Nuclear Inst. and Methods in Physics Research, A 886 (2018) 24–29
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Nuclear Inst. and Methods in Physics Research, A
journal homepage: www.elsevier.com/locate/nima
Live event reconstruction in an optically read out GEM-based TPC
F.M. Brunbauer a,b,*, G. Galgóczi a, D. Gonzalez Diaz a,c, E. Oliveri a, F. Resnati a,
L. Ropelewski a, C. Streli b, P. Thuiner a, M. van Stenis a
a CERN, 385 Route de Meyrin, 1217 Meyrin, Geneva, Switzerland
b Technische Universität Wien, Karlsplatz 13, 1040 Wien, Austria
c Uludağ University, Özlüce Mahallesi, 16059 Bursa, Turkey
a r t i c l e i n f o a b s t r a c t
Keywords: Combining strong signal amplification made possible by Gaseous Electron Multipliers (GEMs) with the high
GEM detectors spatial resolution provided by optical readout, highly performing radiation detectors can be realized. An optically
Micro pattern gas chambers read out GEM-based Time Projection Chamber (TPC) is presented. The device permits 3D track reconstruction by
Optical readout combining the 2D projections obtained with a CCD camera with timing information from a photomultiplier tube.
Scintillation Owing to the intuitive 2D representation of the tracks in the images and to automated control, data acquisition
Reconstruction
Time projection chambers and event reconstruction algorithms, the optically read out TPC permits live display of reconstructed tracks
in three dimensions. An Ar/CF4 (80/20%) gas mixture was used to maximize scintillation yield in the visible
wavelength region matching the quantum efficiency of the camera. The device is integrated in a UHV-grade
vessel allowing for precise control of the gas composition and purity. Long term studies in sealed mode operation
revealed a minor decrease in the scintillation light intensity.
© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction volume. MPGDs are at the core of multiple ongoing detector upgrades
and developments. The TPC of the ALICE experiment is currently
The universality of Time Projection Chambers (TPCs) [1] makes undergoing an upgrade to employ a GEM-based readout [5], while the
them an attractive detector concept for a wide range of applications. ATLAS collaboration is developing a muon spectrometer readout based
To permit an accurate 3D reconstruction of particle tracks, the 2D on Micromegas, which can sustain high particle fluxes and is capable
projection on the endcap of a TPC must be recorded with possibly of achieving high spatial resolution [6]. In view of directional dark
high spatial resolution. Additionally, the drift time information used matter search experiments, optically read out GEM-based TPCs have
to calculate the Z-coordinate of a particle track must be obtained with been suggested and investigated as a candidate technology for providing
possibly high time resolution. accurate information about electron and nuclear recoil tracks [7].
To enable the detection of various types of radiation from low- Aiming at high spatial resolution in the readout of the endcaps of
energy X-rays to highly ionizing alpha particles, robust signal am- a TPC, optical readout is an attractive alternative to commonly used
plification technologies such as Gaseous Electron Multipliers (GEMs) electronic readout concepts. Optical readout is based on the recording
can be employed [2]. This variety of MicroPattern Gaseous Detec- of scintillation light emitted in the gas in the active volume and has
tors (MPGDs) is composed of thin perforated foils with a conductor– previously been shown to allow for high spatial resolution [8]. The
insulator–conductor structure and allows for high electron multiplica- optical readout concept provides a good track recognition capability
tion factors by avalanche amplification in high electric field regions in as already demonstrated in imaging chambers based on parallel-grid
the GEM holes with typical diameters of tens of micrometers. Employing chambers [9]. Coupling MPGDs such as GEMs with optical readout
multiple GEMs as consecutive amplification stages allows high effective allows for high signal amplification as well as good position resolution
charge gain factors [3] while still operating the detector in a stable and comes with the additional advantage of high signal-to-noise ratios
regime. MPGD amplification stages such as triple-GEMs are therefore and the inherent insensitivity to electric noise.
a high potential technology for TPC endcaps [4] providing high signal Optically read out TPCs employing combined readout with cameras
amplification and detection of weakly ionizing particles in the active and Photomultiplier Tubes (PMTs) have previously been developed
* Corresponding author at: CERN, 385 Route de Meyrin, 1217 Meyrin, Geneva, Switzerland.
