Purification and Monitoring in large LAr Time Projection Chamber detectors

Liquid Argon Time Projection Chambers (LAr TPCs) are able to reconstruct tracks of particles as they traverse the medium by collecting the scintillation light and ionization charge produced as they move (C. Rubbia, CERN EP-INT 77-8). Ideally, all electrons should be drifted to the anode plane, but in practice, part of the ionization charge is lost, due to the presence of electronegative impurities in the liquid. A charged particle at the minimum of ionization losses will produce in LAr, after some initial local recombination of the ionization, about 20000 electrons every 3mm of travel path. This small amount of charge has then to drift for a few meters, taking times of the order of milliseconds, in order to reach the anode plane where it is read out by low noise amplifiers. Impurities along the drift path will absorb the electrons and reduce the signal-to-noise ratio. Commercial liquid argon is contaminated by oxygen at a few ppm level and it must be then further purified in order to be used in LAr TPC detectors.The removal of electronegative impurities from liquid argon down to the sub-ppb level and the possibility to reach electrons lifetimes of several milliseconds is fundamental in order to build large LAr TPC detectors. This high purity figure can be achieved by removing the air present inside the cryostat before filling ( cryostat evacuation or gas argon flushing) and by filtering LAr at the time of the filling and during operation thanks to recirculation systems. The gaseous argon resulting from boil-off can also be re-condensed and purified. The cryostat tightness is also essential in order to limit the introduction of new impurities from the atmosphere. Past operation records have shown that an electron lifetime up to 15 ms (20 ppt oxygen equivalent contamination) could be achieved. The construction of very large LAr detectors has strengthened the need for purification systems and monitoring. Large cryostats are also going to be constructed with cheaper techniques inherited from the Liquified Natural Gas ( LNG) tanks industry ( LBNF cryostats) . These tanks cannot stand the atmospheric pressure and be evacuated before filling, but the air has to be removed by flushing pure argon gas (piston purge). While in most of the detectors built up to now the ionisation electrons were read out inside the liquid Argon itself, the dual-phase LAr TPC design adds the possibility of amplifying the electrons in the gas phase above the liquid in order to compensate the drift losses over very long distances (up to 12m). In addition to the purification methods and purity assessment in the liquid and gas phases, the dual-phase design requires as well precise measurements of the liquid argon level and assessment of the thermodynamic conditions of the LAr and of the gas phase, and monitoring of the detector elements at the interface in between gas and liquid as well as the one immersed in the liquid with cryogenic cameras. The design of modular slow control systems acquiring the information from all the sensors and their scalability to wider dimensions is also an important aspect in view to build very large detectors.

R&D activities in the framework of AIDA2020 WP8

WP8 fosters knowledge sharing and common tools in the neutrino community as regards state-of-the-art in very large cryogenic liquid detectors. The construction of liquid argon detectors at the 10 kton scale is an essential ingredient of the future international long-baseline neutrino program unifying the European and USA efforts. The DUNE detector, whose final design consists of four large LAr TPCs, each of them with a 10 kton mass, aims at precision measurements of the neutrino oscillation parameters, the study of atmospheric and solar and supernova neutrinos, as well as pushing the limit of proton decays for a variety of final states.

WP8 activities focus on the most challenging aspects related to this detector development. One of these aspects, studied within the networking activity of Task 8.2, concerns the large size scaling of systems for the assessment of the detector conditions including purity assessment, temperature, pressure and liquid level controls and visual inspection of the detector conditions. These large cryogenic detectors foresee also the application of industrial techniques for the cryostat construction (based on the design of cryogenic tanks to LNG carriers) which had never been applied so far to particle detectors. The main aspects of the WP8 networking activity have been then including:

  • Reviewing: purity assessment and detector control methods
  • Development of large scale monitoring systems for LAr detector
  • Development of high accuracy LAr level controls
  • Development of cryogenic cameras
  • Purification and recirculation of noble gases and liquids, and development of devices and techniques for purity monitoring
  • Assessment of purity and stability of thermodynamic conditions in industrial LNG vessels

WP8 used as test-bench the 3x1x1 m3 dual-phase liquid argon TPC prototype, running at CERN in 2017 and the related test infrastructure at CERN, from which specific R&D results have been obtained. The 3x1x1 prototype has been the first constructed by the CERN Neutrino Platform with the GTT membrane cryostat technlogy ( video, development for large LAr detectors, neutrino platform cryogenics activities). As a example cases, we will mainly refer in the following to the results obtained in that context and to the solutions envisaged for the dual-phase ProtoDUNE detector, expected to start data taking in 2019. The reason for this choice, apart from the size and recent construction of these detectors, is the cryogenic design based on commercial LNG carriers, that has the potential to expand to much larger volumes. So these detectors were fundamental milestones to gain experience with large-scale LAr vessels and large readout systems.

