neutronSPHERE · Neutron Spectroscopy with a Spherical Proportional Counter for precision measurements in deep-underground laboratories

Neutron Spectroscopy with a Spherical Proportional Counter for precision measurements in deep-underground laboratories

Neutron spectroscopy is an invaluable tool for a broad range of scientific, industrial, and medical applications; spanning from direct Dark Matter (DM) searches and neutrino-less double-beta decay to non-destructive measurements of materials and medical imaging or cancer treatment. For example, in underground laboratories, neutron induced backgrounds caused by cosmic ray muons and natural radioactivity may mimic the DM signal, reducing experimental sensitivity. A dedicated, precise, and in-situ measurement of neutron flux would be a valuable tool for characterizing and mitigating neutron background. Despite several attempts towards an efficient neutron spectroscopy system, such measurements remain cumbersome and detailed neutron spectra are sparse both in scientific laboratories and industrial sites. To-date the most widely used method relies on the 3He(n,p)3H reaction, providing high efficiency for thermal neutrons and very low efficiency in γ-rays. However, 3He-based detectors only provide combined slow and fast neutron flux measurements, while energy measurements for fast neutrons is plagued by the so-called “wall effect” - the recoiling proton escapes the gas volume. Moreover, the popularity of such detectors led to a disproportionate demand for 3He with respect to its supply, which resulted in a dramatic price increase. All existing alternatives (BF3-based proportional counters, 10B lined tubes, Bulk scintillators, 6Li coated Ar-filled detectors) have major disadvantages. neutronSPHERE, aims to develop an inexpensive, simple, robust, and reliable fast neutron spectroscopy system particularly sensitive in the 1-20 MeV range. This can be achieved using the Spherical Proportional Counter (SPC), a high-gain large-volume gaseous detector, filled with nitrogen. Neutron detection relies on the 14N(n,a)11B and 14N(n,p) 14C processes, exhibiting cross-sections similar to the 3He(n,p)3H reaction for fast neutrons. Nitrogen provides high efficiency for fast neutron detection without need for moderation, has low sensitivity to γ-rays, avoids toxic or corrosive materials, and can be deployed at sites with strict safety requirements. Any potential wall effect is significantly suppressed due to the high atomic number of nitrogen. Given that the SPC can be made in large volumes, the detector would be sensitive also to thermal neutrons, despite the much lower thermal neutron absorption cross section of nitrogen. neutronSphere performed progress beyond the state of the art for spectroscopic neutron measurements. The project succeeded to operate efficiently the detector at record pressures and electric fields, limiting the wall effect, and demonstrating improved efficiency compared to 3He-based detectors. Following neutronSphere, the proposed method is mature not only for DM related background measurements, but for industrial applications as well.

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