High-resolution extreme ultraviolet spectroscopy of G191-B2B: structure of the stellar photosphere and the surrounding interstellar medium

We have continued our detailed analysis of the high-resolution (R= 4000) spectroscopic observation of the DA white dwarf G191-B2B, obtained by the Joint Astrophysical Plasmadynamic Experiment (J-PEX) normal incidence sounding rocket-borne telescope, comparing the observed data with theoretical predictions for both homogeneous and stratified atmosphere structures. We find that the former models give the best agreement over the narrow waveband covered by J-PEX, in conflict with what is expected from previous studies of the lower resolution but broader wavelength coverage Extreme Ultraviolet Explorer spectra. We discuss the possible limitations of the atomic data and our understanding of the stellar atmospheres that might give rise to this inconsistency. In our earlier study, we obtained an unusually high ionization fraction for the ionized He ii present along the line of sight to the star. In the present paper, we obtain a better fit when we assume, as suggested by Space Telescope Imaging Spectrograph results, that this He ii resides in two separate components. When one of these is assigned to the local interstellar cloud, the implied He ionization fraction is consistent with measurements along other lines of sight. However, the resolving power and signal-to-noise available from the instrument configuration used in this first successful J-PEX flight are not sufficient to clearly identify and prove the existence of the two components

The silicon photomultiplier-based camera for the Cherenkov Telescope Array small-sized telescopes

The Cherenkov Telescope Array (CTA) is the next generation ground-based gamma-ray astronomy observatory, planned to comprise two arrays of imaging air Cherenkov telescopes (IACTs) located in the northern and southern hemispheres. Three telescope sizes are required to cover the CTA gamma-ray energy range from 20 GeV to 300 TeV. An array of several tens of Small-Sized Telescopes (SSTs) at the southern site situated in the Andes at Paranal in Chile, will provide unprecedented sensitivity above 1 TeV and up to 300 TeV, and offer the highest angular resolution of any instrument at these energies. Following a down selection from three prototype telescopes, the design finally selected for SST comprises a dual mirror Schwarzschild–Couder optic with a 4.3 m diameter primary mirror and a 1.8 m secondary mirror imaged by a silicon photomultiplier (SiPM)-based camera with a ∼9°field of view (FoV). The dual mirror optics produce a smaller plate-scale aplanatic focal plane allowing a small, low-cost camera to be employed, compared to that required for the conventional single mirror Davies-Cotton IACT design. The camera comprises an array of 2048 SiPM pixels, configured as 32 sensor and electronics modules each with an 8 × 8 pixel2 tile populated with 6 × 6 mm2 SiPM pixels. Full waveform capture on every channel is provided by the TARGET ASIC which performs the dual function of event triggering and waveform digitization of the full camera at 1 GSample/s. We describe the finalized SST camera design including its optimization for the production phase of the project anticipated to begin in 2023

Extensive air shower tracker using Cherenkov detection

Cosmic rays continuously bombard Earth’s atmosphere triggering cascades of secondary particles. Many constituents progress to reach the surface and capturing these events can intrigue and awe young curious minds, opening them to the amazing world of physics. Cloud chambers are an established method of revealing the subatomic world; frequently used by universities to introduce cosmic rays to visitors and prospective students, they provide a fascinating real-time display of the ‘ghostly’ particles showering upon those viewing. Using the Cherenkov radiation detection technique, we have developed a novel, compact, Extensive Air Shower (EAS) particle tracking method that enhances the cloud chamber visualisation of cosmic ray interactions towards a digital audience. Once digital, live event interaction can be streamed to multiple display devices presenting an immediate illustration of the event that showered in that location. Our instrument hardware is built around Cherenkov-optimised silicon photomultiplier sensors. Each single detection unit monitors particle event rate and tracks incident angle by measuring Cherenkov intensity. By operating multiple detection units in one location, we can record time correlated air shower events to monitor and collate information on the primary cosmic rays. We introduce first results, illustrating instrument response and EAS rate variations, compiled from the initial running period of our development instruments. We present intensity spectra, compare with simulation, and describe the instrument response due to sensor location, Cherenkov intensity, mean muon energy and detector acceptance angle. With further development towards low-cost readout electronics, we aim to build a networked array of trackers, located around the campus, to expand data gathering ability and scientific potential.</p

A normally-distributed crosstalk model for silicon photomultipliers

Optical crosstalk (OCT) in silicon photomultipliers (SiPM) occurs when photon detection in a microcell leads to the production of further photons that are also detected. Various models have been considered to predict experimental data with varying degrees of success. In this paper, we introduce the Normally-Distributed Crosstalk Model (NDCM), where the probability of triggering additional microcells is given by a 2-d normal distribution with a standard deviation of : a device-specific parameter representing OCT photon propagation path length in terms of microcell pitch. Monte Carlo (MC) simulations of NDCM are compared to existing models and experimental data from the CHEC-S camera developed for the Cherenkov Telescope Array, which suggests that OCT occurs with a ≈ 5 microcells in this device

Picosecond imaging at high spatial resolution using TOFPET2 AISC v2d and microchannel plate detector

