A resistive ACHINOS multi-anode structure with DLC coating for spherical proportional counters

The spherical proportional counter is a gaseous detector used in a variety of applications, including direct dark matter and neutrino-less double beta decay searches. The ACHINOS multianode structure is a read-out technology that overcomes the limitations of single-anode read-out structures for large-size detectors and operation under high pressure. A resistive ACHINOS is presented, where the 3D printed central component is coated in a Diamond-Like Carbon (DLC) layer. The production and testing of the structure, in terms of stability and resolution, is described. Further applications in fundamental physics and industry are also discussed.


Introduction
In its simplest form, the spherical proportional counter [1], shown in Figure 1(a), consists of a grounded, spherical, metallic vessel filled with an appropriate gas mixture and a spherical anode of radius approximately 1 mm at the centre. The anode is supported by a grounded metallic rod, which also shields the wire used to apply a positive voltage to the anode and read out the signal. The electric field, which varies as 1/r 2 in an ideal spherical proportional counter, allows the ionisation electrons produced through particle interactions in the gas volume to drift to the anode. Within approximately 1 mm from the anode an avalanche occurs, providing signal amplification. Experiments using this detector include NEWS-G [2], searching for dark matter particles, and R2D2 [3], searching for neutrino-less double beta decay.
For large detectors, or detectors operating in high pressure, the small ratio of electric field strength to gas pressure (E/P) increases the probability of electron attachment and recombination. In the single anode configuration, increasing the E/P ratio can be achieved by either increasing the anode voltage or the anode size, both of these options lead to an increased discharge probability .
ACHINOS, a sensor structure composed of several anodes at a radius r S from the centre, as shown in Figure 1(b), has been proposed to overcome this challenge [4]. With ACHINOS, the avalanche gain is determined by the anode radius r A and voltage V, while the electric field at large radii is the collective electric field of all the anodes, determined by V, r S , and the number of anodes. Additionally, it is possible to read out each anode individually, allowing the three-dimensional reconstruction of the ionisation tracks.
The anodes are maintained in position by means of a central support structure. This structure need to be constructed using resistive materials, as has been the case with previous read-out technologies [5].

ACHINOS with "resistive glue" coating
As discussed in Reference [4,5], the ACHINOS central electrode needs to be constructed with a high resistivity material. 3D printing provides a convenient means to produce a high-precision structure, however, currently 3D printing with insulators and conductors is more widely available. Thus, an appropriate coating needs to be applied. Initially, an Araldite adhesive mixed with copper powder was explored, with main benefits being the relatively low radioactivity of Araldite 2011 [6] and the possibility to control the resistivity of the coating by changing the relative amounts of Araldite and copper. It was found that mixtures containing 20% to 50% w/w copper powder were appropriate. An example is shown in Figure 2. While such structures produced promising results, it was found that the coating layer was susceptible to damage from discharges.

ACHINOS using DLC coating
DLC is a form of amorphous carbon containing both the diamond and the graphite crystalline phase. DLC coating [7], thanks to its excellent surface resistivity, in addition to structural, chemical and thermal stability, offers a novel method for producing high quality resistive materials for gaseous detectors [8,9].
The central structure was constructed using 3D printing with different substrates, including resin, nylon, and glass, as shown in Figure 3(a). DLC was deposited using magnetron sputtering. Several batches of DLC-coated structures were produced, with a range of coating thickness between 360 nm and 720 nm. The measured resistance between two anti-diametric points on the surface, ranged from 300 MΩ to 10 GΩ. The DLC-coated resin 3D printed structure was installed onto a copper rod and electrically connected to it using a conductive Araldite copper mixture, as shown in Figure 3

Experimental results
The ACHINOS structure with 11-anodes, each 1 mm in diameter, shown in Figure 3(b) was installed in a 30 cm diameter spherical proportional counter that operated in sealed mode. An 55 Fe source was installed inside the detector, the position of which could be adjusted without opening the detector. The source principally emits a 5.9 keV X-ray when it decays to 55 Mn by electron capture. The experimental set-up is shown in Figure 4. Signals are read out by a CREMAT CR-110-R2 charge sensitive preamplifier [10] and passed to a CALIbox digitiser with a dynamic range of ±1.25 V with a maximum sampling frequency of 5 MHz was used.

