detection of recoil neutron in nDVCS H, H, E, E (x,ξ,t) ~ x-ξ t (Q2) e e’ gL* x+ξ n n’ g Detected in forward CLAS Not detected ed→e’ng(p) Detected in FEC, IC nDVCS: strongest sensitivity to E PID (n or g?) via TOF + angles to identify the final state Ji’s sum rule → access to quark orbital momentum 2Jq =  x(H+E)(x,ξ,0)dx 8/4/08 - S. Niccolai – IPNO

Neutron kinematics in nDVCS Neutron momentum (GeV/c) Neutron q (°) Average neutron momentum 0.3-0.5 GeV/c GPD-based event generator (free neutron) Ee = 11 GeV CD Most of the neutrons at q~60° Thanks to Marion Mac Cormick 8/4/08 - S. Niccolai – IPNO

GEANT4 simulation estimate and maximize detection efficiency for neutrons resolution on TOF to separate neutrons and g or find an alternative way to separate neutrons and g x z y l R r Realistic geometry, following design for CTOF: solid composed by trapezoids Segmentation: Radial, to determine interaction point TOF  p (5 layers in r, 3 cm thick) Azimuthal, to determine f (30 slices) q can be obtained via z=1/2veff(t1-t2) Dimensions R= 39 cm r = 24 cm l = 50 cm 8/4/08 - S. Niccolai – IPNO

GEANT4 simulation estimate and maximize detection efficiency for neutrons resolution on TOF to separate neutrons and g or find an alternative way to separate neutrons and g Realistic geometry, following design for CTOF: solid composed by trapezoids Segmentation: Radial, to determine interaction point TOF  p (5 layers in r) Azimuthal, to determine f (30 slices) q can be obtained via z=1/2veff(t1-t2) Possible to add layers of another material 8/4/08 - S. Niccolai – IPNO

Neutron efficiency Detector material: scintillator Generated neutrons with pn=0.1-1.0 GeV/c, q=90°, f=13.5° (center of 6th f slice) Efficiency: Nrec/Ngen Nrec= number of events in 6th f slice having Edep>Ethreshold (r: first good hit only) 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) Light quenching effect taken into account by reducing Edep for protons by a factor 5 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

TOF resolution Kyoungpook’s measurement Simulation (Raffaella De Vita For each f, r slice: TOF = (t1+t2)/2 t1 = tofGEANT+ tsmear+ (l/2-z)/veff t2 = tofGEANT+ tsmear+ (l/2+z)/veff veff=16 cm/ns (value used in GSIM) tofGEANT= average of times of all «steps» z = average of z positions from all steps tsmear = smearing factor: Gaussian centered at 0 with s= t0/√Edep (MeV), t0 = 92 ps (deduced from Kyoungpook’s measurement at 6 MeV) for 1st r layer, for other layers t0 = 200 ps Simulate time distribution of the scintillator light Introduce spread due to light transmission in the bar Account for transmission from the scintillator to the pmt photocathode (different size, lightguide...) Account for conversion to photoelectrons (q.e. =20%) Include additional time spread due to PMT transit time and amplification Kyoungpook’s measurement Simulation (Raffaella De Vita & Marco Mirazita)

TOF resolution: results s = 90 ps s = 122 ps s = 138 ps s = 175 ps Neutrons, pn= 1 GeV/c s = 164 ps Threshold = 2 MeV Resolution is worse with smaller threshold

TOF resolution: results s = 54 ps s = 82 ps s = 82 ps s = 76 ps Photons, pg=1 GeV/c s = 69 ps n/g separation possible up to pn<1 GeV/c At 3s overlap between n and g

The spaghetti option: KLOE Active material: 1.0 mm diameter scintillating fiber Core: polystyrene, r=1.050 g/cm3, n=1.6 High sampling structure: 200 layers of 0.5 mm grooved lead foils (95% Pb and 5% Bi). Lead:Fiber:Glue volume ratio = 42:48:10 Conceived as an electromagnetic calorimeter, it turned out to be very efficient for neutrons Could this solution be viable for us? Studies underway… 50% more efficient than a scintillator, measured with neutron beam (Uppsala) and reproduced by simulation (FLUKA) See Anna Ferrari’s talk for details on KLOE spaghetti calorimeter 8/4/08 - S. Niccolai – IPNO

