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Projects 2013

The 2013 R&D AO was arbitrated by the Steering Committee on 6 March, after examination of the dossiers by the SRDC. The results are as follows:

 

ATLAS-DTBB: New digital electronics to generate ATLAS L1 trigger primitives

P. Schwemling (Irfu/DPhP)

The LTDB (LAr Trigger Digitizer Board) is an upgrade of the ATLAS calorimeter Level-1 trigger system. It is designed to cope with the increased luminosity of the LHC. In order not to increase the trigger rate abow the present allocated bandwidth of 20 kHz for the calorimeter level 1 electromagnetic trigger while maintaining high efficiency for electromagnetic objects (electrons and photons), the upgraded trigger has to be more selective agains jets. This can be achieved by adding more discriminating variables in the trigger decision. The present system (see figure 1, top) is based on the summation of the energy deposited in Δη × Δφ = 0.1 × 0.1 trigger towers, with no information on longitudinal shower development retained and no information about the nature of the shower, electromagnetic or hadronic. To overcome these limitations and give more information to build the trigger decision, the LTDB (figure 1, bottom) keeps the longitudinal information on the shower development, and the granularity of the sommation is significantly improved: in the first and second samplings of the calorimeter, that carry most information about shower development, it is 0.025 in the η direction instead of 0.1.

Figure 1- Granularity of the present calorimeter L1 trigger system (top) and of the upgraded calorimeter L1 trigger system , based on the LTDB (bottom)

In the new system, a trigger tower of 0.1 × 0.1 is divided into 10 independant sums of elementary detector cells. These sums are called supercells in the following. In addition, the LTDB digitizes the signal coming from the supercells and sends the digitized results over highspeed (5Gb/s) digital optical links to the central trigger system, where the decision to keep or reject the data of each event is taken.

Description of the work achieved:

The first step of the project was to develop the building blocks for the analog section of the LTDB, i.e. the analog chain going from the FrontEnd crate baseplane to the LTDB ADCs, and creating also the summed signals needed for normal operation of the TBB, that are sent back to the backplane. The design was based only on the radiation hard components used in the present front-end electronics. It features a minimal number of active components, allowing to use minimal space on the PCB, dissipating power in the analog section to a minimum, minimizing the noise injected in the system and the signal transit time. After amplification and conversion from single ended to differential, and before being digitized, the signals are subjected to a mild shaping, in the form of a single pole of 15 ns, to slow down somewhat the signal, filter out high frequency noise and avoid aliasing problems at the digitization stage. After studying several possible realizations of the pole, the best option in terms of noise and cross-talk turns out to be the active RC, which is what we have finally implemented.

The digital mezzanines, connected each to the mother board by two connectors, one for signal transmission, and the other for powering, are based on a COTS 12 bits low power ADC, a Cyclone V ALTERA FPGA to configure the ADCs, read them out and format the data. Standard ATLAS clock cleaners and PLLs for clock distribution and reception have been used. Optical communication with the LTDB readout system is done with standard CERN designed VTTX transceivers running at 5 Gbs/s. Two versions of the mezzanines have been designed and produced, and both have been successfully mated to the analog motherboard. After having defined the schematics of the analog section of all channels, we have designed and produced a 64 channels test-board. This step allowed us to prove that we were able to fit all the components on the space available on the board. After that step, we have proceeded to the design of a complete LTDB demonstrator board.

In parallel with the design of the demonstrator board, we have also built a mechanical demonstrator to test the insertion in an ATLAS Front-End crate. We also used this mechanical board to check successfully that the mechanical dimensions were all correct. The board has been characterized at CERN using the same setup as the one used for the US demonstrator. The tests that were done include connectivity tests, linearity measurements, noise and cross-talk measurements. The measurements showed its performances were more than adequate for integration into ATLAS and for maintaining the triggering capability of the present system.

To resume:

  • The LTDB demonstrator based on the options we promote, i.e. an analog motherboard plus digital mezzanines, has been successfully built, tested at CERN and integrated on the ATLAS detector.
  • All signals needed for normal operation of the present trigger system are correctly generated by the LTDB demonstrator. This was the main condition for the collaboration to accept the integration of the demonstrator on the detector.
  • A common architecture for the final LTDB was found following discussions with US colleagues.

Publications:

 

DACTOMUS : Diagnostics and compact beam transport for multistages laser plasma accelerator

N. Delerue (LAL)

Laser plasma acceleration based on the laser wakefield mechanism is capable of creating accelerating longitudinal electric fields of up to some 100 GV/m. Studies with laser systems of a few tens of TW to PW have demonstrated electron trapping and subsequent acceleration in non-linear interaction regimes called blowing or bubble regimes. Acceleration fields of the order of 10-100 GV/m are typically obtained over a few millimetres, and it has been shown that over a scale length of one centimetre they produce clusters of several GeV. Although the ultra-high acceleration gradient regimes of laser plasma accelerators are extremely attractive for the development of accelerators to push the energy frontier of high-energy physics, several aspects, related to the performance of the laser systems and the mastery of acceleration physics, tend to favour acceleration regimes with lower gradients (1-10 GV/m). The energy of the electrons accelerated in these quasi-linear regimes can be increased by several successive laser plasma acceleration steps. The objective of this type of study is to demonstrate the feasibility of laser plasma accelerator schemes producing electron clusters with controllable parameters, and evolving to higher energy by adding acceleration stages. In this context, the objective of the DACTOMUS (Diagnostic And Compact beam Transport fOr MUltiStaged laser plasma accelerators) project is to develop a compact transport and focusing system and associated diagnostics for an electron beam generated in a laser plasma accelerator.

