- Email: [email protected]
Status of the development of large area photon detectors based on THGEMs and hybrid MPGD architectures for Cherenkov imaging applications
Nuclear Instruments and Methods in Physics Research A 824 (2016) 139–142
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Status of the development of large area photon detectors based on THGEMs and hybrid MPGD architectures for Cherenkov imaging applications M. Alexeev a,b, R. Birsa c, F. Bradamante c,d, A. Bressan c,d, M. Büchele e, M. Chiosso a,b, P. Ciliberti c,d, S. Dalla Torre c, S. Dasgupta c,d, O. Denisov a, V. Duic c,d, M. Finger f,g, M. Finger Jr.f,g, H. Fischer e, M. Giorgi c,d, B. Gobbo c, M. Gregori c, F. Herrmann e, K. Königsmann e, S. Levorato c, A. Maggiora a, A. Martin c,d, G. Menon c, K. Steiger c,h, J. Novy f,g, D. Panzieri a,i, F.A. Pereira j, C.A. Santos c,n, G. Sbrizzai c,d, P. Schiavon c,d, S. Schopferer e, M. Slunecka f,g, F. Sozzi c, L. Steiger c,h, M. Sulc h, S. Takekawa a,b, F. Tessarotto c, J.F.C.A. Veloso j, N. Makke c,d a
INFN - Sezione di Torino, Torino, Italy University of Torino, Torino, Italy INFN - Sezione di Trieste, Trieste, Italy d University of Trieste, Trieste, Italy e Universität Freiburg, Physikalisches Institut, Freiburg, Germany f Charles University, Prague, Czech Republic g JINR, Dubna, Russia h Technical University of Liberec, Liberec, Czech Republic i University of East Piemonte, Alessandria, Italy j i3N-Physics Department, University of Aveiro, Aveiro, Portugal b c
art ic l e i nf o
a b s t r a c t
Available online 27 November 2015
We report about the development status of large area gaseous single photon detectors based on a novel hybrid concept for RICH applications. The hybrid concept combines Thick Gaseous Electron Multipliers (THGEMs) coupled to CsI, working as a photon sensitive pre-amplification stage, and Micromegas, as a multiplication stage. The most recent achievements within the research and development programme consist in the assembly and study of 300 300 mm2 hybrid photon detectors, the optimization of front-end electronics, and engineering towards large area detectors. Hybrid detectors with an active area of 300 300 mm2 have been successfully operated in laboratory conditions and at a CERN PS T10 test beam, achieving effective gains in the order of 105 and good time resolution (σ ¼ 7 ns); APV25 front-end chips have been coupled to the detector resulting in noise levels lower than 1000 electrons; the production and characterization of 300 600 mm2 THGEMs is ongoing. A set of hybrid detectors with 600 600 mm2 active area is envisaged to upgrade COMPASS RICH-1 at CERN in 2016. & 2015 Elsevier B.V. All rights reserved.
Keywords: THGEM Micromegas Hybrid detector Micro-pattern gas detectors RICH
1. Introduction RICH-1 is a Ring Imaging Cherenkov detector [1] within the Large Angle Spectrometer at the COMPASS experiment [2] at CERN. It is based on a mirror focused configuration [3] and comprises, therefore, n
Corresponding author. E-mail address: [email protected] (C.A. Santos).
http://dx.doi.org/10.1016/j.nima.2015.11.034 0168-9002/& 2015 Elsevier B.V. All rights reserved.
