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Neutron damage study

Neutron damage study. Hlab meeting 11/21 K.Shirotori. Introduction. Conduction band. Typical energy spacing of hypernuclei is a few 10 keV . ⇒ Only Ge detector can measure these spacing : Energy resolution ～ 2keV

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Neutron damage study

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  1. Neutron damage study Hlab meeting 11/21 K.Shirotori

  2. Introduction Conduction band • Typical energy spacing of hypernuclei is a few 10 keV. ⇒ Only Ge detector can measure these spacing : Energy resolution ～ 2keV • Energy of γ rays are transformed to a lot of electron-hole pairs by EM interaction. ⇔It must be cooled because of the thermal excitation. • Neutron damage in Ge crystal traps the electron and decrease the number. ⇒Energy resolution gets worse. Small energy gap 3eV ⇒Electron easily move to conduction band and e-hole pair is produced. e- Valence band γ × × n p e- e+ (hole) × × Bias V 1 pair/3eV w/o loss

  3. Effects of neutron damage Before After

  4. Neutron flux in experiments

  5. Purpose • FIFC report for DAY-1 experiment at J-PARC • To estimate the number (flux n/cm2) of neutron hitting Ge detectors by Geant4 simulation and experimental data (E566) • To check the degree of damage • Visible damage: ～109 /cm2 • Critical damage: ～1010 /cm2

  6. GEANT setup • Physics List • Electro-Magnetic • γ→Compton, Photo Electric Effect, e+e- production • e→Multiple Scattering, Ionization, Brems, Annihilation(e+) • μ→Multiple Scattering, Brems, Pair Production, Ionization • その他→Ionization, Multiple Scattering • Decay • Hadron • Low energy elastic • Low & high energy inelastic scattering (<20 GeV)

  7. Event of Simulation Downstream degrader Hyperball Pb collimeter Adjustable degrader upstream degrader K- B1 FV LC B2 B3

  8. Neutron production position E509 Deg, B3, 10B • Geをヒットしたneutronの生成された場所を示す。 • E509ではメインはhyperballが取り囲んでいるdegrader, B3, 10Bターゲットだが、ビームライン上のカウンターやdegraderからも来ている。 • For E509, 660MeV  was generated • For E556, 1050MeV + was generated LC Deg Ge target 20cm CH2 E566

  9. Neutron Number • 100000 (E509) and (E566) were generated • E509  1311 neutrons for one Ge • 0.01311 neutron per beam • E566  672 neutrons for one Ge • 0.00672 neutron per beam ⇒8.74×109 /Ge @ 1.3×1012 π+ beam

  10. Estimation from p cross section • Total cross section p at p=1GeV/c • =~25mb, (elastic ~10mb) • 20cm CH2 d=0.93(g/cm3) • N(H) = 0.93*20*2/14*6*1023=15.9*1023個/cm2 • N(C) = 0.93*20/14*6*1023=7.97*1023個/cm2 • For p • Nscat/Nin=0.04 • For C (assume (C) = (p)*A2/3) • Nscat/Nin=0.10 • Target is so thick, there are many reaction events.

  11. Number of neutron from E566 data • Run140～543 @ Beam trigger • All sum of single Ge ADC spectrum • W/o Timing gate • Ge live time 0.55 • DAQ live time 0.84

  12. ADC spectrum w/o TDC timing gate (Single ADC sum) Number of peak (2.31±0.22)×103 →193±18 /Ge (12 single Ge worked)

  13. 300×(693 keVピーク数) (中性子数/cm2 )= (Ge検出器の体積 [cm3]) Estimation of neutron flux • 公式を使用 (Geの体積) = π×3.52×7=269.4 cm3 (Geの表面積) = π×3.52 = 38.48 cm2 (単純に正面から中性子が入ったと仮定) (中性子数) = (8.27±0.77)×103個/Ge

  14. Neutron 数 : Run140～534 • (中性子数/Ge) = (8.27±0.77)×103 • π+ビーム数 : 1.24×1012 • Prescale factor 5.0×105 • (中性子数/Ge) = (4.14±0.39)×109 • (中性子数/π+/Ge) = (3.34±0.31)×10-3 ⇒(w/ Dead time) = (7.59±0.70)×10-3 Ge 0.55, DAQ 0.84 総π+ビーム 1.3×1012 ⇒(9.87±0.91)×109 /Ge

  15. Triggerによる比較 • 単に(π+, K+)をビームと同数だとみなして計算 • Beam triggerと(π+, K+) trigger Beam : (9.87±0.91)×109 /Ge (中性子/π+) = (7.59±0.70)×10-3 (π+, K+) : (5.41±0.09)×1010/Ge (中性子/(π+, K+)) = (4.16±0.09)×10-2

  16. Summary • Beam triggerで見積もった中性子数はSimulationと合う ⇒8.71×109 /Ge(Sim:0.00672⇔Exp:0.00759 /π+) Beam • (9.87±0.91)×109 /Ge • Flux (2.55±0.24)×108 /cm2/Ge (π+, K+)のみ • (5.41±0.09)×1010 /Ge • Flux (1.41±0.02)×109 /cm2/Ge • 文献によると • 測定可能な損傷 : ～109 /cm2 • 深刻な損傷 : ～1010 /cm2 ⇒損傷の現れ具合とほぼ一致する

  17. Temperature dependency of energy resolution

  18. Result (3rd measurement)

  19. Neutron damaged Ge det.FWHM/FWTM vs Temperature

  20. During the irradiation (experiments) line shapes/neutron tail depends sensitively on Ge crystal temperature and also on counting rate. Thus Ge detectors must be kept cold below 85K during the course of an experiment for ~108 n/cm2neutron flux. Once Ge crystal temperature raises, the effect is irreversible unless thermal cycled and annealed. Annealed detector’s resolution/residual tail is NOT so sensitive to Ge crystal temperature. Worsening of Ge resolution with increasing Ge crystal temperature is due to the increased thermal leakage current. Response to FIFC

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