  • ELISA electron source

Within the framework of the DACTOMUS project, the instrumentation required for focusing and diagnosing electron bunches was built and tested with an electron source generated by a compact laser plasma accelerator operating in the non-linear regime. Control of the electron injection into the accelerator structure is achieved by controlling the ionisation of nitrogen at impurities in a dihydrogen gas.

The electron source (or first stage) is produced by ionisation-controlled electron trapping and acceleration in the laser-driven wakefield inside a gas cell. A photograph of the gas cell and an example of the electron density profile are shown in Fig. 1. The pressure inside the gas cell can vary from 100 to 500 mbar and the length of the gas cell can vary from 0 to 10 mm. Fluid simulations were performed with OpenFOAM and SonicFoam to obtain a realistic profile of the gas density in the cell. The density decreases at the edges of the gas cell along the laser propagation axis, which has an impact on the non-linear focusing of the laser at the cell entrance.

Figure 1 - Photograph of the ELISA gas cell on the left. The density profile is shown on the right.

The density profile is then used as an input for PIC (Particle-In-Cell) simulations with WARP. The results show that the position of the laser waist in the gas cell has a significant impact on the electron beam quality. Most of the electron beam charge for energies above 30 MeV is in the 50-100 MeV range. The energy peaks at 68±11 MeV. A transport line downstream of the gas cell optimised for an intermediate energy of 72 MeV was experimentally tested.

  • Transfer line

The objective of the transfer line downstream of the gas cell is to characterise the properties of the electron beam such as the energy distribution and angular divergence. The transfer line must be:

  1. compact: the total length should be less than one metre
  2. made of permanent magnets
  3. not very sensitive to the pointing of the electron beam.

Furthermore, the implementation implies a minimum value of the distance between the first magnet and the gas cell of 160 mm. We chose to use a triplet instead of a doublet for focusing in order to reduce the beam size in the quadrupoles (and thus the chromatic effects). A dipole is then inserted downstream of the triplet to perform the energy separation. The movement of the dipole is motorised, allowing measurements to be made with the triplet alone to check the laser pointing axis and the electron beam pointing axis. The triplet consists of two 80-millimetre-long focusing quadrupoles and one 120-millimetre-long quadrupole. The length of the quadrupoles was chosen to use the same cubic permanent block of 40 × 40 × 40 mm3 . Angular clipping at 18.8 mrad (5 mrad) is achieved by adding a collimator to the entrance of the triplet with diameter φ = 6 mm (φ = 1.5 mm, respectively). The magnetic fields of the triplet and the dipole were simulated using Opera software. The field map was measured at LLR. The triplet is located in the same vacuum chamber as the gas cell, located in the centre of the chamber. A second vacuum chamber contains the mobile dipole and the lanex screen. The distance between the dipole and the triplet was imposed by the constraint of a separate chamber.

Figure 2 - Implementation of the transfer line in the UHI100 experimental area

The experiments showed that the centroid of the electron beam was well stabilised on the lanex screen from one shot to the next. As expected, a beam pinch is experimentally observed at the position corresponding to the reference energy of 72 MeV. Several measurements were made for different plasma conditions, for example by varying the gas pressure in the gas cell.

Publications:

 

INGMAR: Irradiation of ice and meteorites analysed by VIS-IR reflectance

R. Brunetto (IAS)

Space weathering processes affect atmosphere-less bodies in the Solar System. They include irradiation by solar wind, galactic cosmic ions, electrons, UV and X-rays, and bombardment by micrometeorites. These processes cause variations in the optical properties of small Solar System bodies surfaces, affecting efforts to draw connections between specific meteorites and asteroid types. They have been widely studied for the Moon and S- and V-type asteroids, but little is known about carbonaceous asteroids weathering in space as previous studies have struggled to define a general spectral trend among dark surfaces. The INGMAR (“IrradiatioN de Glaces et Météorites Analysées par Réflectance VIS-IR”) project aims at developing an experimental setup dedicated to VIS-IR spectroscopic analysis of solids of astrophysical interest to be coupled to different irradiation platforms. In particular, in the first phase of the project, meteorites, carbons, and ices that are analogs of carbon-rich asteroid surfaces will be irradiated (40 keV, as a simulation of solar wind) at the SIDONIE platform (CSNSM, Orsay) and analyzed by reflectance (micro-)spectroscopy.

