An upgrade of the camera focal plane of a Schwarzschild–Couder Telescope prototype (pSCT) for the Cherenkov Telescope Array (CTA)
The CTA Observatory
The analysis of the features of ultra high energy cosmic photons allows to study the nature of astrophysical phenomena of ultra high energy and to address unsolved problems of modern physics, like the cosmic ray propagation and the nature of the dark matter.
The goal of the CTA project is to use techniques already employed by the current generation of Imaging Air Cherenkov Telescopes (IACTs), based on the detection of UV Cherenkov light produced in the atmospheric showers. The aim is to improve the flux sensitivity by at least one order of magnitude in the core energy range and to extend the energy range beyond 100 TeV.(Fig. 2).
To achieve this goal, CTA foresees the installation of telescopes with different dimensions and acceptances in two arrays (Fig. 1) , The array construction is intended to be fully complete by 2025 as open observatory for a large scientific community.
The Istituto Nazionale di Fisica Nucleare (INFN) is involved in the development of a solution for the Cherenkov photon cameras in collaboration with the Fondazione Bruno Kessler
[2](FBK) to develop SiPM sensors sensitive to UV light and proposed to equip the focal planes of some of the CTA telescope designs.
www.cta-observatory.org
The Cherenkov Telescope Array (CTA) will be the next generation of ground-based observatory of very high energy gamma ray sources.
The Italian Institute of Nuclear Physics (INFN) is involved in the R&D effort for the development of a possible solution for one of the Cherenkov photon camera designs, working on replacing the Hamamatsu MPPC S12642-0404PA-50 with more UV sensitive ones from Fondazione Bruno Kessler (FBK). INFN is currently developing the preamplifiers and the carrier boards for the SiPM chips that interface with the mechanics of the camera.
To test the feasibility and the performance of SiPM cameras, a focal plane camera prototype module, upgraded with High Density NUV – SiPMs, produced by FBK, with a micro cell of 30 μm x 30 μm and 6 mm x 6 mm area, is being assembled.
In this work, we describe the SiPM carrier boards, the assembly process and the qualification tests (IV curves and dark count) performed, before and after assembly, on the focal plane modules to qualify the procedures.
The SiPM sensors
Silicon PhotoMultipliers (SiPM) are arrays of microcells of Avalanche Photo Diodes (APD) connected in parallel and operated in Geiger mode. They are characterized by gain factors of ~10
6, timing of the order of ~ns, with recovery times comparable to those of conventional phototubes. This features make SiPMs ideal to equip the telescopes cameras of CTA.
SiPM FBK p
+-n of 6x6 mm
2dimensions with 30x30 μm
2microcells, sensitive to near UV light and breakdown voltages of
~28V, called High Density NUV (NUV-HD), have been selected to assemble the camera focal plane for a Schwarzschild – Couder Telescope prototype update. The sensors have been characterized both with pulsed sources and in dark conditions. They show a noise rate less than 100 kHz/mm
2, an excellent signal-to-noise ratio for single photons and a Photon Detection Efficiency greater than 40% in UV region. Details on the SiPMs performances are given elsewhere in this conference
[8].
Module design and prototypes
The module and mechanical design and the front-end electronics (Fig. 3) were adapted to equip the focal plane of the Prototype Schwarzschild-Couder Telescope (pSCT), which will be operated at the VERITAS site in Arizona at the end of 2016.
The focal plane of the pSCT will be equipped with 16 matrices made of 64 NUV-HD SiPMs.
Each matrix is made by four independent modules (quadrants) equipped with 16 NUV-HD SiPMs.
Module prototypes made of 16 NUV-HD SiPMs have already been assembled and tested (Fig. 4). The characterization confirmed a very good uniformity, better than 10%, to light signals and noise.
Each unit has been designed to cover a 26.9 × 26.9 mm
2area, corresponding to 53.8 × 53.8 mm
2area of each matrix. Each 64 SiPM matrix will be arranged in a “chessboard” geometry with a matrix-matrix distance of 200μm. The mechanical layout of the 16 NUV-HD module PCB is shown in Fig.4. With this configuration, the matrix pitch is 54 mm, while the sensor pitch is 6.75 mm, equally distributed all over the camera area. Each matrix is readout by a dedicated front end board to amplify, shape and digitize the photon detector signal, described in [9].
Before the placement of SiPMs, the PCB will be thermally coupled to the copper blocks used as mechanical structure of the PCB and to radiate excess heat produced by the electronics. This step is crucial to achieve the best possible alignment between the open surface of copper planes and the focal planes modules in the x, y and z coordinates. Custom mechanical holders will be produced with holes and position pins to achieve a high accuracy for the alignment in the xy plane.
