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Posts Tagged ‘material’

This article appeared in Fermilab Today on Nov. 26, 2014

The plastic scintillator extrusion line, shown here, produces detector material for export to experiments around the world. Photo: Reidar Hahn

The plastic scintillator extrusion line, shown here, produces detector material for export to experiments around the world. Photo: Reidar Hahn

Small, clear pellets of polystyrene can do a lot. They can help measure cosmic muons at the Pierre Auger Observatory, search for CP violation at KEK in Japan or observe neutrino oscillation at Fermilab. But in order to do any of these they have to go through Lab 5, located in the Fermilab Village, where the Scintillation Detector Development Group, in collaboration with the Northern Illinois Center for Accelerator and Detector Design (NICADD), manufactures the exclusive source of extruded plastic scintillator.

Like vinyl siding on a house, long thin blocks of plastic scintillator cover the surfaces of certain particle detectors. The plastic absorbs energy from collisions and releases it as measurable flashes of light. Fermilab’s Alan Bross and Anna Pla-Dalmau first partnered with local vendors to develop the concept and produce cost-effective scintillator material for the MINOS neutrino oscillation experiment. Later, with NIU’s Gerald Blazey, they built the in-house facility that has now exported high-quality extruded scintillator to experiments worldwide.

“It was clear that extruded scintillator would have a big impact on large neutrino detectors,” Bross said, “but its widespread application was not foreseen.”

Industrially manufactured polystyrene scintillators can be costly — requiring a labor-intensive process of casting purified materials individually in molds that have to be cleaned constantly. Producing the number of pieces needed for large-scale projects such as MINOS through casting would have been prohibitively expensive.

Extrusion, in contrast, presses melted plastic pellets through a die to create a continuous noodle of scintillator (typically about four centimeters wide by two centimeters tall) at a much lower cost. The first step in the production line mixes into the melted plastic two additives that enhance polystyrene’s natural scintillating property. As the material reaches the die, it receives a white, highly reflective coating that holds in scintillation light. Two cold water tanks respectively bathe and shower the scintillator strip before it is cool enough to handle. A puller controls its speed, and a robotic saw finally cuts it to length. The final product contains either a groove or a hole meant for a wavelength-shifting fiber that captures the scintillation light and sends the signal to electronics in the most useful form possible.

Bross had been working on various aspects of the scintillator cost problem since 1989, and he and Pla-Dalmau successfully extruded experiment-quality plastic scintillator with their vendors just in time to make MINOS a reality. In 2003, NICADD purchased and located at Lab 5 many of the machines needed to form an in-house production line.

“The investment made by Blazey and NICADD opened extruded scintillators to numerous experiments,” Pla-Dalmau said. “Without this contribution from NIU, who knows if this equipment would have ever been available to Fermilab and the rest of the physics community?”

Blazey agreed that collaboration was an important part of the plastic scintillator development.

“Together the two institutions had the capacity to build the resources necessary to develop state-of-the-art scintillator detector elements for numerous experiments inside and outside high-energy physics,” Blazey said. “The two institutions remain strong collaborators.”

Between their other responsibilities at Fermilab, the SDD group continues to study ways to make their scintillator more efficient. One task ahead, according to Bross, is to work modern, glass wavelength-shifting fibers into their final product.

“Incorporation of the fibers into the extrusions has always been a tedious part of the process,” he said. “We would like to change that.”

Troy Rummler

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In a previous post, I had described how we use photons to map the material in a detector. Here I will mention a complementary way using particles such protons, pions, neutrons, etc. (these particles are collectively known as hadrons).

Hadrons interact with matter differently than photons; the latter interact purely via the electromagnetic force, whereas the former do so mainly via the strong force. The likelihood of hadrons interacting in matter is quantified by a property called the “interaction length; more about this later.

Just as a photon can convert when it travels through material, a hadron can interact and produce what we call a “secondary interaction”. In a way, this is the same idea as when the two proton beams at the LHC collide. Let’s say I have a proton that was created in the primary collision. As it travels out through the detector, it can interact with another proton in a nucleus in, say, the silicon detector. At times, this secondary interaction will have two or more charged particles emerging from it; at other times, one may have only one charged particle coming, e.g., one pion and two neutrons, or, the initial proton may just suffer a small deflection, etc.

If the secondary interaction has two or more charged particles coming out of it, we can use our software to check if the daughter particles come from the same spatial point. If they do, we have a vertex describing the location of the secondary interaction. The spatial distribution of these secondary vertices will give us a map of the material in the detector. I am currently working on this project and preliminary results are very promising.

As I wrote in the previous post, the likelihood of photon conversions in a material can be quantified by a property called “radiation length”; this depends on the intrinsic properties of the material such as atomic number, i.e., number of protons in the atom, and also atomic mass, which is proportional to the number of protons and neutrons in the atom. Since photons interact via the electromagnetic force, “radiation length” has to depend on the charge of the nucleus, i.e., the atomic number. In contrast, the strong force makes no distinction between a proton and a neutron, thus, “interaction length” has no dependence on the atomic number, but only on the atomic mass. The latter length also has some dependence on the energy of the incident particle. Although, we can derive from one from the other, it can be tricky. Since every material in our simulation package has to be described with a radiation and an interaction length, material maps made using photons and hadrons serve as very good checks on our understanding.

— Vivek Jain, Indiana University

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