Since 2007 we have led an ongoing radiation damage program at the LANSCE facility at Los Alamos National Laboratory. This effort is organized by Sally (who submits and defends proposals to a review panel at LANL) and Martin and involves all members of our group in biannual testbeam operations. Work at the beamline is followed by dosimetry and activation measurements and detailed study of radiation-induced modifications of device characteristics, all done by us in our UNM lab. The runs in September and December 2012, September 2013, February 2014, and October 2014 each involved approximately 100 unique prototypes. This work we do facilitates research by us and by scientists at 15 other institutes on ATLAS and CMS upgrades, RD42 diamond, RD53 readout chips, and RD50 silicon technologies. In our recent runs, the devices under test included silicon and diamond sensors, readout electronics chips and custom ASICs, VCSEL drivers, monolithic active pixel sensors, power supply switching elements, specialized cables, miniaturized peltier coolers, and upgrade calorimetry elements. In 2013 Haley and Martin added a customized system to provide vortex airflow cooling to silicon sensors undergoing temperature-critical annealing studies. Aidan is implementing a system of linear stages this year to allow us to scan larger devices in the beam. Igor developed the October 2014 run plan. Neil is taking over from Prabi the operation of our gamma spectrometer for nuclide activation measurements. We all take shifts and participate in device fixturing, assembly, and measurements. Our 2014 collaborative paper with UCSC derives from this effort. We propose to continue to lead LANSCE irradiations for the foreseeable future (at least through 2019) to promote technology developments for the LHC and other Energy Frontier applications.
We have developed a new method for real time measurement of charged particle beam profile and fluence. This technique was pioneered by Martin and implemented by Aaron (in software) and Prabi and Haley (in hardware). Aaron reported on it at DPF2013, and our NIM publication followed in 2014. We now use this diode array technique routinely in our Los Alamos irradiations. Neil updated the configuration this year to allow simultaneous readout of multiple arrays at different points in the stack of devices under test.
Sally coordinates on-demand irradiations of prototype detector technologies at the Sandia Annular Core Research Reactor (ACRR) and Gamma Irradiation Facility (GIF). This program was spearheaded by Sally and required over a year of negotiation of an Inter-Institutional Visitors Agreement between Sandia and UNM. In spring 2014 the GIF operators implemented a new radioactive source configuration specifically to provide uniform exposure to our large devices. In 2014 we applied the ACRR to about 20 tracking and calorimetry technologies requiring exposures from 1014 to a few times 1016 1-MeV-n-eq/cm2. We used the GIF especially for exposures of new pixel readout integrated circuit structures in our role as members of RD53. Martin collaborates with LBNL on this development of 65nm readout for future LHC pixel upgrades, and he is responsible for testing the transistors before and after GIF exposures.
Martin, Nelly, and Sally are collaborating with Gian-Franco Dalla Betta of the University of Trento to improve the performance of silicon pixel sensors of the 3D geometry, which are used for particle tracking. "3D" differs from traditional planar silicon processing by orienting the electrodes perpendicularly to the wafer surface, permitting much smaller inter-electrode spacing. The result is lower depletion voltage, faster charge collection, and thus generally higher radiation tolerance. This technology, already in the ATLAS-IBL baseline, may meet even more critical needs at future hadron colliders. We measure the leakage current, capacitance, breakdown voltage, and charge collection efficiency of several design variations related to the electrode column structure. We recently published first measurements on proton-irradiated devices as "Characterization of New FBK Double-Sided 3D Sensors with Improved Breakdown Voltage". During 2014 we exposed samples of a further evolved design to gammas and neutrons at Sandia as well as high fluence protons at Los Alamos. We are preparing a paper on the results, for submission to JINST approximately November 2014. Nelly's honors thesis research will investigate 3D design options that are tolerant beyond 2 x 1016 1-MeV-n-eq/cm2.
