BRIKEN

The BRIKEN (Beta-delayed Neutron Measurements at RIKEN) international collaboration is providing a new window into our stardust origins by making the first-ever decay measurements of many of the rare neutron-rich isotopes central to heavy element formation in stars. 

BRIKEN, a TRIUMF collaboration, is based at the RIKEN Nishina Center for Accelerator-Based Science in Wako, Japan.  

The collaboration has created the world’s most efficient neutron detector for astro- and nuclear physics by pooling the international participants’ neutron detector resources and using the world’s most neutron-rich beams from RIKEN’s Radioactive Isotope Beam Factory. 

Neutron-rich isotopes usually undergo beta-delayed neutron decay; the nucleus first emits an antineutrino and a beta particle (a high-energy electron), followed by the emission of a neutron. Commissioned in 2016, BRIKEN measures two key characteristics of these decays: the half-life, or how long it takes for half the nuclei to decay; and the neutron emission probability, the ratio of how often a neutron is actually emitted in each 100 decays. For example, of 100 nuclei of iodine-137 (¹³⁷I), on average 7.65 decay by the emission of a beta-delayed neutron and 92.35 decay just by the emission of a beta particle. 

This data is a critical input parameter for astrophysical models of the rapid neutron capture process, or r-process, in supernovae and binary neutron star mergers, events which produce about half of the elements heavier than iron. Of the 3000 neutron-rich nuclei predicted to exist only about 300 have been experimentally measured. 

In 2017, BRIKEN’s first run measured 268 neutron-rich nuclides, including measuring the neutron emission probability of 180 of them for the first time, and for about 60 of them the half-life was measured for the first time. In total, the collaboration aims to measure about 400 exotic nuclei.  

The BRIKEN collaboration involves about 70 scientists and engineers from Japan, Canada, Spain, the United States, the United Kingdom, Hungary, Poland, Hong Kong, South Korea, and Vietnam.

Located three floors underground at the RIKEN Nishina Center, BRIKEN is a van-sized experimental set-up. The BRIKEN array consists of three independently operating detector components.  

First, exotic, neutron-rich nuclei are produced at RIKEN’s Radioactive Isotope Beam Factory. Beams of uranium-238 (²³⁸U) are accelerated to high energies by several ring cyclotrons and fired at a beryllium target, resulting in a diverse mix of nuclei, all lighter than uranium.  

This mix of nuclei is sent through the 60-meter-long BigRIPS fragment separator where a combination of magnets and slits separates-out a cocktail of nuclei within a mass range requested by researchers and sends them to BRIKEN.  

In BRIKEN’s core, the isotope cocktail beam is stopped and embedded in the Advanced Implantation Detector Array (AIDA), a stack of six, double-sided silicon-strip detectors. The implanted neutron-rich isotopes decay emitting a beta particle. AIDA records the time between implantation and beta particle detection, enabling BRIKEN researchers to determine an isotope’s half-life. 

To measure the number of neutrons emitted in these decays, the implantation detector is surrounded by BRIKEN’s neutron detector. The one-meter square neutron detector consists of up to 160 tubes filled with helium-3 (³He) gas, the tubes embedded in a large block of high-density polyethylene, a plastic. The hydrogen in the plastic is a neutron moderator: the beta-delayed neutron decays produce neutrons too high in energy to interact with the ³He; the hydrogen in the plastic slows the neutrons to an energy at which they interact with the ³He more efficiently, producing an electrical signal. The timing of the neutron detections is correlated with the beta decays to determine the isotope’s half-life and the probability of neutron emission. 

BRIKEN can be used with the addition of two high-purity germanium detectors on either side to measure the gamma-radiation (high-energy photons) also emitted during the decay. These photons provide information on the neutrons’ energy, and thus an isotope’s nuclear structure. The germanium detectors operate similarly to those used in TRIUMF’s GRIFFIN and TIGRESS experiments. 

TRIUMF scientist Iris Dillmann and her group are core collaborators in the BRIKEN project. The group proposed two of the presently 6 experimental proposals. While a fraction of the pre-existing neutron detector electronics came from a grant that Iris held while she was leading a Helmholtz Young Investigators Group in Germany, her present Canadian NSERC grant supported important upgrades of this electronics for the operation in the BRIKEN array. Dillman’s TRIUMF group has spent several months in Japan during the setup, commissioning, and following the experiments.