Nuclear Inst. and Methods in Physics Research, A 892 (2018) 114–121 Contents lists available at ScienceDirect Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima Metal-core pad-plane development for ACTAR TPC J. Giovinazzo a, *, J. Pibernat a , T. Goigoux a,1 , R. de Oliveira b , G.F. Grinyer c,d , C. Huss a , B. Mauss d , J. Pancin d , J.L. Pedroza a , A. Rebii a , T. Roger d , P. Rosier e,2 , F. Saillant d , G. Wittwer d a b c d e Centre d’Etudes Nucléaires de Bordeaux Gradignan (CENBG) - UMR 5797, CNRS/IN2P3 - Université de Bordeaux, Chemin du Solarium, F-33175 Gradignan, France CERN Micro Pattern Technologies, EP-DT department, CERN 1211 Genève 23, Switzerland Department of Physics, University of Regina, Regina, SK S4S 0A2, Canada Grand Accélérateur National d’Ions Lourds (GANIL), CEA/DRF - CNRS/IN2P3, Bvd Henri Becquerel, 14076 CAEN, France IPN Orsay, Université Paris Sud, CNRS/IN2P3, 91406 Orsay, France ARTICLE Keywords: TPC Pad-plane Active targets INFO ABSTRACT With the recent development of active targets and time projection chambers (ACTAR TPC) as detectors for fundamental nuclear physics experiments, the need arose for charge collection planes with a high density of readout channels. In order to fulfill the mechanical constraints for the ACTAR TPC device, we designed a pad-plane based on a metal-core circuit with an conceptually simple design and routing for signal readout, named FAKIR (in reference to a fakir bed of nails). A test circuit has been equipped with a micro mesh gaseous structure (micromegas) for signal amplification and a dedicated readout electronics. Test measurements have been performed with an 55 Fe X-ray source giving an intrinsic energy resolution (FWHM) of 22 ± 1% at 5.9 keV, and with a 3-alpha source for which a resolution of about 130 ± 20 keV at 4.8 MeV has been estimated. The pad-plane has been mounted into a reduced size demonstrator version of the ACTAR TPC detector, in order to illustrate charged particle track reconstruction. The tests preformed with the X-ray and the 3-alpha sources shows that results obtained from pads signals are comparable to the intrinsic result from the micro-mesh signal. In addition, a simple alpha particle tracks analysis is performed to demonstrate that the pad plane allows a precise reconstruction of the direction and length of the trajectories. 1. Introduction The need for time projection chambers (TPC) in low energy nuclear physics experiments has considerably increased in the last decade. Such instruments may be used as active targets, like the MAYA detector , where the gas volume plays the role of the nuclear reaction target and of the detector for reaction products. They may also be used as thick stopper for radioactive ions produced at fragmentation facilities and as detector for the decay products of the implanted ions, as in the case of the TPC  used for the studies of 2-proton radioactivity and possibly other exotic decay modes. Since the requirements are very similar for the various nuclear physics cases, the ACTAR TPC collaboration started the development of a second generation TPC aiming to fulfill the requirements for a broad physics program including reactions, structure and decay studies. The principle of operation of the ACTAR TPC detector is to collect the ionization signal from the charged particles traveling through the active volume on a collection plane made of pads, so that the signal on all pads gives directly a 2D projection (, ) of the particles tracks. In addition, the readout of the pads signal is performed by the GET electronics [3,4]: the signal on each pad is sampled in time, which allows a digitization of the signal along the 3rd dimension . The pad size is a compromise between the granularity for tracks reconstruction and the number of electronics channels. The ACTAR TPC project aims to construct two TPC chambers sharing the same electronics: a ‘‘cubic’’ version for nuclear reactions (where the reaction products may have long transverse tracks) with a pad-plane of 256 × 256 mm2 , and a ‘‘cuboid’’ version with a 128 × 512 mm2 , better adapted for decay studies at fragmentation facilities where the ions implantation depths * Corresponding author. E-mail address: [email protected] (J. Giovinazzo). 1 Present address: CEA Saclay, IRFU, SPhN, 91191 Gif-sur-Yvette, France. 