UC Berkeley

The GRETA collaboration has developed a large position-sensitive HPGe detector array for both stable and radioactive beam experiments, called the Gamma-Ray Energy Tracking IN-Beam Array (GRETINA). The array’s primary purpose is for high-efficiency, high-precision gamma-ray spectroscopy. Its position sensitivity allows experimenters to sequence photon tracks via Compton kinematics. This tracking is used, among other things, to estimate the emission angle of a gamma ray relative to its parent nucleus’s velocity. This is key for spectroscopy because it gives a way to correct for Doppler shifts in photon energy.

Because GRETINA can measure both energy and position of individual photon interactions, it also opens the door for gamma-ray imaging. I sought to use these capabilities to measure the lifetimes of short-lived nuclear states produced by in-beam experiments. The current approach to this problem – called the “Recoil Distance Method” (RDM) – requires large amounts of beamtime, a resource which is in short supply. With gamma-ray imaging, on the other hand, we can theoretically extract these lifetimes “for free”, using the same data taken during spectroscopy runs. Where RDM generally requires multiple measurements to find a lifetime, imaging delivers full de-excitation curves from a single run. Lifetimes can be extracted directly by fitting to these curves.

The research presented here provides a feasibility study of two gamma-ray imaging techniques: traditional Compton imaging, and a new method based on Doppler shifts. Where Compton imaging depends on the first two hits in a photon track, “Doppler-Shift imaging” requires only the first. This is extremely promising, as I found that GRETINA’s position resolution greatly limits the fidelity of Compton imaging. Doppler-shift imaging is more

sensitive to detector energy-resolution, but largely insensitive to position resolution. In both cases, finite imaging resolution causes an imaged de-excitation curve to deviate from the ideal, purely exponential shape. To improve accuracy, curve fitting must account for the detector imaging response.

Many parameters affect the overall imaging response of GRETINA. I evaluated several of them here through a series of detailed Geant4 simulations, and explored several data cuts to improve imaging quality. In general, Doppler-shift imaging offers 5-6x better imaging resolution and 2-3x better imaging efficiency. With data filters, Compton imaging resolution

can deliver comparable resolution at the expense of 85-90% of counts.

The end goal of this imaging work is to understand how imaging response translates to precision in lifetime measurements. Imaging resolution, counting statistics, and the lifetime of the nuclear state in question all have significant effects on this precision. I studied each parameter using detailed simulations built in Geant4, calculating sensitivity ranges for lifetime measurements using gamma-ray imaging. I found that even very poor imaging resolution can be corrected for if the detector response is accurately characterized. For example, with 25.0 mm imaging resolution and 50K samples per lifetime curve, we can achieve < 5% relative uncertainty on measured lifetimes between 29 ps and 5.2 ns at 0.3c parent velocity.

However, while the simulations are extremely encouraging, an experimental validation with 92 Mo was ultimately unsuccessful. At 35 ps, the lifetime of the isotope’s 740 keV transition is on the lower edge of our sensitivity range – especially when combined with the very low 0.085c recoil velocity of the parent nucleus. I also ran into complications from Compton background counts in the experimental spectrum, which made it very difficult to

characterize the imaging response reliably. Knowing this, however, we now have a better idea of the requirements for using gamma-ray imaging in lifetime experiments.