Work achieved:

Ion irradiation experiments of the Allende meteorite has been performed, using 40 keV He+ and Ar+ ions at the SIDONIE platform, as a simulation of solar wind irradiation of primitive bodies surfaces. We used different fluences up to 3x1016 ions/cm2, corresponding to short timescales of ~103-104 years in the main asteroid belt. Samples were analyzed before and after irradiation using visible to far-IR (0.4 – 50 µm) reflectance spectroscopy, and Raman micro-spectroscopy. Similarly to what is observed in previous experiments, results show a reddening and darkening of VIS-NIR reflectance spectra. These spectral variations are however comparable to other spectral variations due to viewing geometry, grain size, and sample preparation, suggesting an explanation for the contradictory space weathering studies of dark asteroids. After irradiation, the infrared bands of the matrix olivine silicates change profile and shift to longer wavelength (Fig. 1, left), possibly as a consequence of a more efficient sputtering effect on Mg than Fe (lighter and more volatile species are preferentially sputtered backwards) and/or preferential amorphisation of Mg-rich olivine. Spectral variations are compatible with the Hapke weathering model. Raman spectroscopy shows that the carbonaceous component is substantially affected by irradiation: different degrees of de-ordering are produced as a function of dose, to finally end with a highly disordered carbon. All observed modifications seem to scale with the nuclear elastic dose. We performed an extension of this work, by irradiating pellets of the CM Murchison meteorite. Results show that the reddening/darkening trend observed on silicate-rich surfaces is not valid for all carbonaceous chondrites, and that the spectral modifications after irradiation are a function of the initial albedo.

In parallel to the irradiation activity, we have developed a micro-reflectance analysis of extraterrestrial particles. The reproducibility of the technique, at the ~20 μm scale, was evaluated to estimate measurement uncertainty. For calibrations, natural San Carlos olivine 20-100 m particles were used. The reflectance spectra of isolated particles were measured one by one using different microscope objectives. A second calibration was performed applying the same technique to terrestrial microparticles prepared by our colleagues of the GIADA instrument team (Rosetta mission). Some particles were covered with a thin layer of aromatic carbon (~ 200-500 nm). Thanks to spectral modeling, results will be used to constrain the effects of carbon inclusions in the reflectance spectra in the VIS-NIR spectral range.

Figure 1 - Left : The IR confocal microscopic (spot ~15 µm) reflectance spectra on Allende's matrix before (black) and after irradiation at maximum fluence for Ar+ (red) and He+ (blue) beams. Right: Comparison of the reflectance spectra of objects having experienced distinct extents of space weathering: our three Hayabusa particles (in black, blue and red), the ground-based spectra of asteroids Itokawa (in grey diamonds) and Lick (in red dots), and the laboratory spectrum of the Alta'ameem LL5 meteorite (in grey) reported by Hiroi et al. (2006).

Thanks to these calibrations, we were able to apply the micro-reflectance technique to three extraterrestrial grains returned from asteroid Itokawa by the Japanese Hayabusa mission. We characterized the mineralogy and the extent of space weathering of the three Itokawa particles provided by JAXA to our consortium. Identification of the minerals, characterization of their elemental compositions and measurements of their relative abundances were led through Raman spectroscopy. Reflectance spectra in the visible and near-IR wavelength constrain the mineralogy of the grains and allow direct comparison with the surface of Itokawa (Fig. 1, right). The spectra reflect the extent of space eathering experienced by the three particles. The NIR-VIS reflectance (incidence = 45°, light collection at e=0°) spectra of the three particles, in particular the 1-μm band, are consistent with the presence of both olivine and pyroxene detected via Raman.

The good results obtained on Itokawa and GIADA grains allowed us to measure (for the first time in this wavelength range) micro-reflectance spectra of two Antarctic micrometeorites of possible cometary origin. For both micrometeorites we measured an albedo compatible with that of cometary nuclei. The in-situ coupling between the spectrometers and the irradiation line in the vacuum chamber has been made. A complete renewal of the SIDONIE platform has been performed at the same time. The setup is completely operational. The extension to icy samples has also been performed.

Publications:

 

HARD: Ultra low noise HEMT cryo electronics for bolometers (Edelweiss)

C. Nones (Irfu/DPhP)

The main purpose of HARD was to provide EDELWEISS III (and in prospect a larger next-generation search in collaboration with the US-led experiment SuperCDMS) with an improved sensitivity to particle Dark Matter candidates (WIMPs) in the low-mass region. In order to achieve this result, a new cold front-end electronics was mandatory, which has been designed in the present program. The conventional cold JFET was replaced by a lownoise High Electron Mobility Transistor (HEMT), capable to provide a noise reduction by a factor 3, followed by a complete amplifying circuit placed at 1 K. This upgrade concerns a few EDELWEISS-III readout lines. To fully exploit this improvement, a new detector conception is proposed, aiming at a lower intrinsic capacitance of the readout electrodes of the ionization channel. A cryogenic test facility has been developed for the HARD program, using an existing novel liquid-free dilution refrigerator located at the IRAMIS/SPEC laboratory. In this facility, we performed a systematic characterization of HEMTs produced custom at LPN, and full amplifiers based on these components were developed. The cross-section sensitivity to WIMPs in the mass range 5-10 GeV has been improved by at least 2 orders of magnitude, increasing dramatically the discovery potential of EDELWEISS III and setting the bases for a better detector technology for the EDELWEISS-SuperCDMS common search.