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The 3x1x1 LNG membrane cryostat prototype at CERN.

Cryogenic system

Large scale cryogenic detectors require the development, construction and operation of state-of-art cryogenic systems capable of purifying large amounts of fluids and maintaining a stable thermodynamic environment inside the cryostat. Their main tasks are:

  • Displace the air from inside the cryostat by filling it with pure argon gas to the level that the main contaminants (oxygen, moisture and nitrogen) are reduced to the part-per-million (ppm) level. Subsequently cool down and fill the cryostat with liquid argon in a uniform and controlled manner
  • Ensure a stable thermodynamic environment inside the cryostat (fluctuations within a few mbars on the pressure and 1-2 kelvin on the temperature)
  • Keep the electronegative impurities in the liquid below the 100 ppt level.

The above cited constraints are much more stringent than what is commonly required by industrial standards. As example, the liquid argon which is provided by industry is pure only at the ppm level. Therefore innovative developments are required and cryogenic systems with a relatively high level of complexity have to be constructed in close collaboration with specialized industries. In the specific case of the 3x1x1 detector the cryogenic expertise from CERN was also a key input. As example we show in Figure 1 the piping and instrumentation diagram used for the operation of the 3x1x1 m3 TPC, together with a picture of a custom designed liquid recirculation pump system.

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Left: piping and instrumentation diagram of the 3x1x1 detector. Right: an example of a custom development: the liquid argon centrifugal pump contained in its own vessel that can be accessed for maintenance without polluting the main volume. Click on the image to see the full scale version

One innovative concept developed and successfully tested here for the first time is indeed the operation of a submerged centrifugal pump ( ACD-cryo TC34.2 ) confined at the bottom of a closed vessel fixed to the cryostat, called pump tower, which has a 350 mm diameter and a 3.5 m length. A liquid nitrogen heat exchanger with a cooling power of 2 kW is present inside the pump tower to compensate the heat generated from the pump. This unique design allows for the pump to be extracted for maintenance without polluting the argon of the main volume. Another attractive feature of the system is that possible waves and turbulence introduced by the pump are confined inside the tower and avoid perturbing the liquid level in the active region. The pump tower communicates with the main cryostat liquid and gas volumes via two 25 mm diameter ports. Their openings can be controlled from the exterior via two long stem cryogenics valves to regulate the flow of liquid and to equalize the pressures between the pump tower and the main volume.

Purification and recirculation of noble gases and liquids

Purity during purging phase

To clean the detector from impurities present in air, purging the cryostat with gas argon before cooling it down and filling it with Liquid Argon is essential. LNG membrane tanks, of which the 3x1x1 was a first prototype, cannot be evacuated since the corrugate membrane inside the cryostat would not stand atmospheric pressure. Before filling and prior to cool down, the cryostat is then purged with gas argon purified by passing it through an oxygen getter to remove air from inside the tank. The purge is first performed in open-loop and then in closed-loop at a later stage. During the purge in open-loop the gas argon injected in the cryostat is free to exhaust to air through a venting valve. When the purge in closed-loop starts, the venting valve is closed, so that the gas cannot escape to the atmosphere. Instead, the injected gas argon is extracted from the vessel through a double diaphragm pump and subsequently purified by a getter before going back into the cryostat. The getter (a commercial SAES MicroTorr MC4500) removes $\rm H_2O,\;O_2,\;CO,\;CO_2,$ and $\rm H_2$ down to <100ppt. The concentration of impurities, namely moisture, oxygen, and nitrogen, is constantly monitored through Residual Gas Analysers (RGAs) at various sampling points in the cryogenic system. Independent devices are used to trace nitrogen, oxygen and moisture.To compensate for the sampled gas, the system also contains a make-up gas line which injects purified gas argon. During the purge in open-loop the ratio of nitrogen to oxygen (3.7 for air) may give hints to whether or not air leaks are present in the system. Once the levels of $\rm H_2O,\; N_2\; and\; O_2$ are below $\sim$50 ppm, the second purge in closed-loop may start. Cool down starts once the levels of the water, oxygen, and nitrogen impurities have fallen to a few ppm. Figure 1 shows the evolution of the concentration of impurities due to moisture, Nitrogen and Oxygen during the purging phase of the 3x1x1 detector in winter 2017.

