Film emulsions may help determine if elusive particles change "flavor" as they travel
Aug. 25, 1999: Can a subatomic particle change its identity like a shape-shifter in a science fiction movie? If the particle, the neutrino, indeed oscillates among the three possible "flavors," it means that neutrinos have a small mass that may account for some of the missing mass in the universe.
To help answer this, scientists may use something like a vault full of safe-deposit boxes. The valuables they hold will be plates of lead and photographic film waiting for a neutrino to make a near-impossible collision with the nucleus of a lead atom and leave a debris trail in the film that betrays its presence.
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In the 1950s and '60s scientists faced a bewildering array of particles coming from particle accelerators as they pushed to ever higher energies. Order was offered in the 1960s when several scientists proposed what is now called the Standard Model. In it, six types of quark (and corresponding anti-quark) are the building blocks for heavy particles. Mesons (middleweight particles) are made of two quarks (or antiquarks). Baryons (heavyweights, including protons and neutrons in the nuclei of atoms) are made of three quarks (or antiquarks).
Right: Everything in the universe comes from combinations of 12 types of particles and four forces in the Standard Model. The MINOS experiment will focus on the three types of neutrinos and whether each oscillates to become one type then another. The Standard Model is rounded out by four bosons: W and Z (weak force), photon (electromagnetic force), and gluon (strong force). The neutrino was a "desperate remedy" proposed in 1930 by Wolfgang Pauli when physicists realized that some mass or energy was lost in certain types of radioactive decay. Pauli suggested that the missing energy was carried away by a particle that could not then be detected. Enrico Fermi named it the neutrino - "little neutral one." They stayed invisible until 1956 when detector methods had advanced. By 1962, scientists had identified the electron and muon neutrinos. As scientists learned to detect neutrinos, they discoverd a curious fact: some neutrinos seemed to be missing. Even though only one neutrino in several million is ever detected, scientists expected to snare a few dozen out of the countless numbers flooding to Earth from the fusion reactor inside our sun. In fact, immense underground detectors measured only half as many neutrinos as calculated. A possible answer to this "solar neutrino deficit" had been offered in 1957 by Bruno Pontecorvo in the USSR. He suggested that neutrinos might also change "flavor" (one of several terms physicists use as convenient expressions in describing subatomic particles; the terms bear no real relationship to the particle's physical condition). Such a flavor change would not only account for the low numbers of neutrinos detected from the sun, but also have implications for the mass of the universe. The Main Injector Neutrino Oscillation (MINOS) experiment is designed to investigate this possibility. |
Electrons,
buzzing in clouds around the nucleus, are in a separate category
called leptons (lightweights). There are only six leptons: electrons,
muons, and taus, plus three corresponding neutrinos. Leptons are
their own fundamental particles. Like quarks, leptons are believed
to be fundamental particles with no underlying structure.
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Photographic emulsions are the world's oldest radiation detection technique, dating back to 1896 when Henri Becquerel discovered radiation when he accidentally left a piece of uranium ore atop some fresh film. In an age of electronic detectors, emulsion technology has been all but discarded. But it survives at NASA's Marshall Space Flight Center, which now is sharing its expertise with Fermilab, west of Chicago, where Marshall acquired the skill more than three decades ago.
"They came to us because they heard we had experience with emulsions," said Dr. Mark Christl, a cosmic-ray scientist at NASA/Marshall. Christl and Walter Fountain, both with years of experience in making and analyzing emulsion plates, lent some of their experience to the U.S. Department of Energy's Fermilab which is managing the Main Injector Neutrino Oscillation (MINOS) experiment.
"We have the only full-service emulsion laboratory in the United States," Christl said.
MINOS will be one of the longest, completely terrestrial, particle physics experiments yet, stretching from a proton accelerator at Fermilab to an iron mine in Soudan, Minn. High-energy protons will be slammed into a target at Fermilab. The debris from those collisions will be filtered through steel and Earth. An array of instruments will look for signs of neutrino oscillation. And that's just the first 1.5 km (5,000 ft). A detector will measure the mu-neutrinos produced in the target.
The Soudan iron mine is 730 km (453 mi) to the north-northwest of the Main Injector portion of MINOS at Fermilab. The Soudan is tailor-made for the oscillation experiment. The 730 km of Earth's crust will block everything except the neutrinos coming from Fermilab. And with the neutrinos traveling near the speed of light, they'll have just enough time to oscillate to another "flavor."
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The primary detector in the mine will be, ironically, iron, cast into 486 massive steel plates interspersed with electronic particle detectors.
As a supplement, to help confirm the results of the main detector, the MINOS science team is looking at adding a front-end imaging detector that uses photographic emulsions.
The emulsions are strips of photographic film, similar to strips that NASA/Marshall supplied for the petawatt laser fusion experiments at Lawrence Livermore Laboratory. Each strip is composed of a 500 micron-thick plastic substrate, 1 cm wide and 12 cm long (0.4x4.7 in), coated on both sides with a 50 micron-thick emulsion. The emulsion is about 20 times thicker than what is found on film you use in your camera. It's specially made in terms of silver halide content and grain size for nuclear track detection. But other than that, it's a lot like black & white film, as fine as the best photographic film.
