HALIMA integrates high resolution gamma spectroscopy with fast timing in one setup. Eight HPGe detectors are mounted on a central ring in a horizontal configuration to capture gamma ray energies, while three surrounding rings of LaBr3(Ce) detectors provide timing information. The digital data acquisition system and high voltage supply are positioned behind the array to process detector signals.
To reduce Compton continua in the LaBr3(Ce) spectra, the team built a compact CsI(Tl) based anti Compton shield for each scintillation detector. Each LaBr3(Ce) crystal is surrounded by four separate CsI(Tl) crystals read out with silicon photomultipliers, forming an active veto against scattered events. This shield increases the peak to total ratio by a factor of about 1.5, improving spectral quality for lifetime analysis.
A solar cell array serves as a detector for fission fragments to enhance selectivity for neutron rich nuclei. After fission, the recoiling fragment travels about 8 cm before reaching the solar cells, which act as implantation detectors and record the fragments. This configuration allows event by event correlation between specific fission fragments and their associated gamma rays, effectively selecting isomer specific events and strongly suppressing background.
The HALIMA system supports coincidence schemes that connect fission fragment or beta particle signals with gamma rays in both HPGe and LaBr3(Ce) detectors. Using the FF/beta Ge LaBr3(Ce) LaBr3(Ce) coincidence method, the researchers can measure lifetimes from a few picoseconds to several hundred nanoseconds, covering many isomeric transitions in neutron rich nuclei.
In a commissioning run with a 252Cf source, the group measured the lifetimes of excited states in 134Te, 138Ba and 132Te. The results matched literature values, confirming the performance of the HALIMA setup and the analysis procedures for lifetime extraction. The same experiment also identified excited states populated in 252Cf fission whose lifetimes had not been reported previously, extending nuclear structure data for these isotopes.
The project received support from the National Natural Science Foundation of China, the Natural Science Foundation of Guangdong Province, an International Atomic Energy Agency coordinated research project, and several regional and national programs in China. The authors report that the data and methodology are openly accessible to promote wider collaboration in neutron rich nuclear structure research.
Research Report: HALIMA: a hybrid array for lifetime measurement of neutron-rich nuclei at IMP
Related Links
Shanghai Institute of Applied Physics
Understanding Time and Space
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