Description of the achieved results:

  • HEMTs production at LPN

HEMTs for HARD were produced at LPN. In the first part of the project, existing HEMTs have been used. They are based on an AlGaAs/GaAs heterostructure grown by MBE (Molecular Beam Epitaxy). It consists of a GaAs buffer layer, a 20 nm AlGaAs spacer layer which is much thicker than that employed in commercial HEMTs (between 2 and 5 nm), a Si d-doping layer, then a 15 nm undoped AlGaAs barrier layer, and finally a 6 nm undoped GaAs cap layer. HEMTs with various gate lengths and gate widths were fabricated and individually packaged in a ceramic SOT23. A dedicated HEMT production for HARD has been performed in the second part of the project, optimized for an input capacitance of 100 pF. It has to be underlined that HEMTs have no operating temperature limit and their power consumption can be as low as 30 μW. A voltage noise as low as 0.46 nV/√Hz at 1 kHz has been obtained with an input capacitance of about 100 pF. The 1/f noise that was penalizing HEMTs for low frequency applications has been dramatically reduced and becomes negligible above 1 kHz. These transistors are very suitable for high impedance low-temperature detector.

  • Above-ground tests @ CSNSM

Many tests have been done at CSNSM in order to characterize HEMTs in terms of noise performances.

  • Production and test of a new amplification board @ IRFU/SEDI

A new voltage amplifier board capable to read 4 ionization channels and 2 heat channels has been produced and testes at 4K by the SEDI group involved in the project. The board prototype, named CRYOHARD, was designed to be connected on one FID EDELWEISS detector at the 1K stage of the LSM cryostat. The main characteristics of this amplifier board are:

  1. Very low operation temperature, down to ~ 1K
  2. Very low noise. The amplifier noise will be as close as possible to that of the input HEMT, i.e. 0.5 nV/√Hz and at 4.2 K, with a gatesource capacitance Cgs=100 pF
  3. High input impedance, optimized for low threshold charge collection. (The amplifier input capacitance will be proportional to that of the input HEMT with a proportionality factor between 1 and 0.2)
  4. Band-width: from DC to ~ 20 kHz
  5. Very low power consumption: of the order of 1 mW/channel
  6. Gain: between 10 and 100, and temperature-stable in time (~ 1%)

All these properties have been matched during a test done at IRFU/SEDI when the amplifier board has been tested inside a LHe dewar.

  • Development of a new test facility at IRAMIS/SPEC

Thanks to HARD funding, we have been able to instrument the Helium Free Dilution Refrigerator at the Cryogenic Lab of IRAMIS/SPEC (Fig. 1). We have equipped it with 6 complete temperature measurement channels and temperature regulation (MM3, MGC3), and a vibration insulator system (Newport, auto-leveling). The cryostat exhibits 200 uW at 100 mK and cools down in 15 hours from room temperature down to 20 mK (base temperature being about 10 mK). One advantage of this installation is that it can run several ionization and heat/light composite bolometers thanks to the large number of reading channels installed inside. The acquisition system was also developed in the P2IO framework. This installation was used as a test-bench for HEMT-based electronics.

Figure 1 - IRAMIS/SPEC low temperature facility for detector and electronics tests.
  • Production, test, and improvements of a final-design amplifier board

In the second part of the project, a 4-channel amplifier board based on HEMTs was designed, fabricated and tested by IRFU (SEDI and SPP). After a general check at 4 K in SEDI, the card was tested in the aforementioned IRAMIS/SPEC facility during three lowtemperature runs. The card was fully and successfully integrated with the standard EDELWEISS readout used in LSM. 

  • Fabrication of a FID 200g detector to be coupled to the HARD amplifier board

The HEMT readout requires special low-capacitance detectors (<20pF). These have been developed at CSNSM starting from HPGe crystals with a mass of 200 g. These detectors present the same electrode structure as the ones presently used in EDELWEISS (so called Full InterDigit – FID – version), which allows us to fully reject surface events by preserving a large fiducial volume, but they are equipped with special very thin interleaved charge collecting electrodes (Fig. 2), permitting to lower the energy threshold well below 100 eV when used with appropriate low-capacitance HEMTs and cabling.

Figure 2 - A low-capacitance Ge detector fabricated at CSNSM.

Publications:

 

HighSPID: Low-energy light particle identification by shape analysis for very high granularity Si stripped multi-detectors (Spiral2 phase2)

Y. Blumenfeld (IPNO)

The aim of the HIGHSPID (HIgh Granularity HodoScope for Particle Identification) project was to build and test a prototype electronic chain for reading information from a Si-Strip detector in order to measure the energy and identify in mass and charge of light charged particles (from protons to Li) by pulse shape analysis. This system is used for multi-particle detectors based on Si-Strip technology, and in particular for the GASPARD detector deployed in the next generation ISOL radioactive beam facilities such as HIE-ISOLDE, SPES and SPIRAL2. The electronics developed also equip Si detectors dedicated to the measurement of atomic clusters such as those accelerated at the Orsay tandem.

Description of the completed work:

In order to solve the problem of particle identification, the technique of pulse shape analysis for light particles has been studied. It is based on the digitisation of the charge and/or current signal with a frequency that must be adapted to the rise time of the recorded signal. Several experiments were carried out at the Orsay Tandem. For this purpose, a 500μm thick DSSSD made from an nTD wafer to ensure good uniformity of resistivity was purchased. The N side of the detectors faced the beam to increase the PSD possibilities. Four bands on each side of the detector were read by PACI preamplifiers designed and built at the IPN Orsay.

The first test showed that very simple observables such as the maximum of the current signal can provide enough information to identify the light particles. The effect of the detector bias was also studied. Without filtering, the best compromise is obtained with the detector depletion bias where the PSD is of good quality without affecting the energy resolution. However, we have shown that by using a bipolar filter, PSD can also be obtained at the nominal (over depletion) bias.

