The ALICE Time Projection Chamber (TPC) is the main tracking detector of the central barrel. Through the study of hadronic observables, the TPC provides information on the flavour composition of the collision fireball and on its space-time extent at freeze out. In conjunction with the ALICE Transition Radiation Detector (TRD) and the Inner Tracking System (ITS) it is used to study vector meson resonances, charm and beauty through the measurement of leptonic observables.
The ALICE TPC is a 90m3 cylinder filled with gas and divided in two drift regions by the central electrode located at its axial centre. The field cage secures the uniform electric field along the z-axis.
Charged particles traversing the TPC volume ionise the gas along their path, liberating electrons that drift towards the end plates of the cylinder. The necessary signal amplification is provided through an avalanche effect in the vicinity of the anode wires strung in the readout.
Moving from the anode wire towards the surrounding electrodes, the positive ions created in the avalanche induce a positive current signal on the pad plane. This current signal, which is characterised by a fast rise time (less than 1 ns) and a long tail with a rather complex shape, carries a charge that, for the minimum ionising particle, is typically 5-10 fC. The readout of the signal is done by the 557.568 pads that form the cathode plane of the multi-wire proportional chambers located at the TPC end plates.
The design of the field cage of the ALICE TPC is based on a novel construction principle to adapt the detector to the specific running conditions with heavy ion collisions at the LHC. The high particle densities make it necessary that the field cage keeps instrumental (systematic) errors at a minimum in order not to impair the sensitive pattern recognition and resolution capabilities of the detector as a whole. Although a classical cylindrical geometry - optimum for colliding beam experiments - was chosen the other features of the device differ largely from any other field cage.
Separated by the central HV electrode, the field cage has two detection volumes with an inner/outer diameter of 1.2/5 m and a drift length of 2.5 m each. The total sensitive detector volume is 88 m3, filled with a gas mixture of Ar-CO2 (88-12). With a drift field of 400 V/cm, this gas represents the optimum in terms of charge transport (velocity and diffusion), signal amplification (order 104) and transparency for traversing particles. Hence, the field cage has to sustain a maximum potential of 100 kV at its central electrode. Consequently and in line with the requirements of ALICE, the field cage vessels were built from light materials, yet with sufficient mechanical rigidity for such a large apparatus. A composite honeycomb sandwich structure was thus chosen for its favorable stability/mass ratio. Material of aerospace quality has been used such as aramide-based honeycomb cores (Nomex) and foils of Tedlar. A principal element of our design philosophy is to 'contain' the actual field cage volume by a protective CO2 gas envelope provided by two additional cylinders called the inner and outer containment vessels. Containment of the drift volume is essential for personnel and operational safety. It also allows a substantial reduction in material traversed by particles. Another unique feature of the field cage is its internal potential defining system designed to provide a highly uniform electric field with radial distortions of no more than one part in several thousands. The entire potential strip network is suspended from 18 support rods mounted equidistantly over 360 degrees, 31 mm away from the cylinder walls. Although this system compromises physical acceptance it benefits from a substantial improvement in field uniformity due to the absence of charges accumulated between strips if glued on an insulating surface.
Readout chambers (LHC Run 1 and Run 2)
The ALICE TPC readout chambers were specially designed to cope with the high track density expected in heavy ion collisions at LHC. The pad size (granularity) of the inner chambers, i.e. those closest to the beam-beam interaction diamond, has been optimized (7 x 4.5 mm2) to the point that a signal induced in the pads after amplification at the anode proportional wires is just visible above the electronic noise (S/N>20).
Detail of the pad side of a readout chamber showing the segmentation into small pads, the three wire grids (anode, cathode, gating) and their contact strip.
This requires a very careful design of the front-end electronics to minimise the noise there, as well as a very high gas amplification (>several thousands). The high gas amplification implies copiously produced positive ions which, if not blocked by a gating grid, would distort the drift field significantly.
Thus the electrostatics of the gating grid, in particular at the chamber boundaries, had to be carefully designed to keep the ion feed back below the 10-4 level. After all optimisation steps it was nonetheless not a priori certain whether the chambers would work stably in a high track density environment. The chambers were subjected to secondaries from heavy beams mocking up LHC-conditions. These tests, as well as long term stability investigations with radioactive sources, have shown that the chambers are well suited to work in the ALICE environment.
The signals from the pads are passed via flexible Kapton cables to 4356 front-end cards located some 10 cm away from the pad plane. In the front-end card a custom-made charge sensitive shaping amplifier transforms the charge induced in the pads into a differential semigaussian signal that is fed to the input of the ALTRO chip.
Each ALTRO contains 16 channels that digitise and process the input signals. Upon arrival of a first level trigger, the data stream is stored in a memory. When the second level trigger (accept or reject) is received, the latest event data stream is either frozen in the data memory, until its complete readout takes place, or discarded. The readout takes place, at a speed of up to 300 MB/s, through a 40-bit-wide backplane bus linking the front end cards to the readout controller unit. The ALTRO chip is a mixed-signal custom integrated circuit designed to be one of the building blocks of the front-end electronics for the ALICE TPC. In one single chip, the analogue signals from 16 channels are digitised, processed, compressed and stored in a memory ready for readout. The Analogue-to-Digital converters embedded in the chip have a 10-bit dynamic range and a maximum sampling rate of 40 MHz.
Click here to read the Technical Design Report for the TPC
The TPC shows excellent performance. The straightforward pattern recognition (continuous tracks) make TPCs the perfect choice for high-multiplicity environments, such as in heavy-ion collisions, where thousands of particles have to be tracked simultaneously. The picture shows an event display of a peripheral PbPb collision (with not too many particle tracks for best viewing) recorded in November 2010 (first PbPb collisions provided by the LHC).