a gaseous radiator, a mirror system and a photon detection system which consists of 576 Multi Anode Photon Multipliers (MAPMTs) [4], covering the most central area of the detection system (25% of its sensitive area), and 12 MultiWire Proportional Chambers (MWPCs) [5] of 600 600 mm2 each, surrounding the MAPMTs. The phase II of the COMPASS physics programme [6] foresees the operation under increased rates, which requires the maintenance of the performance of RICH-1 over future years at the present level, in
140
M. Alexeev et al. / Nuclear Instruments and Methods in Physics Research A 824 (2016) 139–142
more challenging conditions. While MAPMTs can cope with an increased rate, the MWPCs suffer significant ageing under higher photon fluxes and their performance is limited under such conditions. Moreover, MWPCs are intrinsically slow-response detectors and faster ones are needed. In order to enhance the performance of the photon detectors of RICH-1, an upgrade of four of the MWPCs installed in the peripheral area is planned, for early 2016. The investigation of photon detectors for the aimed RICH-1 upgrade resulted in a novel detector concept that will be presented: the Hybrid detector [7]. It is the outcome of eight years of an extensive research and development programme, consisting of the development and characterization of THGEM structures; study of multi-layer THGEM detectors; evolution from triple-layer THGEM configurations to a hybrid concept that merges two Micro Pattern Gas Detectors (MPGD) architectures; optimization of front-end electronics; and engineering towards large area photon detectors.
2. THGEM based photon detectors THGEM [8] structures consist of a drilled set of printed circuit board (PCB) that undergoes a chemical etching process. The application of a voltage difference to the two copper sides of the PCB, in a proper gas, allows charge originated from gas ionization to be multiplied through an avalanche process. THGEMs can be arranged in multi-layer configurations and photon sensitivity can be achieved by depositing a CsI reflective photocathode on the upper THGEM layer. Exhaustive characterization of 30 30 mm2 single layer THGEM detectors allowed the study and optimization of the THGEM parameters (hole diameter, pitch, thickness and rim) [9], as well as their production procedure. The stacking of these structures in multi-layer detectors permitted the study of the detector's working principles and the effectiveness of THGEM staggering as a solution to suppress the ions, originated by the gas ionization, that reach the CsI layer (IBF – Ion back flow) [10], and that are potentially harmful for its life span and for the performance of the detector [11]. Further, triple-layer THGEM detectors have been successfully operated in laboratory and beam tests [12]. The scaling from small sizes to 300 300 mm2 active area THGEMs imposed a big challenge in the development of large THGEM-based detectors [13]. The thickness non uniformity observed in some PCB sheets, with thickness variations up to 15% resulted in gain variations throughout the same THGEM of up to 40%. Therefore, in order to assemble a 300 300 mm2 THGEM detector, the selection of the THGEMs with the best performance and the most uniform gain corresponds to a critical aspect. In this regard, the thickness of tens of PCB sheets was measured to assure thickness variations smaller than 5% for the THGEM production. The produced THGEMs were then subjected to individual characterization through the study of the maximum ΔV applicable throughout the whole THGEM, and the study of the gain uniformity using soft X-rays. Triple-layer THGEM detectors, of the same active area, have then been successfully assembled and operated in laboratory tests. The detector's performance has also been evaluated at test beams, collecting single photon events of Cherenkov light. A pile of events is shown in Fig. 1, where beam particle events are also visible in the center of the Cherenkov corona.
3. The hybrid photon detector The hybrid photon detector concept appears as an evolution of the triple-layer THGEM detectors, resulting in a gaseous detector that merges THGEMs with Micromegas [14]. This MPGD architecture can be coupled with other amplification structures such as GEMs [15] or
Fig. 1. Cherenkov ring obtained by superimposing events in a 300 300 mm2 triple-layer THGEM detector [16].
Fig. 2. Scheme of the hybrid detector concept comprising a dual layer THGEM configuration (thickness: 0.4 mm; hole diameter: 0.4 mm; pitch: 0.8 mm), and a Micromegas (128 μm over a multi-pad anode).
