arXiv subjects

P. Knights

Publications and source records attributed to P. Knights.

At least 19 recordsLinked to original sources

arXiv2026

Reducing simulation-related emissions in rare-event searches through optimised event biasing

Rare-event search experiments continue to extend their sensitivities to unprecedented levels. This requires increasingly small backgrounds, and shielding schemes that can suppress external backgrounds by several orders of magnitude. Moreover, detailed simulations are required to attain a good understanding of these experimental backgrounds. Studies have suggested, however, that simulations and computing-related tasks contribute approximately 10\% to the average particle physicist's carbon footprint. With backgrounds frequently below 0.01 counts per kg of target per keV of energy, simulations require more computing resources; growing a rare-event researcher's computing-related carbon footprint. Event biasing is often applied in shielding simulations as a remedy, yet there is a lack of systematic guidance on how to maximally benefit from them. An optimisation study for the importance-splitting biasing technique is discussed, focused on balancing statistical precision with simulation CPU-time, and how this can benefit the average researcher's carbon footprint.

hep-ex↗

arXiv2026

Response of a nitrogen-filled spherical proportional counter to mono-energetic neutrons

Neutron spectroscopy is an invaluable tool for a wide range of scientific and industrial applications, however, current approaches suffer from limitations that restrict their field of applicability. A safe and inexpensive alternative approach to neutron detection and spectroscopy is the use of a nitrogen-filled spherical proportional counter that exploits the $^{14}\rm{N(n,p)}^{14}\rm{C}$ and $^{14}\rm{N(n},\alpha)^{11}\rm{B}$ reactions. The neutron spectroscopy capabilities of the detector are demonstrated using beams of mono-energetic neutrons. A nitrogen-filled spherical proportional counter, operating at a pressure of $1$ bar, is exposed to neutrons with energies from $0.75$ MeV to $2.75$ MeV at the Tandem accelerator of the National Centre for Scientific Research ``Demokritos'' in Athens. A linear energy response is observed, within the statistical precision of the measurements.

hep-ex↗

arXiv2026

Optimisation of importance sampling implementation in rare-event shielding simulations using Geant4

Rare-event search experiments demand ever-lower backgrounds, and the resulting radio-pure materials and suppressive shielding make related background simulations computationally expensive without event biasing. We present a method for optimising the widely used importance splitting and Russian roulette technique for rare-event searches, in which a whole number of equal-width importance layers fill the shielding geometry. Balancing relative uncertainty against execution time, we optimise the layer thickness for both $\gamma$-rays and neutrons, and across different shielding materials and thicknesses. Expressed as a multiple of the primary particle's mean free path, the optimal thickness generalises across energies for a given particle type and yields a physics-based prediction applicable to any shielding design. The same trend reveals that over-biasing is possible, with relative uncertainty worsening at high layer counts for fixed execution time. The optimal configuration gives an $\mathcal{O}$(20) gain in computational efficiency over the unbiased case.

physics.ins-det↗

arXiv2026

Hydrofluoric acid-free titanium etching for rare-event searches

Rare-event search experiments require construction materials with high radiopurity to minimise background contributions. Thanks to its high mechanical strength, low density, machinability, and commercial availability in relatively radiopure forms, titanium is a suitable material for structural elements in rare-event searches. In such applications, a chemical etching stage is typically performed to remove surface contamination or to prepare the surface for further treatment. However, due to its chemical resistance, the etching of titanium conventionally requires hydrofluoric acid, posing serious health and safety concerns that are further exacerbated in deep underground laboratory settings. An alternative approach is proposed, which uses sulphuric acid. Grade 1 titanium samples were etched in 20\% and 40\% sulphuric acid solutions at 20$^\circ$C and 40$^\circ$C for up to 24\,h. The effects of etching were quantified through mass change measurements, surface roughness analysis, and scanning electron microscopy. Sulphuric acid effectively etches titanium, with up to $3.5\,\pm\,0.3$ mg/cm$^2$ of titanium removed for an unagitated solution of 40\% sulphuric acid at $40^\circ$C for 24\,h. Furthermore, sulphuric acid is shown to be effective at etching at lower concentration and temperature. The formation of a passivation layer during the etching may enable control of the total mass removed.