2 Present address: Institut de minéralogie, de physique des matériaux et de cosmochimie, UMR 7590 - UPMC/CNRS/IRD/MNHN, 4 place Jussieu, 75005 Paris, France. https://doi.org/10.1016/j.nima.2018.03.007 Received 22 October 2017; Received in revised form 26 February 2018; Accepted 4 March 2018 Available online 8 March 2018 0168-9002/© 2018 Elsevier B.V. All rights reserved. J. Giovinazzo et al. Nuclear Inst. and Methods in Physics Research, A 892 (2018) 114–121 have a large dispersion and the decay products may have short track lengths (as for 2-proton radioactivity studies [6,7]). For the pads, the pitch has been chosen to be 2 × 2 mm2 , which corresponds to 16 384 pads for both geometries . For signal amplification, the pad-plane is equipped with a bulk micromegas . For the readout of the signal collected on the pads, there are 2 options: either the pad-plane is totally in the gas volume, which requires an interface that transports the pads signals outside the chamber, or the plane is used as the interface between the inner and outer sides of the gas volume. Due to the number of channels and in order to limit the number of connexion elements and the linear capacitance per channel, the latter option was chosen. Nevertheless, this imposes some constraints on the printed circuit board (PCB) on which the pads are designed. The PCB must then be part of a flange of the gas chamber, and fulfill the sealing conditions. In addition, since the chamber may be used with a wide range of gas pressure, the PCB must be able to resist the deformation caused by the pressure difference between the inside and the outside of the chamber, that may deform or even damage the micromegas or destroy a standard epoxy pad-plane. A possible solution is to glue the PCB on a metallic flange with only a few holes for small connectors that concentrate the signals of pads for a large surface. This requires a sophisticated routing for signals from pads on the inner side of the PCB to the connectors on the outer side. While it has been tested successfully for a small size demonstrator of ACTAR TPC , it requires a specific study for the full size detector. We studied another solution to this problem by considering a PCB built on a metal core in order to stand the mechanical constraint, with a direct connection through the circuit from the pads to a connector with a 2 mm pitch (the size of 1 pad). Despite being conceptually simple and elegant, the manufacturing of the PCB is not straightforward, and Section 2 explains the process that we developed in order to realize the metal-core pad-plane. Measurements have been preformed with an 55 Fe X-ray source and a 3alpha source with a test set-up. The measurements of the global signal collected on the micromegas mesh (or micromesh) are used to extract the intrinsic resolution of the plane and these are described in Section 3. In Section 4, we present the measurements performed with the electronics connected to the pads, and illustrate the track reconstruction with the ACTAR TPC demonstrator. Fig. 1. Schematic view of the metal-core PCB, with a direct connection from the pad to the signal readout connector, with the same pitch as the pads spacing. Each pad is connected to a pin of the connectors. The pins at the end of pads rows are used to ground the structure and the ring around the pads. A 128 μm bulk micromegas, with pillars every 4 mm is added on top of the pads. ∙ the resulting stack is drilled again, at a diameter of 1 mm, inside the holes previously filled with resin; ∙ the pads (and ground ring around the active area) are etched on both sides of the plane; ∙ the copper surfaces (pads, ring) and the holes are metallized (20 to 30 μm); ∙ a protection solder mask is applied around the pads; ∙ the connectors with pins every 2 mm (commercially available) are inserted and wave soldered (this part of the process has been realized by an external company, FEDD company ); ∙ final grinding and polishing are applied; The thickness of metal core (4 mm) covered with a PCB is 4.29 mm. Once the PCB is complete, a 128 μm bulk micromegas  is added to the pads side for signal amplification. This final step is also done at CERN PCB workshop. The resulting pad-plane is shown in Fig. 2. 