THGEMs, and has the intrinsic capability of blocking ions while being transparent to electrons [14]. Altogether, the hybrid THGEM – Micromegas detector, schematized in Fig. 2, makes use of a double layer THGEM þCsI configuration (photon sensitive pre-amplification stage), and a bulk Micromegas. The selected THGEM parameters are thickness of 0.4 mm; hole diameter of 0.4 mm; and pitch of 0.8 mm, while the Micromegas has a 128 μm amplification gap. Overall, the hybrid concept results in a robust detector with high photoelectron extraction efficiency, fast signals, high gain, photon feedback suppression, and time stability. Additionally, the misalignment of the holes of the two THGEM layers [10], together with the ion blocking capability of the Micromegas, makes the hybrid detector effective in the IBF suppression. The hybrid detector also uses a capacitive multi-pad anode, where a set of pads at the surface of the anode is at nominal voltage, and a
M. Alexeev et al. / Nuclear Instruments and Methods in Physics Research A 824 (2016) 139–142
second set of pads, inserted into the anode fibre glass, collects signals induced by RC coupling. The implementation of the capacitive anode grants a better protection of the front-end electronics. 3.1. Study and development of the hybrid detector The development of the hybrid detector concept underwent several stages following the evolution of active area of THGEM structures. The coupling of a 30 30 mm2 dual-layer THGEM detector with a Micromegas [7] proved the validity of the hybrid detector, and gains in the order of 105 and 106 were obtained, in laboratory conditions, using soft X-rays and UV photons respectively. The evolution towards 300 300 mm2 hybrid detectors relied in the selection of THGEMs and Micromegas with uniform gain within their active area. In this regard, and in addition to the THGEMs characterization, also Micromegas of the same sensitive area were characterized to study their gain uniformity, with gain variations below the level of 10%. The laboratory study of 300 300 mm2 hybrid detectors, under a Ar=CH4 (70:30) atmosphere, using an analogue readout chain (charge sensitive pre-amplifier þamplifierþ MCA), while being irradiated by either soft X-rays or UV light showed the capability of the device in reaching, stably, effective gains in the order of 105, which matches the performance of these 300 300 mm2 sized detectors with 30 30 mm2 ones. 3.2. Detector performance in BEAM To study the performance of the hybrid detector in beam conditions, a dedicated test at the T10 facility of the PS accelerator at CERN was performed. The detector was coupled to a fused silica radiator to create a Cherenkov corona centered in the sensitive area of the detector [16]. The detector's performance under such conditions, evaluated by using an analogue readout system, matched the performance in laboratory conditions, with gains in the order of 105 (Fig. 3) when detecting Cherenkov photons, and similar electrical stability. Additional studies allowed the investigation of the effect of the drift field in the suppression of charged particles signals (Fig. 4), as well as its influence in the photon yield of the detector. Results showed that charged particles can be significantly suppressed with negative or mild positive fields. Above 100 V/cm their collection by the detector is enhanced. Simultaneously, the photon yield is maximized by an equally mild positive drift field (approximately 100 V/cm). Additionally, a digital readout system, based on CMAD front-end chips and DREISAM cards [17], was used to reconstruct the Cherenkov ring over the anode pads and study the time distribution of the events (Fig. 5), which resulted in a very good time resolution ðσ 7 nsÞ.
141
3.3. Detector & APV25 Despite the successful operation of the hybrid detector with the aforementioned digital readout system, complementary tests were performed to study the electronic noise level of APV25 front-end chips [18] when coupled to the capacitive anode of the hybrid detector. As these chips are already in use as front-end electronics of the RICH-1 MWPCs [19], this study aimed to evaluate the possibility of using the existing readout electronics in the final detectors. Preliminary results of the coupling of the hybrid detector to one APV25 chip, in which each ADC channel corresponds approximately to 250 electrons, indicate noise levels not higher than 1000 electrons for each of the 128 channels of the APV25 chip.
4. Towards large area detectors The final detectors that will be used for the 2016 RICH-1 upgrade are meant to cover an area of 600 600 mm2 each. The coverage of this active area is to be achieved by merging pairs of THGEMs or Micromegas with 300 600 mm2 in the same plain. This design concept aims to facilitate the procurement of a bunch of THGEMs with good thickness uniformity. Therefore, the thickness measurement of 50 PCB sheets of 700 700 mm2 has been performed, through an automated process, in 36 36 equidistant points of each sheet. The distribution of the thickness variation, δthickness, for the 50 PCB sheets, is presented in Fig. 6, showing that 43 of the 50 pieces have thickness variations smaller than 4%. Photon Yield
1
Amplitude for beam particles
0.8 0.6 0.4 0.2 0
-1000
-500
0
500
Fig. 4. Cherenkov photon yield and sensitivity to charged particles, normalized to an arbitrary scale, as a function of the applied electrical drift field, for a 300 300 mm2 hybrid detector.