A follow-up experiment was carried out with the same detector and electronics, except that the digitisers were replaced by the state-of-the-art WaveCatcher digitisers, built at the LAL and funded by the HIGHSPID grant. They offer more channels (64) at a sampling rate of 1GHz with an acquisition rate 10 times higher than the MATACQ previously used. The measured response was intended to focus on the discrimination between 3He and 4He particles. Preliminary analysis showed that the discrimination between 3He and 4He could be achieved satisfactorily.

In summary, the best observable for the PSA technique was investigated and showed that the best results can only be obtained with observables based on the current signal. Therefore, both charge and current signals are needed in the GASPARD electronics. We therefore submitted a 9-channel ASIC version of the PACI preamplifier funded by the HIGHSPID grant. This ASIC was tested with the prototype detector purchased with the HIGHSPID funds. After solving some crosstalk problems, excellent performance was obtained with an electronic resolution of about 10 KeV. 

The minimum sampling rate required for digital electronics was also investigated by reducing the number of samples from 1GHz down to 100MHz. Below 200MHz, the quality of discrimination is considerably reduced. The best sampling rate for the GASPARD electronics would be close to 500MHz. In order to move in this direction, readout tests were carried out using cards developed at IRFU/SEDI. The iPACI load signals were digitised by an "ASAD" board with minor modifications to match the iPACI signals. The iPACI current signals were digitised by SAMPIC boards. A schematic view of the installation is given in the figure below:

Schematic view of the installation for reading the iPACI chip

Publications:

 

MICROMEGAS Imager: 2D Micromegas with segmented grid for neutron imaging (NTOF)

F. Gunsing (Irfu/DPhN)

The objective of this project was to develop and test a working prototype neutron detector with the development of the low mass XY segmented mesh and anode microbulk detector and the on-board data acquisition system. The mesh strips provide the Y information, while the anode strips provide the X information. This results in a very low mass device with good energy resolution capabilities. Such a detector is virtually "transparent" to neutrons, which is ideal for in-beam neutron measurements such as flux and profile monitoring with minimal beam disturbance. The proposed development greatly simplifies and accelerates the 2D microbulk production procedure, making the construction of large-area microbulk detectors economically feasible and suitable for rare event experiments such as the search for dark matter. Mesh segmentation requires the development of an on-board data acquisition system with the ability to trigger on each channel. This acquisition system is based on the GET (Generic Electronics for TPC) system.

Description of the work carried out:

The characterisation of a neutron beam is essential for most applications. One of the challenges in studying neutron-induced reactions is to determine both the number of incident neutrons and their spatial distribution. Neutron beams are usually used with a diameter of a few centimetres and their intensity is often not uniform in space. The total number of neutrons is usually measured using a beam flux monitor, counting the number of neutrons incident with a low mass reaction chamber. In this case, experiments can be performed downstream of the detector.    

  • Development of the XY microbulk detector

For this project, an initial development of the segmented mesh and anode was carried out at CERN's Microbulk laboratory, with the mesh and anode each divided into a number of 20 strips. Three different prototypes were built and tested in order to find the optimal design in terms of the diameter of the mesh holes and their corresponding spacing, as well as the spacing between the strips at the mesh. The aim of this optimisation was to ensure the feasibility of the construction with an acceptable loss of energy resolution (~12% on the optimal prototype with a 55Fe source instead of the 11% limit achievable using microbubble technology). The production yielded satisfactory results in terms of feasibility and performance. Subsequently, two final Microbulk detectors were designed and produced, consisting of 60x60 strips on a 6x6 cm2 area, where each strip was spaced at 40 µm and with a repeat distance of 1 mm. The mesh was created using an etch of 60 µm diameter holes with a pitch of 100 µm. These final Microbulks were attached to a PCB ring for mounting inside the detector chamber. This configuration is shown in Figure 1.

Figure 1 - The 6x6 cm2 microbulk mounted on the PCB ring. The 60 strips on the front face are clearly visible.
  • Development of the acquisition system

The AGET acquisition system was implemented in its reduced CoBo configuration mode, capable of acquiring 256 channels, which is sufficient for an XY detector. A number of 4 AGETs on an AsAd board were used as the main acquisition board. Special front-end boards were designed and manufactured, in order to properly connect the mesh and anode strips to the AsAd board for strip reading, to provide high voltage to the mesh strips and to protect the AGET chips from potential electrical discharges in the detector. A picture of the detector in the chamber is shown in Figure 2.

Figure 2 - The detector in the chamber containing the XY microbulk. The printed circuit board and the front boards for the mesh and anode strips are visible, as well as the grounding that has been done.
  • Detector performance

The detector performance was tested with X-rays, using a 55Fe source (EKα= 5.9 keV, EKβ=6.5 keV). The detector chamber was filled with a gas mixture of 95% argon - 5% isobutane (iC4H10) at atmospheric pressure, circulating at a constant flow rate of 6 NL/h. The best resolution in this case was found to be 13%. This energy resolution value is very close to the theoretical limit for proportional meters with this gas. The characterisation of the detector was completed by the gain and transparency curves for different gases. After the characterisation with X-rays, the detector was successfully used under real conditions at the n_TOF and GELINA facilities, for the extraction of the neutron beam profile. The use of standard cross sections and the neutron time-of-flight technique allows the reconstruction of the relative energy distribution of the neutron beam flux and profile. This is an essential ingredient for the absolute normalisation of cross-section measurements at a given energy. 

