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Development and Application of an eV Neutron Polarization for Parity Violation Studies at CSNS Back-n Beamline

Xu Qin, Tianhao Wang, Xuanbo Chen, Changdong Deng, Yongce Gong, Zenghang Huang, Wei Jiang, Zhengquan Liu, Guangyuan Luan, Haotian Luo, Qiuyue Luo, Yongjia Lv, You Lv, Nikolaos Vassilopoulos, Xichao Ruan, William Michael Snow, Kang Sun, Sepehr Samiei, Jian Tang, Shilin Wang, Hongyi Wu, Xiaomin Xiong, Xinyu Yuan, Junpei Zhang, Mofan Zhang, Qiwei Zhang, Qingbo Zheng, Ruirui Fan, Xin Tong

Abstract

The dynamic enhancement of symmetry-breaking effects in neutron-nucleus resonances provides a sensitive testing ground for Time-Reversal Invariance Violation (TRIV). Exploiting this mechanism, the Neutron Optics Parity and Time Reversal Experiment (NOPTREX) seeks to elucidate the origin of the universe's baryon asymmetry. Critical to this effort is the precise measurement of Parity Violation (PV) asymmetries, which is essential to calibrate the nuclear parameters required for future TRIV experiments. To facilitate these studies, we developed an eV polarized neutron at the Back-n white neutron beamline of the China Spallation Neutron Source (CSNS). Neutron polarization is generated by an in-situ Spin-Exchange Optical Pumping (SEOP) $^3$He filter. Spin manipulation is performed by an adiabatic spin flipper, while spin polarization is preserved over the flight path by a vacuum transport system equipped with a solenoidal guide field. Experiments successfully measured an asymmetry of approximately $7.8 \pm 2.4$ (stat.) $\pm 0.3$ (sys.) % at the 0.747 eV p-wave resonance of $^{139}$La. These results are in agreement with previous results on this resonance and validate the system's capability for PV measurements.

Development and Application of an eV Neutron Polarization for Parity Violation Studies at CSNS Back-n Beamline

Abstract

The dynamic enhancement of symmetry-breaking effects in neutron-nucleus resonances provides a sensitive testing ground for Time-Reversal Invariance Violation (TRIV). Exploiting this mechanism, the Neutron Optics Parity and Time Reversal Experiment (NOPTREX) seeks to elucidate the origin of the universe's baryon asymmetry. Critical to this effort is the precise measurement of Parity Violation (PV) asymmetries, which is essential to calibrate the nuclear parameters required for future TRIV experiments. To facilitate these studies, we developed an eV polarized neutron at the Back-n white neutron beamline of the China Spallation Neutron Source (CSNS). Neutron polarization is generated by an in-situ Spin-Exchange Optical Pumping (SEOP) He filter. Spin manipulation is performed by an adiabatic spin flipper, while spin polarization is preserved over the flight path by a vacuum transport system equipped with a solenoidal guide field. Experiments successfully measured an asymmetry of approximately (stat.) (sys.) % at the 0.747 eV p-wave resonance of La. These results are in agreement with previous results on this resonance and validate the system's capability for PV measurements.
Paper Structure (15 sections, 13 equations, 12 figures)

This paper contains 15 sections, 13 equations, 12 figures.

Table of Contents

  1. Introduction
  2. Back-n white neutron source
  3. eV neutron polarization
  4. In-situ ³He system
  5. Adiabatic spin flipper
  6. Physical field design
  7. Neutron spin behavior in magnetic Fields
  8. Simulation of flipper efficiency
  9. Vacuum polarization transport system
  10. Detector Polarization Preservation
  11. Experimental Results
  12. Measurement of the asymmetry of ^139La
  13. Measurement about the spin flipper efficiency
  14. Verification of Polarization Transmission and Its Impact on Measurements
  15. Summary and outlook

Figures (12)

  • Figure 1: Layout of the Back-n beam line at CSNS.
  • Figure 2: Schematic of the high-energy polarized neutron system, showing the key components such as the in-situ^3He system, spin flipper, and polarization transport system. These components work in tandem to ensure efficient polarization of neutrons for high-precision parity violation measurements.
  • Figure 3: Fitting result for the $^{3}\mathrm{He}$ polarization measured on Back-n
  • Figure 4: (a) Mechanical assembly of the part polarization, comprising the in-situ^3He system, the spin flipper, and the vacuum polarization transport system. The entrance and exit of the flipper are defined at 80cm from its geometric center. The labels 1 and 2 denote the geometric center ($z=0cm$) and the exit plane ($z=80cm$) of the adiabatic spin flipper, respectively. The axial separation between the ^3He system and the flipper entrance is 18.63cm, while the distance from the flipper exit to the transport system is 20.7cm. (b) Spatial distribution of the magnetic field vectors, illustrating the field topology in both the flipper-on and flipper-off states.
  • Figure 5: (a) and (b) show the magnetic field distribution on the cross-section of the mechanical structure depicted in Fig. \ref{['fig:Mechanical assembly']}(a), corresponding to the spin flipper being in the flipper-on and flipper-off states, respectively.
  • ...and 7 more figures