500 χ2 / ndf Constant
Counts
Slope
97.03 / 95 -0.04741 ± 0.00095
10
90.09 / 24 458.4 ± 8.4 -766.6 ± 0.1 7.208 ± 0.089
300 200 100
1
0
χ 2 / ndf Constant Mean Sigma
400
5.185 ± 0.027
Counts
102
1000
Drift Field [kV/cm]
50
100
150
200
Charge [fC] Fig. 3. Signal amplitude spectrum of Cherenkov photons, using a hybrid detector of 300 300 mm2 with a gain of 105 .
0
-950
-900
-850
-800
-750
-700
-650
-600
Time [ns] Fig. 5. Time distribution of detected Cherenkov photon events in a 300 300 mm2 hybrid detector, with σ 7 ns.
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M. Alexeev et al. / Nuclear Instruments and Methods in Physics Research A 824 (2016) 139–142
10
capable of single photon detection and presenting stable gains in the order of 105. Additionally, the study of the electronic noise level of the APV25 front-end chips when coupled to the detector has proven the good chip performance, allowing the use of the existing readout electronics of COMPASS RICH-1. The RICH-1 upgrade will then proceed by adopting hybrid detectors as a replacement for the existing MWPCs. The mass production of the final THGEMs is currently ongoing, aiming for the assembly and installation of the new Photon Detectors for the 2016 COMPASS physics run.
Frequency distribution of δthickness of 50 PCBs
Frequency
8 6 4 2 0
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
δthickness [%] Fig. 6. Distribution of the thickness variation (δthickness) for a set of 50 700 700 mm2 sized PCBs.
χ2 / ndf Constant Mean Sigma
250
Counts
200
12.28 / 10 266.1 ± 9.6 471.2 ± 0.1 1.935 ± 0.045
Acknowledgments The author F. A. Pereira is supported by a Doctoral Grant from FCT, Reference SFRH/BD/81259/2011.
References
150 100 50 0
420
430
440
450
460
470
480
490
500
Thickness [μ m]
Fig. 7. Distribution of a thickness measurement for 36 36 points of a PCB with δthickness ¼ 2:6%, and a theoretical thickness of 470 μm.
Fig. 7 shows the thickness distribution of the 36 36 measured points of a PCB sheet with δthickness ¼ 2:6%. The mean measured value of 471:2 μm is quite close to the expected thickness of the PCB (400 μm plus 2 copper layers of 35 μm each), with small variations along the area of the sheet: σ ¼ 1:93 μm. Four THGEMs with an active area of 300 600 mm2 have been produced and tested, with results matching the ones obtained for 300 300 mm2 THGEMs, validating the size scaling, and allowing the mass production of 300 600 mm2 sized THGEMs, which is currently ongoing. A future characterization of the produced structures will allow the selection of the THGEMs with best performance for the assembly of the final detectors for the RICH-1 upgrade.