physics.ins-det↗

arXiv2025

Overlap-aware segmentation for topological reconstruction of obscured objects

The separation of overlapping objects presents a significant challenge in scientific imaging. While deep learning segmentation-regression algorithms can predict pixel-wise intensities, they typically treat all regions equally rather than prioritizing overlap regions where attribution is most ambiguous. Recent advances in instance segmentation show that weighting regions of pixel overlap in training can improve segmentation boundary predictions in regions of overlap, but this idea has not yet been extended to segmentation regression. We address this with Overlap-Aware Segmentation of ImageS (OASIS): a new segmentation-regression framework with a weighted loss function designed to prioritize regions of object-overlap during training, enabling extraction of pixel intensities and topological features from heavily obscured objects. We demonstrate OASIS in the context of the MIGDAL experiment, which aims to directly image the Migdal effect--a rare process where electron emission is induced by nuclear scattering--in a low-pressure optical time projection chamber. This setting poses an extreme test case, as the target for reconstruction is a faint electron recoil track which is often heavily-buried within the order(s)-of-magnitude brighter nuclear recoil track. Compared to unweighted segmentation regression, we demonstrate OASIS's novel overlap region-targeted loss function weight to be the single most important training weight for improving intensity and topological reconstructions of the low-energy electron tracks that tend to be most dominated by pixel overlap. Averaging over eight training campaigns, we further show the addition of overlap-targeted weights to improve median intensity reconstruction errors from -41.1% to -13.3% for these low-energy electrons. These performance gains demonstrate OASIS as a generalizable methodology for recovering obscured signals in overlap-dominated regions.

hep-ex↗

arXiv2025

Design of high-strength, radiopure, electroformed copper-based alloys for rare-event searches: Impact of layer configuration on heat treatments

State-of-the-art and next-generation rare-event search experiments rely on detector materials with stringent requirements on radiopurity and mechanical performance. Additive-free electroformed copper offers exceptional radiopurity, but is limited in mechanical strength, motivating the exploration of application-specific copper-based alloys. Early investigations, based on direct experimentation, explored the synthesis of CuCr alloys through electrodeposition and thermal processing. Subsequently, modeling tools based on the thermodynamic and kinetic properties of the alloy compositions were employed, which led to specific proposals for improved thermal processing. Moreover, the systematic application of computational thermodynamics to materials design further motivated the investigation of CuCrTi alloys, in addition to CuCr alloys. This materials design approach has shaped a trajectory towards designing high-performance, radiopure copper-based alloys, minimizing lengthy and costly trial-and-error. In this work, we explore the impact of initial layer configuration on the effectiveness of heat treatments, paving the way toward manufacturable, radiopure, multicomponent alloys for future low-background experiments.

physics.ins-det↗

arXiv2025

Titanium for rare-event searches: Hydrofluoric acid-free etching

Rare-event search experiments require construction materials with high radiopurity to minimise background contributions. Thanks to its high mechanical strength, low density, machinability, and commercial availability in relatively radio-pure forms, titanium is a suitable material for structural elements in rare event searches. To remove surface deposits on materials used, a chemical etching stage is usually performed. However, the chemical resistance of titanium means that, conventionally, such etching is done with hydrofluoric acid. Hydrofluoric acid presents serious health risks to users, and such hazards are compounded in the case of construction in deep underground laboratories. An alternative chemical etching using sulphuric acid is presented. This is demonstrated to etch titanium, removing 3.7 $\mu$m of material from the surface over the course of 20 hours. Scanning electron microscopy with back-scattered electron spectroscopy was used to study the surface and contamination of the titanium, demonstrating the removal of surface contaminants after etching. The proposed method is a potential alternative to those currently employed.

physics.ins-det↗

arXiv2024

The ionization yield in a methane-filled spherical proportional counter

Spherical proportional counters (SPCs) are gaseous particle detectors sensitive to single ionization electrons in their target media, with large detector volumes and low background rates. The $\mbox{NEWS-G}$ collaboration employs this technology to search for low-mass dark matter, having previously performed searches with detectors at the Laboratoire Souterrain de Modane (LSM), including a recent campaign with a 135 cm diameter SPC filled with methane. While in situ calibrations of the detector response were carried out at the LSM, measurements of the mean ionization yield and fluctuations of methane gas in SPCs were performed using a 30 cm diameter detector. The results of multiple measurements taken at different operating voltages are presented. A UV laser system was used to measure the mean gas gain of the SPC, along with $\mathrm{^{37}Ar}$ and aluminum-fluorescence calibration sources. These measurements will inform the energy response model of future operating detectors.

physics.ins-det↗

arXiv2024

Search for Light Dark Matter with NEWS-G at the Laboratoire Souterrain de Modane Using a Methane Target

