The Art of Folding in High Energy Physics
In physics, measurement shall always minimize the influence on what is to be measured, even though this can never fully be accomplished. Nonetheless, optimization is always possible with proper technology, and this is exactly the approach of the Origami module.
Application: Belle II Experiment
The Origami concept was developed at HEPHY for the future Belle II experiment, where silicon strip detectors measure the tracks of penetrating particles that were created during the collisions of electrons and positrons. The colliding energy - and thus the energy of the resulting particles - is about three orders of magnitude lower than at the Large Hadron Collider (LHC), which means that the particles are much more prone to deflection. Consequently, the material budget in the detector must be reduced to a minimum, which is ensured by our light-weight construction.
The Origami module is built using silicon sensors, which have implanted strips (rotated by 90° on one side with respect to the other) in order to exactly detect the location of particle penetration. This also involves special amplifier integrated circuits, which are connected to the strips and measure the signals of the hit strips. In a classical arrangement, those amplifiers would be arranged on both sides, which would not only imply a double-sided electronics infrastructure, but also duplicate the cooling pipes. Even though the power dissipation of a single chip is only about 1/3 of a Watt, the total consumption of almost 2000 chips will add up to about 650W in a volume comparable to a tramper backpack - without cooling, the detector would quickly overheat.
The trick of the Origami module is that all amplifier chips are aligned in a row, such that a single cooling pipe can service them all. The electrical connection to the strips on the bottom side is made by flexible circuits, which are bent around the edge of the sensor. The cooling of the chips is done by liquid CO2 at a temperature of -20°C and a pressure of 20bar through a pipe made of stainless steel with an outer diameter of 1.6mm and a wall thickness of just 0.05mm - cooling also saves material.
The mechanical structure is made of two ribs made of carbon fiberlaminated around a styrofoam core. The resulting composite is extremely light, but very rigid - similar materials are used for aeronautic and space applications. However, in our case there is an additional requirement which is the radiation resistance. Surprisingly, even mechanical components can fail in that aspect. In the fall of 2010, we learned from irradiation tests that two Styrofoam materials from different vendors behave quite differently: one was shattered into pieces, while the other one remained unaffected.
Layout in Belle II
Four layers of Origami modules are arranged around the beam pipe in the Silicon Vertex Detector of the Belle II Experiment, where the particle collisions occur. The modules are combined to "ladders", which consist of several silicon sensors. In order to cover the same angular region in each layer, the ladders get larger towards the outside. As the energy of the electron beam is higher than that of the positrons, the particles generated in collisions have a so called "boost" towards one direction which is called "forward". Consequently, the detector is slightly asymmetric and in the forward region there are slanted sensors in the three outermost layers. This not only improves the measurements in that region (particles at very shallow angles would have to traverse more material), but also reduces costs: if all ladders were straight, considerably more sensors and readout channels would be needed.
Made at HEPHY
The design and development of the Silicon Vertex Detector of the Belle II Experiment is done at HEPHY involving the departments of Semiconductor Detectors, Electronics and Machine Shop. This work essentially consists of four subjects:
- Silicon Sensors
- Readout Electronics
The silicon sensors with their structures are designed in-house with the respective CAD tools; they are manufactured by highly specialized companies and tested in our clean room, before they are assembled into modules.
In parallel, various electronic readout systems were developed and built through several years, which can be connected to one or more detector modules in order to acquire measurement data with a computer.
The mechanical design of the detector is also a core competence of HEPHY, and it naturally implies the manufacturing of prototypes in our machine shop. The cooling system is closely related; and it is also being designed in-house in collaboration with CERN and the Max Planck Institute of Physics in Munich.
Prototyping and Testing
The prototype modules built at HEPHY are tested in our lab with a radioactive source. Certain properties, however, can only be determined with a highly energetic particle beam, as it is available at CERN. Thus, we are guest users there once a year in order to perform a so called "beam test". Moreover, irradiation campaigns are conducted to test electronic components and materials for their suitability in the experiment. In fall 2010, such an irradiation was performed in parallel to a beam test, such that the properties of detector modules could be measured in the beam before and after the irradiation. The results of this effort will be used to iterate the design of the components such that an optimum detector will finally be installed in the Belle II Experiment.