Presentation of results and publications:

The results of the detector development and testing, as well as the neutron beam measurements, have been presented at international meetings and conferences [1-3].

[1] M. Diakaki, « A transparent XY-MicroMegas neutron beam profiler », 66th Lindau Nobel Laureate Meeting 2016, Lindau, Germany.

[2] « A new transparent XY-MicroMegas neutron beam profiler », 4th International Conference on Micropattern Gaseous Detectors (MPGD) 2015, Trieste, Italy.

[3] Several presentations at RD51 and n_TOF collaboration meetings.

[4] M. Diakaki et al., "Development of a novel segmented mesh {MicroMegas} detector for neutron beam profiling", Nucl. Instr. Meth. A 903 (2018) 46

 

PRIVAT (Integrated Regional Platform for the Validation of Taus Algorithms)

A. Zabi (LLR)

The CMS (Compact Muon Solenoid) experiment aims to study the results of proton collisions produced by the LHC (Large Hadron Collider) at CERN. The characterisation of the Higgs boson sector and the search for new physics will require the full capacity of the LHC. The main objective of the PRIVAT project was to find a solution for efficiently triggering electrons/photons and tau leptons during the LHC's Run 2 (2015-1018) and Run 3 (2020-2022) experiments. After several upgrade steps, which have now begun with the first extended shutdown, the LHC will be able to achieve collisions at 13.5 TeV in the centre of mass and at more than 1x1034 cm-2 s-1 of instantaneous luminosity. The average number of collisions per run will reach 50, 'Pile-Up', and will therefore exceed the design parameters of the machine. Under these intense conditions, not only must the calorimetric signals be identified, but an effective pile-up mitigation technique must be implemented to correctly determine their energy. Improved algorithms will allow the accurate reconstruction of L1 objects with improved resolutions. A novel approach to the triggering technique was proposed and called "Time Multiplexed Trigger" (TMT). It is based on the recent microTCA technology and relies on the installation of a high-speed optical link to retrieve information for all sub-detectors. The objective of the project was to develop sophisticated algorithms that can be implemented as firmware in Xilinx Virtex 7 FPGAs.

Description of the work performed:

 

  • Development of an electron and tau lepton seeker

The LLR team has a long experience in the design and development of triggers. Bearing in mind the limitations of the current system, the approach considered was to first develop the best possible algorithms given the improved granularity of the Phase 2 triggers. A stand-alone analysis was developed to work with both the data from the first LHC run and the Monte Carlo simulated data. The real data allowed a direct comparison of performance with the Run 1 trigger architecture and the simulated data helped to test the robustness of the algorithms against severe stack-up conditions. The group succeeded in producing algorithms to efficiently select electrons, photons and tau leptons by introducing an innovative method of dynamic calorimeter clustering. The detailed study of the deposition energy, or 'footprint' of the particles in the detector, was used to optimise the clustering parameters, including the energy thresholds.

The electron and photon search algorithm was based on the clusters produced with the information from the electromagnetic calorimeter (ECAL). After a detailed study of the off-line tau lepton energy distributions in ECAL and the Hadronic Calorimeter (HCAL) for the different decay modes, we concluded that the same dynamic clustering can be used with the following modifications: clusters should be reconstructed by adding the ECAL and HCAL energy and nearby clusters should be merged to improve the 3-leg decay signals. Unprecedented performance was obtained for both algorithms.

  • Expected performance

The efficiency compared to the Run 1 system is shown in Figure 1. The performance achieved for electrons and photons includes a factor of 4 improvement in angular resolution, up to 30% improvement in energy resolution, and a factor of 2 reduction in rate. The tau algorithm in step 1 could not achieve 100% efficiency at the plateau whereas the trigger in step 2 does. The superiority of this approach is therefore fully demonstrated by these results.

Figure 1- Performance of the electron and tau lepton finder algorithms for the Run II trigger.
  • Firmware implementation and binary emulation

The firmware implementation was the main challenge of this project, as these very sophisticated algorithms need to be optimised to fit into a single Virtex 7 FPGA. A software emulator was developed based on the standalone analysis to more accurately simulate the trigger response. A large server has been purchased to handle the 8 hours of compilation required to build the firmware. The platform is also convenient for sharing pattern files easily between physicists and engineers. Based on the CACTUS website, a central repository structure located at CERN was developed. The VHDL design has been optimised by implementing generic components and sharing resources between algorithms. Test models are used to verify functionality.

  • A new development platform at LLR

P2IO's financial support has enabled the group to purchase the equipment necessary to build a microTCA test bench. The platform has an MP7 board that is used to test the developed algorithms. In order to optimise the synthesis steps and the VHDL implementation steps in a firmware, a powerful server was added with the necessary hard disk. The MP7 is delivered with a complete software package that allows communication with the board. Script commands can be used to write test patterns to memory and capture the result via DAQ blocks.

Performance was beyond expectations and the firmware implementation progressed as expected. The use of the platform allows verification of functionality as well as synchronisation performance, which is a key factor in estimating the latency of the final system.

