Endeavors- Dark Energy, Neutrinos, and Other Strangeness

8/3/2019 Endeavors- Dark Energy, Neutrinos, and Other Strangeness

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Research and Creative Activity The University of North Carolina at Chapel Hill

endeavorsSpring 2003

DARK

ENERGY,NEUTRINOS

and otherstrangeness


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ont

ent

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Hidden away underground, in a bunker-like warren packed withgauges and wires and machines, stands a hulking beast known as an Enge spectrograph, whichteases out the shrapnel from nuclear reactions. Art Champagne bolted this monster togetherfrom scavenged parts, painted the carcass two species of blue (Tar Heel and Blue Devil), thenmounted the hardware to roll on a track enameled Wolfpack red. For all who have seen the

frenzy that erupts when any two of these colors clash on a basketball court, it is hard to imaginethem peacefully sharing a basement together. But they do. And Triangle Universities NuclearLaboratory (TUNL) is one place in North Carolina where the rivalries of Tobacco Road pale toinsignificance in the face of, well, the universe.

As heretical as this may seem to some of our fans, we should say, in all earnestness, thatCarolina is a better school because of Duke. We are also a better school because of N.C. State.And these two fine universities are better because of Carolina. In several fields of science, we areinseparably one research enterprise, one powerhouse taking on the oversized jobs that none of uscould do alone. Some of those jobs are in physics and astrophysics.TUNL is a place scientists goto recalculate the age of the universe. Where they simulate the nuclear reactions that power thesun. Where they contemplate the nature of dark energy.

Out in the airy sunlight, on leafy green campuses full of spirited young women and mengoing about their studies and having their fun, let the rivalries thrive. But down here in the lab,

where particles explode into the tiniest fractions of physical existence, bragging rights may wellbe the silliest concept of all. Everything we are, right down to the iron in our blood and theoxygen in our lungs, came from the stuff of exploding stars. As Carl Sagan put it, We are star-stuff. And here, in this place, we are one.

The Editor

Spring2

003

Jason Smith

2003, Endeavors magazine, University of North Carolina at Chapel Hill


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Deep inside the labyrinth of theTriangle Universities Nuclear Laboratory,wrench-wielding physicists smash particlesand ponder the riddles of the universe.

stories by Angela Spivey photos by Steve Exum



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Tom Clegg keys in the security code

that sends the elevator down to thebig vault. The doors open, he clips

on a radiation detector, then he starts thetour in the NASA-like control room wherea technician named Paul chews on an unlitcigar. Darting through doors, ducking to

avoid pipes, Clegg leads the way througha maze of metal cages, concrete walls, andendless wires. Suddenly, he stops. He pointsout a towering sea-foam-green machine.This green thingits called a tandem Vande Graaff acceleratormakes the workhappen. Here, work means hurling atomicnuclei at each other to cause a reaction.

This is the Triangle Universities NuclearLaboratory (TUNL), one of the largestuniversity-based nuclear physics labs in thenation. Most of it sits underneath a park-ing lot on the campus of Duke University.

There, we said itDuke. Three universi-tiesDuke, Carolina, and N.C. Statecre-atedTUNL in 1965 and jointly operate it,sharing space, equipment, and resources.Clegg, professor of physics and astronomyat Carolina, has been busting stuff aparthere for years.

Someone calls out a warning: theyreturning on the accelerator. When theaccelerators on, Clegg says, dependingon which beam is going, radiation levels can

be high and you have to be careful whereyou walk. Very basically, heres how theseexperiments work. The researchers take apiece of thin, postage-stamp-sized foil andcoat it with some elementsay, silicon. Thiselementits nucleus, to be more exactiscalled the target. A machine called an ion

source produces a beam of ionized hydrogenprotonsprotons that have lost an electron,thereby gaining an electric charge. Variouslenses and steerers direct the ion beam tothe accelerator, which ramps the particlesup to the desired speed.

