- Email: [email protected]
A new generation optical module for deep-sea neutrino telescopes
Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S139–S141
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A new generation optical module for deep-sea neutrino telescopes P. Kooijman a,b,c, E. Berbee a, R. de Boer a, H. Boer Rookhuizen a, E. Heine a, J. Hogenbirk a, M. de Jong a, H. Kok a, A. Korporaal a, S. Mos a, G. Mul a, H. Peek a, P. Timmer a, P. Werneke a, E. de Wolf a,b, a
Nikhef, Science Park 105, 1098 XG Amsterdam, The Netherlands University of Amsterdam, Institute for High Energy Physics, Science Park 904, 1098 XH, Amsterdam, The Netherlands c University of Utrecht, Institute for Subatomic Physics, Princetonplein 5, 3584 CC Utrecht, The Netherlands b
On behalf of the KM3NeT Consortium a r t i c l e in fo
abstract
Available online 6 May 2010
A design is presented for a new type of optical module for deep-sea neutrino telescopes. The module consists of a glass pressure resistant sphere containing 31 small 3 in. photomultiplier tubes oriented so as to provide a maximal coverage in solid angle. In addition to the photomultipliers the sphere contains all the electronics for production of the high voltage and for the readout of the signals of the photomultipliers. The power consumption of the module is kept to a mere 7 W. A heat transfer system conducts the excess produced heat from the electronics to the ambient sea water. The Optical module forms a complete stand-alone detector that is connected to the outside world via a single optical fibre and two copper conductors to provide electrical power. & 2010 Elsevier B.V. All rights reserved.
Keywords: Multi-PMT Optical module Self contained 3 in. PMTs
1. Introduction In the framework of the KM3NeT design study we have investigated the use of a novel optical module consisting of a single glass pressure sphere containing many small 3 in. diameter photomultiplier tubes. This contrasts with the conventional approach of fitting the largest possible photomultiplier in a pressure sphere. The original idea for producing such a sphere was put forward by the late Esso Flyckt at the first VLVnT workshop held in Amsterdam in 2003 [1]. This multi-PMT configuration has several advantages. First of all the total photocathode area that can be fitted in a standard 17 in. glass pressure vessel is significantly larger than with large photomultipliers. In fact the 31 photomultipliers that are fitted in the sphere (see Fig. 1) have a photocathode area equal to three 10 in. or almost four 8 in. phototubes. The configuration provides a highly segmented photocathode area that allows for photon counting at low light intensity to be performed by the counting of hit photomultiplier tubes. Simulations have shown that this gives an advantage in the reduction of background from the 40K decays, which is always present in detectors deployed in seawater.
Corresponding author at: Nikhef, Science Park 105, 1098 XG Amsterdam, The Netherlands. Tel.: + 31 205925123; fax: + 31 205925155. E-mail address: [email protected] (E. de Wolf).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.04.141
The optical module provides a more uniform response as a function of the angle of incidence of the photons (see Fig. 2). The response to downward going tracks is enhanced and so the major background in deep-sea neutrino telescopes due to atmospheric muons can be more readily identified and removed. The number of high pressure transitions is reduced as fewer optical modules are required for the full neutrino telescope and furthermore failure of a photomultiplier or its high voltage unit will only marginally degrade the performance of an optical module.
2. Layout constraints Having decided to put a large number of photomultipliers inside a single sphere it is natural to also place all electronics necessary for the operation of the photomultipliers and their readout inside the single sphere. Placing all this inside the relatively confined space requires an optimization of the design in terms of power consumption, heat generation and transport. In order to create a viable system the cost of the readout electronics and high voltage supply must also be kept under control. In this design the photomultipliers are readout using the timeover-threshold technique. This allows for the identification of each hit photomultiplier by a single bit in a time stream. The communications with the shore is provided by a point-to-point fibre optic system utilizing on-shore continuous wave lasers. The data from the PMTs are then put on the fibres using reflective modulators in the optical module. This removes the necessity
S140
P. Kooijman et al. / Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S139–S141
Fig. 1. The multi PMT optical module a 17 in. sphere containing 31 photomultipliers and all necessary readout electronics.
Fig. 3. High voltage generation board with signal collection and slow control distribution.
