Report (2) on JPARC/MLF-12B025 Gd(n,  ) experiment TIT, Jan.13, 2014 For MLF-12B025 Collaboration (Okayama and JAEA): Outline 1.Motivation and Thermal neutron reaction 2.MLF-12B025 Experiment 3.Data Spectrum Multiplicity M=2 events 4.Calibration 5.Summary 1 A03 公募研究 「ガドリニウムの熱中性子吸収反応でのガン マ線相関測定」

1.Motivation:A 200-ton Gd tank at Kamioka A new Gd-loaded water Cherenkov detector, sensitive to neutrons The detector is about to operate this month.  Mark’s talk. Purpose of research is to give the fundamental new data of Gd(n,  ) capture reaction, such as multiplicity, Energy spectrum and correlations. Number & Shape?

0) History of thermal neutron reaction 1934:Fermi et al., Neutrons can be thermalized through H(n,n) and slow neutrons are effective in disintegrating other nuclei. Theoretical development of slow neutron nuclear reactions by H.Bethe (1935)/Breit-Wigner (1936). 1938:Harn-Strassman, discovery of U fission by slow neutrons. 1942: Fermi, the liberation of the nuclear energy.  1938: E.Fermi, Nobel Prize in Physics for discovery of nuclear reactions brought about by slow neutrons.  1944:O.Harn, Nobel Prize in Chemistry for discovery of the fission of heavy nuclei. 3

Neutron-nucleus reaction at the thermal neutron energy with large cross sections Reaction Cross-section (barn) Resonance Energy E 0 (eV) n+p  d+  (2.2MeV) n+ 3 He  p+ 3 H+(0.8MeV)5333 n+ 6 Li   + 3 He+(4.8MeV) 940 n+ 10 B   + 7 Li+(2.3MeV) 3837 n+ 113 Cd  114 Cd*   meV n+ 149 Sm  150 Sm*   meV n+ 155 Gd  156 Gd*   meV n+ 157 Gd  158 Gd*   meV n+ 161 Eu  162 Eu*   meV Cf. n+ 235 U  A+B (Fission ） 583

Gd (Z=64) Gd is one of the few nuclei (Cd, Sm, Gd, Eu) which have the resonance in the thermal neutron energy. The largest neutron capture cross section(σ) among all stable nuclei. Applications Physics: Neutrino experiments (Double Chooz, Daya Bay, RENO) Industry: Control rod and burnable poisson at Nuclear Reactors. Medicine: Gadolinium contrast agents in MRI (large  ) 5

Neutron capture reaction Gd(n,  ) Element Cross section [barn] Gd Gd Proton Thermal energy 25meV E n =1 meV 1 eV 1 MeV

113 Cd, 149 Sm, 151 Eu, 157 Gd (n,  ) MeV

2. Experiment Gd(n,  ) (2012B025) Purpose of the experiment is to measure the multiplicity, the energy and correlations of the  -rays from Gd(n,  ) reaction at the thermal neutron resonance. Data were taken during March 15-17, 2013 (3 days). *Beam 1.3 × n/(s · m 2 ) in the neutron energy range of meV range *Trigger = >100keV in one cluster and no energy deposit in BGOs. *Event rate =16K Hz. TOF and energy of 14 Ge crystals are recorded for each event. Calibration data ( 60 Co, 137 Cs) for spectrum corrections.

JPARC-MLF BL04 (ANNRI) and Ge array 9

ANNRI Solid angle coverage=22%, Veto counter coverage=50% Ge cluster= 7 hexagonal Ge crystals BGO veto counters, surrounding Ge counters BGO This case =1 cluster, 3hits

3. Data TOF (21m) = Neutron energy Single energy spectrum Multiplicity M=1, 2 events 11

3. Time-of-Flight and Neutron Spectrum Neutron capture resonances are clearly seen (below 1200μsec). Background spectrum is normalized by the number of proton-pulses. Background in the thermal region is negligible ~1% (above 5000μsec). Data (Gd, 10μm thick) Background (Empty) TOF [μsec] Counts Data (Background subtracted) Breit-Wigner(Normalized to 1st. resonance) thermal region

Breit-Wigner Formula Calculation for Gd cross section (σ) Data (Flux×σ) 31.4meV 2,85eV Gd resonance parameters 10meV 100meV 1eV 10eV

Analysis Procedure We merge a group of neighbering hit crystals and calculate the energy for each group (=multiplicity M). E1E1 E2E2 E3E3 E1E1 E2E2 E3E3 E1E1 E2E2 M up =1,M down =1 (M total =2) events ☞角度相関 ~180° クラスター上 vs クラスター下 E 1 [MeV] E 2 [MeV] Sum 7.9MeV( 157 Gd) we see some strong correlations.

Multiplicity M=1-5 are seen. M=1: ~10 9 M=2: ~10 8 M=3: ~10 6 M=4: ~10 4 M=5: ~ Data (Black histogram) MC (color)

Total energy spectrum : Black(Gd) vs Background (Empty) Some gammas from Aluminum (beam pipe) 7724keV. Background is less than 1% for most of the energy region, Especially below 1 MeV.

M=2 analysis (example 1) Require one γ-ray energy to be 6750±50keV. The other γ-ray energy is plotted. A clear correlation between 6750keV and 1187keV is seen. 1187keV decays to the ground state, either directly (45%) or through 79-keV state (54%). *Only 1108-keV peak is seen since 79keV is below the detector threshold (100keV). Energy [keV] Yield [counts] 1187 keV 1108 keV Compton edge of 1187keV Compton edge of 1108keV 0 +, g.s 2 +, , 7937 Level of 158 Gd

M=2 analysis (example 2) Energy [keV] Yield [counts] 181 from from from from from from from 1187

4. Calibration data taken with 60 Co, 137 Cs sources We must reproduce this data spectrum with Monte Carlo. 19

5. Monte Carlo (geometry) and Gd(n,g) Reaction Models 20

Total energy spectrum ： Data(Black)Vs. Monte Carlo （ Red,Green ） -----Preliminary (Ongoing work) Data statistics = 10 9 events. Histograms are normalized by the total number of events.

Summary We took data (3.5x10 9 events) of Gd(n,  ) reaction. Multiplicity=1-5 hits are observed. – M=1 (~10 9 events), M=5 (a few hundred events). We implemented the detector geometry into the Monte Carlo program (Geant4). We now try to understand the energy response of Ge crystals using calibration data (Co-60, Cs-137) and MC. After we can reproduce the calibration data (Co-60, Cs- 137) using MC, we will compare data with MC using several Gd(n,  ) reaction models. 22

The  -ray energy spectrum Black (sum) vs Red (M=1) 23

1. n+A  n+A （ Elastic ） n+A   +B (  production) n+A  C+  (Capture) n+A  D+E (Fission) 基底 C( 複合核 ) E 結合 エネル ギー 数 MeV

Quadrupole Moment (Q)

JPARC-MLF Neutron Source Intensity: 1.3 × n/(s · m 2 ) in the neutron energy range of meV range (300kW operation). 27

Gd(n,  ) at RENO