PID in the TPC is based on simultaneous measurement of momentum and ionisation strength: for a particle with a given momentum the ionization depends only on its charge and mass, which are the parameters needed for PID. Inside the ALICE TPC, the ionization strength of all tracks is sampled up to 159 times, resulting in a resolution of the ionization measurement as good as 5%. The figure below shows exemplarily the TPC particle identification (PID) performance.
In the figure the functional dependence of the ionization strength on the momentum is shown as lines together with data. The particle separation capabilities are obvious: especially at low momenta the different particle families are well separated.
The described PID methods allows also the observation of rare antinuclei (with negative charge z >1). In the shown data sample, they were enhanced by the use of an offline trigger, which includes the PID information of the ALICE Time of Flight system. In this way, ten anti-4He nuclei were identified.
The countries that participated in the construction and operation of the TPC are: Denmark, Germany, Hungary, Slovakia, Sweden, Switzerland, Norway,Poland,
Upgrade of the TPC Readout (during LS2)
The upgrade of the TPC aims at an increase of the data-recording rate for Pb-Pb interactions by a factor of about 100. The MWPC readout chambers were replaced with Gas Electron Multiplier (GEM) detectors, and the new front-end electronics enable continuous readout.
The expected lead-lead collision rate during LHC Run3 (2022 onwards) is 50 kHz. This corresponds, on average, to a collision every 20 μs. In the ALICE TPC, a 90 m3 gas volume where ionization electrons take 100 μs to drift the full 2.5 m distance to the Readout Chambers, the equivalent of 5 events will overlap, which suggests that in order to record the information of all charged particles of all collisions, a continuous readout technique should be used.
This poses two severe problems for the Multi Wire Proportional Chambers used for the readout of the TPC during LHC Run1 and Run2. These chambers are composed of an anode wire grid, where the electron amplification occurs, sandwiched between a cathode wire grid and a flat pad plane, where the signals are read out. But in addition, on top of this structure there is another wire grid, called the gating grid, which makes it possible for the detector to perform at high event rates and multiplicities. Introduced in the LEP era (ALEPH, DELPHI), the gating grid allows one to prevent electrons from non-triggered events to reach the amplification region, by applying an alternating voltage on its wires. Upon a trigger, the gating grid is quickly switched to a flat potential thus allowing ionization electrons of this particular event to reach the anode wires and induce a signal onto the pads. Now, the crucial role of the gating grid is to close itself just after all electrons from our event have reached the anode grid, such as to trap the positive ions produced in the avalanches and prevent that they invade the drift volume. In the same way, charge from non-triggered events is never amplified and thus no extra positive ions are produced. This mechanism allows one to keep the drift volume relatively clean of slow drifting ions (160 ms full drift time) which otherwise would build up a considerable space-charge density and lead to important distortions of the electric drift field. For example, if the current TPC would be run, without switching the gating grid, at 50 kHz Pb-Pb, tracks would appear distorted by as much as 1 m. But 1 m distortions are obviously too much to correct for. If we were to use the gating grid, the maximum trigger rate would be determined by the 100 μs for the drift of the electrons (gating grid open) plus another 180 μs for the corresponding ions to reach it (gating grid closed); this is about 3 kHz, much lower than the 50 kHz the LHC is expected to provide. So a gating technique is not possible.
Furthermore, the amount of charge reaching the anode wires would lead to the saturation of the amplification field in their vicinity, thus affecting the uniformity of the gas gain. Now, remember here that one of the functions of the ALICE TPC is particle identification through measurement of the specific energy loss of all charged particles; but if the gain is modified by fluctuating space-charge in the amplification region, the dE/dx determination will be seriously affected. So wire chambers altogether won’t do the job.
Looking for alternative solutions for the readout chambers, an obvious choice e micro-pattern gaseous detectors, GEMs. These Gas Electron Multipliers, foils with lots of tiny holes introduced by F. Sauli in the 90’s, are capable of coping with the rates and multiplicities expected and provide ‘intrinsic ion blocking’. However, standard configurations of triple GEMs do not provide sufficient ion blocking. We define the Ion Back-Flow (IBF) as the number of positive ions, after amplification, escaping back into the drift volume per initial primary electron. In order to keep the track reconstruction distortion to a bearable level, of order of 10 cm, the IBF from a GEM structure should be 1 % or below. Standard triple GEMs achieve about 5%. After an intensive R&D programme, an IBF below 1% has indeed been achieved by using non-standard configurations of stacks of 4 GEMs, with hole pitches different from standard, and with voltages and fields different from standard. It turns out that the minimization of IBF enters in competition with the energy resolution, i.e. the precious dE/dx performance, and by careful optimization of the GEM structure a good compromise has been reached.
TPC readout chamber with GEM foils during assembly
But what about the stability of such an arrangement? GEMs have been optimized for years in order to be robust against discharges. Although we have departed substantially from the standard configuration, it turns out that the sharing of the gain between four, rather than three, GEMs provide, at our odd fields, the same discharge probability (about 10-8 for alpha particles) as the standard device. It should be noted here that, for various reasons, the operating gas of the upgraded TPC is Ne-CO2-N2 (90-10-5), where the addition of N2 has proven to strengthen the stability.
So we think we have a concept that guarantees charged-particle momentum determination and excellent particle identification through dE/dx. And we keep the beautiful field cage of the TPC.
One more change that had to be undertaken was that the pad plane has now switched roles from being a cathode to, in the GEM case, an anode. This means that the polarity of the signal will be negative. This ‘little detail’, and the need to read out all pads continuously, leads to the necessity to redesign and build a new set of front-end electronics.
During the LHC Long Shutdown 2 (LS2) the TPC was removed from the ALICE cavern, all readout MWPC chambers were removed and new ones, using GEMs were installed.