2. The FAKIR (bed of nails) metal-core PCB for the pads plane 3. Intrinsic resolution Since the pad-plane PCB represents the interface between the interior of the gas chamber and the exterior at atmospheric pressure, any pressure difference will result in a deformation of the plane, depending on the surface of the PCB. For the full size detector with the 128×512 mm2 pad-plane (‘‘cuboid’’ geometry), the expected deformation is in the order of 0.1 mm with a 4 mm thick high resistance aluminum core. In order to get the same order of deformation for the 256 × 256 mm2 pad-plane (‘‘cubic’’ geometry), a 7 mm thick core of stainless steel is required. The calculations have been performed with the ANSYS® software . Since the development has been carried out on the small size ACTAR TPC demonstrator (64 × 128 mm2 ), the prototype presented here has been built with a 4 mm thick high resistance aluminum core. The principle of the direct connection between the pads and the connectors to the readout electronics is illustrated in Fig. 1. Most of the manufacturing process has been carried out at the CERN PCB workshop. The main steps of this process are as follows: In order to check the performances of the pad-plane, we first measured the signal collected on the micromesh, regardless of the pads individual signals, using a very simple test set-up. The tests presented in this paper have been performed with P10 gas (90% argon and 10% methane), at various pressures. 3.1. Test set-up For these measurements, the pads were grounded and a high voltage ℎ was applied on the micromesh. This voltage depends on the signal to measure: it was typically on the order of −500 to −600 V for the 5.9 keV X-ray of an 55 Fe source (limited by sparks that appear around −650 to −700 ) and on the order of −350 V for the 3-alpha source, with alpha particles energies around 5 MeV (the HV is reduced here to avoid saturating the micro-mesh preamplifier and the pads signal). An electrode located a few cm above the active area defines the active volume by applying a high voltage around −1000 V. The P10 gas pressure was 1 bar and 400 mbar for the 55 Fe X-ray source and the 3-alpha source, respectively. The test set-up is shown in Fig. 3. The ionization electrons produced by charged particles in the active volume drift towards the micromesh, where the signal is amplified by an avalanche process. The micromesh is connected via a decoupling ∙ the metal plate is drilled to obtain 1.5 mm diameter holes every 2 mm on the whole surface that will contain pads; ∙ the holes are filled with an epoxy resin, that is used to insulate the pads connection from the metal core; ∙ the PCB layers (25 μm Krempel adhesive, 75 μm polyimide and 18 μm copper) are added on both sides of the plate; 115 J. Giovinazzo et al. Nuclear Inst. and Methods in Physics Research, A 892 (2018) 114–121 Fig. 2. View of the top side of the pad-plane showing the pads (left) and of the bottom side with the pin connectors for signal read-out (right), that led to the FAKIR name referring to the fakir bed of nails. The prototype has 32 × 64 pads of 2 × 2 mm2 . Fig. 3. Test set-up of the pad-plane. The left picture shows the pad-plane mounted on the flange of the gas chamber. The inner side is visible, with the pads defining the active area, and the micromesh. The right picture shows the drift electrode (a thin mesh) held by a plastic frame, located 2.5 cm above the pad plane. The active volume is between the drift electrode and the pad-plane. On the picture, a source is located few mm above the drift electrode. capacitor to a charge sensitive pre-amplifier (CSA). The output of the CSA is shaped (2 μs) with a standard spectroscopic amplifier, and digitized by a 12-bit ADC of a VME-based acquisition system. 3.2. Resolution with an 55 Fe X-ray source In order to test the behavior of the detector, we performed measurements with a standard X-ray source of 55 Fe, located on top of the drift electrode (see Fig. 3 right). The main peak in the mesh signal histogram (Fig. 4) corresponds to the photopeak conversion of the 5.9 keV X-ray from the source. The other visible peak corresponds to the escape peak of the Argon atoms present in the P10 gas, with an energy of 3.2 keV. A fit to the energy spectrum with the approximation of 2 gaussian components for the main peaks and a continuous background gives a full width at half maximum of 21.5 ± 0.5% at 5.9 keV, which is comparable to the resolution obtained with other micromegas-based detectors . Fig. 4. Mesh signal measured for 5.9 keV -ray from an 55 source. The plot shows the 5.9 keV peak resulting from the -ray conversion to ionization electrons. The small peak corresponds to the 3.2 keV signal resulting from the escape peak of Argon atoms. The fit to this spectrum gives an energy resolution (FWHM) of 21.5 ± 0.5%. 