The NEWS-G direct detection experiment uses spherical proportional counters to search for light dark matter candidates. New results from a 10 day physics run with a $135\,\mathrm{cm}$ in diameter spherical proportional counter at the Laboratoire Souterrain de Modane are reported. The target consists of $114\,\mathrm{g}$ of methane, providing sensitivity to dark matter spin-dependent coupling to protons. New constraints are presented in the mass range $0.17$ to $1.2\,\mathrm{GeV/c^2}$, with a 90% confidence level cross-section upper limit of $30.9\,\mathrm{pb}$ for a mass of $0.76\,\mathrm{GeV/c^2}$.

hep-ex↗

arXiv2024

Transforming a rare event search into a not-so-rare event search in real-time with deep learning-based object detection

Deep learning-based object detection algorithms enable the simultaneous classification and localization of any number of objects in image data. Many of these algorithms are capable of operating in real-time on high resolution images, attributing to their widespread usage across many fields. We present an end-to-end object detection pipeline designed for real-time rare event searches for the Migdal effect, using high-resolution image data from a state-of-the-art scientific CMOS camera in the MIGDAL experiment. The Migdal effect in nuclear scattering, crucial for sub-GeV dark matter searches, has yet to be experimentally confirmed, making its detection a primary goal of the MIGDAL experiment. Our pipeline employs the YOLOv8 object detection algorithm and is trained on real data to enhance the detection efficiency of nuclear and electronic recoils, particularly those exhibiting overlapping tracks that are indicative of the Migdal effect. When deployed online on the MIGDAL readout PC, we demonstrate our pipeline to process and perform the rare event search on 2D image data faster than the peak 120 frame per second acquisition rate of the CMOS camera. Applying these same steps offline, we demonstrate that we can reduce a sample of 20 million camera frames to around 1000 frames while maintaining nearly all signal that YOLOv8 is able to detect, thereby transforming a rare search into a much more manageable search. Our studies highlight the potential of pipelines similar to ours significantly improving the detection capabilities of experiments requiring rapid and precise object identification in high-throughput data environments.

hep-ex↗

arXiv2024

Measurement of scintillation from proportional electron multiplication in liquid xenon using a needle

Charge amplification in liquids could provide single-phase xenon time projection chambers with background discrimination and fiducialisation capabilities similar to those found in dual-phase detectors. Although efforts to achieve the high electric field required for charge amplification and proportional scintillation in liquid xenon have been previously reported, their application to large-scale detectors remains elusive. This work presents a new approach to this challenge, where - instead of the thin wire approach of previous studies - a needle-like high-voltage electrode is employed to demonstrate proportional charge amplification and secondary scintillation production in liquid xenon. This is an important milestone towards the development of an electrode structure that could be utilised in a large-scale, single-phase time projection chamber with dual read-out.

physics.ins-det↗

arXiv2023

First operation of an ACHINOS-equipped Spherical Proportional Counter with individual anode read-out

The multi-anode sensor ACHINOS revolutionised the capabilities of the spherical proportional counter by enabling large-size, high-pressure, operation and TPC-like reconstruction capabilities through individual anode read-out. First measurements with an individually read out ACHINOS are performed, which enables improved calibration and response homogenisation. Experimental results demonstrating the improvement in energy resolution brought by the individual anode calibration are presented. These are complemented by detailed simulation studies on the effect of sensor design and manufacturing imperfections, and how they may be corrected both in hardware and analysis.

physics.ins-det↗

arXiv2023

Performance of a spherical high pressure gas TPC for neutrino magnetic moment measurement

The measurement of neutrino magnetic moment larger than $10^{-19}\mu_B$ would be a clear signature of physics beyond the standard model other than the existence of massive Dirac neutrinos. The use of a spherical proportional counter detector filled with gas at 40 bar located near a nuclear reactor would be a simple way to perform such a measurement exploiting the developments made on such a technology for the search of dark matter and neutrinoless double beta decay. Different targets can be used just by replacing the gas: xenon, CF$_4$ and argon were compared and the sensitivity in one year of data taking could reach the level of $4.3 \times 10^{-12} \mu_B$, $6.5 \times 10^{-12} \mu_B$, and $8.5 \times 10^{-12} \mu_B$, respectively.

hep-ex↗

arXiv2023

Exploring light dark matter with the DarkSPHERE spherical proportional counter electroformed underground at the Boulby Underground Laboratory

