When protons or ions collide with targets or each other, they produce lots of new particles. Some of these are wanted, some are unwanted, but whatever their desired status – they need to be well understood. Neutrons, which belong to these products, are a particular challenge. Their characteristics such as their energy, direction and number are a pain to measure, but knowing them well brings many advantages to various branches of science and its applications. A novel experimental approach based on a new neutron spectrometer recently tested by members of the Laboratory for High-Energy Physics at the University of Bern in collaboration with Politecnico di Milano and its spin-off company Raylab yields promising results that appear to be even more versatile than expected.
“Neutrons are strange animals,” says the University of Bern’s Saverio Braccini. Their behaviour and characteristics dramatically depend on their production circumstances; every detail of the nuclear reaction with which they are produced as well as the materials in the environment have an influence on their energy, direction and number and, consequently, the way they interact with matter. “The situation is often so complicated that they discourage almost everybody to study them,” Braccini says. A method that would reliably tell scientists what kind of neutrons they are dealing with would make many scientists’ lives easier. The good news is that this method and an appropriate instrument seems to exist. Based on a new compact neutron spectrometer developed by Raylab, a spin-off company from the Politecnico di Milano in Italy, a novel experimental approach to precisely assess neutron fields in a wide energy range was recently tested by members of Braccini’s group at the University of Bern in collaboration with Politecnico di Milano and Raylab.
But first things first – why is it important to know all these details about the neutrons so well? It’s also a question of impact on humans. Neutrons make things radioactive, posing a danger to people and causing damage in materials. Knowing exactly what radioactive dose they produce, for example as secondary particles produced in proton therapy, helps to adjust the treatment to the patient; knowing the damage they cause in materials (for example particle detectors) helps to correctly estimate the material’s lifetime and possibly improve it, and knowing how much radioactivity they produce in accelerator facilities helps to safely dispose of parts when the accelerator is decommissioned.
The new instrument is not only called DIAMON (“Direction-aware Isotropic and Active neutron MONitor with spectrometric capabilities“), it also has the rough shape of a cut diamond. Its semiconductor-based sensors measure neutron spectra, the directions of the particles and the field quantities in real time using a special code, potentially replacing the traditional complex and time-consuming methods of spectrometers and time-of-flights measurements. It was tested with the medical cyclotron at Bern at low energies and at the CERF facility at CERN (“CERN-EU high-energy Reference Field“) at high energies. Both facilities provide neutron fields, making it possible to compare the data from DIAMON with the reference and simulations. The explored energy range is so big that “it virtually spans two planets”, as Braccini describes it.
This versatility and ease of use opens doors to a range of applications. A long-term goal is the production of a fully characterised neutron beam using a medical cyclotron that can be used to study nuclear reactions, for the production of radionuclides for diagnostics and therapy (theragnostics) in particular. They could allow to cure certain types of cancers that cannot be treated in other ways. Particle physics also benefits from controlled neutron beams: in some particularly harsh environments, scientists need to make sure that their sensitive equipment can withstand the high levels of radiation they are exposed to. Irradiating prototypes with a well-understood neutron beam can simulate years of exposure, making it possible to predict and alleviate future damage. This would ideally complement the already ongoing radiation hardness studies with proton beams at the Bern medical cyclotron.