3.3. Resolution with a 3-alpha source Since the ACTAR TPC detector is designed for nuclear physics, involving charged particles ranging from protons to heavy ions, the mesh signal has also been measured with a 3-alpha source. For this measurement, the source was collimated and located between the drift electrode and the pad-plane, so that the alpha particles trajectories are along a plane parallel to the pads. The source primarily emits alpha particles at 5157 keV (for 239 Pu), 5486 keV (for 241 Am) and 5805 keV (for 244 Cm), but each source component also emits lower energy alpha particles. For the analysis of the mesh signal spectrum (Fig. 5), the peaks were fit taking into account all components of the source. A resolution (FWHM) of 150 ± 5 keV for an alpha particle energy of 5.5 MeV has been obtained (3.1%). It should be noticed that we consider here that the signal corresponding to the full energy deposit is collected on the pad plane, despite a small fraction of this signal may be lost while the particle travels through the collimator. A more Fig. 5. Mesh signal measured for a 3-alpha source. The average energies of the peaks are 5148.8 keV for 239 Pu, 5478.4 keV for 241 Am and 5794.9 keV for 244 Cm. A fit to this spectrum gives an energy resolution (FWHM) of 150 ± 5 keV at 5.5 MeV. 116 J. Giovinazzo et al. Nuclear Inst. and Methods in Physics Research, A 892 (2018) 114–121 accurate procedure is presented in Section 4.4.1 for the measurement with the full demonstrator (with a uniform electric field cage and the pads electronics, see Section 4.1). ∙ a time sampling (write) frequency = 50 MHz; ∙ a sampling depth of 512 data per channel (giving a total sampling time of 10.24 μs); ∙ a peaking time = 502 ns for the shaper; ∙ a dynamic input range of 240 fC or 1 pC depending on the measurement (X-ray or alpha particles). 4. Demonstrator characterization In the previous section, the tests were performed only with the signal measured on the micromesh. In order to characterize the signals from the individual pads, the read-out electronics needs to be connected. The source measurements (alpha particles and X-ray) presented in this section have been performed with the ACTAR TPC demonstrator (2048 pads) equipped with the GET electronics [3,4]. 4.2. Calibration and analysis procedure In order to evaluate the detector capabilities concerning energy measurement, it is necessary to sum the signal amplitudes from the pads that collect the charges from a charged particle track. This requires to gain match the whole amplification and processing channels of all pads: the amplification from the avalanche at the micromegas on one side, and the read-out electronics on the other side. Ideally, this could be performed by injecting the same amount of charges on each single pad. Practically, this can hardly be achieved. An alternative calibration method is presented in [10,13]. First, a strongly collimated X-ray source is used to scan the pad plane, with all pads grounded, to estimate the variations of the micromesh-pad distance from the shift of the Xray signal amplitude measured on the micromesh. Then the electronics channels are gain matched with a fast voltage pulse applied on the micromesh. The induced signal is measured on each pad and corrected for the amplitude differences due to the pad-mesh distance variations. Nevertheless, this procedure could not be applied in the current measurement, since we do not have a scanning table available. A partial scan (about 30% of the pads) could be achieved, indicating an inhomogeneity of less than 2% of pads-mesh distance (for a 128 μm thick micromegas). The incompleteness of the scan does not allow a pad by pad correction of the micromesh amplification. As a consequence, we performed the calibration using only the induced signal from pulses applied on the mesh, considering it allows a simultaneous gain matching of the micromesh amplification and of the read-out electronics. It is only an approximation because the signal amplitude variations due to pads-mesh distance inhomogeneities is different in the case of an induced signal (variation as inverse of distance) and in the case of the avalanche amplification (exponential variation) of real particles measurement. Since the pads-mesh variations are rather small (about 2% compared to the 5 to 10% variations of electronics gain), this simplified procedure gives fairly good results. The pulser gain matching is illustrated in Fig. 7. The plot of a single pulser event signal on the pads (Fig. 7c) clearly shows that there is an important detector side effect on the induced amplitude. For this reason, the pads located on the border of the collection plane were excluded from the energy analysis presented in this section. In addition to the pulser gain matching calibration, some corrections are applied to the time sampled data registered for each pad. We used a rather simplified procedure with respect to the recommendations of . The raw data from each pad (before gain matching) were corrected according to the fix pattern