(updated: october 2016)
contact@CENBG: J. Giovinazzo
In order to study the 2-proton radioactivity, we use a Time Projection Chamber (TPC) in order to perform the tracking of the emitted protons. The nuclei of interest are implanted inside the active gas volume of the TPC, where the protons are emitted. The gas is ionized along their trajectories. With a uniform electric field in the gaz chamber, the ionization electrons drift towards a 2-dimensions collection plane (X-Y) and the 3rd dimension is is the drift time towards the collection plane.
The first TPC is based on on a collection plane with 2 sets of orthogonal strips (X and Y), and provides four 1D distributions along the 2D axis: the measured charge and the collection time along (X and Y). This technology allowed for the first direct observation of the 2-proton radioactivity, but it also showed some instrumental limits for trajectories reconstruction and concerning acquisition dead-time issues.
That is the reason why we started the development of a second generation TPC. This development is performed within the ACTAR TPC collaboration.
The purpose of this new TPC is to get a full 3D digitization of the charge deposit (along particles tracks) in the gas volume. Concerning the collection plane, we developped a pads (2x2 mm2) plane (instead of strips), in order to measure directly a 2D projection of the signal (X-Y). For each pixel (pad), a time sampling of the signal gives the distribution of the collected charge along the 3rd dimension (Z). The readout electronics for the time sampling of the pads signal is developped by the GET collaboration (General Electronics for TPCs). The principle is illustrated in figure 1.
Figure 1: Principle of the 3D digitization of the charge distribution in the active volume. The ionization along the particles tracks drift towards a X-Y pad plane, and the Z dimension distribution is measured with the time sampling of the signal measured on each pad.
The ACTAR TPC detector
The development in the ACTAR TPC collaboration aims to the realization of 2 detectors based on the same principles, with 2 geometries. The reaction chamber is based on a square pad plane (128x128 pads of 2x2 mm2) for nuclear reaction studies. The decay chamber is based on a rectangle pad plane (64x256 pads of 2x2 mm2) for nuclear decay studies. Both detectors share the same electronic: 16384 readout channels.
In the particular case of the 2-proton radioactivity studies, due to the very short half-life of nuclei, a special mode of the GET electronics will be used. For each channel, the memory that stores the sampled signal is split in 2 halves, one use for the measurement of the ion implantation and the other one for the protons emission, in order to avoid missing the decay because of the processing time of the implantation event.
Pad plane prototype: CENBG option
Two demonstrator detectors are build in the ACTAR TPC collaboration, with a smaller size (2048 pads) that the final detector. Each demonstrator is based on a different option for the pad plane technology: the GANIL demonstrator uses a standard PCB, while we developped a new type of pad plane with a metal-core PCB. Both pad plane are equipped with a micromegas for signal amplification.
The challenge for the pad plane design is the pads density of the 2D collection plane. In addition, the plane is at the interface between the chamber interior (gas) and the outside world (air), and must be able to stand pressure differences up to 1 bar at least.
The GANIL option, based on a standard PCB requires reinforcement armatures on the flenge, which imposes small connectors and a complex routing of the PCB from pads to connectors. This routing becomes extremly delicate in the case of the final detector with 16384 pads.
The CENBG option is based on a conceptually very simple design, with a direct connection from the pads to connectors with a pitch of 2 mm. This implies that the PCB itself has to support the mechanical constraints, and we proposed to build a PCB with a metallic core (either high resistance aluminum or stainless steel), as shown in figure 2. This development in carried out in collaboration with CERN PCB workshop.
Figure 2: Design of the detection plane based on a metal-core PCB. The copper circuit itself (orange) is insulated from the metallic core (grey) with a resin (green). Connectors with a 2 mm pitch are inserted and soldered, then the micromegas is added on top.
The prototype of the pad plane (figure 3) has been tested with 6 keV X-rays from a 55<\sup>Fe source and with alpha particles from a triple alpha source (with energies around 5 to 6 MeV. See note on the prototype tests.
Figure 4: Prototype for the collection pad plane with metal-core PCB: the left picture shows the side in the gas volume with the micromegas on it, and the right picture shows the side with connectors to the readout electronics.
The development also includes the circuits connecting the pad plane to the GET electronics. A solution with flexible PCB has been chosen and designed at CENBG (see figure 4). These connectors also hold the protection circuits (ZAP) for the GET electronics.
Figure 4: Flexible circuits (brown) connecting the detector (right side connectors) to the AsAd boards of the GET electronics (left side). The right side PCB also contains the protections for the electronics.
ACTAR TPC demonstrator at CENBG
The demonstrator has been fully equipped with the GET electronics (early 2016). Figure 5 shows the ensemble, and figure 6 shows the pad plane and the drift cage (made at GANIL) that creates the uniform electric field in the active gas volume.
Figure 5: ACTAR TPC demonstrator at CENBG.
Figure 6: Drift cage (from GANIL) on top of the collection pad plane, that defines the active gas volume in the TPC chamber.