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Evolution of the impurities measured in the gas during open, closed loop piston purge and cooling down. The measured temperatures inside the gas near the bottom, middle and top of the main cryostat volume are indicated in blue.

Cool down

A cool down procedure is necessary in order to slowly bring the cryostat from room temperature to a temperature close to liquid argon before filling and avoid so thermal shocks. The cooling of the 3x1x1 cryostat was performed spraying the detector and the cryostat with Argon droplets. A mixture of LAr and GAr at 87 K is inserted into a system of atomizing nozzles located at the corner of the cryostat. The nozzles generate a spray which allows the tank to be cooled down uniformly and slowly, thereby preventing thermal shocks to the components inside. This method was successfully tested for the first time at FNAL in the 35-ton cryostat. The cooling power provided by the nozzles must compensate for the total heat input to the cryostat via the passive insulation and other internal heat sources (e.g. internal cables, electronics, injection of warm gas, recirculation pump, etc.), as well as provide enough cooling power to reach 87 K. The temperatures in various parts of the cryostat and detector components is monitored during cool down thanks to a network of temperature probes read out by the slow control system, as shown in the example in the figure below:

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Temperatures monitoring at various heights inside the cryostat during the cool down phase.

While cooling down takes place, the levels of the water, oxygen, and nitrogen impurities drop even further, as impurities “freeze” and the out-gassing rate decreases. Once the cool down stage has completed, the filling stage of the cryostat can begin. Especially during filling monitoring the level of the liquid is crucial: this is performed with level meters and cryogenic cameras.

Purifying the liquid argon

The liquid argon inside the cryostat, which is operated at near atmospheric pressure, evaporates at a rate that depends on the insulation quality and on the total heat input provided to the system. Both the evaporated gas, so called boil-off, and the liquid are constantly recirculated and purified in a closed loop. The boil-off argon is pumped out by a gas recirculation pump and sent to a commercial gas getter before going into a condenser. Once re-condensed, the boil-off argon is then sent to phase separator, which separates the gaseous and liquid argon: the gas is returned to the condenser, whereas the liquid goes back to the tank. In the case of malfunction with the gas recirculation pump, the boil-off gas goes through an emergency line towards the condenser, by-passing the getter. Such “dirty” liquid argon is not sent back to the tank, but is instead redirected to the purifier (the same one used to purify the recirculated liquid argon) and only then sent back to the cryostat. Inside the cryostat, a submersed LAr pump recirculates the liquid argon, which is then purified outside the tank in the “purifier valve box”. The mechanism of the purifier has been pioneered by the ICARUS Collaboration. It relies on a molecular sieve to remove water by physical absorption and on aluminium coated copper pellets to remove oxygen through the following reaction:

$O_2+2Cu \rightarrow 2CuO$

while regeneration of the filters involves the reaction below:

$CuO + H_2 \rightarrow Cu+H_2O$

For the purpose, the purifier valve box is equipped with regeneration gas ($H_2$) inlet and an outlet pipes. Hydrogen must be warmed up to 200 ˚C for the above reaction to take place. The heater is also placed inside the purifier valve box. To monitor the behaviour of impurities in the gas, the same Residual Gas Analysers used during purging are used. Monitoring the liquid purity is a bit more tricky as there are no commercial devices that can measure impurities in liquid and down to the ppt level. Custom designed systems, such as the purity monitor developed at UCL, are the solution.

Techniques and devices for purity monitoring

Three main methods can be used in order to assess liquid argon purity during the initial phase of its operations:

  • Observation of the long component of the scintillation light
  • Measurement of the attenuation of the electron ionization signal during the drift in a dedicated "slow drift" device (purity monitor)
  • Measurement of the strength of the electron signal at different distances from the anode for long muon tracks.

