Spaghetti Served at CERN

Scientists Develop a New Particle Absorber for the LHCb Experiment at CERN, NUST MISIS reports

A group of scientists from NUST MISIS has developed a new technology for the production of an absorber for the electromagnetic particle calorimeter for the LHCb experiment at CERN (Geneva, Switzerland). The purpose of the experiment is to explain and understand why antimatter disappeared during the evolution of the Universe. The unique properties of a special lead-antimony alloy and a cassette technology for replacing light-emitting fibers will allow meeting strict technical requirements, simplifying maintenance and extending the life of the calorimeter.

To answer the question of why the Universe consists mainly of matter, and not of equal parts of matter and antimatter, scientists accelerate streams of elementary particles to speeds close to the speed of light and make them collide in special chambers. During the collision of beams at gigantic energies, many particles and antiparticles are born. At the same time, the picture of their decays is slightly different and it is this difference that scientists are trying to find. To do this, each particle must be detected during decay, and then its track must be traced and its energy measured.

To do this, particle streams are passed through a number of different detectors, including calorimeters (devices for measuring the energy of electrons and hadrons), which are part of the LHCb detector complex at CERN. The modern electromagnetic LHCb calorimeter comprises “shish kebab” type modules — alternating layers of a plastic scintillator (a substance that absorbs and re-radiates energy into a light flash) and a lead absorber.

The introduction of the program of the increased luminosity of the LHCb experiment involves the collection of statistics in modes of increased radioactivity due to an increase in the number of particle collisions per second. This imposes some serious requirements on the load and radiation resistance of the experimental systems, as well as the materials from which they are made.

To improve the radiation resistance and increase the service life of the calorimeter, the researchers proposed to use a new “spaghetti” geometry, which consists of scintillation fibers oriented in the secondary particles’ motion direction. The fibers are enclosed in an absorbent matrix — a metal absorber.

A group of researchers and engineers from NUST MISIS has developed and created a prototype of a new “spaghetti” type absorber, which is made of a special lead-antimony Garth alloy by casting.

“The uniqueness of the alloy composition provides zero casting heat shrinkage, minimum viscosity and required hardness while maintaining high density, which is necessary to localize the electron shower in the absorber. Heat shrinkage is a very important parameter since the manufacturing tolerances for such products are very small — up to 50 micrometers. As a result, we have produced an extremely accurate and monolithic block,” said one of the main developers of the technology, senior researcher at the Laboratory of Multifunctional Magnetic Nanomaterials, Associate Professor of the Department of Functional Nanosystems and High-Temperature Materials, Ph.D. Dmitry Karpenkov.

The scientific group made a square-edged item with a side length of 30.25 mm and a total length of 320 mm, which is a metal matrix with 121 longitudinal channels with a diameter of Ø 2.2 mm. The wall thickness between these channels should be strictly 0.55 mm along the entire length. The developers emphasize that the pitch between the fibers optimized to achieve the required energy resolution of the order of 10% / √E due to the detailed simulation

The developed technology is based on filling a graphite mold with the preinstalled metal rods with liquid melt. The use of an alloy of lead and antimony, which has a minimum viscosity for this system and practically zero heat shrinkage during crystallization, provided filling of all technological voids of the workpiece and withstanding the strictest requirements for linear tolerances and the shape of the final product. After the filling, the rods have been mechanically removed. To facilitate this process, each of the rods was pre-coated with graphite grease.

The proposed technology significantly speeds up the production and assembly time of modules of this type. Due to the new design solution — a cassette assembly scheme — partial operational replacement of degraded scintillation fibers in an electromagnetic calorimeter during the annual technical shutdown of the experimental complex will be possible.

The developed prototype of the lead absorber-based calorimeter module was tested on a test electron beam from the SPS accelerator at CERN. During the tests, it was filled with scintillation fibers based on polystyrene produced by the NRC “Kurchatov Institute” IHEP, Protvino.

“The properties and characteristics of the created prototype were tested on laboratory beams at CERN. As a result, a prototype of the ‘spaghetti’ type module can be effectively used in the new LHCb electromagnetic calorimeter. At the moment, it is necessary to solve some design problems, but even now we can say that the concepts of ‘shish kebab’ and ‘spaghetti’, which have fundamentally different designs, can effectively complement each other in the LHCb experiments,” concluded the leading scientific expert of the experiment, Andrei Golutvin.