Fig. 2. The angular acceptance of the optical module. The lower (dashed) curve is for 31 3 in. photomultipliers, whereas the upper (solid) curve also incorporates the extension cones.
of having communication lasers in the optical module and so significantly reduces the local power dissipation. The readout system is described in detail in a contribution to this conference [2]. The PMT with which all tests so far have been performed is the Photonis XP53X2B. It is a photomultiplier with an outer diameter of 76 mm with a photocathode diameter of 72 mm. The entry glass is of the concave–convex type. The curvature of the front face matches the inner curvature of the 17 in. glass sphere. The concave inner surface of the front face is designed for optimum timing response. The dynode structure is of the box-linear type with a large first dynode area in order to create insensitivity to the Earth’s magnetic field. Photonis has unfortunately ceased to manufacture photomultipliers. Both Hamamatsu and ETEL Ltd are now providing prototypes of similar construction. The photomultipliers have a maximum high voltage requirement of 1500 V and run at a gain of a few 106. This is advantageous in terms of total anode charge collected during the expected more than 10 years operation of the KM3NeT telescope. It reduces gain variations through aging to a minimum. Further processing of the signal electronic amplification will,
however, be necessary. A bespoke Cockroft–Walton type high voltage unit has been designed to feed the 3 in. photomultipliers. This unit has been miniaturized to the extent that it fits within the circular pin structure at the base of the PMT. By careful design the power consumption gas has been reduced to a value of 4.5 mW. The transition to the time-over-threshold signal is done immediately on the high-voltage base by incorporating an amplifier-discriminator. So effectively a digital signal leaves the photomultiplier. This amplifier-discriminator adds a further 25 mW to the power (see Fig. 3). A recent development that is presented in a separate contribution to this workshop is the effective extension of the photocathode area of the tube by affixing an extension cone to the outer circumference of the glass front of the photomultiplier [3]. Fig. 2 shows the increase in performance for the 31 photomultiplier optical module.
3. Mechanical layout and thermal properties The 31 photomultipliers are arranged 19 in the lower hemisphere and 12 in the upper hemisphere. To locate them inside the sphere they are held in a foam core. The use of foam has an additional advantage that it isolates the photocathode thermally from the heat producing elements in the optical module. In between the glass and the photomultiplier (and the foam) Wacker Siligel two component gel provides for the necessary optical contact. The foam is chosen stiff enough to hold the photomultipliers, but sufficiently flexible to allow
P. Kooijman et al. / Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S139–S141
S141
for the deformation of the glass sphere by the hydrostatic pressure. Optimization of the foam is being performed at present. To transport the heat to the outside of the sphere and to house the necessary electronics an aluminium mushroom shaped conductor has been designed. The converter board that houses the necessary DC/DC conversion circuitry is in fact housed completely within the aluminium in order to avoid electromagnetic interference. The optical modulator and a field programmable gate array and any other frontend electronics are housed on the storey logic board. This is placed in close proximity to the cooling mushroom (see Fig. 4). Space is also foreseen for a compass tiltmeter, a piezo acoustic transducer and a light flasher system. The overall power consumption is o 6:5 W. Thermal tests have been performed with the system, surrounded by 15 1C water. The maximum temperature registered was at the FPGA position on the storey logic board where 28 1C was recorded. The photocathode remained at a temperature of 16–18 1C.
Acknowledgements This work is part of the research program of the ‘Stichting voor Fundamenteel Onderzoek der Materie (FOM)’, which is financially supported by the ‘Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)’ and through the EU-funded FP6 KM3NeT Design Study Contract no. 011937. References
Fig. 4. Aluminium cooling mushroom showing the space reserved for electronics printed circuit board (above) and the 3D design (below).
