Project
LATTES: an innovative detector for very high energy gamma ray astrophysics in the southern hemisphere

Code PTDC/FIS-PAR/29158/2017 e POCI/01-0145-FEDER-029158

Beneficiary Entity

LIP - Laboratório de Instrumentação e Física Experimental de Partículas

Project summary

Summary

The goal of this project is to ensure the Portuguese participation in the design and prototyping of a detector able to monitor the Southern gamma-ray sky above 50 GeV, filling in the energy gap between satellites and ground arrays.

The observations of gamma-ray telescopes in the last decade changed radically our perception of the Universe. High-intensity flares with an energy spectrum extending beyond the GeV have been observed. The next step will be led by instruments able to survey continuously large portions of the sky, and sensitive to the energy gap between satellites and ground arrays (50 GeV - 0.5 TeV). Present and planned large field-of-view (FoV) gamma-ray observatories are located in the Northern hemisphere, missing in particular the galactic center. While a wide FoV Southern observatory will surely be built, the question is whether it will be similar to the existing extensive air shower (EAS) arrays, or an innovative solution able to cover the energy gap. The technological challenges of lowering the energy threshold of ground arrays have been considered nearly impossible to overcome, as they stem from the physics of air showers.

LATTES is currently the only wide FoV observatory planned for the Southern hemisphere. Such an instrument will be a powerful time-variance explorer, able to issue pointing alerts to imaging atmospheric Cherenkov telescopes (IACTs) and thus fully complementary to the Cherenkov Telescope Array (CTA). It will collect abundant data and play a fundamental role in the search for emissions from extended regions, as the Fermi bubbles or dark matter annihilation regions. To overcome the huge technology issues involved, LATTES proposes an innovative concept: a compact EAS array of hybrid detector units combining two autonomous resistive plate chambers (RPCs) with a water Cherenkov detector (WCD). RPCs provide very good space and time resolution, while the WCD ensures trigger efficiency and background rejection. Combined, they bring good sensitivity all the way down to 50 GeV. LATTES will cover an area of at least 20,000 m2 and be placed about 5,000 m above sea level. The proposed solution is conceptually and technologically innovative but relies on well-grounded R&D in which LIP had a leading role.

The LATTES concept has been proposed by scientists from Portugal (LIP), Brazil (CBPF) and Italy (Universities of Padova, Rome and Udine). The priority of the LATTES international team is to develop the concept in its different dimensions, bringing it in the next five years to the point in which it is mature for construction of a full-scale observatory. The present proposal will allow Portugal to maintain its key role in this ambitious project, funding, the required detector R&D, and the development of the simulation and performance evaluation framework. This implies namely contributing to the construction of two full-size prototype detector units.

Achieved Results

The main goal of this proposal was to pave the way towards the construction of the next-generation wide field-of-view gamma-ray observatory, able to monitor the whole Southern hemisphere sky and to cover the flux sensitivity energy gap between satellite-borne and ground-based experiment. The proposed experiment, named as LATTES in memory of Cesar Lattes, a famous Brazilian experimental cosmic ray physicist, was planned to be a compact array of small detector units to be placed at an altitude of 5000 m and covering an area of 20000 m^2.

An innovative hybrid detector unit concept was explored: two autonomous resistive plate chambers (RPCs) placed on top of a water Cherenkov detector (WCD) with reflective white walls. The WCD would serve essentially as a high-performance calorimeter and the RPCs would provide accurate timing for the arrival of the shower secondary particles to the ground. The former would allow triggering on low energy showers, effectively lowering the energy threshold, while the latter would ensure a good geometric reconstruction, essential to identify the astrophysical source and reduce the hadronic background. The results obtained with the simulation studies demonstrated that this detector concept is able to reach with good sensitivity energies around 100 GeVs, thus covering the existing energy gap.

In the meanwhile, parallel efforts to design the next gamma-ray observatory in the Southern hemisphere appeared all around the world. We had a central role in the effort which led to the unification of most of these proposals being, in particular, responsible by the organization in May 2019 of the meeting in Lisbon where it was decided the formation of the SWGO collaboration - a three year R&D project to design the optimal gamma-ray wide-field experiment to be placed in South America covering an area of 80000 m^2. Dissemination and outreach activities are also an integrated part of our work.

In detail:
1. Preliminary studies on the detectability of extended sources (Fermi Bubbles) and transients (GRB and flares) were conducted. The complementarity between an experiment similar to LATTES but with an area four times higher and the planned high sensitivity narrow field of view Cherenkov Telescope Array observatory (CTA) was explored. The obtained results gave origin to a peer-reviewed publication.
2. An end-to-end simulation was developed to evaluate LATTES baseline performance. A fast simulation was created to significantly increase the number of events that will be possible to simulate, allowing thus to explore the high-energy region. First studies on the inclusion of a sparse array were made. The background rate due to atmospheric muons (and small showers) was also evaluated.
3. Classical algorithms for the event angular and energy reconstruction as well as for gamma-hadron separation, were implemented. The obtained performances at 1 TeV energies were similar to the best present experiment (HAWC), but the sensitivities at low energies were significantly better. This work led to one publication.
5. A small hypobaric chamber was designed and built with the purpose to test in the future RPC prototypes able to operate efficiently at lower pressure equivalent of high-altitude (~5000 m).
6. Detailed 1 D thermal simulations of the detector units placed in the Atacama site were developed in collaboration with the IST Mechanical Engineering department. The 3D model of such simulations is under development. A test setup to study the freezing of the water in the WCDs was built and validated using pure water.
7. One RPC hodoscope demonstrator for outreach purposes was built and installed in the Lousal Mine, in southern Portugal which is today a Science Centre. A webpage with information about the project was created (www.lip.pt/experiments/lattes/).

8. Preliminary studies on the use of artificial neural networks to explore the signal patterns of showers at the ground were conducted in collaboration with the computer science departments of the University of Coimbra. A significant improvement of the gamma/hadron discrimination at energies of 1 TeV was achieved which led to a publication of a seminal paper.

9. A new detector concept was proposed for SWGO. The station is a small water Cherenkov detector with four photomultipliers at the bottom. The PMT signal time trace was analysed with a convolutional neural network, developed in collaboration with the computer science department of the Granada University, allowing to tag extensive air shower muons in the station. With this method it is possible to achieve an excellent gamma/hadron discrimination for energies of ~ 1 TeV, similar to those achieved by experiments with much bigger (and costly) stations. These results gave origin to a peer-reviewed publication.

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Support under

Reforçar a investigação, o desenvolvimento tecnológico e a inovação

Region of Intervention

Centro  /  Lisboa

 

Funding

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Dates

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