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

Periodic Reporting for period 1 - neutronSPHERE (Neutron Spectroscopy with a Spherical Proportional Counter for precision measurements in deep-underground laboratories)

Reporting period: 2020-09-11 to 2022-09-10

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.
Five work packages facilitated the completion of the neutronSphere goals.
The simulation framework previously developed by the host was further enhanced in terms of realism of the detector response to ionizing radiation and was used to optimize the detector parameters and simulate physical processes in the SPC. A wealth of information provided, including simulated detector pulses that are directly comparable to measurements.
Two 30cm diameter spheres were used for neutron spectroscopic measurements. Two 11-anode ACHINOS sensors (read out in 2 channels) were constructed and characterized, responding to α-particles and γ-rays from radioactive sources on a wide range of energies. The detector was operated with different gases and pressures, providing guidance for the operational parameters during neutron measurements. A custom-made filter, to remove gas impurities, built and characterized. Finally, work performed towards the individual readout of an 11-anode sensor, with improved resolution.
Data-taking campaigns performed in the graphite stack facility at the UoB for the detector characterisation using thermalised and fast neutrons from an AmBe source. Thermal neutron detection confirmed by collecting the 625keV recoil energy of the (n,p) reaction in the detector. Furthermore, the Rn decay chain emanated by a commercial filter, was used as a direct energy reference. The efficiency of the method was estimated by simulating the thermalisation process on the graphite stack. Further measurements up to the record pressure of 1.8bar N2 confirmed the detection capability of the SPC.
The capability of the SPC to perform fast neutron spectroscopy confirmed with the campaign at the MC40 cyclotron facility at the UoB. Series of measurements performed with the detector filled with 1 and 1.5 bar N2. The detector’s response on fast neutrons was ascertained with the use of energy moderators. Simulation studies described the experimental data, providing insights on the signal generation processes. This made possible further exploitation of the simulation to emulate hadron-therapy environments and develop a method to measure neutron-induced doses in patients and personnel.
The deployed SPC at Boulby was upgraded by the commission of an 11-anode ACHINOS sensor and a custom-made filter with low radon emanation. After the detector characterisation, the SPC was used to detect fast neutrons from a 251Cf source and without any neutron source, recording only background radiation from the cavern.
The fellow regularly presented the progress in meetings of the NEWS-G collaboration and of the Birmingham Instrumentation Laboratory for Particle Physics and Applications. Furthermore, the fellow communicated the scientific results in several national and international conferences, meetings and workshops.
neutronSphere addresses head-on a particle physics instrumentation challenge: neutron spectroscopy. Despite several attempts towards an efficient neutron spectroscopy system, such measurements remain cumbersome and detailed neutron spectra is sparse both in scientific laboratories and industrial sites. The neutronSPHERE project provides a unique alternative to 3He-based detectors for neutron spectroscopy by using the SPC, a high-gain large-volume gaseous detector, filled with nitrogen. The detection principle exploits the 14N(n,α)11B and 14N(n,p) 14C processes, and exhibits all major advantages of 3He-based detectors, and goes beyond that to provide fast neutron spectroscopy at an affordable price.
Τhe neutron background measurements provide a lasting contribution to DM searches in two of the deepest underground laboratories (Boulby, SnowLab). The method can also be used in industrial and medical applications as a safe and cost-effective method for fast neutron measurements.
neutronSphere advanced the instrumentation of SPC and can potentially deliver the next-generation detectors for rare-event searches. DarkSPHERE, a future 3m diameter SPC, has been proposed to be installed at Boulby. DarkSPHERE can also act as a probe for more detailed neutron background measurements, supernova neutrino detection and neutrinoless double beta decay searches.
The Birmingham Gaseous Detector Laboratory, significantly developed under neutronSphere, strengthen Birmingham's research capabilities, and promoted gaseous detector applications in physics and industry.
neutronSphere produced a series of publications in peer-reviewed physics journals. Results were presented in international and national conferences, topical workshops, and meetings.
The fellow at Boulby underground lab, operating the detector
The detector in front of the graphite stack, responding to thermal neutrons
Schematic of the spherical Proportional counter
Gas purification filters under construction
The multi anode ACHINOS sensor
Gas purification filters under construction
The detector at the cyclotron beamline, responding to fast neutrons