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2024-09-12: GSI FRS-Ion-Catcher – Analysis for the Double Alpha Decay Experiment

A new experimental campaign to measure a predicted nuclear decay mode, double alpha decay, was conducted at the FRS Ion Catcher in 2022. In July 2024, the data analysis group met in Orsay (France) to summarise the interim results and facilitate further progress.

A new experimental campaign to measure a predicted nuclear decay mode, double alpha decay, was conducted during a 4-month offline run at the FRS Ion Catcher in 2022. Within this experiment, ions of 224Ra produced by a 228Th source were measured to detect the simultaneous emission of two alpha particles. (The concept of the experiment is explained on the FRS IC web page, performance details are outlined in a recent article: Varga et al., NIM A 1063, 169252 (2024).)

In July 2024, the data analysis group met in Orsay (France) to summarise the interim results and facilitate further progress. Two geometric models used to describe the detector hit pattern, one Geant4-based and another custom-developed, were cross-checked, reaching full agreement on the hit pattern, see the picture below for details. The custom analytical model is employed to determine the configuration of the detection setup in great detail and to track the stability of the implantation spot position over a long data-acquisition run.

Along with the geometry investigation, time and energy calibration of the data is ongoing. Once the analysis procedure is finalised, it will be applied to a combined data set. The data set is blinded in the energy region corresponding to double alpha events. An accurate simulation of the expected background will give this measurement strong potential to discover a new radioactive decay mode. 

The double-sided silicon strip detector registers alpha and beta particles. It consists of 16 p-strips and 16 n-strips with a width of 3 mm, oriented perpendicularly and forming a matrix of 16x16 pixels. Two models are designed to reproduce the hit pattern which comprises a spherically symmetric distribution from a point source and a shadow from the foil holder (y = 9-11). Monte Carlo simulations account for all major physical processes, but they are computationally demanding. The analytical model calculates solid angles, which enables the estimation of the geometric efficiency for each detector pixel. It operates significantly faster than Monte Carlo simulations and allows for nearly real-time verification of beam stability. Charged particle hit pattern: experimental data (left) and its comparison to simulated data obtained from Monte Carlo simulation with Geant4 (middle) and analytical model (right). The double-sided silicon strip detector registers alpha and beta particles. It consists of 16 p-strips and 16 n-strips with a width of 3 mm, oriented perpendicularly and forming a matrix of 16x16 pixels. Two models are designed to reproduce the hit pattern which comprises a spherically symmetric distribution from a point source and a shadow from the foil holder (y = 9–11). Monte Carlo simulations account for all major physical processes, but they are computationally demanding. The analytical model calculates solid angles, which enables the estimation of the geometric efficiency for each detector pixel. It operates significantly faster than Monte Carlo simulations and allows for virtually real-time verification of the stability of the ion implantation spot.