Subatomic Physics – Fast Timing Array

Facility for low spin structure studies of exotic states and nuclei

The facility comprises of eight 2”x”2 LaBr3(Ce) detectors, which are  scintillation detectors bearing good efficiency for photon detection (~3%) and excellent timing resolution (~<400ps).  In addition high speed digitisation (500MHz) allows for the detectors to be utilised and integrated into existing detector systems.

The project started in 2017 with commissioning tests and measurements utilising short-lived radioactive sources produced at iThemba LABS.  An array of the eight detectors confirmed previously reported measurements from literature to provide confidence in the measurement system.  Almost immediately new techniques utilising the extremely fast timing resolution were developed, resulting in new ways to utilise these type of detectors.

Both 123Te and 67Zn are known to have several states with half-lives ranging few hundred ps to a few µs. Many of these half-lives have only been measured with indirect techniques and could suffer from large systematical errors. Many of these states are populated by decay of 123I and 67Ga which was provided by the Isotope Production facility at iThemba LABS. The measurements were done using the setup described in subsection 4.1 and γγ events from cascades where the state of interest is populated and subsequently de-excited was identified. A time spectra is produced by using the time of the populating γ-ray as the start and the time of de-exciting γ-ray as the stop. The data sets are currently being analyzed and preliminary results for the half-life of the 3/2− state in 67Zn has been found extracting the decay half-life to be 1.090(71) ns.

Outside of the laboratory, applications to environmental measurements have been proposed for the array. Possible applications of LaBr3(Ce) for low activity measurements such as infield environment studies have been investigated. These investigations used IAEA reference samples of KCl, uranium ore and thorium ore. Data were collected over a predefined period followed by a background run. The final spectra of each reference samples is then obtained by subtracting the spectra obtained during the background run from the source run. Preliminary results show that this technique can reliably be used.

The detector array has subsequently been coupled to the AFRODITE spectrometer, complementing the high energy resolution Clover detectors with the fast-timing capabilities of the LaBr3(Ce) detectors.  Several experiments have been undertaken and new results expected.