noise channels to reduce coherent noise. After calibration, a linear baseline was subtracted to the samples on a channel-by-channel basis, in order to remove the baseline fluctuations. In the analysis, we limited the information extracted from the pads sampled signal to the amplitude of the signal pulses (the maximum value) and the time of this maximum in the sampling window. Several procedures were tested to determine the maximum signal time and amplitude: smoothing the signal to reduce high frequency noise, fitting locally the peak around the maximum with a parabolic shape and using different ranges for the baseline correction. No significant improvement has been observed. Finally, a software threshold was set to determine whether the pads are considered as hit or not. Only the hit pads are then used for the event analysis. For example, the total signal measured for a single event is the sum of the amplitudes of the hit pads. 4.1. ACTAR TPC demonstrator set-up The demonstrator installation is presented in the top part of Fig. 6. The pad plane was mounted on the top flange of the gas chamber. A simple gas regulation unit controlled the gas pressure and flow. For the readout of the pad signals by the GET front-end electronics, we developed specific connectors using a kapton flexible printed circuit (FPC), shown in Fig. 6. The capacitance for each channel, from the FPC and its connection boards, is about 25 pF. Due to the high density of pads, the surface of the front-end boards is larger than the padplane surface, and a direct connection was not possible. The FPC is shielded in order to protect the propagated signals from electromagnetic disturbances. The FPC length is on the order of 20 cm, with a controlled impedance, allows to keep the equivalent capacitance seen from the front-end of GET relatively low, and consequently to keep the input noise as low as possible, despite the distance between the pad-plane and the electronics. For the measured signals, this noise depends on the settings of the GET electronics, such as the peaking time (or shaping time), the dynamic range (or gain), or the sampling frequency, A detailed study of this issue is beyond the scope of this paper but an illustration of this noise will be given in Ref. , in the least favorable case of the highest gain of the channel pre-amplifier (120 fC dynamic range). In the conditions of the present measurements (see further), the noise increase is at maximum 25% of the intrinsic noise of the GET electronics (few coder units of the 12 bits ADCs with a 1 V full input dynamics). The active volume of the demonstrator includes a drift field cage manufactured at GANIL (bottom-right part of Fig. 6). The purpose of this cage is to provide a homogeneous electric field inside the chamber. The top of the drift cage is the cathode, made of a metal grid on a PCB, on which a negative high voltage is applied to repeal the ionization electrons towards the pad plane. In order to create a uniform field, 20 μm diameter wires are soldered every millimeter on side frames, with a voltage gradient (typically 100 V/cm) from the pad-plane (or micromegas) to the cathode voltages. To obtain this gradient, resistors are placed between neighbor wires. More details are provided in Ref. . This drift cage was used in the measurements performed to characterize the tracking capabilities of the detector, which was achieved using alpha particle trajectories in the active volume. The alpha source was placed on the side of the active volume in order to measure tracks of the particles crossing the volume. In the case of X-ray, since the charge deposit is almost point-like, we used the set-up presented in Fig. 3 in order to keep the conversion and drift volume small, and place the source so that it faced the center of the pad-plane. For the demonstrator measurements, we also kept the VME-based data acquisition system (see Section 3.1) to register the mesh signal amplitude in coincidence with the pads signal. The purpose of this VME acquisition is to monitor that the global conditions remained stable, for example by monitoring the energy resolution of the mesh signal. The VME and GET acquisitions were synchronized by forcing a common dead-time, and both systems were trigged by a leading edge discriminator output from the mesh signal. For the measurements presented here, the following settings of the GET electronics were used: 117 J. Giovinazzo et al. Nuclear Inst. and Methods in Physics Research, A 892 (2018) 114–121 Fig. 6. ACTAR TPC demonstrator set-up. The left photograph shows the flexible connections that are needed between