Reminder of WG2 goals
The primary goals of WG2 are to assess MGPD performance, define common test standards and solve
or get around related physics issues. Its role is also to understand better the phenomena at play in gas
detectors. Major progress has been made in this direction in close relation with other working groups.
Over the last 5 years, a wide variety of characterisation results have been reported in WG2. Benchmark
performance as well as sensitivity to various kind of particles have been measured with different
MPGD technologies. Beside compiling results, WG2 is also the natural place to discuss and agree upon
test standards which enable comparison and eventually selection of best suited detectors for a given
application. At the beginning of the collaboration, sparking, ageing and charging-up were identified as
the main physics issues limiting the performance and a wider spread of MPGDs. Dedicated studies
were carried out to reveal the underlying mechanisms and eventually find solutions. In this section, the
WG2 activity and achievements are summarised, connections to other working groups are underlined.
Common characterisation
Several RD51 groups, by producing and sharing results, have shed a new light on basic phenomena in
gases which are discussed below. Sometimes at the interface between working groups, they contributed
to the definition of new design rules of MPGDs and to the development and validation of software
tools.
The process of signal amplification by electron avalanche impacts a lot on the performance of the
detector (timing, gas gain, stability, signal to noise ratio, ageing etc...). Avalanche development could
be studied in several ways, for instance, by measuring the relation between gas gain and applied
voltage. Using measurements performed in a large variety of geometry and experimental conditions,
mechanisms such as UV photon feedback and Penning effect could be quantitatively studied, providing
important inputs to Monte Carlo programs.
In GEM-like geometries, the electrodes are not facing the avalanche region and photon feedback is
naturally suppressed. This advantage allows operation in pure gases and thus imaging of cherenkov
radiation (RICH) and search of Dark Matter (DM) by conversion of cherenkov/scintillation light on
photo-sensitive coatings. In addition, DM experiments require a large target mass and thus the use of
liquid argon or xenon. Operation in cryogenic conditions is still very challenging : the necessary large
increase in electric field makes the detector stability very sensitive to the electrode surface quality. For
this reason, double phase configurations with extraction of primary electrons from the liquid to the gas
phase are investigated. In 2011, this concept has been successfully applied to a liquid argon TPC
prototype read out by a LEM (Large THGEM) and strips (Fig. 1 right).
Electron multiplication and primary ionisation are statistical processes which set a limit to the energy
and spatial resolution. Related fluctuations should be minimised and are being measured with improved
precision by means of GridPix detectors (a combination of a Micromegas grid on top of a CMOS pixel
chip acting as anode). Thanks to an unprecedented granularity and a high sensitivity to single electrons,
GridPix was used to measure some moments of the gas gain distribution (as well as the Fano factor)
and to discriminate between various models of avalanche development.
In order to optimise pixel detectors for high tracking accuracy, the detailed path of electrons must be
predicted. But at the vicinity of the MPGD holes, field gradients and diffusion make this task difficult.
In cascaded structures, the field configuration is even more complex and calculating the path
probabilities of electrons remains even more challenging. Ion drift is also of interest in single photon
detectors to minimise backflow to the photo-cathode. For the time being, these probabilities are
inferred from current measurements under continuous irradiation of the detectors. They are necessary
inputs to validate field solver programs and microscopic tracking algorithms. From the measurements
compiled in WG2 it appears that in most MPGD geometries, close to 100% of the primary electrons
can be transferred from the drift to amplification region and are multiplied by avalanche. Ion backflow
can be reduced easily to the percent level (compulsory for an LC-TPC). Lower fractions are possible in
Micromegas of smaller pitch and in GEM/THGEM stacks too, probably because of the misalignment
between foils.
GEM-THGEM foils can easily be coated with photo-sensitive materials (e.g. CsI). Assembled in a
stack, they provide enough gas gain to detect single photo-electrons. These gaseous photo-multipliers
(GPM) reach similar efficiencies as traditional PM tubes and have no sensitivity to magnetic fields. But
to preserve the photo-cathode from ion impact, innovative and complex designs have recently been
proposed (COBRA, flower-THGEM) and are being characterised. Another challenge is to efficiently
extract and detect single photo-electrons. The hole pitch of THGEMs being relatively large, photoelectrons liberated between holes can be lost due to the particular shape of the field in this region (Fig.