The first method gives an indirect measure of the LAr purity during an initial phase (for impurities of the ppm level), and is based on the scintillation light produced by incoming particles and detected by photodetectors inside the cryostat (e.g. photomultiplier tubes in the 3x1x1 WA105 detector). Purity monitors may cover the range going from the initial detector operation at low purity up to a purity measurements corresponding to several ms electrons lifetime. They are compact devices which may be installed in diffent places inside the cryostat in order to check the uniformity of LAr purity. The measurement of ionization along tracks, as a function of the drift, is a complementary technique to purity monitors and it can be used also for very long lifetimes when the purity monitors may be saturated.

Scintillating light

Scintillation in liquid argon is composed of a fast and a slow component ($\tau_f$ and $\tau_s$ respectively). A more detailed discussion can be found in the ProtoDUNE -DP/WA105 TDR. Variations in the time constants due to electronegative impurities are significant for the slow component which is related to de-excitation of triplet states. To a first approximation, the quenching effect of impurities can be described by the following equation (from JINST vol 5, p 5003,2010):

 $\frac{1}{\tau^*} = \frac{1}{\tau} + k[\rho]$ \\ $\Rightarrow \tau^* = \frac{\tau}{1+k\rho}$

where $\tau^*$ is the value of the time decay constant when impurities are present, $\tau$ is the same constant, but for zero impurities (which is about 1.6us), $\rho$ is the concentration of impurities and $k$ is the quenching rate constant, measured, in case of oxygen contamination, to be k(O2) = 0.54 ± 0.03 ppm−1 µs−1. From the above equation and given $\tau_s&amp;gt;\tau_f$ , it follows that, for the same amount of impurities, $\tau_s$ will be more affected than $\tau_f$. This method is sensitive at the beginning of operation of the detector when impurities evolve from the ppm to the ppb level and then gets quickly saturated.

The following figure shows the average of several waveforms on a photomultiplier of the 3x1x1 prototype. Data are taken in conditions of high liquid argon purity (about 4 ms) and therefore a fit to the slow component of the scintillation light provides a result compatible with the one in absence of impurities.

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Measurement of the slow component of the scintillation light in high purity conditions.

Another measurement peformed with light, similar to sampling the ionization charge along tracks, is related to the measurement of the S2 light which is produced in the extraction and multiplication region in a dual-phase detector. This light is proportional to the ionization along the track and it is produced as soon as the charge deposited along the track drifts to the extraction and multiplication region. The fact that the S2 light is constant as a function of the drift shows that there is not significant charge attenuation along the drift.

The following figure shows the sum averaged waveforms from 3 PMTs acquired with the 3x1x1 prototype TPC, operated at a drift field of 0.5 kV/cm (i.e., a drift velocity of∼1.6 mm/μs ) and extraction fields in liquid above 2 kV/cm (corresponding to≥3 kV/cm in GAr with the liquid in the middle of the extraction gap).

S2 extraction new.jpg
Sum of averaged waveforms from the 3 negative based PMTs showing the prompt and proportional scintillation components. The prominent peaks centred at around 200 us correspond to the prompt scintillation in liquid (S1) and the continua extending for ~600 us after are from the proportional scintillation in gas (S2). The higher S2 yield is clearly visible when the LEMs are polarised (red curve).

The black curve shows the measurements taken without LEM field, while the red curve corresponds to those acquired with the LEM field of 26 kV/cm. The first peaks centred at around 200μs are from the primary scintillation in liquid and the flat continuum is from the proportional scintillation in the gas. The proportional scintillation signal lasts until the farthest electrons each the extraction region. The maximum time extension of the secondary light signal is therefore comparable to the maximum electron drift time. As can be seen from the figure, the secondary light contribution extends for about 600μs, which is the expected time for an electron drifting over 1 m in a field of 0.5 kV/cm. The almost flat S2 continuum qualitatively indicates that the liquid argon purity is sufficiently high so that there is no substantial attenuation in the extracted ionising charge over the entire drift distance.

Purity monitors

Purity monitors can then ensure direct purity assessment during the detector purification and operation covering an intermediate range where the electrons lifetime in LAr goes from a few to several milliseconds. These devices are based on the comparison of a stimulated ionization signal at its production and after a long drift in a confined region with a weak drift field. Being relatively compact devices, purity monitors can be installed at different heights in the detector in order to check for the effectiveness of recirculation and possible stratification of impurities.

