Speaker
Description
The recent measurement of the Neutron Star Merger event by LIGO [1] and subsequent optical measurements have revealed that Neutron star mergers are probably one of the primary sites for the r-process of nucleosynthesis [2]. An important source of uncertainty in the r-process models is the nuclear data input [3], especially important is the neutron capture cross-section which is directly observable for only a handful of nuclei close to stability.
The Oslo Method provides an alternative, indirect route to constrain the neutron capture cross-sections by providing the nuclear level density (NLD) and $\gamma$-ray strength function ($\gamma$SF) which are important in Hauser-Feshbach calculations. The method requires experiments where the $\gamma$-ray distribution is measured as a function of excitation energy. This has been achieved for several years with transfer reactions with light ion beams, eg. $\mathrm{p}$,$\mathrm{d}$,$^{3}\mathrm{He}$,$\mathrm{\alpha}$, at the Oslo Cyclotron Laboratory and more recently in $\beta$-decay experiments [4]. A new class of experiments have recently joined the 'Oslo Method family', namely the inverse kinematics experiment. The NLD and $\gamma$SF of $^{87}\mathrm{Kr}$ was successfully extracted from a experiment with a $^{86}\mathrm{Kr}$ beam hitting a deuterated polyethylene target at iThemba LABS in early 2015. With the addition of inverse kinematics we are now able to probe the NLD and $\gamma$SF of virtually any nuclei that can be accelerated in the lab. The $\gamma$SF and NLD of $^{87}\mathrm{Kr}$ will be presented together with Hauser-Feshbach calculations of the neutron capture cross-section of $^{86}\mathrm{Kr}$.
New developments are also happening at the Oslo Cyclotron Laboratory with the implementation of the Oslo SCintillator ARray (OSCAR), consisting of 30 large volume LaBr$_{3}$(Ce) detectors (3.5x8"). OSCAR is a substantial upgrade from the old CACTUS array (NaI-detectors) both in terms of improved energy and time resolution and efficiency. To compliment the new array, we have replaced the old analog data acquisition system to a new state-of-the-art digital system consisting of seven Pixie-16 digitizers (2 x 500 MHz, 5 x 250 MHz), allowing for much higher throughput. The new array will allow for new innovations in experimental techniques that could allow for extraction of NLD & $\gamma$SF above the neutron separation energy and discrimination between neutron and $\gamma$-rays.
[1]: B. P. Abbott et al., Phys. Rev. Lett. 119, 161101 (2017)
[2]: E. Pian et al., Nature 551, 67-70 (2017)
[3]: M. R. Mumpower et al., Prog. Part. Nucl. Phys. 86, 86 (2016)
[4]: A. Spyrou et al., Phys. Rev. Lett. 113, 232502 (2014)