[1] E. de Wolf (Ed.), Workshop on Technical Aspects of a Very Large Volume Neutrino Telescope in the Mediterranean Sea, 5–8 October 2003, ISBN 906488-026-3. [2] J. Hogenbirk, et al., A photonic readout and data acquisition system for deep-sea neutrino telescopes, in: Accelerators, Spectrometers, Detectors and Associated Equipment, VLVnT09 Proceedings, Nuclear Instruments and Methods in Physics Research, Section A, Athens, Greece, 2009. [3] O. Kavatsyuk, et al., in: Accelerators, Spectrometers, Detectors and Associated Equipment, VLVnT09 Proceedings, Nuclear Instruments and Methods in Physics Research, Section A, Athens, Greece, 2009.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A new generation optical module for deep-sea neutrino telescopes P. Kooijman a,b,c, E. Berbee a, R. de Boer a, H. Boer Rookhuizen a, E. Heine a, J. Hogenbirk a, M. de Jong a, H. Kok a, A. Korporaal a, S. Mos a, G. Mul a, H. Peek a, P. Timmer a, P. Werneke a, E. de Wolf a,b, a
Nikhef, Science Park 105, 1098 XG Amsterdam, The Netherlands University of Amsterdam, Institute for High Energy Physics, Science Park 904, 1098 XH, Amsterdam, The Netherlands c University of Utrecht, Institute for Subatomic Physics, Princetonplein 5, 3584 CC Utrecht, The Netherlands b
On behalf of the KM3NeT Consortium a r t i c l e in fo
abstract
Available online 6 May 2010
A design is presented for a new type of optical module for deep-sea neutrino telescopes. The module consists of a glass pressure resistant sphere containing 31 small 3 in. photomultiplier tubes oriented so as to provide a maximal coverage in solid angle. In addition to the photomultipliers the sphere contains all the electronics for production of the high voltage and for the readout of the signals of the photomultipliers. The power consumption of the module is kept to a mere 7 W. A heat transfer system conducts the excess produced heat from the electronics to the ambient sea water. The Optical module forms a complete stand-alone detector that is connected to the outside world via a single optical fibre and two copper conductors to provide electrical power. & 2010 Elsevier B.V. All rights reserved.
Keywords: Multi-PMT Optical module Self contained 3 in. PMTs
1. Introduction In the framework of the KM3NeT design study we have investigated the use of a novel optical module consisting of a single glass pressure sphere containing many small 3 in. diameter photomultiplier tubes. This contrasts with the conventional approach of fitting the largest possible photomultiplier in a pressure sphere. The original idea for producing such a sphere was put forward by the late Esso Flyckt at the first VLVnT workshop held in Amsterdam in 2003 [1]. This multi-PMT configuration has several advantages. First of all the total photocathode area that can be fitted in a standard 17 in. glass pressure vessel is significantly larger than with large photomultipliers. In fact the 31 photomultipliers that are fitted in the sphere (see Fig. 1) have a photocathode area equal to three 10 in. or almost four 8 in. phototubes. The configuration provides a highly segmented photocathode area that allows for photon counting at low light intensity to be performed by the counting of hit photomultiplier tubes. Simulations have shown that this gives an advantage in the reduction of background from the 40K decays, which is always present in detectors deployed in seawater.
Corresponding author at: Nikhef, Science Park 105, 1098 XG Amsterdam, The Netherlands. Tel.: + 31 205925123; fax: + 31 205925155. E-mail address: [email protected] (E. de Wolf).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.04.141
The optical module provides a more uniform response as a function of the angle of incidence of the photons (see Fig. 2). The response to downward going tracks is enhanced and so the major background in deep-sea neutrino telescopes due to atmospheric muons can be more readily identified and removed. The number of high pressure transitions is reduced as fewer optical modules are required for the full neutrino telescope and furthermore failure of a photomultiplier or its high voltage unit will only marginally degrade the performance of an optical module.
2. Layout constraints Having decided to put a large number of photomultipliers inside a single sphere it is natural to also place all electronics necessary for the operation of the photomultipliers and their readout inside the single sphere. Placing all this inside the relatively confined space requires an optimization of the design in terms of power consumption, heat generation and transport. In order to create a viable system the cost of the readout electronics and high voltage supply must also be kept under control. In this design the photomultipliers are readout using the timeover-threshold technique. This allows for the identification of each hit photomultiplier by a single bit in a time stream. The communications with the shore is provided by a point-to-point fibre optic system utilizing on-shore continuous wave lasers. The data from the PMTs are then put on the fibres using reflective modulators in the optical module. This removes the necessity
S140
P. Kooijman et al. / Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S139–S141
Fig. 1. The multi PMT optical module a 17 in. sphere containing 31 photomultipliers and all necessary readout electronics.
Fig. 3. High voltage generation board with signal collection and slow control distribution.