Fig. 2: Differential sensitivity to a pointlike gamma source for CTA sites, compared with
actual IACTs and space experiments [4,5]
1000-01000 1000- 0 1000
Fig. 1: Possible configuration of the CTA telescopes in the
southern site.[3]
REFERENCES
[1] Progetto Premiale ‘’Telescopi Cherenkov made in Italy Teche.it’’
[2] https://www.fbk.eu
[3] G. Maier et al., arXiv:1508:06042
[4] www.cta-observatory.org (2015.05.05) [5] T. Hassan et al., arXiv:1508:06075
[6] C.R. Benn, S.L. Ellison, New Astronom. Rev., 42, 503 (1998) [7] K.J. Meager for the CTA collaboration, arXiv:1407:3271
[8] D. Simone et al., SiPM based camera for the Cherenkov Telescope Array, this conference
[9] C.Bonavolontà et al., Development of a charge preamplifier to improve NUV-HD SiPM performance, this conference
Acknowledgemnts
Authors wish to thank the support received by the agencies and organizations in Funding Agencies at www.cta- observatory.org
1 km
LST: Large Size Telescope MST: Medium Size Telescope SST: Small Size Telescope
Ener gy (T eV) 2- 10 1- 10 1 10 2 10
-1)
-2 s Flux Sensitivity (erg cm
2 ´ E
14- 10 13- 10 12- 10 11- 10 10- 10
LST MST SST
CTA South 50h CTA North 50h
Fermi 10 years
Veritas 50h Magic 50h
HAWK 1 year HAWK 5 years
Differential Sensitivity
5 bins per decade in energy
Fig. 3: SiPM NUV-HD 6x6 mm2
SiPM = 6,24 x 6,24
Dimensioni Modulo = 53,8 x 53,8 Spazio x Bonding= 0,51
Passo di ripetizione = 6,75 Distanza SiPM-bordo = 0,16
A4
V.Postolache
R1
CTA MODULE
PESO: $PRPSHEET:{Peso}
$PRPSHEET:{Fine}
FR4 MODULE 6 x 6 x SiPM Separati
FOGLIO 1 DI 1 SC ALA:2:1
N. DISEGNO TITOLO:
REVISIONE NON SC ALARE IL DISEGNO
MATERIALE:
DATA FIRMA
INFN PERUGIA
DISEGNATO
SBAVATURA E BORDI NETTI INTERRUZIONE DEI
NOME
VERIFIC ATO APPROVATO FATTO
FINITURA:
ANGOLARE:
QUALITA' SE NON SPECIFICATO:
QUOTE IN MILLIMETRI FINITURA SUPERFIC IE:
TOLLERANZE:
LINEARE:
15/02/2016
6,24
0,16 6,24
0,51
6,75
53,80 0,51
53,80
0,16
54x54mm2 module equipped with 64 NUV-
HD SiPM
Bias (V) 2626.52727.52828.529
Current (nA)
210 310[01][02][03][04][05][06][07][08][09][10][11][12][13][14][15][16]
Fig. 4: Matrix layout, 16 NUV-HD SiPM module prototype, IV curves for16 SiPM and the design of the SiPM carrier board.
Prototype equipped with 16 NUV-HD SiPM
In order to speed up the SiPM placement and glueing on the PCBs, a “pick & place” machine is used to first distribute the
conductive glue to the PCB top layer metal pads and with a vacuum suction cup to take the sensors out of the custom holder and place them on the PCB.
A preliminary test to validate the accuracy of the pick & place machine has been run using a previous production of NUV SiPM with 3.5 × 3.5 mm2 area. 64 SiPMs have been placed by the machine on a custom PCB used for a previous project ∼ .
The residuals with respect to the nominal positions and the rotation of the sensor with respect to the coordinate system are reported in Fig. 5, the accuracy to which the sensors have been placed amounts to 30μm, with a spread in the angular ∼ resolution of 0.5◦. From the maps of the matrices, it is clear that the outliers in the distributions are dominated by a cluster in ∼ the top-rightmost corner. The preliminary tests are in conclusion encouraging. Additional tests will be run using more solid PCB produced to hold 6 × 6 mm2 sensors and NUV-HD SiPMs.
The SiPM matrices will be electrically tested after the bonding step to ensure that all the SiPMs are properly working with an automatic test procedure, by using a multiplexer.
The final step of the assembly is the epoxy dispensing over the SiPM to protect bonds and surface. The epoxy used to cover the modules has to have UV transparency properties to minimize the signal light absorption and mechanical properties for efficient dispensing. Since the UV light absorption probability increases with depth, the epoxy layer has to be dispensed with the highest possible level of uniformity all over the module, avoiding border effects.
Preliminary tests of epoxy deposition and transmittance are being done and are quite encouraging.
Fig. 5: Top Left and Top Center: distribution of residuals with respect to the nominal SiPM size
Top Right: distribution of sensor rotations with respect to the absolute coordinate system. Bottom: the same quantities are shown as function of the sensor position in the matrix.