E-mail address: florian.brunbauer@cern.ch (F.M. Brunbauer).
https://doi.org/10.1016/j.nima.2017.12.077
Received 10 August 2017; Received in revised form 23 December 2017; Accepted 27 December 2017
Available online 29 December 2017
0168-9002/© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
F.M. Brunbauer et al. Nuclear Inst. and Methods in Physics Research, A 886 (2018) 24–29
for the study of rare decay modes and proton spectroscopy [10–12]. after primary electrons have drifted towards and reached the triple-
Using multistep avalanche chambers or GEMs as amplification and GEM and have been amplified in the high electric field regions inside
scintillation stages and operating in Ar–He gas mixtures with small the GEM holes. Due to the optical transparency of the GEM foils of
additives of triethylamine or N2, these previous works successfully about 20%, the PMT may record scintillation light emitted from all
demonstrated the applicability of optical TPCs for imaging of two-proton three GEMs in the multiplication stage with the majority of the detected
decays of 45Fe nuclei [12] or proton spectroscopy of 48Ni, 46Fe and photons originating from the third GEM. The acceptance factor for the
44Cr [11]. A similar readout concept has also been implemented by the detection of secondary scintillation photons by the PMT was determined
DMTPC project, which aims at observing dark matter interactions by to be approximately 6.4 × 10−4 by ray tracing simulations and by taking
determining the energy and direction of nuclear recoils [13]. Optically into account the emission spectrum of the Ar/CF4 gas mixture, the
read out TPCs have also been used in nuclear astrophysics experiments wavelength-dependent transmission of the borosilicate viewport and the
such as studies with gamma-ray beams [14,15]. quantum efficiency of the PMT.
We present a TPC based on a triple-GEM amplification stage optically As signal amplification in the GEM stack happens fast due to high
read out by a high-resolution CCD camera, which provides an image of electric fields inside the GEM holes and transfer fields of about 2
the 2D projection of alpha tracks, and a PMT, which simultaneously kV/cm between GEM foils, this impacts the timing characteristics of
provides timing information to perform 3D track reconstruction. Using the signals observed by the PMT only minimally. The CCD camera
an Ar/CF4 gas mixture with scintillation light emission in the visible facing the bottom of the triple-GEM records integrated images of the
wavelength regime, emitted light can be recorded without the need 2D projection of the particle track on the GEM stack, which acts as the
for wavelength shifters. The operation of the presented device is fully endcap of the TPC. Due to state-of-the-art imaging sensors not being
automated and allows for live 3D reconstruction and display of alpha sensitive to a level where single photon detection is feasible and the
particle tracks. Additionally, the described detector is built to highest low optical transparency of the triple-GEM stack, the CCD camera is not
purity requirements and demonstrates the possibility to realize a sealed capable of recording primary scintillation signals and only records the
TPC based on optically read out MPGDs. much stronger secondary scintillation light emitted during avalanche
amplification in the GEM holes.
2. Experimental methods To maximize the signal-to-noise ratio both in the images recorded
with the CCD as well as in the PMT response, an Ar/CF4 gas mixture with
The drift volume of the presented optically read out GEM-based TPC a composition of 80/20% (by volume) was used. The maximum achieved
is formed by a circular field shaper with a length of 10 cm and a diameter light yield with this mixture was 0.3 photons per secondary electron pro-
of 10 cm, which is coupled to a triple-GEM with an active region of duced during electron avalanche multiplication. The emission spectrum
10 × 10 cm2. These two elements are placed in a UHV-grade vessel to of the scintillation light of this mixture features a pronounced intensity
allow for a highly controlled environment and a pure gas filling. All peak at 630 nm in the visible wavelength regime. Therefore, it can be
components of the detector vessel are sealed by Cu gaskets. Inside the efficiently recorded by the CCD camera, which has a quantum efficiency
gas volume, the usage of low-outgassing materials such as oxygen-free peaking at 75% for 600 nm. The quantum efficiency of the CCD and the
Cu for the field shaper electrodes, ceramic for the GEM frames and PMT at 630 nm were 73% and 11%, respectively. This match between
polyether ether ketone (PEEK) for a holder for the triple-GEM stack the scintillation light emission spectrum and the wavelength-dependent
ensures high gas purity and permits the operation of the TPC in flushed quantum efficiency of the CCD permits high light sensitivity.
as well as sealed modes. Electrical connections are made by clamping Before the initial gas filling, the detector vessel is pumped to a
to avoid the use of solder in the gas volume and all-metal valves and pressure of about 10−6 mbar with a turbomolecular pump. Extended
pressure transducers were used. Two borosilicate viewports 63 mm in pumping for 48 h is used to minimize the residual outgassing from the
diameter on opposite sides of the vessel allow the optical readout of detector elements inside the chamber. Subsequently, the detector vessel
scintillation light in the visible wavelength regime produced in the gas is sealed off with an all-metal valve. Only after extensive purging of all
volume. Behind these viewports, a CCD camera (QImaging Retiga R6) gas lines is the detector filled with the desired gas mixture to a pressure
facing the last GEM of the triple-GEM stack and a PMT (Hamamatsu of 1 bar.