1 left). This effect is now understood and can be minimized with an adequate setting of the fields.
Gas discharges
Gas discharges induce dead time and are potentially harmful to the detector elements and electronics.
As a central topic in WG2, several facts on discharges could be compiled and made available to the
RD51 community. Because the spark probability is a function of the total charge released in the gas,
operation at low gas gain is obviously preferred but not always possible. The ionising background is
also relevant as well as the gas mixture (more precisely the atomic number of the atoms). Electrode
geometry and surface quality are other parameters that can be optimised for low spark rate. It terms of
potential damage to the detector, the electrical energy stored in the mesh or foils rises quadratically
with the applied voltage. Sparks are thus less energetic in gas mixtures with strong Penning effects
which have lower workable voltages.
An important milestone achieved in WG2 is the development of efficient protections against sparks.
For applications where rate capability is not mandatory, external diode networks are fine (T2K-TPC).
In systems with a very large number of channels (LC-DHCAL), the number of passive components
becomes prohibitive but integration of smaller diodes inside the ASICs is investigated. The integration
of resistive elements to readout electrodes can be used to limit the spark current and the resulting
voltage drop. It is an appealing solution for tracking in high rate environments (SLHC-MAMMA) or
when the spread of avalanche signals onto readout electrodes is desirable (LC-TPC).
Several configurations of conductors, resistors and insulators were tested by the MAMMA groups. The
final configuration uses resistive strips capacitively coupled to readout strips through a separating
insulating layer. The prototype behaviour under irradiation is remarkable and shows virtually no
voltage (nor efficiency) drop even at high gains. The LC-TPC Micromegas configuration is more
conventional, the pad plane being coated with an insulating and a resistive layer. Avalanche charge is
evacuated laterally, inducing signals on several adjacent strips. An excellent resolution both in the rφ
plane and drift direction is achieved as well as full protection against discharges. In GridPix detectors,
the insulating layer is absent to fully exploit the high granularity of the pixel readout. High resistivity
materials deposited by wafer post-processing technology have been tried. Protection against discharges
has been demonstrated in several gas mixtures. In multi-stage structures, the gain is distributed over
several foils and the actual gain of each foil is low (a few tens). Also, diffusion between foils reduces
the charge density and thus the spark rate. In applications with constrains on material budget or
thickness, one foil is used. Sparking then matters and resistive GEMs have been developed.
Charging-up and ageing
Insulating materials are always present in MPGDs. Resistive elements can also be incorporated to the
design. Depending on the exact configuration, the avalanche charge may not be readily evacuated to
ground, causing time dependent variations of field and thus gain. MPGDs such as GEM or micro-Bulk
are prone to charging-up but, by a careful choice of the geometry, this effect can be attenuated or even
suppressed. With resistive elements, the time constants can in principle be calculated from the
conductivity, permittivity and geometry. Resistive MPGDs with high rate capability have been built
and successfully tested in the RD51 test beam facility (Bulk Micromegas LC-TPC module).
Radiation tolerance is an attractive feature of MPGDs which makes them competitive with solid-state
sensors for tracking in hard radiation environment like the one at SLHC. Two main ageing studies were
reported in WG2. The first study was performed by the NIKHEF Detector R&D group who exposed a
thin GridPix chamber to an intense strontium source, accumulating a total charge equivalent to more
than 10 years of SLHC. With a drift gap of 1 mm, this very fast detector (called GOSSIP) can measure
track vectors. It is a candidate for the upgrade of the inner layers of the vertex detector at future SLHC.
In view of the upgrade of the endcap muon spectrometer of ATLAS at SLHC, the MAMMA
collaboration conducted irradiation tests of resistive strip Bulk Micromegas. Several tests were actually
done using various particles composing the LHC background (neutrons, gamma, X-rays and charged
particles). In total, the charge equivalent to 5 to 10 years of SLHC was accumulated during the tests.
By constantly monitoring the current of the mesh, no degradation of performance was observed.
Figure 1 : High resolution scan of a THGEM surface
with single photo-electrons revealing the 800 μm pitch
hole pattern and small less efficient zones between holes
(top). Two-dimensional view and strip signals of a muon
traversing the liquid argon Large GEM TPC (right).