1. Review of Micromegas Tracking Detectors for CLAS12 – May 7, 2009 Reviewers: Madhu Dixit, Mac Mestayer Presentations covered the following topics: –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

2. 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.

3. Comments for further study before start of construction: 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

4. 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.

5. 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.

6. 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 3- dimensional parameter space (gap length, drift field, choice of gas) in the trade-off between robust operation, signal size, and resolution.

7. 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.

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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