R375) facing the cathode mesh used to define the drift field are placed To permit live 3D reconstruction of alpha particle tracks in the TPC, a
as shown in Fig. 1. triggering, data acquisition and reconstruction algorithm was developed
The 6-megapixel CCD camera with bulb-mode triggering capabil- and implemented both in hardware as well as software. PMT signals are
ities features a 12.5 × 10 mm2 imaging sensor with pixels of 4.54 × recorded by a digital storage oscilloscope (DSO) (LeCroy WaveRunner
4.54 μm2. The imaging sensor is cooled to −20 ◦C to achieve a low 625Zi, 2.5 GHz, 40 GS/s) and the intense and unambiguous secondary
read noise of 5.7 electrons (RMS) and a dark current rate of 0.00017 scintillation signals are used as a trigger to identify the occurrence of
electrons/pixel/second. Imaging the full active area of the detector, a a new event. These trigger signals are used to stop the exposure of
magnification factor of approximately 10 results in an effective pixel the CCD camera as well as to trigger the data acquisition of the PMT
size of about 45 × 45 μm2 on the imaging plane. The PMT was operated waveform from the DSO and the recorded image from the CCD camera
at a gain of approximately 2 × 106 in order to be sensitive to primary via a microcontroller coupled to a control script executed on a standard
scintillation signals. desktop computer. Once a PMT waveform and the matching CCD image
The employed GEM foils feature holes with a diameter of 70 μm have been transferred to the computer, a reconstruction algorithm
and a hole pitch of 140 μm, which are chemically etched into the Cu- automatically performs a 3D reconstruction of the particle track, stores
polyimide-Cu composite material by a double-mask etching technique. the data for later offline analysis and displays a 3D representation of the
For highly ionizing alpha particles, the triple-GEM was operated at event on-screen.
moderate gains of several 103. A stainless steel mesh-cathode is used The exposure time of the recorded CCD images depends on the time
together with the field shaper composed of multiple ring-shaped Cu between subsequent events and varies from image to image due to the
electrodes connected with high-voltage resistors to define the electric employed bulb mode triggering. Therefore, the noise level due to the
drift field in the active volume of the TPC. The presented setup permits dark current of the pixels of the CCD camera, which depends on the
drift fields of up to 500 V/cm and can also be used with significantly exposure time, is different for each image. For the used imaging sensor,
lower drift fields of tens of V/cm due to the low level of outgassing the read noise of 5.7 electrons (RMS) is much more significant than the
and the resulting high gas quality in the detector vessel minimizing the dark current rate of 0.00017 electrons/pixel/second for typical exposure
significance of attachment and loss of primary electrons. times of less than 1 s.
The PMT facing the cathode is used to record both the primary Alpha particles originating from the decay of 220Rn were used for the
scintillation signal as well as the secondary scintillation light emitted demonstration of the track reconstruction capabilities of the presented
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F.M. Brunbauer et al. Nuclear Inst. and Methods in Physics Research, A 886 (2018) 24–29
Fig. 1. (a) Schematic of GEM-based TPC read out by PMT and CCD camera. The 10 cm diameter field shaper and the triple-GEM with an active area of 10× 10 cm2 are placed inside of a
UHV-grade vessel while the readout elements are placed behind borosilicate viewports. (b) Setup with optically read out TPC, oscilloscope for PMT readout, turbomolecular pump and
pressure gauges for cleaning the chamber for high-purity operation.
Fig. 2. Typical PMT waveform exhibiting primary and secondary scintillation pulses of Fig. 3. Distribution of times 𝑡1 between primary scintillation and leading edge of
an alpha particle in the TPC. The times 𝑡1 and 𝑡2 between the primary scintillation and the secondary scintillation. Electron drift times for alpha particles originating along the length
leading and trailing edges of the secondary scintillation pulse are used to determine the of the active conversion volume are reflected in the distribution. A pronounced peak
Z-coordinates of the particle track. Inset: CCD image of alpha track with signal-to-noise around 1.1 μs corresponding to electrons drifting along the entire length of the drift
ratio of 48 and visible Bragg peak. volume of 10 cm is attributed to the accumulation of decaying nuclei on the cathode.
device. A 228Th source was used to flush 220Rn into the detector vessel the secondary scintillation pulse are extracted. The distribution of drift
where decays of 220Rn resulted in 6.4 MeV alpha particles with a times 𝑡1 between the primary scintillation and the leading edge of the
random distribution of orientations and spatial origins. A number of secondary scintillation pulse is shown in Fig. 3.