the pads (on the chamber top flange) and the front-end electronics. The right picture shows the drift cage built at GANIL in order to provide a homogeneous electric field in the active volume of the TPC. A high voltage on the order of 2000 V is applied on the drift electrode (on top of the cage). The uniform field is obtained with horizontal wires (20 μm diameter, 1 mm spacing) soldered on the epoxy frames, with resistors between neighbor wires. Fig. 7. Illustration of the pulser calibration of the pads (gain matching). The plot (a) shows the amplitudes of the signal for all channels (2048 pads) with various values for the height of the pulse injected on the micromesh. The alignment resulting from a linear calibration performed on all channels with these data is show in plot (b). The signal measured on pads results from the induced current generated on pad inputs. The plot (c) corresponds to the pads signal amplitudes for a single event, according to the position of the pads on the collection plane. The charge injected on the pads from the border of the plane is different from the other pads, due to border effects inducing a different the mesh-pad capacitance, indicating that the pulser calibration may be used for gain alignment of the corresponding pads. 4.3. 55 Fe 4.4. Triple-alpha source test measurement X-ray source test measurement For the measurement with the 3-alpha source, the drift cage shown in Fig. 6 was used, in order to obtain a constant electric field in the active volume to perform a real tracking of the alpha particles. In this section, the alpha source is located on the side of the drift volume, and roughly collimated (see Fig. 9). An example of an alpha particle detection event is presented in Fig. 10. The amplitude of the signal collected on the pads is proportional to the energy loss of the particle along its trajectory. Fig. 10a represents the projection of the energy loss (Bragg peak) on the 2D pad-plane (- dimensions). According to the analysis procedure presented in Section 4.2, we limit the timing analysis to the position (on time axis) of the maximum amplitude of the sampled signal of each pad. This information is used to analyze the 3rd dimension () of the track. Measurements were performed with pads signal readout, in the same configuration as described in Section 3.2 with an 55 Fe source. The source was located on top of the test setup as presented in Fig. 3. For the analysis of the energy distribution (summed pads amplitude), the events were selected with the following conditions: the hit pattern did not extend to the borders of the pad plane (1 pad width border rejection) and the multiplicity of hit pads was less than 6 (that removes events such as cosmic rays). The fit of the signal amplitude distribution, under the same conditions described in Section 3.2 gives a resolution (FWHM) of 22.1 ± 0.7% (see Fig. 8), which is comparable to the previous measurement of the mesh signal with grounded pads. Since the mesh signal was also measured in parallel, the same data for the mesh signal give a resolution of 23.0 ± 0.8%. A possible reason for the slightly better resolution with pads compared to mesh signal may be the gain matching procedure that corrects to some extend the amplification variations from pad to pad, which cannot be achieved with the mesh signal. The difference is not really significant. 4.4.1. Alpha particle energy correction Despite the fact that the energies of the emitted alpha particles are known, it does not correspond exactly to the signal measured in the active volume. The particles first travel across a gas volume outside the drift cage, in which the ionization signal is not collected (dead zone). In order to estimate the effective resolution of the detector, it is necessary to correct for this energy deficit. 118 J. Giovinazzo et al. Nuclear Inst. and Methods in Physics Research, A 892 (2018) 114–121 Fig. 8. 55 -ray source measurement: (a) example of the signal amplitude (in coder units) measured on the pads for a single event; (b) distribution of the pads hits for the measurement; (c) distribution of the summed signal of hit pads for all events that fulfill the selection criteria (see text). This is achieved using a 4 [14,15] simulation, that takes into account the entire gas volume of the chamber around the limited active volume, the source collimator and the source extension. The main adjustable parameters are the distance between the source location and the active volume, and the gas pressure. The Penelope ‘‘physics list’’ of the 4 toolkit is used in the simulation. The Livermore ‘‘physics