New straw tracker for the NA62 experiment
The design of the NA62 straw tracker is driven by the requirement of low multiple scattering and high efficiency over the full acceptance. An ultra-light straw tracker operated in vacuum was chosen for this purpose. Extensive R&D was performed in order to manufacture straws in large quantities using ultrasonic welding. The NA62 straw tracker is a collaboration between Dubna and CERN and two chambers are constructed in each site. At CERN, PH-ESE has developed the front-electronics and readout system, and the detector has been designed and developed by PH-DT.
Straw trackers are small drift chambers; particles passing through a thin tube ionize gas molecules inside, which send signals as they collide with a wire. They are extensively used in high-energy physics experiments, as they allow high resolution position measurement of charged particles. In NA62, the straw tracker is designed to measure the momentum and direction of charged tracks.
Straw tube chambers work similarly to multiwire proportional chambers, but, instead of high voltage being supplied along a plane of wires, high voltage is maintained between a metal coated tube (or straw) that is grounded, while a sense wire in the center is under HV. Straw tube chambers can be comfortably operated in higher rate environments and are generally considered to be more reliable, since a single broken wire can easily be isolated and only impacts its own readout channel.
The NA62 tracking system consists of four straw chambers 2.1 m x 2.1 m with a total of 7168 straws each having a length of 2.1 m and a diameter of 9.85 mm. In each chamber, the straw tubes are positioned in four different views to measure four coordinates for every charged particle. Each straw plane has a 12 cm wide opening for the beam in the middle. In order to measure the momentum of the charged particles, the four chambers are arranged either side of a high aperture dipole magnet that provides a vertical magnetic field of 0.36 T.
The four “views” of straws in one of chamber. A cap in each straw allows for the beam to pass without interacting with the straws.
The choice of the straw material has been a compromise among many different requirements, i.e. radiation length, permeation of gases, mechanical properties, adhesion to metal coating, bonding with epoxy, and the ability to be ultrasonically welded into a tube. The straws were manufactured from 36 μm thin PET foils coated with two thin layers of Cu and Au to provide electrical conductance and reduce permeation. For the wire, gold-plated tungsten was used.
NA62 straws manufactured from 36 μm thin PET foils coated with two thin layers of Cu and Au. They can withstand the high pressure exerted by the gas inside the experiment’s vacuum tank - an impressive 100 metres long by 2.5 m diameter.
Once manufactured, straws are glued, equipped with end plugs and leak tested. Every straw is uniquely identified, and characteristics such as inner diameter, gas tightness, straw tension and straw straightness are controlled and recorded. One of the main mechanical challenges in straw chambers is how to center the anode inside the straw. If the wire is not at the center of the straw or the straw is not straight, the effective electrical field seen by electrons drifting towards the anode wire is distorted, and will affect the gas gain and energy resolution of the straw. A final verification is made by taking some events from cosmic rays with every straw.
Straw ends are glued to an aluminium frame and are visually inspected before a leak test is performed.
Finally, one of the key components of the chamber is the so-called active web, a multi-layer, flex-rigid printed circuit board, which carries the high voltage to the wire and transmits the signal to the front-end electronics. The electronics is based on CARIOCA chips – similar to those used by the LHCb experiment - and a TDC with FPGA of <1 ns intrinsic resolution. Finally, two Straw Readout Boards per view receive the data through serial links and order the data before sending them to the PC farm with 2 Gbit/s over optical links.
The final quality control for each chamber consists of recording events from cosmic rays. The goal is to have 100% working channels.
The team with the first NA62 straw detector module in Building 154
The installation of the NA62 straw tracker in the experimental area started earlier this month. The last chamber, manufactured in JINR, will be installed in August. The system will be ready for data-taking in October 2014.
You can view a photo-story covering the main steps of the construction of the NA62 straw detector: here.