CTA: The glowing blue makeup of our universe

© Proposed CTAO Telescopes, Courtesy of CTA. Three classes of telescopes are required to cover the full CTAO energy range (20 GeV to 300 TeV). The SCT, second from the left, is being proposed as an alternative medium telescope.

A vision for more

Almost twenty years ago, visionary scientists devised a plan to push the technological boundaries of existing ground-based gamma-ray telescope detectors (H.E.S.S., MAGIC, and VERITAS). These telescopes detect the particle showers released by gamma rays reaching our atmosphere. These ultra-high energy particles create a blue flash of “Cherenkov light”, helping us to address some of the most perplexing questions in astrophysics. Their desire was to create unprecedented technology in terms of sensitivity and accuracy. Thus, was born the idea of the next generation of telescopes, Cherenkov Telescope Array (CTA) surpassing its predecessors in terms of number, size, and technology.

Fast forward to today, CTA is now a multinational, worldwide project with more than 60 telescopes divided between the northern and the southern hemispheres. Still in development, the first array in the Northern Hemisphere will focus on the study of extragalactic objects at the lowest possible energies, and a second array, in the Southern Hemisphere will cover the full energy range and concentrate on galactic sources.

Succeeding in creating such a considerable feat demands innovative technology among many things. Many actors proposed ideas and prototypes over the years aiming to find solutions to overcome the various challenges posed by such an ambitious project. Hamamatsu Photonics, present from the very early scientific discussions, competed with its leading technology - the Photomultiplier Tube (PMT). This was only the start of a journey with many obstacles to face, starting with the customization of a product to exact specifications and trialing it until its success. Not to mention designing different and improved technologies such as Hamamatsu’s groundbreaking MPPC^® (Silicon photomultiplier).

The key is blue

The particularity of CTA is its sensitivity to the highest-energy gamma rays. Gamma-ray bursts are powerful events generated from some of the most violent environments in our Universe, such as supernova explosions. In fact, these are a trillion times more energetic than visible light*. Identifying them is not an easy task, as they do not reach the earth’s surface. However, when passing through the atmosphere, they produce subatomic particle cascades. As these particles descend into our atmosphere, they lose energy and emit Cherenkov radiation. The ultraviolet and finally visible (mostly blue) fraction caused by secondary particles can then be detected from the ground by the telescopes’ high-speed camera.

CTA’s telescopes differ in size and in number as they aim to capture all the various energy spans. First, their large mirrors collect the glowing blue light, which is then detected by the high-speed PMT camera system. The light flashes are so faint and short (a few billionths of a second), it is not possible to see without this highly sophisticated technology. The PMTs detect the light flashes and amplify them in a compact, low noise, and wide dynamic range gain block. The information is then converted into a measurable electronic signal.

Early discussions projected the need for 200,000 PMTs to fulfill the whole range of telescopes. This demanded a colossal undertaking in terms of providing consistent quality to meet custom specifications. PMTs are mostly made by hand, and their assembly is an arduous task. Therefore, manufacturing and testing such a large number throughout each phase is both time-consuming and costly. One of the major pain points was also achieving high sensitivity through high quantum efficiency in order to detect the faintest light signals. This challenge limited the maximum gain of the PMT. Timing, as mentioned, is also crucial; any signal delay means missing or providing distorted data. To maintain the time window low, PMTs need to be fast and small with a highly efficient photocathode.

In addition, the after-pulse generated created a source of wrong signals that had to be minimized. Identifying the root of these challenges lead to a better understanding of materials and procedures in the manufacturing process.

Enabled by the EU FP7 program for the CTA preparatory phase, many institutes, universities, and their laboratories, in collaboration with Hamamatsu, have been involved in improving these PMT properties. In parallel, given some of the technological challenges surrounding PMTs, the development of highly sensitive light detectors based on semiconductor technology, the MPPC^®* was requested. The shape, dimensions, and microcell size of the MPPC^®s, were evaluated from a fully customized hexagonal structure to a more typical square with 75 x75 μm cells. The technology required did not exist, yet scientists collaborating with Hamamatsu researchers worked together to find the optimal solution.

*Silicon photomultiplier made by Hamamatsu Photonics

Improving technology over decades of collaboration

Over nearly twenty years of collaboration, scientists and Hamamatsu continue to push design ideas for both PMTs and MPPC^®s.

The evaluation of the PMT design was reviewed and adjusted many times creating, for example, the SBA (Super bialkali) photocathode with increased quantum efficiency compared to common bialkali photocathodes. The idea was to replace the 1” PMT with a newly developed one, which limited amplification while still maintaining the advantage of time speed. As there was less gain needed, one dynode was removed.

As the MPPC^® manufacturing process is quicker (using fewer parts, which can be mass-produced and lightweight materials), they have proven to be more cost-effective in large-scale production. Over the years, their technology improved drastically on many levels. One example is how Hamamatsu limited the crosstalk by adding trenches, effectively reducing the possibility of fake signals. Another is how the pulses improved over time.

Currently, as CTA telescopes are in their advanced stages of development, thousands of PMTs and MPPC^®s are being produced and tested. Although no CTA telescopes are currently finished, a camera prototype is already in operation, delivering promising results. The expected results obtained from CTA will provide insights into distant galaxies and extreme particle accelerators in space. Once completed, the hope is to accomplish a completely new view of the night sky and ultimately our universe.

CTA: The glowing blue makeup of our universe

A vision for more

The key is blue

Improving technology over decades of collaboration

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