5. Conclusion The concept of the hybrid THGEM – Micromegas detector has been validated by comprehensive laboratory and beam tests, being
[1] F. Tessarotto, et al., Journal of Instrumentation 9 (2014) C09011. [2] P. Abbon, et al., Nuclear Instruments and Methods in Physics Research Section A 577 (2007) 455; P. Abbon, et al., Nuclear Instruments and Methods in Physics Research Section A 779 (2015) 69. [3] E. Nappi, J. Seguinot, La Rivista del Nuovo Cimento 28 (2005) 1. [4] P. Abbon, et al., Nuclear Instruments and Methods in Physics Research Section A 595 (2008) 23. [5] G. Baum, et al., Nuclear Instruments and Methods in Physics Research Section A 433 (1999) 207. [6] The COMPASS Collaboration, COMPASS-II Proposal, 2010. [7] M. Alexeev, et al., Journal of Instrumentation 8 (2013) C12005. [8] A. Breskin, et al., Nuclear Instruments and Methods in Physics Research Section A 598 (2009) 107; S. Dalla Torre, Nuclear Instruments and Methods in Physics Research Section A 639 (2011) 111. [9] M. Alexeev, et al., Journal of Instrumentation 5 (2010) P03009. [10] M. Alexeev, et al., Journal of Instrumentation 8 (2013) P01021. [11] H. Hoedlmoser, et al., Nuclear Instruments and Methods in Physics Research Section A 574 (2007) 28. [12] M. Alexeev, et al., Nuclear Instruments and Methods in Physics Research Section A 695 (2012) 159. [13] M. Alexeev, et al., Journal of Instrumentation 9 (2014) C03046. [14] Y. Giomataris, et al., Nuclear Instruments and Methods in Physics Research Section A 376 (1996) 29. [15] S. Kane, et al., IEEE Nuclear Science Symposium Conference Record 1 (2001) 265. [16] M. Alexeev, et al., Nuclear Instruments and Methods in Physics Research Section A 732 (2013) 264. [17] P. Abbon, et al., Nuclear Instruments and Methods in Physics Research Section A 587 (2008) 371. [18] M.J. French, et al., Nuclear Instruments and Methods in Physics Research Section A 466 (2001) 359. [19] P. Abbon, et al., Nuclear Instruments and Methods in Physics Research Section A 567 (2006) 104.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Status of the development of large area photon detectors based on THGEMs and hybrid MPGD architectures for Cherenkov imaging applications M. Alexeev a,b, R. Birsa c, F. Bradamante c,d, A. Bressan c,d, M. Büchele e, M. Chiosso a,b, P. Ciliberti c,d, S. Dalla Torre c, S. Dasgupta c,d, O. Denisov a, V. Duic c,d, M. Finger f,g, M. Finger Jr.f,g, H. Fischer e, M. Giorgi c,d, B. Gobbo c, M. Gregori c, F. Herrmann e, K. Königsmann e, S. Levorato c, A. Maggiora a, A. Martin c,d, G. Menon c, K. Steiger c,h, J. Novy f,g, D. Panzieri a,i, F.A. Pereira j, C.A. Santos c,n, G. Sbrizzai c,d, P. Schiavon c,d, S. Schopferer e, M. Slunecka f,g, F. Sozzi c, L. Steiger c,h, M. Sulc h, S. Takekawa a,b, F. Tessarotto c, J.F.C.A. Veloso j, N. Makke c,d a
INFN - Sezione di Torino, Torino, Italy University of Torino, Torino, Italy INFN - Sezione di Trieste, Trieste, Italy d University of Trieste, Trieste, Italy e Universität Freiburg, Physikalisches Institut, Freiburg, Germany f Charles University, Prague, Czech Republic g JINR, Dubna, Russia h Technical University of Liberec, Liberec, Czech Republic i University of East Piemonte, Alessandria, Italy j i3N-Physics Department, University of Aveiro, Aveiro, Portugal b c
art ic l e i nf o
a b s t r a c t
Available online 27 November 2015
We report about the development status of large area gaseous single photon detectors based on a novel hybrid concept for RICH applications. The hybrid concept combines Thick Gaseous Electron Multipliers (THGEMs) coupled to CsI, working as a photon sensitive pre-amplification stage, and Micromegas, as a multiplication stage. The most recent achievements within the research and development programme consist in the assembly and study of 300 300 mm2 hybrid photon detectors, the optimization of front-end electronics, and engineering towards large area detectors. Hybrid detectors with an active area of 300 300 mm2 have been successfully operated in laboratory conditions and at a CERN PS T10 test beam, achieving effective gains in the order of 105 and good time resolution (σ ¼ 7 ns); APV25 front-end chips have been coupled to the detector resulting in noise levels lower than 1000 electrons; the production and characterization of 300 600 mm2 THGEMs is ongoing. A set of hybrid detectors with 600 600 mm2 active area is envisaged to upgrade COMPASS RICH-1 at CERN in 2016. & 2015 Elsevier B.V. All rights reserved.