list’’ has also been tested for comparison, but no significant difference was noticed. In addition, a maximum step size of 0.1 mm is included to avoid long steps with processes such as continuous energy-loss, that may create spurious effects in the Bragg peak. At this stage, the purpose of the simulation was to estimate 2 quantities: the effective mean alpha energy deposited in the active volume, and the intrinsic dispersion (width) of the shifted peaks. Several simulations have been performed on a large range of distances and pressure values around the measurements nominal values to estimate the uncertainty for the energy corrections. Fig. 9. Schematic view of the collimated alpha source located on the side of the active volume. Before entering the active volume delimited by the drift cage, the alpha particles travel through a gas region where their energy deposit is lost (the ionization signal is not collected). The signal collected in the active volume thus corresponds to a degraded energy with respect to the alpha emission energy. ∙ 10 gas pressure = 400 mbar; 4.4.2. Estimated resolution The energy distribution for a measurement with the 3-alpha source is shown in Fig. 11. The measurement conditions are: ∙ micromegas amplification voltage: ℎ = −370 V; drift electrode high voltage: = −2070 V; Fig. 10. Example of the signal collected for an alpha particle. On plot (a), the amplitude measured for each pad of the - plane corresponds to the collected charge (from ionization) showing the Bragg peak of the particle in the gas. On the 3D plot (b), the vertical axis is the time of the pads signal maximum amplitude, that is proportional to location along the -axis where the energy deposit occurs. 119 J. Giovinazzo et al. Nuclear Inst. and Methods in Physics Research, A 892 (2018) 114–121 Fig. 12. Energy loss function 0 built from the simulated Bragg peak to define the energy loss profile model along the trajectory for alpha particles up to = 10 MeV, (with a track length ). As an example, the filled area correspond to ranging from 0.65 to 1, which is the region of interest for alpha particles with a track length = 0.35 ⋅ . The arrow indicates the energy loss profile along the particle track. Fig. 11. Summed pads signal measured for the 3-alpha source. The gray histogram shows all events while the filled histogram (dark red online) contains only events that fulfill the selection conditions (see text). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) ∙ lateral distance between the source and the active volume = 18 mm. It can be compared to the results from the  program: = 47.5 ± 1.5 mm∕μs (the uncertainty includes a variation of ±20 mbar of the gas pressure and of ±5 V∕cm of the electric field). The particles trajectories were analyzed using both the direct information from the detector (energy) and the fit with the model presented previously (tracking). Since the track is approximated by a straight line while the real path of a particles results from many random scatterings on the gas atoms, this introduces few limitations that are not caused by the detector quality itself. The spatial resolution deduced from the track length calculated with fit parameters of measured event is ≃ 3.2 mm for an average track length of 110 mm. This does not correspond to the precision of track reconstruction, since the real tracks have an intrinsic dispersion . If we could estimate this intrinsic dispersion, also determine the √( we)could ( )2 2 track reconstruction precision: = − . We estimated the value from the 4 simulation (considering only the starting and stopping points of the simulated tracks to be consistent with the data), but the result is slightly larger than the value: ≃ 3.3 mm. Nevertheless, these values indicate that the tracking precision may be much lower than this value, of the order of 1 mm, which is consistent with the values obtained with a similar detector . In the case of track angle measurements, this precision will be propagated to the determination of angles. Obviously, the influence of spatial precision on angles is larger for shorter tracks. From angular measurements presented in  and , an angular resolution in the order of 1◦ or less is achievable with such a pad-plane. In addition, ⃖⃖⃖⃖⃖⃖⃖⃖ ⃗ due to ions scattering, the observed track direction 0 1 does not correspond exactly to the emission direction, independently from the points estimate precision. For example, in the simulation with the same conditions as the source measurement, the angle average difference is about 2.3◦ . In this case, the intrinsic reconstruction capabilities of the