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Description
β detected NMR is a method to determine the magnetic moments of short-lived isotopes with a sensitivity inaccessible to conventional NMR. One of the isotopes scheduled to be investigated with β detected NMR at VITO is $^{11}\text{Be}$. It is of interest because it is a single neutron halo nucleus. Measuring the magnetic moment of $^{11}\text{Be}$ with greater accuracy will help to give insight into the nuclear magnetisation distribution of $^{11}\text{Be}$ and thus directly confirm its halo structure.
To enable such measurements, the VITO beamline has undergone multiple major upgrades and extensions in the past, including the installation of a superconducting solenoidal magnet with sub-ppm homogeneity and the ability to measure in liquid samples [1]. The β-detectors are a critical aspect; their purpose is to detect the asymmetrically emitted β-particles from the hyperpolarized decaying isotopes. The detector setup that is currently being developed will consist of two plastic scintillators with silicon photomultipliers. This setup will be able to measure the energies of the detected β particles. For $^{11}\text{Be}$ this is important because the two most intense transitions have opposite β asymmetry parameters and cancel each other out [2]. Measuring only the higher energy decay to the ground state will result in an increased measured β-decay asymmetry.
The trajectory of the β-particles in the magnetic field and their energy defines the minimum dimensions of the detector. Consequently, the two scintillators must be significant in volume. Collecting the scintillation light with good energy resolution poses challenges. Simulations of the light transport within the scintillator volume were performed. Numerous silicon photomultiplier arrangements and geometries were simulated in Geant4. The objective of this optimization is to find a configuration with sufficient energy resolution.
[1] Gins, W. & Harding, Robert & Baranowski, Mikołaj & Bissell, M.L. & Garcia Ruiz, Ronald & Kowalska, Magda & Neyens, G. & Pallada, S. & Severijns, Nathal & Velten, Ph & Wienholtz, Frank & Xu, Zhengyu & Yang, Xiaofei & Zákoucký, D.. (2019). A new beamline for laser spin-polarization at ISOLDE. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. 925. 10.1016/j.nima.2019.01.082.
[2] Levy, C. D. & Pearson, M. & Morris, Gerald & Chow, Kai Hang & Hossain, M. & Kiefl, Robert & Labbé, R. & Lassen, Jens & MacFarlane, W. & Parolin, T. & Saadaoui, Hassan & Smadella, M. & Song, Q. & Wang, Shuangshuang. (2010). Development of the collinear laser beam line at TRIUMF. Hyperfine Interactions. 196. 287-294. 10.1007/s10751-009-0148-9.