alpha particles were therefore not contained in the active drift volume Multiplying the shorter time 𝑡1 between the primary scintillation
of the TPC. In the presented case of alpha particles from the decay of pulse and the beginning of the secondary one by the determined electron
220Rn, partially contained events can be easily identified by the length drift velocity in the drift region yields the Z-coordinate of the track
of their tracks or by their deposited energy and could therefore easily point closest to the GEMs. Equivalently, multiplying the longer time
be excluded from further analysis. More generally, tracks crossing the 𝑡2 by the electron drift velocity provides the Z-coordinate of the track
borders of the active region could be identified and excluded as partially point closest to the cathode. This method introduces some uncertainty
contained events. in the angle of tracks almost perpendicular or parallel to the GEMs.
For parallel tracks, the width of the secondary scintillation pulse is
3. Results and discussion very short, which negatively impacts the determination of Z-coordinates
from the difference in electron drift times. For perpendicular tracks, the
An optically read out GEM-based detector was operated as a TPC to 2D projection of the track is very short and it may not be possible to
demonstrate the live event reconstruction capabilities of the presented determine the orientation of the track from the position of the Bragg
device for alpha particles. An exemplary PMT waveform is shown in peak in the CCD image.
Fig. 2 together with a CCD image of an alpha track. The angular resolution of the presented device could not be deter-
Events without matching PMT waveforms and images are discarded mined as the geometry of the high purity vessel did not allow a selection
from further analysis, which may include exposures showing more than of incident particle angles and relied on a gaseous source with random
a single alpha track or waveforms without an identifiable primary decay origins and orientations. Exemplary schematic representations of
scintillation signal. About two thirds of events did not show identifiable two extreme cases of track angles are shown in Fig. 4.
primary scintillation signals, which is mainly attributed to limited geo- Subsequently, the alpha track is extracted from the CCD image by
metric acceptance and partially contained events, and more than 90% employing a flood-fill algorithm applied to a binary image obtained from
of images showed only a single alpha track. For accepted events with the original grayscale image by a threshold of 5 standard deviations
matching PMT waveforms and CCD images and identifiable primary above the background level. The background level and the standard
scintillation signals, the times 𝑡1 and 𝑡2 as shown in Fig. 2 between deviation of the background intensity are extracted for each image
the primary scintillation pulse and the leading and trailing edges of individually. The flood-fill algorithm identifies all bright pixels in the
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F.M. Brunbauer et al. Nuclear Inst. and Methods in Physics Research, A 886 (2018) 24–29
Fig. 5. The peak of the distribution of reconstructed alpha track lengths representing fully
Fig. 4. Reconstructed alpha tracks visualized inside the field cage with GEMs at the contained tracks agrees with a comparative simulation. Shorter track lengths correspond
bottom with black dots indicating origins of tracks. (a) Long secondary scintillation pulses to partially contained alpha tracks.
characterize tracks nearly perpendicular to the GEMs. (b) Short secondary scintillation
pulses correspond to nearly parallel tracks.
drift times from PMT waveforms might contribute to the widened track
length distribution. For some particle tracks that are almost parallel to
binary image connected to a start pixel on the track. This algorithm the GEMs, the reconstruction algorithm could not accurately determine
excludes individual hot pixels in the recorded images and is thus suited the small time differences between primary and secondary scintillation
to identify dominant tracks and their extent in the images. pulses, which resulted in errors in the reconstructed track lengths.
The X-Y-projection resulting from this analysis is then combined For tracks almost perpendicular to the GEMs, the Z-projections are
with the Z-coordinates obtained from the drift times of the closest and large and even small inaccuracies in the determination of the time
farthest track points to arrive at a 3D representation of the straight differences between primary and secondary scintillation pulses might
alpha track. By comparing energy loss profiles from PMT waveforms of significantly influence the determined track lengths. A comparative
secondary scintillation signals and from line profiles of the pixel value Geant4 simulation of the ionization of an Ar/CF4 (80/20%) mixture by
intensity along tracks in the CCD images, the orientation of tracks can be 6.4 MeV alpha particles yielded a track length distribution in agreement
determined. The unambiguous presence of Bragg peaks in CCD images as with the peak of the experimentally observed distribution.