detector are not a limiting factor. Finally, Fig. 13 shows the correlation between the fit results (energy and track length) and the energy measured from pads signal amplitude. The energy resolution from the fit (function integral) is not as good as the one from the pads signal. This is an indication that improvements in the track model may be studied. The relative resolution for track length is comparable to the one for the measured energy. For the estimate of the energy resolution, the events were selected according to validation conditions on the hit pads pattern: the tracks must start from the side of the pad-plane where the source is located, to reject events that do not originate from the source and they shall not reach the outer border of the pad plane on other sides. The peaks in the energy signal distribution were fit simultaneously, and the fit results were converted into energy unit taking into account the energy loss in the dead zone of the gas estimated by the simulation. A resolution (FWHM) = 135 ± 20 keV was deduced at an alpha particle energy of 4.8 MeV for the center peak (≃ 2.7%). If one considers this resolution as the result of the intrinsic detection resolution and the dispersion of the deposited alpha √ energy (that is estimated by the 4 simulation), then = 2 + 2 and the effective energy resolution of the detector is = 130 ± 20 keV (the uncertainty contains the fit result uncertainty and the variation of the simulation parameters). 4.4.3. Particle track reconstruction An important purpose of the ACTAR TPC detector is to perform tracking of charged particles, mainly protons and ions. In this section we propose to illustrate the track reconstruction from the test measurements with the alpha source. We consider here that the particle travels on a segment, from a start ⃖⃖⃗0 = (0 , 0 , 0 ) to a final point ⃖⃖⃗1 = (1 , 1 , 1 ) in the gas volume. point The energy loss (Bragg peak) along the track is taken from a pattern built from the 4 simulation (Fig. 12), with 2 adjustment parameters: the starting fraction of the pattern ( ) and an amplitude scaling factor ( ) for the fitted track. The ionization signal from this energy loss drifts towards the collection plane. In order to define the 2D pads signal projection amplitude, we consider a gaussian dispersion of the signal along √ both and , direction, with a dispersion width that increases with , where is the drift time (that is equivalent to the drift length along axis). Practically the track parameters estimate is performed by simultaneously fitting the - signal projection and minimizing the time difference between the drift time for the pads from the measured event and the fitted 2D projection. With √ this linear track fitting model, the estimated track length is: = 2 + 2 + 2 , with = 1 −0 , = 1 −0 and = ⋅(1 −0 ) where is the drift velocity of the signal along axis. This drift velocity can then be estimated from the track length dispersion: for a given particle energy, the fitted tracks coordinates (, , ), are expected to be located on a sphere. The estimate is then the value that minimizes the FWHM of the track length distribution. For the measurement considered here, a value = 48.0±1.2 mm∕μs is obtained. 5. Conclusion In the context of the need for a second generation of time projection chambers in nuclear physics experiments, we developed a collection plane for ionization signals with a relatively high density of pads. The concept is based on a very simple design with a direct connection from the pads to the readout connectors. The printed circuit board is built 120 J. Giovinazzo et al. Nuclear Inst. and Methods in Physics Research, A 892 (2018) 114–121 The demonstrator has been used in test measurements for almost two years. After few tens of mounting and dismounting, of pressure changes from 0 to 1 bar, with no problems (damages, leaks,. . . ), we consider this pad-plane has shown its mechanical robustness and reliability. Acknowledgments The research leading to these results have received funding from the European Research Council under the European Union’s Seventh Framework Program (FP7/2007-2013)/ERC grant agreement no 335593 and from the Conseil Régional d’Aquitaine (grant no 2014-1R60402 00003319). Fig. 13. Correlation between the sum signal from the pads (energy) on axis of both plots and the results from the tracks fitting procedure: the total signal from fit function integral on plot (a) and the track length on plot (b). The energies are given in arbitrary units related to the summing of the pads signals in coder units. 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