Publications:

 

SONIM: New miniaturised probes for charged particle detection in molecular imaging based on SIPMs

L. Ménard (IMNC)

The objective of this project was to address the growing interest in biomedical imaging for miniaturised detection systems capable of providing real-time information on the location and kinetics of positron emitting radiotracers. Positron detection has intrinsic advantages over gamma detection in achieving spatial selectivity and sensitivity. In this context, we proposed to develop a new generation of miniaturised probes based on the recent SiPM technology. Indeed, the combination of these photodetectors with dedicated integrated electronics offers the possibility of achieving instrumental breakthroughs. This multidisciplinary project is based firstly on an R&D study aimed at optimising the performance of SiPM photodetectors for the detection of charged particles in a biomedical context. The second objective is to complete this study by developing miniaturised probes specifically dedicated to two applications: (1) an intraoperative beta imaging probe for radiation-guided cancer surgery; (2) an autonomous intracerebral probe for preclinical studies on awake and free-moving animals. The final step was to evaluate the impact of these new systems through specific preclinical protocols.

Results:

  • R&D on SiPM detectors

Optimising the performance of SiPMs for the detection of charged particles requires an understanding of the physical phenomena involved in these detectors such as avalanche multiplication, the origins of the intrinsic noise of SiPMs and associated phenomena, as well as their temperature and bias voltage dependence. In this context, the first task of the project focused on the characterisation of a few SiPMs supplied by two main manufacturers (Hamamatsu HPK and KETEK) in a wide temperature range (-175°C

For this purpose, a cryogenic experimental device dedicated to electrical and optical studies of SiPM devices was designed and built at the LAL, as well as an automatic analysis procedure capable of processing large amounts of experimental data in a short time and providing accurate and fast information on the main parameters of SiPMs and their temperature [1] [2]. Detector properties such as the detection efficiency of PDE photons and the associated Geiger trigger probability have also been evaluated [3,4]. The proposed model fits the shape of the IV curve well over a very wide range of currents from 10-12 A to 10-5 A over the operating range of various devices. Therefore, the IV model can be used as a simple and fast method to determine the SiPM parameters. Comparison of these parameters with those calculated from AC measurements and analysed by the automatic procedure showed good agreement.

  • Intracerebral probes for preclinical neuroscience studies

Behavioural testing and positron emission tomography (PET) neuroimaging in rodents is widely used in neuroscience. Because PET imaging in animals requires general anaesthesia or heavy restraint to immobilise the subject, it cannot be combined with simultaneous behavioural studies. Beta-sensitive intracerebral probes fill this gap by allowing the local concentration of radiolabelled molecules to be measured in awake, freely moving animals. 

The intracerebral probe developed during the project consists of a small, low-noise SiPM device coupled to a scintillating fibre and read by dedicated miniaturised low-power counting electronics. Three SiPM devices were chosen as the most suitable for our application: two small KETEK devices of 0.5 × 0.5 mm2 and a standard Hamamatsu device of 50 × 50 μm2. G gain, DCR and beta sensitivity were measured as a function of Vbias and temperature for each device. The promising preliminary results demonstrate that the beta sensitivity obtained with the KETEK devices can be significantly improved by using a focusing lens between the scintillating fibre and the SiPM or by reducing the thickness of its epoxy protective resin.

  • Development of preoperative positron imaging probes

The objective of the third task of the project was to develop an intraoperative positron imaging probe based on pSiPM technology and to evaluate its ability to perform real-time tumour localisation and postoperative monitoring of the surgical cavity. Two positron imaging probe detection schemes were investigated in order to achieve effective rejection of background from 511 keV annihilation γ-rays while maintaining good positron sensitivity and small probe footprint.

Different designs of positron imaging probes, including scintillator material and thickness, light scattering window, optical reflector and light shielding, were investigated by Monte Carlo simulations and measurements. We found that the detector head design of the first configuration allowing the best compromise between spatial performance, beta sensitivity and minimisation of γ-background contamination was that coupling a 0.1 mm thick scintillator with a 2 mm thick light guide covered with a specular reflector and a 10 µm thick layer as light shielding [6]. The second configuration uses a 0.2 mm thick p-terphenyl scintillator coupled to an 8x8 LYSO:Ce array. This configuration was optimised to achieve the best discrimination between events interacting in the upper and lower scintillators based on clustering methods. The optimally designed site of the two imaging probes has sub-millimetre spatial performance and extremely low distortion over the entire field of view (less than 0.4 mm). The spatial performance and β-sensitivity were also shown to be insensitive to temperature variations thanks to the development of a correction system that adjusts the SiPMs' supply voltage in real time.

The objective of developing a fully operational intraoperative probe in the surgical cavity imposes strong constraints on the probe's compactness (small size, low weight, easy to handle). Miniaturised reading electronics and a mechanical housing have been specially developed for this purpose (fig. 1).

Figure 1 - Positron imaging probe design: SiPM array and miniaturised readout electronics (top), detector head (bottom left) and mechanical housing (bottom middle and right)

Finally, the first imaging probe configuration was evaluated in a preclinical environment with 18F-FDG sources. The measured β-sensitivity was 321 cps/(kBq/ml), which is comparable to the best previously developed beta imaging system. The ability of the probe to detect small radiolabelled tumours was evaluated by simulating a realistic clinical environment with phantom sources. The high beta sensitivity and low intrinsic gamma background sensitivity allowed the detection of tumours as small as 5 mm (39 mg) for an acquisition time of less than 4 s, compatible with the duration of surgery and the absorption parameters of currently available clinical radiotracers, such as 18F-FET and 18F-Choline. Very small tumours (3 mm diameter, 14 mg) can also be detected by increasing the acquisition time to 30s. These results represent a further step towards the development of a fully operational intraoperative imaging probe, designed to increase the accuracy and safety of tumour surgery.

