Transcript Review- Micromegas Vertex Trackers for CLAS12

Review of Micromegas Tracking Detectors for
CLAS12 – May 7, 2009
• Reviewers: Madhu Dixit, Mac Mestayer
• Presentations covered the following topics:
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detector overview: layers, strip pitch, segmentation for central & forward regions
fabrication overview: principles and prototype testing of “bulk” technology
detector simulation: GARFIELD results on drift, diffusion, gain
tracking simulation: particle backgrounds, tracking efficiency and resolution
acceptance and quality assurance: methods to validate component performance
prototype testing: measurements of position resolution, Lorentz angle, gain times
transmission and tracking efficiency for minimum-ionizing tracks; including tests of
curved detectors and tests in magnetic fields
– electronics: overview of requirements for charge and time measurements; options for an
integrated system: amplification/discrimination/digitization/ readout.
• Impressive new pioneering work on curved Micormegas technology and
operation in transverse magnetic fields
Resolution of the charges:
• The simulated performance for resolution, solid angle coverage and
efficiency meet or exceed CLAS12 requirements.
• The design is based upon existing technology, simulated at both the signal
and track-finding level with key parameters verified by prototype tests. The
simulations are consistent with the test results.
• The conceptual plans for detector integration (including safety systems) are
consistent with the overall CLAS12 detector layout.
• The schedule and allocated manpower seem reasonable.
• The group is competent; recognized world leaders in this technology.
We are confident that the group can successfully design and build the proposed
tracking detectors for CLAS12.
Comments for further study before start of
construction:
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Operation in a transverse B-field
Use of bulk micromegas technology
Simulation of background
Beam tests to study effect of highly-ionizing tracks
Segmentation of the wire mesh
X-Y readout in barrel region
Selection of electronics technology
Gas delivery system
Operation in a transverse B-field
• The “barrel” part of the detector will operate in a 5-Tesla magnetic field
oriented transversely to the ion drift direction, causing a shift of the
deposited charge compared to the track position as well as a spatial
spreading of the charge. This “Lorentz angle” effect can be minimized in
three ways: by decreasing the drift (or “conversion”) gap and thus
decreasing the charge spreading and by decreasing the Lorentz angle itself
in two ways: increasing the electric field or by decreasing the electron drift
velocity if the gas.
• The potential problem was quite serious: running in the “standard”
configuration resulted in a 75 deg. Lorentz angle; increasing the resolution
many-fold and reducing the signal size per strip.
• The Saclay group utilized all 3 methods: reducing the drift gap from 3 to 2
mm, increasing the drift field from ~1 to ~6 kV/cm, and choosing a gas
with low drift velocity at these fields.
Implications of design changes
• reducing the drift gap
• Changing from a 3 mm to a 2 mm drift or conversion gaps means that
the number of primary ionization events should decrease from 9 to 6.
The resulting inefficiency estimated from the Poisson statistical
fluctuations should be less than 1%; however the signal size will
decrease for a constant gain.
• increasing the drift electric field
• This will reduce the electron “transparency” by about 50%, resulting in
a decrease of signal size by this amount. Note that this has little effect
on the Poisson fluctuations because the transparency affects all of the
ionization electrons (~20) independently, however it will require
higher gain for the same signal size and possible reduction of the safe
operating range.
• choosing a gas with low drift velocity at high fields
• This should have little consequences other than a small change in the
operating voltage.
Operating in a transverse magnetic
field: conclusions
• The presented design with reduced conversion gap and
increased drift field has been simulated
– simulations indicated a dramatic improvement in resolution and signal
size per strip
• also, it has been tested in a cosmic-ray telescope without
magnetic field
– direct tests with minimum-ionizing particles show successful operation
with high (> 97%) efficiency.
• Our conclusion is that the presented design should work well,
but we note that there might be further optimizations in the 3dimensional parameter space (gap length, drift field, choice of
gas) in the trade-off between robust operation, signal size, and
resolution.
Use of bulk micromegas technology
• The group presented impressive evidence of a rapidly-maturing new
technology.
• The strict dimensional tolerances required for stable operation seem to be
routinely achievable.
• A detailed performance verification protocol has been established.
• All assembly components and procedures are based upon established
industry practices common in the circuit board, wire mesh and thin film
industries.
Simulation of background
• Full simulations done with a GEANT4 code
• Tracks reconstructed in the presence of background
• Results indicate high efficiency and no loss of resolution
• We suggest the following two studies:
1. study the rate and effect of two-layer “punch-through” events in
which a single interaction causes counts in both an X and a Y layer;
these could result in “ghost tracks” which might require additional
global tracking constraints to be eliminated.
2. study the rate of production of very highly-ionizing particles (e.g. low
momentum protons or alpha particles) which might cause sparking;
for example, from elastic ep events or from scattering from nuclear
targets
Beam tests
• consider this as a possibility to better
understand the onset of sparking
• consider cosmic tests in a magnetic field to
verify simulation of resolution and efficiency
Segmentation of wire mesh
• we see two advantages
– smaller dead area in the event of a spark
– possible read-out of the mesh ??
• requires additional high-voltage circuitry
X-Y readout
• 90 deg. stereo orientation for barrel region
– gives best spatial point resolution
– but, it can produce “ghost” track candidates
• study possible “punch-through” events
• study frequency of two real tracks through a segment
• study global-track mitigation
– devise a scheme for flex-cable attachment for the
“Y” layer
Selection of electronics technology
• several alternatives seem feasible
• experience with other large systems
• trade-off between time and charge
resolution?
– time resolution gives background rejection
– charge resolution need be no better than
expected detector resolution
Gas system and utilities
• more detailed conceptual design for gas
system needed
– mixing, monitoring located outside the hall ?
– mixture of active and passive controls needed
• need a concept for location of high voltage
supplies, cables and location of signal
electronics; including cooling