Fig. 2. The angular acceptance of the optical module. The lower (dashed) curve is for 31 3 in. photomultipliers, whereas the upper (solid) curve also incorporates the extension cones.
of having communication lasers in the optical module and so significantly reduces the local power dissipation. The readout system is described in detail in a contribution to this conference [2]. The PMT with which all tests so far have been performed is the Photonis XP53X2B. It is a photomultiplier with an outer diameter of 76 mm with a photocathode diameter of 72 mm. The entry glass is of the concave–convex type. The curvature of the front face matches the inner curvature of the 17 in. glass sphere. The concave inner surface of the front face is designed for optimum timing response. The dynode structure is of the box-linear type with a large first dynode area in order to create insensitivity to the Earth’s magnetic field. Photonis has unfortunately ceased to manufacture photomultipliers. Both Hamamatsu and ETEL Ltd are now providing prototypes of similar construction. The photomultipliers have a maximum high voltage requirement of 1500 V and run at a gain of a few 106. This is advantageous in terms of total anode charge collected during the expected more than 10 years operation of the KM3NeT telescope. It reduces gain variations through aging to a minimum. Further processing of the signal electronic amplification will,
however, be necessary. A bespoke Cockroft–Walton type high voltage unit has been designed to feed the 3 in. photomultipliers. This unit has been miniaturized to the extent that it fits within the circular pin structure at the base of the PMT. By careful design the power consumption gas has been reduced to a value of 4.5 mW. The transition to the time-over-threshold signal is done immediately on the high-voltage base by incorporating an amplifier-discriminator. So effectively a digital signal leaves the photomultiplier. This amplifier-discriminator adds a further 25 mW to the power (see Fig. 3). A recent development that is presented in a separate contribution to this workshop is the effective extension of the photocathode area of the tube by affixing an extension cone to the outer circumference of the glass front of the photomultiplier [3]. Fig. 2 shows the increase in performance for the 31 photomultiplier optical module.
3. Mechanical layout and thermal properties The 31 photomultipliers are arranged 19 in the lower hemisphere and 12 in the upper hemisphere. To locate them inside the sphere they are held in a foam core. The use of foam has an additional advantage that it isolates the photocathode thermally from the heat producing elements in the optical module. In between the glass and the photomultiplier (and the foam) Wacker Siligel two component gel provides for the necessary optical contact. The foam is chosen stiff enough to hold the photomultipliers, but sufficiently flexible to allow
P. Kooijman et al. / Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S139–S141
S141
for the deformation of the glass sphere by the hydrostatic pressure. Optimization of the foam is being performed at present. To transport the heat to the outside of the sphere and to house the necessary electronics an aluminium mushroom shaped conductor has been designed. The converter board that houses the necessary DC/DC conversion circuitry is in fact housed completely within the aluminium in order to avoid electromagnetic interference. The optical modulator and a field programmable gate array and any other frontend electronics are housed on the storey logic board. This is placed in close proximity to the cooling mushroom (see Fig. 4). Space is also foreseen for a compass tiltmeter, a piezo acoustic transducer and a light flasher system. The overall power consumption is o 6:5 W. Thermal tests have been performed with the system, surrounded by 15 1C water. The maximum temperature registered was at the FPGA position on the storey logic board where 28 1C was recorded. The photocathode remained at a temperature of 16–18 1C.
Acknowledgements This work is part of the research program of the ‘Stichting voor Fundamenteel Onderzoek der Materie (FOM)’, which is financially supported by the ‘Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)’ and through the EU-funded FP6 KM3NeT Design Study Contract no. 011937. References
Fig. 4. Aluminium cooling mushroom showing the space reserved for electronics printed circuit board (above) and the 3D design (below).
[1] E. de Wolf (Ed.), Workshop on Technical Aspects of a Very Large Volume Neutrino Telescope in the Mediterranean Sea, 5–8 October 2003, ISBN 906488-026-3. [2] J. Hogenbirk, et al., A photonic readout and data acquisition system for deep-sea neutrino telescopes, in: Accelerators, Spectrometers, Detectors and Associated Equipment, VLVnT09 Proceedings, Nuclear Instruments and Methods in Physics Research, Section A, Athens, Greece, 2009. [3] O. Kavatsyuk, et al., in: Accelerators, Spectrometers, Detectors and Associated Equipment, VLVnT09 Proceedings, Nuclear Instruments and Methods in Physics Research, Section A, Athens, Greece, 2009.