Purity monitors are then a crucial tool for purity assessment in all phases of operation. This motivates the efforts for their specific development in view of the construction of large detectors.

The ProtoDUNE purity monitors

Purity monitors have already been successfully employed in the ICARUS T600 detector and in the 35-ton prototype detector at Fermilab. Both the ProtoDUNE Single-Phase (SP) and Dual-Phase (DP) detectors will also employ purity monitors. The specific development, construction and commissioning of these devices performed at UCL for the ProtoDUNE DP detector. will be described in more details in the next paragraphs.

Design

The purity monitors used in the ProtoDUNE detectors are based on the design of those used by ICARUS; in fact, the signle-phase detector has re-used some old devices removed from it. For the dual-phase detector, some optimisations of the design have been performed, using a COMSOL simulation of the field and determining dimensions and resistances accordingly. The final sketch and dimensions of the Dual-phase purity monitor are shown in the following image:

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Sketch of the purity monitor.
The purity monitors consist of four parallel, circular electrodes: a photocathode held in place by a groove in a stainless-steel disk, two grids (cathode and anode), and a stainless-steel anode plate. The photocathode is a 3 mm Silicon plate (flatness $\lambda$/4) coated with 10 nm Titanium and 200 nm of Gold. A polyimide coated quartz fibre (600 um in diameter) optically couples a Hamamatsu Xenon flash lamp placed outside the cryostat to the photocathode. As shown in the figure below, the fibre-end is held in place at a 20° angle to the cathode plane by a PTFE holder mounted on the edge of the cathode disk (note that the fibre is almost touching the photocathode).

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The Polyimide coated quartz fibre protrudes from the PTFE holder to the gold photocathode. The cathode-grid is placed 18 mm above the photocathode and is electrically connected to the first field shaper by a 50 MOhm resistor (blue in the picture).

Both grids are electroformed Nickel meshes (1.9 mm wire pitch, 100 $\mu$m wire width and 5 $\mu$m wire height, and 87.9% transparency) pinched between two stainless-steel rings (outer diameter 80 mm and inner diameter 60 mm) as shown in the figure below. The volume between the grids contains a set of 15 shaping field stainless-steel rings spaced by 10 mm PFTE spacers and interconnected by a 50 M$\Omega$ resistors to realise a voltage divider chain between the electrode grids and guarantee a unifor drift field. The electrodes and the field shapers are kept in place by three threaded Peek rods and bolts. The purity monitor is housed in a Faraday cage made of an aluminum mesh cylinder to shield the device from electrostatic background.

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One of the two grids before mounting.

High voltages can be independently applied to cathode, cathode-grid and anode (with the anode-grid kept at ground) to establish three electric fields: between cathode and cathode-grid ($E_1$), between the grids ($E_2$), and between anode-grid and anode ($E_3$). The condition for all the drift field lines to by-pass the grids, so that they are fully transparent to the drifting electrons, is given by (see Bunemann):

 $\frac{E_i}{E_j} &amp;gt; \frac{1+\rho}{1-\rho} \;\;\;\;\;\;\;\;\;\;\;\;\;\; with \; \rho = \frac{2 \pi r}{d} = 0.33 $

Where $E_i$ and $E_j$ are fields before and after the grid respectively, $r$ is the wire radius and $d$ the wire spacing of the grid. The above equation implies that $E_1$, $E_2$ and $E_3$ must satisfy:

 $E_3 &amp;gt; 2E_2 &amp;gt; 2E_1$

The purity monitor was successfully tested in vacuum, argon gas and liquid argon at UCL. In order to optimise performance, various values of the electrical field, and different photocathod material (gold, silver, titanium, aluminium) have been tested. The figure below shows as an example the cathode and anode signal in liquid argon at a drift field of 100 V/cm. The two plots represent single waveforms, recorded using a photocatode coated with gold and a silver, respectively.

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Cathode and anode signal in liquid argon at UCL.

The results obtained comparing gold (the material used by ICARUS) and silver are quite interesting. While it was believed that silver could easily be oxydised by exposure to air previous to the installation (and indeed the signal for silver-coating is smaller than that of gold-coating at the beginning of data taking), it was observed that after a few minutes the signal from silver was getting larger, outperforming gold. It has been intepreted as the result of light from the Xenon lamp "cleaning" the cathode material from oxydation, and giving a better result, compatible with the lower work function of silver (4.3 eV vs the 5.1 for gold). The superiority of silver over gold cathode is confirmed for all values of the electric field, as sown in the plot below.

