ESTB Proposal for the ATLAS Inner Tracker (ITK) Upgrade
Test of several pixel sensor technologies
M. Bomben (Paris), P. Grenier (SLAC), S. Grinstein (Barcelona), D. Muenstermann
(Geneva) and J. Weingarten (Gottingen), on behalf of the ATLAS-ITK Pixel Group.
I- Introduction
To extend the physics reach of the LHC, upgrades during the 2022 Long Shutdown
to the accelerator are planned to increase the peak luminosity by a factor 5 to 10
which will enable the experiments to collect up to 3000 fb-1 of data. This, however,
will lead to increased occupancy and radiation damage of the inner trackers,
approaching fluences of a few 1016 neq/cm2 at the innermost layer and still some
1015 neq/cm2 at the outer pixel layers.
The ATLAS experiment plans to introduce an all-silicon inner tracker with the High
Luminosity (HL) LHC upgrade to cope with the elevated occupancy. With silicon, the
occupancy can be adjusted by using the unit size (pixel, strip or short strip sensors)
appropriate for the radiation environment. For radiation damage reasons, only
electron-collecting sensors designs are considered (n-in-p and n-in-n): Beyond a
fluence of about 1015 neq/cm2, trapping becomes the dominant radiation effect and
electrons are trapped significantly less than holes.
ATLAS has put in place a new structure that has in charge the development and
construction of the future tracker, “ITK”, that is composed of 80 institutes.
Several pixel technologies are currently being developed within ATLAS/ITK and in
collaboration with other experiments: silicon 3D, silicon planar, silicon HVCMOS and
diamonds.
Silicon 3D:
Silicon 3D was proposed fifteen years ago to solve the problem of decrease charge
collection efficiency in a large radiation dose environment. Contrary the regular
planar design where electrodes are implanted on the surface of the silicon wafer, in
the 3D design the electrodes penetrate the wafer, from one side to the other (see
Figure 1). Consequently the distance between the electrodes can be optimized and is
much shorter compared the planar design. The charge collection distance is
therefore shorter, leading to small trapping probability and larger charge collection.
In May 2014 ATLAS will install a new pixel layer (Insertable B Layer, IBL), inside the
current detector. Part of it (25%) will be equipped with 3D sensors. It will be the
first HEP detector to be equipped with this technology. The 3D technology has
shown to be working at large irradiation dose. It has to demonstrate reliable
operations at the upgraded LHC luminosity (few 1016 neq/cm2). Distance between
electrodes will have to be optimized. Another challenge will be to reduce pixel size
and wafer thickness.
Silicon Planar:
Planar pixel sensors are the present standard technology for tracking detectors in
high energy physics (e.g. Atlas, CMS). Much experience of designing, optimizing, and
producing silicon sensors has been accumulated and progress towards further
improvements is on-going. Many industrial suppliers and research laboratories are
able to produce considerable quantities of relatively low cost and high yield planar
sensors. Hence, planar sensors are investigated for upcoming generations of
tracking detectors at particle accelerators. The Atlas Planar Pixel Sensor (PPS)
project is a collaboration of 20 European, American, and Asian groups investigating
the possibilities to meet the challenges of radiation hardness, low cost production,
and reducing inactive edges of planar silicon sensors. In particular, large (~4x4 cm2)
sensors ("quad modules") are now of major interest, in an effort of cheap and thin
detectors to cover large areas (~10 m2) of the future tracker outer layers.
Silicon HVCMOS:
CMOS processes exist in a large variety of flavors, among them so-called highvoltage (HV-CMOS) and imaging (CIS, HR-CMOS, DMAPS) processes featuring high
breakdown voltages and moderate to large depletion zones. These in particular
allow for the application of a bias voltage to rely on drift rather than diffusion with
respect to the charge collection thereby enabling much higher radiation-hardness
than earlier MAPS (Monolithic Active Pixel Sensors) approaches. At the same time,
the noise is much reduced thanks to the low capacitance of the small pixels and the
material budget can be reduced thanks to the comparatively thin required active
layer of less than 50 µm. Thinning ASICs (the active sensors) down to ~50 µm is a
standard option of CMOS foundries and therefore cheap and reliable.
CMOS detectors may actually increase the physics potential at HL-LHC thanks to
small pixels size improving spatial resolution and two-track-separation. The small
thickness of the sensor could dramatically reduce the cluster size at high pseudorapidity and so improve the track resolution inside dense high-energy jets.