"They're looking at what kinds of emulsions they might use," Fountain said. "One of the early things they want to do is understand what's involved in pouring emulsions at depth in a mine."
For the Japanese-American Cosmic-ray Emulsion Experiments, ongoing since the mid-1970s, NASA/Marshall made the emulsions shortly before a balloon borne emulsion was due to fly. In a total of 14 balloon flights, the JACEE program flew emulsion chambers near the edge of space for periods ranging from less than a day up to 20 days. The program is wrapping up now and will give way to larger cosmic ray detectors that will spend years aboard the International Space Station or on a satellite.
Trace radiation
"To do a JACEE flight took two months of pouring, drying, and cutting in
darkroom conditions with a crew here," Christl said. "It's not likely we will
take on much of the MINOS effort because they will need 1,000 times what we've
ever done."
MINOS will require a million emulsion plates installed in what will look like a safe deposit box. Metal drawers, 10x10 cm (4x4 in) square and 10 cm (4 in) cm deep will hold dozens of thin plates of emulsion.
Even here the science team will have to be careful. Most metals have trace isotopes that are slightly radioactive, so the science team is exposing emulsions in the mine to candidate materials to see which ones have the lowest intrinsic radiation.
"That is a problem, finding metals that won't contribute to the background radiation. But there will always be some radiation that you can't eliminate," Fountain said. For example, gamma rays are emitted by natural radioactive materials such as radon.
A number of issues have to be addressed, including the best types of emulsions to use, whether to manufacture inside the mine (which requires installing a mini-factory in the mine), or at the surface (which requires some increased risk of radiation plus transport down to the detector gallery), and how quickly the plates must be processed before they oxidize and become useless. Even naturally occurring traces of radon gas could increase the difficulty of finding candidate particle tracks in the emulsions.
And the plates have to be counted. Fountain said the MINOS team may use supermarket-style bar codes to track the emulsions' entire life cycle, from pour to placement in the detector to developing.
One, two, ... one hundred thousand ...
Then someone has to examine the developed emulsions and plot the locations of
hits. This once was the province of graduate students, but should become
automated because of the cumulative size of all the plates involved. It will be
something like combing through dozens of football fields as you look for a
particular set of holes left by cleated shoes.
"The drawback to emulsions is that they require a great deal of work to extract the data," Christl said. "That means manpower or computer power. It takes a lot of patience. It's a challenge." And it's done at the microscopic level. In some cases, the researchers are looking for individual grains in the emulsion.
Yet it is just that feature that makes emulsions so valuable.
"Emulsions have the resolution to detect two closely spaced particles that cannot be resolved by any other method," Fountain said.
"The position resolution is tremendous," Christl said. "That's the power of emulsions. There are many different ways of detecting things, but they're on the order of millimeter resolution. Emulsions are orders of magnitude better."
He cited his own work with the Scintillating Optical Fiber Calorimeter (SOFCAL), an apparatus using plastic fibers that give off flashes of light as cosmic rays pass through. The scintillating fiber technology is also being applied in the FIBERGLAST proposal for a large gamma-ray telescope.
If the fiber were as wide as a football stadium, then you would only know that the particle passed through the stadium. An emulsion would let you tell which seat - both section and row - it struck.
This is crucial in certain branches of particle physics since it's impossible to pull out a particle and put it on a scale to weigh it. Scientists can only measure the tracks they leave - such as liquid gas forced to boil in a bubble chamber, sparks in an ionization chamber, darkened spots in emulsions - and reconstruct the mass and energy of the original particle.
Seeing what develops
In stalking the elusive neutrino, detailed knowledge of the debris locations
will be important to determining just what neutrinos do.
First, though, the emulsions have to get into the mine. Access to the MINOS gallery is restricted to a single elevator that runs on a set schedule, so hopping back to check the emulsions or stepping out for lunch are not ready options. Further, it may be necessary to stay with the gels overnight. So shipping from a separate factory is an option.
"But it increases the fog level [in the emulsions] by the time they're ready to go in place," Fountain said. The standard procedure in the emulsion business is to buy gels in bulk and, just before the experiment, pour the emulsion. "The cosmic ray and background radiation then are minimized. The evidence from a neutrino oscillation is something really difficult to find, so they want to minimize the background level as much as possible."
Christl said the MINOS team is talking with several film manufacturers to see if they could develop the systems that would be needed.
These decisions are still several months away. The ceremonial groundbreaking for the project was held July 20, and many details of the designs remain to be worked out.
When needed, NASA/Marshall will continue to share its expertise in emulsions and cosmic-ray research in several projects, including fusion experiments using the petawatt laser at Lawrence Livermore National Laboratories, and studies of nuclear interactions at the CERN accelerator facility in Europe.