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Amplitude of the cathode signal as a function of the drift field, for gold and silver photocatodes.

Since the tests performed in the UCL lab have lasted only a few hours, there is no guarantee on the stability of this effect over longer periods, or the lack of additional problems. To be on the safe side, in the ProtoDUNE configuration a photocathode coated partially with silver and partially with gold will be used.

Purity determination from long ionisation tracks

Once purity reaches saturation, the signal from MIP will show very little dependence on the distance of the energy deposition from the anode. Another, more direct way to measure the liquid argon purity is by measuring the charge deposited by muon tracks and its attenuation along the drift. This is only possible when the cryostat is fully filled and depends on the availability of muons traversing the TPC (whose frequency is reduced underground). This technique is complementary to the use of purity monitors and can be also helpful in the case of very high purity reached in stable operation conditions, when the information from the purity monitors may be saturated.

The method consists in checking for differences in the signal left by the electrons as a function of the position of the energy deposition. Some example of tracks from the 3x1x1 operation are shown below:

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A cosmic muon producing a long tracks spanning a large part of the detector

After measuring thousands of these tracks, the dependence of the average signal count as a function of the distance from the anode has been determined. The collected charge as a function of drift time is shown in the following figure:

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The scatter plot shows the collected charge per unit length in each view as a function of drift time for through-going muons. The collected charge per unit length is averaged in slices of 10 cm drift distance(corresponding to 62.5 us drift time) and represented with the black markers. This distribution is fitted withan exponential function, overlaid in solid black line, the extracted electron lifetime is presented.

Level controls

Level meters

An important part of task 8.2 is the development of instrumentation capable to monitor the liquid argon level inside of closed cryostat with varying degrees of precision. Chain of temperature probes and custom made cryogenic cameras (described in the next section) provide an estimate of the liquid level. We have also successfully constructed and tested coaxial and parallel plate capacitors capable of measuring the liquid level down to a precision of 100 microns. The reason why such high precision is needed is because the LEM-grid distance is 10 mm and it is needed to know to a much better accuracty in order to adjust the position of this structure with respect to the liquid with sub-mm accuracy. Due to the difference in the relative permittivity of the liquid and gas argon the capacitance of these level meters changes as the function of the depth to which they are submerged. Custom electronics located outside of the cryostat measure the capacitance of each level meter probe. The coaxial level meters implemented on the 3x1x1 have ranges of between 470 and 1200 mm and a sensitivity of around 1 mm. They are used to follow the evolution of the liquid level during filling and emptying of cryostats.

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Pictures of some instruments constructed to monitor the liquid argon level. From left to right: the high-resolution capacitive level meters (a, b), a PCB with temperature probes to measure the gas temperature gradient near the LEMs (c) and one cryogenic camera (d)..

The parallel plate capacitor level meters have a narrow operating range of 25 mm and a sensitivity of better than 100 um. They are intended to provide the monitoring of the level once the liquid reaches the top of the TPC field cage. The function of coaxial level meters is to ensure that the field cage is completely submerged. For dual phase TPCs parallel plates level meters then provide precise information for the adjustment of the CRP with respect to the LAr surface and monitoring the level between the extraction grid and the bottom electrode of the LEMs.

The leveling of the CRP with respect to the liquid surface is performed by adjusting the height of each of the three suspension points based on the feedback provided by the CRP-LMs. The liquid level is measured as the CRP is lowered using the motors by 3 mm in steps of 0.1 mm.

In addition to the installed level meters, the liquid height in the extraction region of the CRP can be inferred by measuring the capacitance between the grid and the bottom electrode of each LEM. Averaging over all 12 LEMs the measured values of this capacitance typically range from 150 pF with the liquid below the grid to around 350pF when the LEMs are submerged. The sensitivity of this technique is illustrated in the figure below, which shows the measured capacitance between one LEM and the grid as a function of the CRP displacement. This method offers the potential advantage of monitoring the liquid level in the CRP extraction region with a 50x50 cm2 granularity and could be used for the CRP level adjustment in the future large-scale detectors where, due to the space constraints, placement of the level meters along the CRP perimeter would not be possible.