Keywords: THGEM Micromegas Hybrid detector Micro-pattern gas detectors RICH
1. Introduction RICH-1 is a Ring Imaging Cherenkov detector [1] within the Large Angle Spectrometer at the COMPASS experiment [2] at CERN. It is based on a mirror focused configuration [3] and comprises, therefore, n
Corresponding author. E-mail address: [email protected] (C.A. Santos).
http://dx.doi.org/10.1016/j.nima.2015.11.034 0168-9002/& 2015 Elsevier B.V. All rights reserved.
a gaseous radiator, a mirror system and a photon detection system which consists of 576 Multi Anode Photon Multipliers (MAPMTs) [4], covering the most central area of the detection system (25% of its sensitive area), and 12 MultiWire Proportional Chambers (MWPCs) [5] of 600 600 mm2 each, surrounding the MAPMTs. The phase II of the COMPASS physics programme [6] foresees the operation under increased rates, which requires the maintenance of the performance of RICH-1 over future years at the present level, in
140
M. Alexeev et al. / Nuclear Instruments and Methods in Physics Research A 824 (2016) 139–142
more challenging conditions. While MAPMTs can cope with an increased rate, the MWPCs suffer significant ageing under higher photon fluxes and their performance is limited under such conditions. Moreover, MWPCs are intrinsically slow-response detectors and faster ones are needed. In order to enhance the performance of the photon detectors of RICH-1, an upgrade of four of the MWPCs installed in the peripheral area is planned, for early 2016. The investigation of photon detectors for the aimed RICH-1 upgrade resulted in a novel detector concept that will be presented: the Hybrid detector [7]. It is the outcome of eight years of an extensive research and development programme, consisting of the development and characterization of THGEM structures; study of multi-layer THGEM detectors; evolution from triple-layer THGEM configurations to a hybrid concept that merges two Micro Pattern Gas Detectors (MPGD) architectures; optimization of front-end electronics; and engineering towards large area photon detectors.
2. THGEM based photon detectors THGEM [8] structures consist of a drilled set of printed circuit board (PCB) that undergoes a chemical etching process. The application of a voltage difference to the two copper sides of the PCB, in a proper gas, allows charge originated from gas ionization to be multiplied through an avalanche process. THGEMs can be arranged in multi-layer configurations and photon sensitivity can be achieved by depositing a CsI reflective photocathode on the upper THGEM layer. Exhaustive characterization of 30 30 mm2 single layer THGEM detectors allowed the study and optimization of the THGEM parameters (hole diameter, pitch, thickness and rim) [9], as well as their production procedure. The stacking of these structures in multi-layer detectors permitted the study of the detector's working principles and the effectiveness of THGEM staggering as a solution to suppress the ions, originated by the gas ionization, that reach the CsI layer (IBF – Ion back flow) [10], and that are potentially harmful for its life span and for the performance of the detector [11]. Further, triple-layer THGEM detectors have been successfully operated in laboratory and beam tests [12]. The scaling from small sizes to 300 300 mm2 active area THGEMs imposed a big challenge in the development of large THGEM-based detectors [13]. The thickness non uniformity observed in some PCB sheets, with thickness variations up to 15% resulted in gain variations throughout the same THGEM of up to 40%. Therefore, in order to assemble a 300 300 mm2 THGEM detector, the selection of the THGEMs with the best performance and the most uniform gain corresponds to a critical aspect. In this regard, the thickness of tens of PCB sheets was measured to assure thickness variations smaller than 5% for the THGEM production. The produced THGEMs were then subjected to individual characterization through the study of the maximum ΔV applicable throughout the whole THGEM, and the study of the gain uniformity using soft X-rays. Triple-layer THGEM detectors, of the same active area, have then been successfully assembled and operated in laboratory tests. The detector's performance has also been evaluated at test beams, collecting single photon events of Cherenkov light. A pile of events is shown in Fig. 1, where beam particle events are also visible in the center of the Cherenkov corona.
3. The hybrid photon detector The hybrid photon detector concept appears as an evolution of the triple-layer THGEM detectors, resulting in a gaseous detector that merges THGEMs with Micromegas [14]. This MPGD architecture can be coupled with other amplification structures such as GEMs [15] or
Fig. 1. Cherenkov ring obtained by superimposing events in a 300 300 mm2 triple-layer THGEM detector [16].
