Hydrogenated Nanodiamond Photocathodes Achieve 4% Efficiency, Demonstrate 10x Superior Robustness, Revolutionizing Single-Photon Detection
Single-photon detection is an essential technology used in everything from basic physics research to medical imaging. In the past, substances like caesium iodide (CsI) have been used extensively in this important detection. Although CsI’s broad wavelength sensitivity and excellent quantum efficiency (QE) in the Vacuum Ultraviolet (VUV) range make it suitable for gaseous detectors, its hygroscopic nature and poor resistance to ion and photon bombardments severely limit its ability to maintain its QE. These materials are frequently characterized as brittle and transient.
A research group headed by F. M. Brunbauer, C. Chatterjee, and G. Cicala, who mainly worked at INFN Trieste in Italy, has effectively shown the possibility of nondiamond materials as a reliable substitute in a significant advancement resolving these constraints. According to their research, nondiamond coatings on detector components exhibit noticeably higher robustness than CsI. This accomplishment implies that nanodiamonds provide a crucial route to the development of single-photon detectors that are more robust and dependable, offering improved longevity and performance, particularly in demanding experimental settings.
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Hydrogenated NanoDiamond: A Robust Alternative
The feasibility of using hydrogenated nanodiamond (HND) and nanodiamond (ND) materials as substitutes for CsI in photon detectors used in gaseous detectors especially those that need fine timing resolution is carefully examined in this study. The researchers successfully developed and tested HND photocathodes, showing complete compatibility with THGEM (Triple-GEM) detectors and notable quantum efficiency.
CsI’s broad band gap (6.2 eV) and low electron affinity (0.1 eV) are responsible for its high QE. Similar attractive properties of nondiamond particles include a similar band gap (5.5 eV) and low electron affinity (0.35 to 0.50 eV). Moreover, ND has good radiation hardness and inherent chemical inertness. Importantly, the electron affinity is further decreased to a negative value during the ND hydrogenation process, enabling the produced photoelectrons to effectively escape without running into an energy barrier at the surface. According to earlier research, HND showed promise as a competitive substitute for CsI, with similar QE values and enhanced robustness.
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Superior Robustness Demonstrated
The remarkable resistance of HND photocathodes against degradation is one of the main conclusions of this current study. According to tests, Hydrogenated Nanodiamond HND photocathodes are substantially more resilient against ion bombardment than CsI, retaining roughly ten times more resistance to degradation under the same circumstances. In particular, ageing tests showed that HND photocathodes their more stable during extended ion bombardment in an Ar/CO2 gas mixture.
HND and CsI photocathodes were initially subjected to a QE measurement before being placed in an irradiation chamber with Ar/CO2 (70/30%) gas to examine the ageing properties. The gas was ionized using an X-ray beam, and the ions that produced moved in the direction of the samples, causing charge accumulation. While HND demonstrated almost an order of magnitude greater robustness. These results address a significant shortcoming of existing CsI-based detectors by confirming that HND provides better stability and a longer lifetime.
Quantum Efficiency and Coating Methodology
Using HND photocathodes, the study team was able to attain a quantum efficiency of roughly 4% at 122 nm. The best freshly hydrogenated samples that have been published in the literature have QE values of about 40% at 120 nm, within a factor of ten of those samples. However, after being hydrogenated and sonicated in 2021, the particular Hydrogenated Nanodiamond HND powder utilized for the measurements had been kept in water for more than two years. This could have changed its characteristics and led to the lower QE seen in comparison to newly made materials.
The team developed a novel method for HND
Coating and hydrogenation. ND grains were treated using the MWPECVD process in a microwave H2 plasma to create the HND powder . A pulsed spray approach was used for the subsequent HND layer coating. The HND grains were dispersed in deionized water, the solution was sonicated for 30 minutes, and a predetermined number of spray shots were used to deposit the solution onto substrates and THGEMs. In order to speed up the evaporation of the water solvent, the substrates were heated to 150 °C. The procedure was carefully regulated, with a 5-second pause between shoots and substrate masking to reduce splash effects and increase surface coverage.
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Seamless Detector Integration
Compatibility with current detecting technology is a major consideration when introducing new materials. The detector response changed very little when THGEMs covered with Hydrogenated Nanodiamond HND layers were characterized, indicating smooth integration. The THGEM’s uncoated and HND-coated tests performed almost identically, with the highest stable operational gain values in each scenario being roughly 50k. This demonstrates that steady gains are maintained and that nondiamond and HND coatings have no detrimental effects on THGEM detector performance.
Additionally, the group was able to create semi-transparent HND photocathodes, which yielded encouraging preliminary findings for detectors that need light transmission, including those employed in picosecond time-resolution settings. According to these results, HND is a material that shows promise for improving the efficiency and dependability of single-photon detection systems.
Future Focus
The advancement of detector technology for upcoming experiments, such as those at Electron-Ion Colliders, depends on this research. To further improve performance, the team is concentrating on streamlining the HND coating procedure. To increase quantum efficiency, current and next research will focus on coating method optimisation and the use of freshly hydrogenated nondiamond powder.
The researchers also intend to build a prototype picosecond detector based on this reliable technology and study how HND photocathodes behave in various gas mixes, including ArCH4. HND is a powerful, practical alternative to CsI, as evidenced by the research’s good findings, which include considerable QE even from long-stored material, full compatibility with THGEM operation, and a 10x higher resilience against ion bombardment.
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