Do neutrinos have mass?
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James Semler (r), Alan Nelson (l) and Ernst deHaas (behind Nelson) work on the balloon under the gaze of curious onlookers. (Photo by Steven Schultz) |
By Steven Schultz
Athletes passing through Jadwin Gym one morning in November couldn't help noticing a giant plastic balloon suspended from the metal rafters.
As they headed into their warm-up routines, a few ventured over to the knot of people at the base of the balloon and popped the question: What is that?
The short answer: a physics experiment.
The long answer is a story about what must be one of the most finely engineered balloons ever made and why Princeton scientists plan to take it to Italy next year and inflate it in a cavern deep in a mountain northeast of Rome.
The balloon is one of Princeton's contributions to a large international research project called Borexino (after the chemical borax, originallybut no longerused in the experiment). The aim is to take a detailed survey of a vast shower of particles that stream out of the sun and pass through Earth. The particles, called neutrinos, are emitted in staggering quantities: face a fingernail toward the sun, and 80 billion of them will pass through it every second. Yet because they leave almost no impact, neutrinos are one of the most elusive and tantalizing particles in physics.
For most of the 70 years since scientists first suspected their existence, physicists believed neutrinos were massless, like the photon, the quantum unit of light. Recent experiments, however, have cast doubt on that assumption.
The answer has profound implications. Assigning mass to neutrinos would force physicists to seriously rework the Standard Model of elementary particle physics. It also could help answer a nagging question about missing mass in the universe; astrophysicists have long puzzled over why all the visible matter in the universe adds up to so much less than they predict. If neutrinos had even a slight mass, they could weigh more than all the visible stars.
"It would mean a new era of physics," says Professor of Physics Frank Calaprice, leader of Princeton's part of the Borexino project. In Jadwin that day in November, Calaprice snapped pictures of the prototype balloon from every angle, lining up members of the research team under the big sphere, like a proud parent at a science fair.
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Nylon Vessel in Jadwin Gym |
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The Borexino project, which also involves scientists from Italy, Germany, Russia, France, Hungary and Poland, is centered at Laboratori Nazionali del Gran Sasso in Italy. The heart of this lab is a series of caverns adjacent to a highway tunnel, nearly a mile beneath Gran Sasso Mountain about 60 miles outside of Rome. It is an ideal spot for studying neutrinos, because the rock shields the detector from other types of radiation and particles that would overwhelm the sensing device. Much of the Borexino experiment is a process of eliminating the "noise" of background radiation and revealing signs of the gossamer neutrinos.
The core of the detector is Princeton's 28-foot balloon, which will be filled with 300 tons of a liquid chemical mixture that produces a flash of light when it interacts with a neutrino. An array of 2,200 ultrasensitive photodetectors will be aimed at the sphere. To prevent false readings from other particles, the balloon will float in an elaborate series of larger spheres and stainless steel housings, each filled with more chemicals or water.
The materials themselves must be uncommonly free of radioactive particles. For example, graduate student Laura Cadonati analyzed many plastics for the balloon before settling on a particular type of clear nylon sheeting that would not disrupt the readings. It was not the sort of information she could get by just calling up the manufacturer, so she devised her own tests.
Clean room construction
And when it's time to build the final version of what the scientists call the Nylon Vessel, the work will take place in an exceedingly clean environment, because common dust contains enough radioactive particles to overshadow any signal coming from neutrinos. Working with grant money from the National Science Foundation, the researchers constructed a $500,000 "clean room" on campus, cutting the level of radioactivity to 10 million times less than in the natural environment. This effort, which took nearly a year, was supervised by technical staff members Richard Fernholz and Fred Loeser.
Following the design of Jay Benziger, professor of chemical engineering, the group is also investing $1.5 million into building a purification plant for the chemical that will fill the balloon.
Even making the prototype of the Nylon Vessel was a significant challenge, explained Alan Nelson, senior technician in the Physics Shop. The balloon is assembled from 36 strips of plastic, each shaped like the peel from an orange slice, glued together at the edges. First the technicians made their own glue (a mixture of some of the plastic in a solvent), then they devised an intricate system for folding the finished work so they could glue each slice onto a flat, clean edge of the next slice.
"We had no idea what would happen after we made the first joint," said Nelson. "This folding process is a crazy system, but it's the only way to glue all these joints together."
First flashes in 2001
The Borexino detector has two main features that make it unique among several worldwide efforts to track neutrinos. First, it can detect extremely faint neutrinos; and second, it will track them in "real time." That is, when a flash occurs, the scientists will know exactly when it happened and even where it was in the detector. Previous experiments could only reveal the total number of neutrinos that passed through the detector over a period of time.
The chances of catching a neutrino in the space of the balloon may seem remote. Neutrinos tread very lightly: one traveling through a light year of solid lead would probably not bump into a single atom. But because the number of neutrinos flowing through the cosmos is so great, the chances turn out to be quite good that over a period of hours a small number will strike a spark of light in the detector.
While one contingent of the Nylon Vessel Group was in Jadwin Gym, other members of the group, including assistant professor of physics Thomas Shutt, Fernholz and Loeser, were in Italy installing a smaller prototype ballooon in the Gran Sasso lab. If all goes as planned, the big balloon will be delivered to Italy later this year (along with several extras, for backup). The scientists hope to glimpse their first flashes of neutrino light in 2001.
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