Gain
The detector was initially filled with 500 mbar of Ar:CH 4 (98%:2%). Data were collected at various anode voltages and pressures, and the amplitude of the 5.9 keV peak recorded, as presented in Figure 5. This shows the proportional response of the detector over a wide pressure range, and demonstrates the ability to achieve a large gas gain of up to 10 4 .

Homogeneity between different anodes
To check the homogeneity of the detector response, the 55 Fe source was moved along the equator of the detector, as shown in Figure 4. The detector was filled with 1000 mbar of Ar:CH 4 (98%:2%) and the anode voltage was set to 2200 V. The amplitude and energy resolution were estimated using the 5.9 keV peak and an example spectrum is shown in Figure 6. A gaussian fit of this yielded an amplitude of (11090 ± 10) ADU1, corresponding to a gas gain of 8.3 × 10 4 , and an energy resolution (σ) of (7.4 ± 0.1)%. The same procedure was repeated for the other measurements, with 1ADU = Analogue-to-Digital Units the results shown in Figure 7(a) for the amplitude and Figure 7(b) for the energy resolution. Counts/100 ADU Figure 6. Energy spectrum from an 55 Fe source measured using a spherical proportional counter filled with 1000 mbar of Ar:CH 4 (98%:2%) and using an ACHINOS. The primary peak has an energy resolution (σ) of (7.4 ± 0.1)%. The second peak, to the left of the main one, is the argon escape peak.
The difference in gain between four of the anodes is not greater than 10%. The fifth anode has an approximately 25% lower gain, and is likely due small variations in the anode radii or distance to the central electrode. Future ACHINOS designs will include individual anode read-out, where an anode-by-anode calibration will be performed. The gain variation for angles in between anodes can be reduced by using more anodes.

Stability
The detector operated stably with no sparks or significant gain variations over approximately 15 hours and the amplitude of the 55 Fe peak was monitored. The detector was operated with 1000 mbar of Ar:CH 4 (98%:2%) with the anode voltage set to 2200 V. The gradual decrease in gain over time is attributed to the introduction of contaminants coming from outgassing and leaks in the vacuum system.

Future developments
In the future, to consolidate the development of ACHINOS, additional R&D effort is planned: • Optimise the 3D printed insulating support • Improve the precision of anode placement • Improve overall symmetry of the structure • Industrialise the sensor manufacturing, with the DLC coating being a significant step in this direction  The number of anodes must be optimised according to the size of the spherical vessel and the needs of granularity of the specific application. This can be guided with a dedicated simulation program [11].
A number of projects where the spherical proportional counter could be used share the require-ment for operation under high gas pressure. To further support this, the use of anodes with radii on the order of 100 µm, is being explored. Such a development would allow higher gas gains to be achieved. For such small anodes, potential surface irregularities result in an increased effect on the energy resolution. As a result, anodes that deviate less from an ideal sphere and have lower surface roughness are being sourced. A second challenge towards smaller anode sizes is the bonding to the read-out wire while minimising exposed contacts which can lead to discharges. Methods of achieving this are being explored with industrial collaborators, at CERN and at CEA Saclay.
A future milestone will be the individual read-out and high-voltage biasing of each anode, which will allow three dimensional reconstruction of the ionisation track. This would also permit the individual calibration of anodes, allowing small differences in gain to be corrected for.
The ACHINOS is already being used in the spherical proportional counter of the NEWS-G collaboration. In the future, the ability to operate in higher pressures, with larger detectors, and with higher gain, increases the applicability of the spherical proportional counter in a number of applications, such as detecting Supernovae [12], fast neutron spectroscopy [13], and measurements of neutrino coherent scattering [14].

Summary
A multi-anode read-out structure for the spherical proportional counter has been developed. ACHI-NOS now incorporates a DLC layer to prevent discharges, and its operation stability has been demonstrated. It was also found that the DLC layer is robust under discharges. The gas gain in various pressures was also studied, demonstrating the ability to operate the detector in a gain of up to 10 4 in 1500 mbar of Ar:CH 4 (98%:2%). The energy resolution has been assessed as a function of azimuthal angle and found to be better than 10% (σ). The difference in gain between different anodes was not greater than 25%, and this can be further improved by using anodes with a smaller deviation from an ideal sphere and lower surface roughness, individually calibrating each anode and the inclusion of more anodes.