First prototypes have already operated after 1015 neq/cm2, but with non-optimized
operation parameters. Further beam tests are required to better establish hit
efficiencies before and after irradiation as well as to identify areas of reduced
efficiency inside the pixel cell.
Diamond:
Diamond detectors are equipping the new ATLAS Beam Diamond Monitor which
will allow the measurement of the luminosity bunch by bunch. It will be installed in
May 2014 in the detector. The diamond sensors are bonded to the same front-end
chip as the IBL pixel (planar and 3D) sensors. Diamond sensors are good candidate
for LHC upgrade since they are radiation hard and have low leakage current. Signal
is however smaller than in silicon. Another challenge is production reliability.
Figure 1: design of planar where electrodes are implanted on the surface of the
wafer and 3D where electrodes are processed through the wafer.
II- Beam tests
Beam tests are crucial for the development of any detector. It is particularly true of
pixel sensors. An intense beam test program has been going in ATLAS since a few
years. Key parameters such as charge collection efficiency, tracking efficiency,
charge sharing between cells can only be measured with the high precision at beam
tests using minimum ionizing particles (MIPs) and a telescope to reconstruct the
particle path. Those parameters are determined for un-irradiated and irradiated
sensors at various radiation doses.
For this proposed beam test, we will be measuring these quantities for various
prototypes of all pixel technologies. Some of the samples will have been previously
irradiated.
Figure 2: Photo of the Eudet telescope: 2 arms and the devices under test in
between.
III- Beam telescope
We are proposing to use the high-resolution ATLAS Pixel Telescope, so-called
“Aconite” which is a copy of the DESY Eudet telescope. It is currently used at DESY.
We will ship it to SLAC in April.
The telescope consists of six planes instrumented with Mimosa26 active pixel
sensors with a pitch of 18.5 microns. Each plane consists of 576x1152 pixels
covering an active area of 21.2x10.6 mm2. A coincidence of four scintillators was
employed for triggering, which resulted in an effective sensitive area of 2x1~cm2.
The tracking resolution is estimated to be 3 microns. The Mimosa26 sensors employ
a continuous rolling shutter for readout. For every trigger signal the telescope
planes integrate hits for 115 micro-s, while the DUTs are sensitive only for 400 ns.
Tracks passing through the telescope during the sensitive time of the DUTs (intime
tracks) are selected in the analysis by requiring that the track has one or more hits
in the other DUTs and the reference plane. Therefore, the size of the reference
sensor and its overlap with the DUTs defines a fiducial region on the DUTs.
The telescope planes are read out by a custom-made VME system, controlled by one
single-board PC per telescope arm. Each of these PCs sends a separate datastream to
a run control PC, using an ethernet connection. The DUTs are read out using the
ATCA/RCE DAQ system developed at SLAC and now widely used at several beam
tests. The system generates a single datastream for all DUTs, which is sent to the run
control PC. On the run control PC all incoming datastreams are merged by the so
called DataCollector. As each datastream contains unambiguous IDs for each trigger,
the streams can be synchronised easily. After merging, the DataCollector saves the
data to disk.
IV- Beam request and beam parameters
We need about week to install the telescope (some beam will be needed to test the
whole apparatus). Experts from DESY and CERN are planned to be here for the
installation.
Then for the actually data taking and test of the various samples, we are proposing
to run the following two weeks. We are expecting about 10 people from Europe and
Japan to participate to the data taking. We will run 24h/day with three shifts.
We would like the highest possible energy, with up to several hundreds of particle
per bunch. Beam size should be ~2cm. One important point is that we will very
likely test irradiated sensors: those have to be kept cold, at all times, including
during data taking. We will have a cold box shipped with the telescope but we will
need dry ice for the cooling.
Dates proposed:
- telescope installation April 28th - May 2nd.
- data taking: May 2nd - May 16th.
V- NOTES
1). ESTB - The way secondary particles are produced at ESTB has the inherent risk
that the full power beam might be delivered to your experiment. This can happen
when the energies between LCLS and the A-line are matched and/or the production
target is removed. So suddenly, instead of a single or a few particles it becomes
possible that up to around 10^9 particles per bunch might be delivered. Please
evaluate the consequences for your experimental apparatus and document them in
the proposal:
As long as this problem is caught relatively quickly, within a few minutes, it would
have no consequences on our apparatus (telescope and devices under test).