Setup for hypernuclear gamma-ray spectroscopy at J-PARC

1. K.Shirotori Tohoku Univ. Japan for the Hyperball-J collaboration J-PARC E13 hypernuclear g-ray spectroscopy experiment Several light hypernuclear g-ray spectroscopy experiments are planned. (4LHe, 7LLi, 10LB, 11LB, 19LF ) Setup for hypernuclear gamma-ray spectroscopy at J-PARC Hyperball-J J-PARC g rays from hypernuclei : Particle-g coincidence Missing mass analysis : Magnetic spectrometer (identification of hypernuclear bound states ) Scattered p- Beam K- g g-ray measurement by Ge detector array Missing mass analysis g-ray measurement (p+, K+ g) @ KEK, (K-, p-g) @ BNL Magnetic spectrometers + Hyperball (K-, p- g) reaction @ J-PARC Optimized magnetic spectrometer + Hyperball-J Upgrade SksMinus Hyperball-J Wall configuration and simulated efficiency Modified SKS (Superconducting Kaon Spectrometer) (half picture) Neutron damage High counting rate (K-, p- ) reaction @ pK = 1.5 GeV/c 1 m SksMinus To analyze 1.4 GeV/c (1.1~1.8 GeV/c) scattered p- • Large acceptance ~100 msr, q=0-20 degree → Efficient tagging of hypernuclei and angular selectivity • Good momentum resolution 2-4 MeV/c (FWHM) → Identification of bound states of hypernuclei (mass resolution ~5 MeV (FWHM) ) 2.7T • Mechanical cooling by Pulse tube • PWO background suppression • Waveform readout for pulse shape analysis ~6% photo peak efficiency @ 1 MeV Previous condition Target ~20 g/cm2 2.2T New Ge detector Pulse Tube refrigerator SDC1~4 : Momentum analysis STOF : Time-of-flight SAC : p- identification SFV : K- beam veto • Large drift chambers • Optimized incident beam angle Mechanical cooler ⇒Suppression of the neutron damage effect with Ge crystal below 85 K (LN2 cooling ~90 K) Beam decay background Veto _ Cooler Ge SMF : m- from K- → m- + n SP0 : p- from K- → p- + p0 SKS for 1.05 GeV/c (p+, K+) reaction (pK = 0.72 GeV/c) 13 cm 10 cm • Cooling power Ge crystal temperature less than 85K when biased • Low mechanical vibration Minimization of microphonics noise • Compact size Realization of the planar arrangement of Ge detectors Simulated performanceAcceptance, Momentum resolution and Particle ID PMT Reaction ID by BAC and SAC (n=1.03) @ trigger • STOF : Time resolution ~150 ps (rms) PWO CsI ~2 keV (FWHM) energy resolution and cooling power of less than 85 K are achieved. Hyperball-J ∞-type element Rejection of K- beam through background SAC ~98% + SFV PWO counter and Waveform readout for high counting rates SAC BAC • Acceptance at 1.4 GeV/c, ~130 msr • Enough angular acceptance, ~20 degree • Momentum resolution at 1.4 GeV/c ~2.1 MeV/c (FWHM) To improve energy resolution and recover rejected events at high counting rates • Baseline shift restoration • Pileup signals separation The PWO crystal has very first decay constant ~6 ns. (~300 ns for BGO) But, small light yields (PWO/BGO = 1 /10) Beam veto (SFV) • Doped PWO crystal • Cooling of PWO below 0℃ Pulse height ADC ⇒ FADC (wave form digitization) (after shaping) Beam decay background veto counters Target 20cm PWO counter types BAC SAC Beam K- decay products make serious background ⇒ Same kinematical region to (K-, p-) reaction Ge energy spectrum 1 MeV g ray No baseline restoration m Beam K Baseline restoration Decay _ 60Co 1.3 MeV g-ray spectrum K- → m-n (63.4%) Muon Filter K- → p- p0 (21.1%) PiZero Veto -type U-type Overlap Muon Filter W-type L-type • In the test experiment, the positron beam is irradiated to Ge detector to make frequent baseline shits by reset signal. • ⇒The restoration of the baseline shits • 3.7 keV⇒ 3.1 keV (FWHM) • 2.6 keV w/o beam The performance of background suppression of the PWO counter by cooling below 0℃ is the same as of BGO counter. p- (absorbed in the iron) m- (passing through) K-→p-p-p+ (5.58%) K-→e-p0n (4.87%) K-→m-p0n (3.27%) K-→m-p0p0 (1.73%) _ Summary _ 86%of m-n events detected 14 % non-rejected (stopped in the iron) by offline analysis Totally > 99.9 % • Over kill for true π ~2.5% 80% of p-p0 events detected 10 sets of 3 mm lead plate and 8 mm scintillation counter layer at 1.5 GeV/c beam. • J-PARC E13 experiment by the (K-, p-) reaction @ pK =1.5 GeV/c • Optimal magnetic spectrometer • New Hyperball system for the high counting rate • Magnetic spectrometer, SksMinus and newly constructed array, Hyperball-J SksMinus performance : More than 100 msr acceptance, 20 degree coverage, ~2 MeV/c momentum resolution Hyperball-J : ~6 % efficiency, Mechanical cooling system, PWO counter, Waveform readout The experiment will be performed at 2009. Contribution of three-body decay is relatively small. Sufficient performance to reduce the trigger rate and background