Gamma-ray polarization as a tool for nuclear structure experiments
Every individual radioactive nucleus has a unique "fingerprint" of gamma rays that it emits when it decays. The energies of these gamma rays tell us the distinct energy levels of the nucleus it decays into, and the angle in between two successive gamma rays gives us information about the angular momenta of the individual states. Additionally, by looking at the direction a gamma ray scatters (and thus its polarization), we can gain information on the parities of the states involved. An undergraduate at TRIUMF recently worked out a way to measure the polarization of gamma rays with GRIFFIN, a high-efficiency, high-resolution gamma-ray detector at TRIUMF. Future development in this project includes: writing a C++ class to easily handle the creation of plots necessary to measure polarization, writing a Java-based web page that will allow experimentalists to calculate the expected polarization signal, running simulations to grasp how many statistics are needed, and adding a feature to a web tool to let experimentalists easily calculate the needed statistics.
Gamma-ray angular correlations as a tool for nuclear structure experiments
The angles between two successive gamma rays emitted from a decaying radioactive nucleus can give us information about the angular momenta of the states involved. This technique has had significant development work done at GRIFFIN, including development of techniques for creating and normalizing the plots, creation of a C++ class to standardize their creation, a web tool for calculating expected angular distributions, the development and testing of a simulation package for these decays (McLean, MSc thesis), and development of two quicker methods for extracting the angular correlation coefficients (Ashfield, Reed thesis, 2017). Future work includes: expanding the web tool to include Monte Carlo simulation so experimentalists can quickly estimate the necessary statistics needed for a measurement and testing the energy and detector configuration dependencies of the reduced simulation methods.
Living up to the hype: energy resolution at GRIFFIN
The individual GRIFFIN crystals have a FWHM resolution of less than 2 keV at 1332 keV full energy. However, to utilize the full, 64 crystal power of GRIFFIN, we need to sum all of the signals together. Non-linearities, temperature drifts, time drifts, and cross-talk effects can all negatively impact the summed spectrum. These effects have all been investigated and characterized. The temperature dependence has had a fix applied for future data, but not verified. A correction for cross-talk has been implemented in the data analysis software. Future work is needed to verify the temperature dependence correction, further characterize the non-linearities, develop a coping mechanism for working for the non-linearities, and develop a technique to fix any time drifts.
Summing corrections for GRIFFIN
GRIFFIN measures the energy that a gamma ray deposits in a single crystal. During analysis, we take that energy information and create a level scheme for the nucleus of interest. We also measure the relative intensities of the gamma rays involved. However, if two gamma rays interact in the same crystal at the same time, GRIFFIN reports the sum of their energies. Unless this summing can be disentangled, it will impact the branching ratios measurements. Calculating the impact of this summing for a general decay scheme is non-trivial, but the math is relatively well researched. Future work includes developing a user-friendly calculation method and tool for this correction, investigating the implications of summing on the full decay scheme, and understanding in which situations summing is an important correction.