BonnerSphären

BonnerSphären, also known as Bonner sphere spectrometers (BSS), are a neutron spectrometry system used to determine the energy distribution of incident neutron fields. The typical setup consists of a fast-neutron detector placed at the center of a set of moderator spheres, usually made of polyethylene, of varying diameters.

The response of each sphere depends on neutron energy; larger spheres moderate fast neutrons to thermal energies differently than smaller spheres. By measuring counts in the central detector for each sphere and applying unfolding techniques, the incident neutron spectrum across a broad energy range can be inferred.

Common detectors are 3He proportional counters or BF3 tubes. Spheres range from a few centimeters to several tens of centimeters in diameter, and the arrangement may include auxiliary materials to influence high-energy response. The system is often calibrated with known neutron fields or modeled by Monte Carlo simulations to generate a response matrix.

Applications span radiation protection and dosimetry in nuclear facilities, reactor physics, accelerator environments, space and aviation radiation assessment, and medical physics research. BonnerSphären are valued for their broad energy sensitivity and relatively simple hardware.

Data analysis involves unfolding the measured sphere counts to estimate the neutron energy spectrum. This requires regularization and prior information to stabilize the solution. While powerful, the method depends on accurate response functions and adequate statistics, and it typically provides a coarse-grained spectrum rather than high-resolution spectra.