This work stands apart: there is almost nothing nearby, with the closest neighbor at a distance of 0.535. Around it lie studies on neutron diffraction, scattering, neutron detection techniques, and scintillator-based detectors. But they all deal either with thermal neutrons or with other gases and geometries. A nitrogen-filled spherical counter for fast neutrons is a pioneer here. Earlier, such detectors were tried with continuous spectra, but no one had calibrated them on a monoenergetic beam.
The detector is an aluminum sphere 30 cm in diameter, filled with nitrogen at a pressure of 1 bar. Inside, the 11-anode ACHINOS sensor consists of small stainless steel balls 1 mm in diameter, positioned 1.4 cm from the center. Each anode has its own preamplifier and digitization. Neutrons were produced at the tandem accelerator in Athens via the reaction 7Li(p,n)7Be. The beam was collimated through holes in borated polyethylene. Neutron energies were 0.75, 1.0, 1.425, 1.8, 2.22, 2.517, and 2.75 MeV. For each energy, pulse amplitudes and rise times were measured.
Neutrons were detected through two reactions: 14N(n,p)14C and 14N(n,α)11B. Each gives its own peak in the amplitude–rise-time plot. For the (n,p) reaction, the peak is broad in time because of the large proton range. For (n,α), the peak is compact. Energy resolution: 3.1–5.0% for the alpha channel and about 7% for the proton channel. The reconstructed energy depends linearly on the beam energy. For the (n,α) reaction, the slope is 1.000±0.015; for (n,p), the slope is 0.83±0.10. These are within statistical uncertainties.
The key difference from neighboring work is the first quantitative calibration of a nitrogen-filled spherical counter with monoenergetic neutrons. Previously, only continuous spectra from sources were available. Now we have reference points. This proves that the detector can act as a spectrometer, not just a counter. In addition, individual readout from each anode allows rejection of background events and improves resolution.
Next, one could increase the nitrogen pressure to boost efficiency and reduce wall effects. There is already experience working up to 1.8 bar. The energy range can be extended: the accelerator provides neutrons up to 20.5 MeV. It would be interesting to test the response to thermal neutrons, where the nitrogen capture cross section is three orders of magnitude lower than that of helium-3, but for some applications this is acceptable.
This work will influence neutron dosimetry in medicine, nuclear material control, and background characterization in underground laboratories for dark matter searches and neutrinoless double beta decay.
Perform measurements at higher nitrogen pressures and with other gas mixtures. Investigate the detector's behavior at energies above 2.75 MeV, where the contribution of the (n,α) reaction grows.
For neutron physics, the problem of accurate spectrometry of fast neutrons in mixed fields remains. A nitrogen-filled spherical counter could provide a cheap solution with good resolution, but the systematics need to be better understood, especially the ballistic deficit for the proton channel.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The paper presents a spherical proportional counter filled with nitrogen and equipped with an 11-anode sensor called ACHINOS, tested on a beam of monoenergetic neutrons with energies from 0.75 to 2.75 MeV. Linear responses were obtained for the 14N(n,p)14C and 14N(n,α)11B reactions, with energy resolutions of 3.1–5.0% for the alpha channel and about 7% for the proton channel.
Where its strength is
For the first time, a nitrogen-filled spherical detector has been quantitatively calibrated on monoenergetic fast neutrons, demonstrating its potential as a spectrometer rather than just a counter. Individual readout from each anode allows rejection of background events and improves resolution.
Where it could go next
Worth trying
Why this may matter
The development of inexpensive and safe nitrogen-based neutron spectrometers could reduce dependence on scarce helium-3 and expand the use of neutron spectroscopy in fundamental and applied research.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.50, against a usual 0.64. It looks like few have drilled nearby.
The detector only reached stable gain 18 hours after the high voltage was switched on. The reason for such a long plateau time is not yet clear, but after that, the gain remained stable.