Fig. 2. Scheme of the hybrid detector concept comprising a dual layer THGEM configuration (thickness: 0.4 mm; hole diameter: 0.4 mm; pitch: 0.8 mm), and a Micromegas (128 μm over a multi-pad anode).
THGEMs, and has the intrinsic capability of blocking ions while being transparent to electrons [14]. Altogether, the hybrid THGEM – Micromegas detector, schematized in Fig. 2, makes use of a double layer THGEM þCsI configuration (photon sensitive pre-amplification stage), and a bulk Micromegas. The selected THGEM parameters are thickness of 0.4 mm; hole diameter of 0.4 mm; and pitch of 0.8 mm, while the Micromegas has a 128 μm amplification gap. Overall, the hybrid concept results in a robust detector with high photoelectron extraction efficiency, fast signals, high gain, photon feedback suppression, and time stability. Additionally, the misalignment of the holes of the two THGEM layers [10], together with the ion blocking capability of the Micromegas, makes the hybrid detector effective in the IBF suppression. The hybrid detector also uses a capacitive multi-pad anode, where a set of pads at the surface of the anode is at nominal voltage, and a
M. Alexeev et al. / Nuclear Instruments and Methods in Physics Research A 824 (2016) 139–142
second set of pads, inserted into the anode fibre glass, collects signals induced by RC coupling. The implementation of the capacitive anode grants a better protection of the front-end electronics. 3.1. Study and development of the hybrid detector The development of the hybrid detector concept underwent several stages following the evolution of active area of THGEM structures. The coupling of a 30 30 mm2 dual-layer THGEM detector with a Micromegas [7] proved the validity of the hybrid detector, and gains in the order of 105 and 106 were obtained, in laboratory conditions, using soft X-rays and UV photons respectively. The evolution towards 300 300 mm2 hybrid detectors relied in the selection of THGEMs and Micromegas with uniform gain within their active area. In this regard, and in addition to the THGEMs characterization, also Micromegas of the same sensitive area were characterized to study their gain uniformity, with gain variations below the level of 10%. The laboratory study of 300 300 mm2 hybrid detectors, under a Ar=CH4 (70:30) atmosphere, using an analogue readout chain (charge sensitive pre-amplifier þamplifierþ MCA), while being irradiated by either soft X-rays or UV light showed the capability of the device in reaching, stably, effective gains in the order of 105, which matches the performance of these 300 300 mm2 sized detectors with 30 30 mm2 ones. 3.2. Detector performance in BEAM To study the performance of the hybrid detector in beam conditions, a dedicated test at the T10 facility of the PS accelerator at CERN was performed. The detector was coupled to a fused silica radiator to create a Cherenkov corona centered in the sensitive area of the detector [16]. The detector's performance under such conditions, evaluated by using an analogue readout system, matched the performance in laboratory conditions, with gains in the order of 105 (Fig. 3) when detecting Cherenkov photons, and similar electrical stability. Additional studies allowed the investigation of the effect of the drift field in the suppression of charged particles signals (Fig. 4), as well as its influence in the photon yield of the detector. Results showed that charged particles can be significantly suppressed with negative or mild positive fields. Above 100 V/cm their collection by the detector is enhanced. Simultaneously, the photon yield is maximized by an equally mild positive drift field (approximately 100 V/cm). Additionally, a digital readout system, based on CMAD front-end chips and DREISAM cards [17], was used to reconstruct the Cherenkov ring over the anode pads and study the time distribution of the events (Fig. 5), which resulted in a very good time resolution ðσ 7 nsÞ.
141
3.3. Detector & APV25 Despite the successful operation of the hybrid detector with the aforementioned digital readout system, complementary tests were performed to study the electronic noise level of APV25 front-end chips [18] when coupled to the capacitive anode of the hybrid detector. As these chips are already in use as front-end electronics of the RICH-1 MWPCs [19], this study aimed to evaluate the possibility of using the existing readout electronics in the final detectors. Preliminary results of the coupling of the hybrid detector to one APV25 chip, in which each ADC channel corresponds approximately to 250 electrons, indicate noise levels not higher than 1000 electrons for each of the 128 channels of the APV25 chip.