Prior to the leveling of the CRP with respect to the liquid argon surface, the readings of the CRP-LMs were calibrated to better than 15% and their pedestal offsets were adjusted within a 1 mm accuracy. The deformation of the CRP frame mentioned does not affect our capacity to immerse the entire grid surface without the liquid touching the LEM plane.

The stability of the Liquid Argon level has been measured over the whole duration of data taking. The result of the measurement over few days is shown in the following figure.

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The stability of the Liquid Argon level over several days, as measured by the level meters

Cryogenic cameras

Four identical digital video cameras capable of operating in liquid argon were tested on the 3x1x1 prototype and installed at various points inside the cryostat. They can provide the visual feedback for the monitoring of the detector filling and inspection of the stability of the liquid argon surface. In addition, they can also be used to detect potential electric discharges.

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A view of the casing and of the cryogenic camera installed in the 3x1x1 prototype

Five LED strips, each approximately 5 m long, provide the necessary illumination inside the cryostat. The cameras and LED lights are turned off and disconnected during data taking since they generate significant noise on the charge readout electronics. They also generate light that could damage the PMTs in operation which must be then switched off during the operations with cameras..

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Images of a 3x1x1m3 TPC inside the cryostat provided by the four cryogenic cameras. Camera a) monitors the functioning of the cryogenic nozzles during cool down. The liquid level during operation can clearly bee seen by cameras b) and c). Camera d) is placed under the ground grid and cathode (clearly visible on the picture) and observes the LEMs.

The cameras are based on the commercially available Raspberry Pi V1 digital camera module. The main selection criterion for the camera model was its demonstrated ability to undergo long-term operations at cryogenic temperatures in absence of a local heat source (which could induce the formation of bubbles) without exhibiting any image degradation. Low power consumption, cost-effectiveness, compact size, and the capability of reading the sensor remotely over a distance of a few meters were also important aspects.

The cameras are contained in a custom-made case composed of a DN40 CF Quartz window, a 20 mm thick spacer flange and a 15 pin SUB-D flange at the back. The system is assembled in an argon atmosphere to avoid development of condensation on the lens once it is cooled to cryogenic temperatures. Each camera is connected via a 15 wire flat flexible cable (FFC) to its own Raspberry Pi computer for image acquisition. Cable lengths of up to 8 m, tested at room and liquid argon temperatures, showed no perceivable image distortions.

Slow Control back-end

All systems recording detector conditions like level, temperature, pressure etc. as well as the position of the readout chambers must be read out from a data acquisition slow-control. In the 3x1x1 prototype, a supervisory system based on the Siemens WinCC Open Architecture software continuously reads the values, reacts to changes, compares them to settings issuing warnings and alarms when necessary. A schematic view of the system is shown in the following fgure:

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A schematic view of the slow-control back-end system used for the 3x1x1 prototype

Four main subsystems are present:

* Process Control System (PCS) to readout and store measurements for the various detector conditions

* Detector safety system (DSS) sets alarms and interlocks even without human intervention

* Detector control system (DCS) deals with the acquisition of the high-voltage values

* CRP motorisation moves the physical position of the readout planes

The following table shows the list of devices monitored by the detector control system, with the number of channels and the operational range. The slow control system of the 3x1x1 prototype has been designed to be scalable to large detectors. The same base design will be used for the 6x6x6 dual-phase ProtoDUNE detector.

Name number of channels resolution range
HV LEM 24 50 pA/10 mV 0-20muA/0-8kV
HV Grid 2 10 nA/1V 0-100 muA/0-12 kV
HV PMT 5 50 nA/100 mV 0-1 mA/0-3kV
HV cathode 1 0.05 %(I)/0.001 %(V) 0-0.5 mA/0-300kV
Coaxial level meters 2 1 mm 470 mm, 1200 mm
Plate level meters 13 100 mum 0-25 mm
CRP step motors 3 100 mum 40 mm
temperature sensors 153 <0.5 K 50-350 K
LED strips 5 - -
Heaters 4 - 100 W
Pressure gauges 4 1% full range 900-1100 mbar
Differential pressure gauges 5 1% full range +/-50 mbar
Cryogenic cameras 4 5 Mpixels sensor -

Thermodynamic condition control

The assessment of these aspects with the 3x1x1 detector has proven that good purity levels (beyond several ms electrons lifetime) can be achieved in LNG tanks (see figure in the purification section above). Developments based on the inclusion of the level meters measurement as a feedback to the cryogenic system showed that stable thermodynamic conditions can be achieved over long periods. These stable conditions include the stability of the liquid surface, of the liquid temperature and of the pressure and temperature gradient in the gas phase.