Publications:

[1] A. Nagai, "Silicon Photomultiplier for Medical imaging - Analysis of SiPM characteristics", Proceedings of Journées de Rencontre Jeunes Chercheurs 2013, p. 43-46

[2] N. Dinu, A. Nagai, A. Para, "Studies of MPPC detectors down to cryogenic temperatures", Nuclear Inst. and Methods in Physics Research A 787 (2015) 275-279.

[3] N. Dinu, A. Nagai, A. Para, "Breakdown voltage and triggering probability of SiPM from IV curves at different temperatures", NIM A, Available online 30 May 2016

[4] A. Nagai, N. Dinu, A. Para, "Breakdown voltage and triggering probability of SiPM from IV curves", submitted to the Conference Record of IEEE NSS 2015

[5] N. Dinu, T. Ait Imando, A. Nagai, et al., "SiPM arrays and miniaturized readout electronics for compact imaging camera". Nuclear Inst. and Methods in Physics Research A 787 (2015) 367-372.

[6] S. Spadola, M.-A. Verdier, et al., "Design optimization and performances of an intraoperative positron imaging probe for radioguided cancer surgery", 2016 JINST 11 P12019.

[7] M.-A.Verdier, S.Spadola, L.Pinot, et al., "Gamma-background rejection method for a dual scintillator positron probe dedicated to radio-guided surgery", Nuclear Inst. and Methods in Physics Research, A 912 (2018) 315–319.

 

THEOS: Cavities with vacuum-compatible deformable mirrors for high-power lasers

N. Leroy (LAL)

The various systems using high optical power or a long baseline are mainly composed of Fabry-Perot cavities, such as gravitational wave detectors or compact high-fluence X-ray sources. Injecting laser sources of more than 100 W into cavities with a fineness of more than a few hundred will lead to the storage of up to 1 MW of power. Such power creates thermal lenses by absorption inside the coatings and substrates of the mirrors that make up the cavity. These thermal deformations modify the geometry of the cavity modes and strongly limit the amount of energy stored. 

A second source of mismatch comes from the modal composition of the injected laser beam. It is very difficult to obtain a pure Gaussian mode: high-order modes are added to the fundamental mode as it propagates through the optics used to match the laser output to the cavity input. The aberrations can be induced by static defects in the optics or thermal defects when the laser power is higher than some 100W. If the mode sent to the cavities contains high-order modes, only the fundamental mode will be coupled into the cavities and a significant part of the power will be lost by reflection (and may also be a source of noise via scattered light). Controlling time-varying or static but unpredictable phase aberrations in a laser beam can be achieved by a so-called adaptive optics system. This consists of 3 elements, a system capable of detecting phase aberrations, a system capable of calculating corrections from the error signal and a system capable of applying them. As thermal aberrations can be time-varying or unknown, an adaptive system appears essential to constantly correct distortions. Such a system, and in particular the corrector device, must meet very high optical quality requirements, be vacuum compatible and not introduce noise into the system.

Description of the work carried out:

The main idea of the thermally deformable mirror (TDM) is to control the optical length of a mirror via the temperature of the substrate. The temperature and its shape are controlled with a resistor network (see figure 1). A full simulation using Matlab was set up to study the two main parameters of temperature control: thickness and thermal conductivity. The main results of this study show that the thickness has a small influence on the response amplitude for the spatial frequency range we are considering to correct. On the contrary, the thermal conductivity is very important, whatever the spatial frequencies to be corrected. A second study using finite element simulation was carried out to test the size of the actuators and the different possible substrates that can be used to build the TDM. The study shows that the shape of the response is independent of the size of the actuators (if they are small enough relative to the thermal diffusion length) and the thickness of the substrate. It allows the optical and thermal properties of different materials to be compared and the TDM response to be defined in each case. The material must have a large amplitude variation while being kept at a reasonable temperature to avoid damage. Materials commonly used in optics, such as fused silicate or BK7, meet this requirement.

Figure 1 - (Left) Schematic view of a thermally deformable mirror (TDM) where the substrate is heated by a resistor network that modifies the optical path and may then be able to correct an incident wavefront. (Right) Arrangement of the 61 actuators used in the TDM.

The second step in this work was to understand how to use such a device in an optical mode matching system. A full amplitude and phase correction requires two TDMs on the installation. A full study was made to define the correct separation distance between the two objects by calculating the full field propagation in the system. It was also decided to use a third TDM in the optical configuration to mimic any defects that the other two mirrors would be able to control. Some preliminary tests have been carried out. The mode is attenuated but some lower order modes also increase. These correspond to misalignment and focus which can be controlled by additional systems in the facility (telescope for focus and tilting mirrors for alignment). We have also set up a better optical cavity to achieve better accuracy in the position and separation of the different high-order modes. This work is being pursued with another project on the CALVA platform that uses this THEOS development.

Publications:

 
#180 - Last update : 10/04 2021

 

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