4. Towards large area detectors The final detectors that will be used for the 2016 RICH-1 upgrade are meant to cover an area of 600 600 mm2 each. The coverage of this active area is to be achieved by merging pairs of THGEMs or Micromegas with 300 600 mm2 in the same plain. This design concept aims to facilitate the procurement of a bunch of THGEMs with good thickness uniformity. Therefore, the thickness measurement of 50 PCB sheets of 700 700 mm2 has been performed, through an automated process, in 36 36 equidistant points of each sheet. The distribution of the thickness variation, δthickness, for the 50 PCB sheets, is presented in Fig. 6, showing that 43 of the 50 pieces have thickness variations smaller than 4%. Photon Yield
1
Amplitude for beam particles
0.8 0.6 0.4 0.2 0
-1000
-500
0
500
Fig. 4. Cherenkov photon yield and sensitivity to charged particles, normalized to an arbitrary scale, as a function of the applied electrical drift field, for a 300 300 mm2 hybrid detector.
500 χ2 / ndf Constant
Counts
Slope
97.03 / 95 -0.04741 ± 0.00095
10
90.09 / 24 458.4 ± 8.4 -766.6 ± 0.1 7.208 ± 0.089
300 200 100
1
0
χ 2 / ndf Constant Mean Sigma
400
5.185 ± 0.027
Counts
102
1000
Drift Field [kV/cm]
50
100
150
200
Charge [fC] Fig. 3. Signal amplitude spectrum of Cherenkov photons, using a hybrid detector of 300 300 mm2 with a gain of 105 .
0
-950
-900
-850
-800
-750
-700
-650
-600
Time [ns] Fig. 5. Time distribution of detected Cherenkov photon events in a 300 300 mm2 hybrid detector, with σ 7 ns.
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M. Alexeev et al. / Nuclear Instruments and Methods in Physics Research A 824 (2016) 139–142
10
capable of single photon detection and presenting stable gains in the order of 105. Additionally, the study of the electronic noise level of the APV25 front-end chips when coupled to the detector has proven the good chip performance, allowing the use of the existing readout electronics of COMPASS RICH-1. The RICH-1 upgrade will then proceed by adopting hybrid detectors as a replacement for the existing MWPCs. The mass production of the final THGEMs is currently ongoing, aiming for the assembly and installation of the new Photon Detectors for the 2016 COMPASS physics run.
Frequency distribution of δthickness of 50 PCBs
Frequency
8 6 4 2 0
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
δthickness [%] Fig. 6. Distribution of the thickness variation (δthickness) for a set of 50 700 700 mm2 sized PCBs.
χ2 / ndf Constant Mean Sigma
250
Counts
200
12.28 / 10 266.1 ± 9.6 471.2 ± 0.1 1.935 ± 0.045
Acknowledgments The author F. A. Pereira is supported by a Doctoral Grant from FCT, Reference SFRH/BD/81259/2011.
References
150 100 50 0
420
430
440
450
460
470
480
490
500
Thickness [μ m]
Fig. 7. Distribution of a thickness measurement for 36 36 points of a PCB with δthickness ¼ 2:6%, and a theoretical thickness of 470 μm.
Fig. 7 shows the thickness distribution of the 36 36 measured points of a PCB sheet with δthickness ¼ 2:6%. The mean measured value of 471:2 μm is quite close to the expected thickness of the PCB (400 μm plus 2 copper layers of 35 μm each), with small variations along the area of the sheet: σ ¼ 1:93 μm. Four THGEMs with an active area of 300 600 mm2 have been produced and tested, with results matching the ones obtained for 300 300 mm2 THGEMs, validating the size scaling, and allowing the mass production of 300 600 mm2 sized THGEMs, which is currently ongoing. A future characterization of the produced structures will allow the selection of the THGEMs with best performance for the assembly of the final detectors for the RICH-1 upgrade.
5. Conclusion The concept of the hybrid THGEM – Micromegas detector has been validated by comprehensive laboratory and beam tests, being
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