The figure below shows the pressure inside the cryostat and the temperatures measured 2.6 cm above the LEMs in various points, over a week of data taking. The measurements under stable conditions indicate a stable pressure Pcryostat= (999.5±1.4) mbar (despite much larger changes of the external pressure). The gas argon temperature is measured in four points, at various different points of the cryostat (but always at the same heigth), so the measured temperatures differ by fractions of a degree. The measured values are around 101 K for all four sensor, with fluctuations of less than a degree.

Pressure-temp.png
Pressure (left) and temperature (right) measurements over a week of stable data taking

References

  • C. Rubbia, The Liquid Argon Time Projection Chamber: A New Concept for Neutrino Detectors,CERN-EP-INT-77-08
  • S. Amerio, et al., Design, construction and tests of the ICARUS T600 detector, Nucl. Instrum. Meth. A527 (2004) 329–410.doi:10.1016/j.nima.2004.02.044.
  • A. Badertscher, et al., Construction and operation of a Double Phase LAr Large Electron Multiplier Time Projection Chamber.arXiv:0811.3384 link
  • B. Aimard et al. A 4 tonne demonstrator for large-scale dual-phase liquid argon time projection chambers 2018_JINST_13_P11003 link

-- MarioCampanelli - 2018-13-11

Topic attachments
I Attachment History Action Size Date Who Comment
PNGpng 311_PID_pump.png r1 manage 603.3 K 2018-12-07 - 09:17 SebastienMurphy  
PNGpng 3x1x1_protype.png r1 manage 1237.8 K 2018-12-03 - 11:33 DarioAutiero The 3x1x1 membrane cryostat prototype at CERN
PNGpng 840-29-163.png r1 manage 201.8 K 2018-12-02 - 17:35 MarioCampanelli  
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JPEGjpg Photo_22-06-2016_14_00_12.jpg r1 manage 2527.3 K 2018-01-25 - 01:24 LauraManenti Pics PrM
JPEGjpg Photo_25-04-2017_17_56_35.jpg r1 manage 1591.9 K 2018-01-25 - 01:24 LauraManenti Pics PrM
PNGpng Pressure-temp.png r1 manage 168.0 K 2018-11-21 - 15:35 MarioCampanelli  
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PDFpdf SampleWaveformGold_70.140.280Vcm.pdf r1 manage 139.6 K 2018-12-12 - 20:23 MarioCampanelli  
JPEGjpg SampleWaveformSilver_100.200.400Vcm.jpg r1 manage 92.4 K 2018-12-07 - 10:56 MarioCampanelli  
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PNGpng Waveforms.png r1 manage 121.6 K 2018-12-12 - 20:31 MarioCampanelli  
PNGpng Waveforms2.png r1 manage 121.6 K 2018-12-13 - 12:40 MarioCampanelli  
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JPEGjpg cathodescan.jpg r1 manage 52.8 K 2018-12-06 - 20:49 MarioCampanelli  
JPEGjpg cooldown.JPG r1 manage 65.7 K 2018-12-13 - 10:29 DarioAutiero Cool down temperatures monitoring
PNGpng cryostat-cryo-level.png r1 manage 70.5 K 2018-11-21 - 11:42 MarioCampanelli  
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JPEGjpg impurities_vs_time.jpg r1 manage 95.1 K 2018-11-13 - 11:43 MarioCampanelli  
PDFpdf impurities_vs_time.pdf r1 manage 44.1 K 2018-01-24 - 17:59 LauraManenti impurities vs time in the 311
JPEGjpg light_slow.JPG r1 manage 36.3 K 2018-12-13 - 11:16 DarioAutiero light slow component
JPEGjpg prompt_LAr_GAr.jpg r1 manage 274.2 K 2018-12-03 - 14:49 MarioCampanelli  
JPEGjpg purmonsketch.jpg r1 manage 163.7 K 2018-01-25 - 00:50 LauraManenti sketch purity monitor
JPEGjpg purmonsketch.jpg.jpg r1 manage 534.4 K 2018-12-07 - 18:21 MarioCampanelli sketch purity monitor
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