well as in PMT waveforms, as shown in Fig. 2, allows for an orientation The trend of the secondary scintillation light intensity for radiation
of tracks and permits the identification of the start and end points of events with a known constant energy was recorded over an extended
alpha tracks. period of time while the detector volume was sealed without gas
The drift velocity in the employed gas mixture for different electric recirculation to investigate the possibility of operating the detector in
fields was determined from the distribution of drift times and the length sealed mode. 5.9 keV 55Fe X-rays were aimed into the active volume of
of the field shaper. The drift times of a number of alpha particles from the UHV-grade chamber through a thin metallic window in the position
a gaseous source in the field shaper were measured. The known length where otherwise the viewport for the PMT was located. The X-ray source
of the field shaper was subsequently divided by the longest observed was placed directly in front of the thin metallic window facing the
drift time to obtain the drift velocity. For a drift field of 400 V/cm in cathode of the detector. The integral of the secondary scintillation light
the used Ar/CF4 (80/20%) mixture, a drift velocity of 8.2 cm/μs was signal was computed from the acquired PMT waveforms. Energy spectra
obtained. This value is slightly lower than the drift velocity of 10.2 as shown in the inset in Fig. 6 were compiled from the acquired PMT
cm/μs in an Ar/CF4 (90/10%) mixture measured by P. Colas et al. [16], signals. The energy spectra displayed both escape and full energy peaks
which may be explained by the larger fraction of CF4 in the presented of 55Fe with an energy resolution of 32% FWHM at 5.9 keV. The trend
case. The conversion factor from pixels in the CCD image to spatial X-Y of the scintillation light intensity was investigated by recording energy
coordinates was determined by counting GEM holes in a line profile of spectra in intervals of 60 min and plotting the position of the full energy
the pixel value intensity in a CCD image and dividing the length of the peak over time. Operating the detector for almost six days in sealed
line profile in pixels by the distance obtained from the number of holes mode, the scintillation light intensity decreased by approximately 5%.
and the known pitch of 140 μm between individual holes. Fig. 6 shows the minor decrease in light intensity over time, which
The automated triggering, data acquisition and 3D reconstruction is attributed to a slight contamination of the gas in the chamber as
algorithm, which was developed and employed to combine the informa- a result of outgassing from the GEM foils and other materials used in
tion from the CCD camera and the PMT into a live display of alpha parti- the detector assembly. Nevertheless, the small signal loss over several
cle events in the TPC, enables the recording and reconstruction of events days suggests that a sealed TPC based on the presented concept is
at a rate of approximately 2 Hz. Since the used 220Rn source achieved feasible.
a low rate of alpha decays, this was sufficient to record a reasonable
fraction of the events. In the presented setup, the rate-limiting factor 4. Conclusion
was the communication between the data acquisition devices and the
desktop computer running the control and reconstruction algorithms. A TPC integrated in a high-purity vessel allowing for extended sealed
The finite active drift volume of the TPC results in a number of mode operation is presented. Primary scintillation of alpha particles in
alpha particles not being fully contained. At atmospheric pressure, the the Ar/CF4 gas mixture was recorded by a PMT. A triple-GEM multipli-
distribution of reconstructed alpha track lengths shown in Fig. 5 was cation stage was employed to amplify the primary electrons and achieve
observed. secondary scintillation light signals strong enough to be read out by a
The peak between 45 and 50 mm corresponds to alpha tracks fully CCD camera. The 2D projection imaged by the camera was combined
contained in the drift volume while shorter recorded track lengths result with depth information obtained from the time between primary and
from partially contained events. The broadening of the peak may be due secondary scintillation signals recorded by the PMT to enable 3D event
to inaccuracies in the determination of the track length for tracks nearly reconstruction of 6.4 MeV alpha tracks. A hardware-based triggering
parallel or perpendicular to the GEMs. While the high spatial resolution mechanism was combined with device control, data acquisition and
of the CCD images provides good measurements of the lengths of the event reconstruction algorithms to fully automate data taking and 3D
2D projections of tracks, inaccuracies in the determination of electron track reconstruction. This permits live display of alpha tracks in the
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F.M. Brunbauer et al. Nuclear Inst. and Methods in Physics Research, A 886 (2018) 24–29
and nuclear astrophysics experiments. Moreover, the shown possibility
to operate the device in sealed mode for extended periods of time
introduces the possibility to realize portable TPC-like detectors relying
on MPGDs for signal amplification and optical readout for effective
data acquisition. With an integrated calibration mechanism or by using
getters to extend the achievable time in sealed mode operation with
acceptable decrease in signal intensity, versatile portable devices may
be feasible. The high dynamic range and the good spatial resolution of
optically read out GEM-based TPCs make them well suited for providing
topology information for a variety of events.
Acknowledgment
The authors gratefully acknowledge support in designing the pre-
sented device by Christophe Bault (CERN, Geneva, Switzerland).
Fig. 6. The trend of the full energy peak position obtained from secondary scintillation
light spectra over six days displays minor signal degradation. Inset: 55Fe energy spectrum References
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