Microchannel plate-based detectors provide advantages over their solid-state counterparts for applications where a combination of virtually zero noise photon-counting, large format, short wavelength sensitivity with high resolution timing and imaging are required. Solar-blind applications, Cherenkov detectors for high energy physics, and UV astronomy are such fields. The application to Cherenkov detection is particularly demanding, requiring time resolution below 100 picoseconds combined with imaging and high throughput. This requires the readout to combine high spatial and temporal event resolution at high rates necessitating a parallel readout approach. We describe a readout design comprising a pixellated readout array instrumented using multi-channel fast timing electronics and incorporating charge centroiding to achieve sub-pixel spatial resolution. We present experimental results of the relationship between time-over-threshold and signal amplitude and a centroiding image obtained with an MCP detector using a pixellated readout geometry. The readout was optimised for a fast electronics implementation based on the TOFPET system, a multi-channel all-in-one ASIC originally designed for time-of-flight PET using silicon photomultipliers

Modelling the behaviour of microchannel plates using CST particle tracking software

Photon counting detectors are essential for many applications, including astronomy, medical imaging, nuclear and particle physics. An extremely important characteristic of photon counting detectors is the method of electron multiplication. In vacuum tubes such as photomultiplier tubes and microchannel plates (MCPs), secondary electron emission (SEE) provides electron multiplication through an accelerating field across the dynode(s). A significant electron cascade can be observed in these structures which are routinely used in industry and research. Both devices have been thoroughly tested experimentally. Developing new MCP designs can be expensive and time consuming so the ability to simulate new structures will provide many advantages to instrument designers and manufacturers. There are, however, significant challenges in accurately simulating MCPs, with many geometrical variables to consider as well as material SEE properties. The SEE process is probabilistic, and with MCPs having a very high gain, significant computational resource is required to simulate the resulting electron output for a model. In our research we illustrate how this can be achieved by developing an MCP model using Computer Simulation Technology (CST) Studio Suite software. The model consists of a charged particle source, a small seven-pore MCP structure (including electrodes, resistive and emissive surfaces), as well as the readout anode, with appropriate potentials applied to the components of the model. We present simulation results from the modelled MCPs, demonstrate electron multiplication performance, and compare these results with those predicted by theory. Our goal is to expand this model and identify optimum MCP parameters, for various science applications, using novel materials to optimise detector performance

Progress towards a 256 channel multi-anode microchannel plate photomultiplier system with picosecond timing

Despite the rapid advances in solid state technologies such as the silicon photomultiplier (SiPM), microchannel plate (MCP) photomultipliers still offer a proven and practical technological solution for high channel count pixellated photon-counting systems with very high time resolution. We describe progress towards a 256 channel optical photon-counting system using CERN-developed NINO and HTDC ASICs, and designed primarily for time resolved spectroscopy in life science applications. Having previously built and demonstrated a 18 mm diameter prototype tube with an 8×8 channel readout configuration and <43 ps rms single photon timing resolution, we are currently developing a 40 mm device with a 32×32 channel readout. Initially this will be populated with a 256 channel electronics system comprising four sets of modular 64 channel preamplifier/discriminator, and time-to-digital converter units, arranged in a compact three dimensional configuration. We describe the detector and electronics design and operation, and present performance measurements from the 256 channel development system. We discuss enhancements to the system including higher channel count and the use of application specific on-board signal processing capabilities. © 2011 Elsevier B.V

Using quantum entangled photons to measure the absolute photon detection efficiency of a multi-pixel SiPM array

Spontaneous parametric down-conversion (SPDC)of a visible pump photon is the generation of two less energetic, quantum entangled photons (QEPs), often in the near infrared (NIR), using a non-linear crystal e.g. beta barium borate. Since the detection of one QEP predicates the existence of its entangled twin, QEPs have previously been used to measure the absolute photon detection efficiency (PDE), η(λ), of a detector under test by measuring time-coincident events with an additional trigger detector, allowing evaluation of η DUT (λ)without recourse to a calibrated reference detector. In this paper, the QEP absolute PDE measurement technique is outlined, and an extension of this technique is proposed to measure η(λ)for pixels on a multi-pixel array where each pixel provides an individual signal output. By treating all pixels in a multi-pixel array as indistinguishable, Monte Carlo simulations show that the symmetry of the measurement allows η(λ)to be determined for each pixel. A route towards experimental measurements using this technique with a 64-pixel SiPM array combined with a 64-channel waveform digitiser module is outlined

Operation of microchannel plate PMTs with TOFPET multichannel timing electronics

We describe an experimental programme to evaluate TOFPET multichannel timing electronics using microchannel plate PMTs in single photon counting mode. Time resolution measurements were made using: (i) the on-board electronic stim signal; (ii) a Photek PMT210 high speed single anode MCP photomultiplier detector, and; (iii) imaging with a PMT240MA multi-anode MCP detector using a pixelated multi-layer ceramic readout. Experimental measurements using an electronic stim with the ASIC electronics gave a time resolution of 43 ps rms. Detector timing of the PMT210 detector was evaluated using a 40 ps wide pulsed laser with amplitude walk correction using the time over threshold capability of the TOFPET electronics. Single photon timing resolution of better than 100 ps rms was demonstrated. Furthermore, 256 discrete pixel imaging has been demonstrated by coupling a multi-anode pixelated MCP detector to the TOFPET system