Comparison of efficiencies (GEANT4) Simulation underestimates KLOE’s data by 5-8%: maybe light quenching is overestimated 8/4/08 - S. Niccolai – IPNO

Neutron/photon separation with spaghetti calorimeter Photon, no lead Photon, scintillator + lead Studied the number of hits per event with Edep>threshold, for neutrons and photons 8/4/08 - S. Niccolai – IPNO

Neutron/photon separation with spaghetti calorimeter Threshold = 2 MeV Neutrons Photons But we still need good time resolution to determine q angle of the neutron (required angular resolution needs to be estimated) Can it be achieved with one fiber? Very different multiplicities for n and g!!! “Lasagna”: 75 scintillator layers (1.3 mm each) + 74 lead layers (0.7 mm each) BUT Eff(g) = 99% Eff(n) = 20% →We need to estimate COUNT RATES for n and g 8/4/08 - S. Niccolai – IPNO

Neutron detector: requirements Whichever solution will be chosen for the neutron detector (layers of scintillators or spaghetti), there are the following issues: limited space upstream and downstream, due to the presence of the light guides for CTOF → no space for additional light guides to “escape” from the high magnetic field light collection in the high magnetic field BUT, compared to CTOF, the requirement on TOF resolution is less stringent: from preliminary simulations, a time resolution twice as bad as the one currently achieved in KNU and Jlab measurements can still be good enough to separate photons from neutrons for neutron momentums up to 1 GeV Can SiPM be the solution? We need photodetectors insensitive to magnetic field, providing decent timing resolution

SiPM: characteristics SiPM (proposed by Sadygov and Golovin in the ’90) matrix of tiny microcells in parallel / each micro-cell = Geiger Mode-APD + Rquench output signal is proportional to the number of triggered microcells Al ARC -Vbias Back contact p n+  Rquenching h p+ silicon wafer Front contact Solution considered at Orsay/LAL is a matrix of SiPMs: larger area covered reduce noise by requiring time coincidence of several SiPMs within the matrix General view Photo-sensitive side 25 µm PROS: Insensitive to magnetic field High gain (106) Good intrinsic timing resolution (30 ps/pixel) Good single photoelectron resolution CONS: Very small active surface (1mm2) → small amount of light collected Noise Thanks to Nicoleta Dinu (LAL) for the images and the infos 8/4/08 - S. Niccolai – IPNO

Tests on photodetectors with cosmic rays at Orsay “Trigger” PMTs (Photonis XP2020) “Trigger” scintillators (BC408) 1cm thick Scintillator bar (NE102) 80cm (length) x 4 cm (width) x 3 cm (thickness) Drawing by A. Maroni 8/4/08 - S. Niccolai – IPNO

The current setup Photonis XP20D0 8/4/08 - S. Niccolai – IPNO

Electronics & DAQ are in place The data taking is underway Plan: Measuring TOF resolution with reference PMTs Substituting PMT at one end with SiPMs (matrix of SiPMs will be constructed, collaboration with neighbors at LAL for electronics) …and let’s see! 8/4/08 - S. Niccolai – IPNO

Conclusions and to-do list CTOF and Neutron detector could coexist in one detector, whose first layer can be used as TOF for charged particles when there’s a track in the central tracker, while the full system can be used as neutron detector when there are no tracks in the tracker. Neutron detection is necessary for the measurement of nDVCS (Ji’s sum rule) Detection of DVCS recoil neutrons with ~15% of efficiency and n/g separation for p≤1 GeV/c seems possible from simulations, using scintillator as detector material A spaghetti calorimeter could give higher efficiency while permitting to use hits multiplicity as PID method, but it increases photon efficiency a lot…and angular resolution? To do list: use complete CLAS12 simulation and realistic event generators for signal (nDVCS) and backgrounds (ed→enp0(p)) to define needed resolutions (q,f) study the time resolution for the spaghetti option evaluate count rate for signal (n) and background (g), to understand if high photon detection efficiency is a problem or not Hardware: tests on timing with SiPM planned for the spring at Orsay, tests with ordinary PMs already underway 8/4/08 - S. Niccolai – IPNO