Diamond is another sensor substrate option with potential for extreme radiation hardness in future particle tracking applications. During the period 2012-2014, Rui, Martin, and Sally carried out a study of the resistivity of polycrystalline diamond sensors as a function of temperature and charged particle fluence. Any effect of radiation on the resistivity propagates to the leakage current and could influence assessments of the material properties that depend on leakage current, such as active volume and charge collection distance. This is relevant to contemporary implementations of diamond detectors (including luminosity monitors at the LHC) as well as options for inner volume particle tracking in the future. Our 2014 paper reports no resistivity degradation up to fluences relevant to the HL-LHC. Sally presented "Recent Results on Diamond Radiation Tolerance" at IPRD2013.
Haley, Rui, and Martin conducted a study of the effect of humidity on reverse breakdown in silicon sensors of the 3D geometry, as a function of radiation damage up to fluences relevant to the HL-LHC. Reverse breakdown becomes an increasingly important parameter as detectors age and bias voltages must be raised to maintain adequate signal size. While humidity studies have been carried out previously for a few planar silicon geometries, data on humidity effects upon the 3D geometry are not in the literature. Our study shows that breakdown correlates with humidity and manifests at edges and surfaces not directly correlated with electrodes. Haley presented this, her honors thesis result, at a seminar at UNM in May 2014. Martin is preparing an electrostatic simulation to validate the results.
Martin and Neil are developing an ultra-low-mass interconnect for use in future colliders. This design (not the same as the one described in the ATLAS Upgrade section above) must have ultra-low mass because the cable will attach directly on top of its sensor (this is not typically done) and maintain constant impedance over 2 meters, a challenge for flex technology. Haley and Neil populated several prototypes that are now being assembled with sensors and electronics by our collaborators at SLAC and U.C. Davis.
Aaron collaborated with UCSC on a paper on microstrip electrode readout noise that was published in 2013. The team studied an application appropriate to cases for which beam-delivery and detector-occupancy characteristics permit a long shaping-time readout of long daisy-chained ladders of fine-pitch sensors read out by a single front-end amplifier. They found that network effects significantly mitigate the amount of readout noise contributed by the detector load.
Igor is a member of the ATLAS Large Eta Task Force, which is predicting the physics gains possible during Phase-II (year 2022 and beyond) of improved detector performance at pseudorapidities in the range 2.5 to 5. There is potential for significantly increased acceptance in over a dozen processes of high interest, including H → 4μ and vector boson fusion jets from HWW events. The ITK simulation is central to the effort, as extended tracking coverage is the primary driver for everything outside the tracker. Thus Igor's objective is development of a charge collection model for ITK including this possible expansion. The model will describe the charge generated due to energy loss (dE/dx) along the track path in the sensor's depleted volume. Igor is incorporating into the model effects including (1) impact of Lorentz angle on charge drift, (2) position smearing due to a thermal diffusion, (3) noise on top of the charge signal, (4) cross-talk induced from other pixels, and (5) radiation damage effects including partial depletion, charge trapping, and increased shot noise, leading to hit efficiency degradation. To simulate a time-over-threshold measurement, a time stamp for each pixel above threshold is computed, taking into account charge-dependent timewalk and trigger-dependent offsets. The unprecedented pile-up and radiation in the Phase-II era renders earlier models insufficient.
In 2015, Igor will be developing the baseline charge model and leading a small team implementing it into the ITK simulation software. He will use this software to analyze detector performance for a spectrum of configurations of pixel pitch, sensor thickness, and so forth. He will play a leading role at the test beam runs to collect the experimental data needed for the model development. By 2016, Igor will be extending and refining the model through comparisons to his beam test l data. In 2017 he will finalize the radiation damage factors and lead the preparation of this portion of the ITK TDR. Year 2018 will bring the start of pixel pre-production with associated effort by Igor on beam tests and further simulation to optimize engineering and cost questions. While this will surely benefit ATLAS, the effort on the charge model is sufficiently general to be applied to other future tracker applications as well. The effort will require half of his time throughout 2015-19.