arXiv subjects

Derek Strom

Publications and source records attributed to Derek Strom.

4 recordsLinked to original sources

arXiv2026

EON-SII: Design of a transportable picosecond stellar intensity interferometer for compact-star astrophysics

Stellar intensity interferometry (SII) measures correlations in photon-arrival fluctuations recorded by telescopes observing bright celestial sources. It can resolve angular scales far smaller than those accessible to a single optical telescope and is largely insensitive to atmospheric turbulence. After the first demonstration of SII on Sirius in 1956, Hanbury Brown and Twiss used the technique to measure the diameters of 32 stars. More recently, VERITAS, MAGIC, H.E.S.S., and CTAO's LST-1 have revived the method, although observations remain restricted to bright targets because of their optical design, optimized for gamma-ray astrophysics, rather than SII. We present EON-SII, the design and performance of a two-telescope intensity interferometer intended to extend the SII technique to compact targets at magnitudes of about V=8.5 up to V=10.7. Each transportable telescope has a 4-m diameter mirror, approximately 9m2 collecting area, an actively aligned 18-panel primary mirror, and Cassegrain optics specified to concentrate at least 90% of the light within 3 arcsec. A fibre-free spectrograph covers 400-550 nm at R~7000-8000 and is designed to provide of order 1000 statistically independent spectral channels.

astro-ph.IM↗

arXiv2026

Direct comparison of SiPMs and PMTs in operation with a bright background and prospects of using SPADs as truly digital sensors

The use of silicon photomultipliers (SiPMs) alongside conventional photomultiplier tubes (PMTs) is a remarkable technological development in modern ground-based very high energy gamma-ray astronomy. SiPMs exhibit comparable or even higher photon detection efficiencies (PDEs) than PMTs. The sensitivity of a PMT matches well the spectral shape of Cherenkov radiation from extended air showers. In contrast to a PMT, the sensitivity of a SiPM is shifted toward longer wavelengths, where the intensity of light of night sky (LoNS), considered as unwanted noise, increases significantly. It is obvious that a SiPM with a higher PDE will indeed measure more Cherenkov light than a PMT, but it will also detect significantly higher LoNS noise; the question is which factor will predominate in the signal-to-noise-ratio (SNR). To compare the performance of a PMT with that of a SiPM, we built SiPM-based modules and installed these and operated in parallel in the imaging camera of the 17 m diameter MAGIC telescope. Our long-term studies show that SiPM, despite their higher PDE, can deliver only a comparable to PMT performance. As already the name SiPM suggests, we use these semiconductor sensors analogously to classical PMTs: We amplify their small signals, digitize, and calibrate the converted amplitudes. Although SiPM is essentially a digital sensor, its common-anode design does not allow one to directly profit from it. Numerous arrays of single-photon avalanche diodes (SPADs) are being developed in various laboratories worldwide. Unlike SiPM, SPAD arrays digitize the incident photons from the outset and count their number. We will dwell on the potential further developments of SPADs.

astro-ph.IM↗

arXiv2019

Quantum efficiency enhancement of bialkali photocathodes by an atomically thin layer on substrates

We report quantum efficiency (QE) enhancements in accelerator technology relevant antimonide photocathodes (K2CsSb) by interfacing them with atomically thin two-dimensional (2D) crystal layers. The enhancement occurs in a reflection mode, when a 2D crystal is placed in between the photocathodes and optically reflective substrates. Specifically, the peak QE at 405 nm (3.1 eV) increases by a relative 10 percent, while the long wavelength response at 633 nm (2.0 eV) increases by a relative 36 percent on average and up to 80 percent at localized hot spot regions when photocathodes are deposited onto graphene coated stainless steel. There is a similar effect for photocathodes deposited on hexagonal boron nitride monolayer coatings using nickel substrates. The enhancement does not occur when reflective substrates are replaced with optically transparent sapphire. Optical transmission, X-ray diffraction (XRD) and X-ray fluorescence (XRF) revealed that thickness, crystal orientation, quality and elemental stoichiometry of photocathodes do not appreciably change due to 2D crystal coatings. These results suggest optical interactions are responsible for the QE enhancements when 2D crystal sublayers are present on reflective substrates, and provide a pathway toward a simple method of QE enhancement in semiconductor photocathodes by an atomically thin 2D crystal on substrates.

physics.app-ph↗

arXiv2018

Free-standing bialkali photocathodes using atomically thin substrates

We report successful deposition of high quantum efficiency (QE) bialkali antimonide K2CsSb photocathodes on graphene films. The results pave a pathway towards an ultimate goal of encapsulating technologically-relevant photocathodes for accelerator technology with an atomically-thin protecting layer to enhance lifetime while minimizing QE losses. A QE of 17 % at ~3.1 eV (405 nm) is the highest value reported so far on graphene substrates and is comparable to that obtained on stainless steel and nickel reference substrates. The spectral responses of the photocathodes on graphene exhibit signature features of K2CsSb including the characteristic absorption at ~2.5 eV. Materials characterization based on X-ray fluorescence (XRF) and X-ray diffraction (XRD) reveals that the composition and crystal quality of these photocathodes deposited on graphene is comparable to those deposited on a reference substrate. Quantitative agreement between optical calculations and QE measurements for the K2CsSb on free suspended graphene and a graphene coated metal substrate further confirms the high quality interface between the photocathodes and graphene. Finally, a correlation between the QE and graphene quality as characterized by Raman spectroscopy suggests that a lower density of atomistic defects in the graphene films leads to higher QE of the deposited K2CsSb photocathodes.

cond-mat.mtrl-sci↗