We present the conceptual design and the physics potential of DarkSPHERE, a proposed 3 m in diameter spherical proportional counter electroformed underground at the Boulby Underground Laboratory. This effort builds on the R&D performed and experience acquired by the NEWS-G Collaboration. DarkSPHERE is primarily designed to search for nuclear recoils from light dark matter in the 0.05--10 GeV mass range. Electroforming the spherical shell and the implementation of a shield based on pure water ensures a background level below 0.01 dru. These, combined with the proposed helium-isobutane gas mixture, will provide sensitivity to the spin-independent nucleon cross-section of $2\times 10^{-41} (2\times 10^{-43})$ cm$^2$ for a dark matter mass of $0.1 (1)$ GeV. The use of a hydrogen-rich gas mixture with a natural abundance of $^{13}$C provides sensitivity to spin-dependent nucleon cross-sections more than two orders of magnitude below existing constraints for dark matter lighter than 1 GeV. The characteristics of the detector also make it suitable for searches of other dark matter signatures, including scattering of MeV-scale dark matter with electrons, and super-heavy dark matter with masses around the Planck scale that leave extended ionisation tracks in the detector.

hep-ex↗

arXiv2022

ACHINOS: A Multi-Anode Read-Out for Position Reconstruction and Tracking with Spherical Proportional Counters

The spherical proportional counter is a versatile gaseous detector with physics applications ranging from rare event searches to fast neutron spectroscopy. In its simplest form, the detector operates with a single channel read-out, and uses pulse-shape information to reconstruct the interaction radius, which is used for background discrimination and target volume definition. Recent developments in the read-out instrumentation have enabled the use of a multi-anode read-out structure, ACHINOS. The multiple anodes provide information about the interaction position which, coupled with the radial information, can be used to reconstruct an ionisation track. This ability has implications for several applications of the detector, for example, background discrimination in rare event searches.

physics.ins-det↗

arXiv2022

Purification Efficiency and Radon Emanation of Gas Purifiers used with Pure and Binary Gas Mixtures for Gaseous Dark Matter Detectors

Rare event searches require extreme radiopurity in all detector components. This includes the active medium, which in the case of gaseous detectors, is the operating gas. The gases used typically include noble gas mixtures with molecular quenchers. Purification of these gases is required to achieve the desired detector performance, however, purifiers are known to emanate 222 Rn, which is a potential source of background. Several purifiers are studied for their O 2 and H 2 O purification efficiency and Rn emanation rates, aiming to identify the lowest-Rn options. Furthermore, the absorption of quenchers by the purifiers is assessed when used in a recirculating closed-loop gas system.

physics.ins-det↗

arXiv2022

Fast Neutron Spectroscopy with a High-pressure Nitrogen-filled Large Volume Spherical Proportional Counter

We present a fast neutron spectroscopy system based on a nitrogen-filled, large volume gaseous detector, the Spherical Proportional Counter. The system has been successfully operated up to gas pressure of 1.5 bar. Neutron energy is estimated through measurement of the 14N(n,a)11B and 14N(n,p)14C reaction products. These reactions have comparable cross sections and Q-values with the 3He(n,p)3H reaction making nitrogen a good alternative to 3He use for fast neutron detection. Two detectors were built at the University of Birmingham and are currently used for the measurement of fast and thermal neutrons in the University of Birmingham and the Boulby underground laboratory, respectively.

hep-ex↗

arXiv2022

Neutron spectroscopy: The case of the spherical proportional counter

Neutron spectroscopy is an invaluable tool for many scientific and industrial applications, including underground Dark Matter searches. Neutron-induced backgrounds produced by cosmic ray muons and the cavern radioactivity can mimic the expected Dark Matter signal. However, existing neutron detection methods have several drawbacks and limitations, thus measurements remain elusive. A promising new approach to neutron spectroscopy is the use of a nitrogen-filled spherical proportional counter that exploits the $^{14}$N(n,$\alpha$)$^{11}$B and $^{14}$N(n, p)$^{14}$C reactions. This is a safe, inexpensive, effective and reliable technique. In this work, the latest instrumentation developments are incorporated in a compact detector operated at the University of Birmingham (UoB) with high gain at gas pressure up to 1.8\,bar. We demonstrate spectroscopic measurements of thermalised and fast neutrons respectively from an $^{241}$Am-$^9$Be source and from the MC40 cyclotron facility at UoB. Additionally, the detector response to neutrons is simulated using a framework developed at UoB and compared with the experimental results.

physics.ins-det↗