We send beams of particles at the tar-gets, Clegg says. Our targets are othernuclei, and we bang them together reallyhard, and shrapnel comes out. The shrapnelare pieces of the struck nucleus, and how thatstuff comes out tells us something about theforces that hold it together.

Clegg is simplifying, of course. Dozensof machinesflux meters, control boxes,dehumidifiers, compressors and filters and valveswork together. Climbing atop asilver vacuum pump, Clegg points out oneof the red, plate-sized magnets that helpdraw the beam where the scientists want itto go. And as Clegg leads the way throughthe cages and pumps and wires, a continuousroaring, a little quieter than a bus, comesfrom the many vacuum pumps that keep air

out of the beam line. If the ionized particlehit normal air, theyd lose their charge andslow to a stop.

TUNL (pronounced like tunnel) haslong been known for this tandem-accel-erator setup, which uses an elaborate array

of ion sources and beam lines to producevarious types of particles and send themaround the vault to one of many target stations. For instance, in the late 1980s Cleggled a team of twenty students and facultyin building an ion source that lets themcontrol the spin orientation of the beamparticles. Protons, it seems, can spin eitherclockwise or counterclockwise. Each oneis an extremely tiny magnet, Clegg saysKnowing their spin orientation from thebeginning of the experiment allows controof the magnets orientation. These ions with

a known spin are said to be polarized. Cleggion machine is known as the most intensesource of dc (continuous stream) polarizedpositive hydrogen and deuterium ions in theworld. Other ion sources atTUNL producebeams of unpolarized protons, deuteronsand neutrons.

Training students to crawl

Students helped build much of this equip-ment. This laboratory has a reputation fortraining students to crawl around the appa-ratus, Clegg says. When something breaks

a graduate student will probably fix it. Theyknow how to solve problems as they arise,says Hugon Karwowski, professor of physicsand astronomy. Usually about thirty gradstudents are working at TUNL at any onetime. The lab has produced thirty to fortyCarolina Ph.D.s, says Ed Ludwig, professorof physics and astronomy and the first Caro-lina scientist to work there. Most studentshere work through entire experiments, frombuilding equipment to analyzing results.

Left: Tom Clegg flanked by watertanks in TUNLs underground vault.

Facing page: A vacuum tube glows

in the Laboratory for ExperimentalNuclear Astrophysics at TUNL.

2003, Endeavors magazine, University of North Carolina at Chapel Hi


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Cleggs, Karwowskis, and Ludwigs workhas focused on spin dependence in few-bodyphysics, which means studying the mag-netic forces that hold two or three protonstogether. Protons are charged positively,

and, Clegg explains, positive charges usuallyrepel one another. But inside a nucleus, theystick together. So theres an attractive forceinside the nucleus which is stronger than therepulsive force of the charge, Clegg says.What holds nuclei together? What is thenature of that force?

Since its early days, when most of theresearch focused on the big accelerator,TUNL has continued to grow. When thewhole group meets, there is standing room

only in the buildings single conference room.One of TUNLs newest additionscreatedand constructed by Carolina scientistsisthe Laboratory for Experimental Nuclear Astrophysics (LENA)one of only three

dedicated nuclear astrophysics (the nuclearphysics of stars) labs in the nation.

Star simulation

So Clegg finishes the whirlwind tour,hands you off to the LENAguys, and dis-appears into the innards of the vault. ArtChampagne walks you to a separate buildingwhere he, Christian Iliadis, and a group ofstudents simulate the reactions that powera star. LENA is another metal maze; walk

in and you might find Johannes Pollanenwho just finished his senior year at Carolinatrying to find on which table beside which

roll of tape and box of lugs he laid his screw-driver. LENAis on a smaller scale than thebig vault, though, and the two accelerator(one of which is painted purple) operate athigh intensity (number of protons hittinga target per second) but comparatively lowenergyone accelerates the beam over a dif-ference of one million volts, the other overa difference of 200,000. The insides of starwork at accelerating energies much lowerthan that5,000 to 20,000 volts. But thosereactions can take ten billion years, so tostudy them the researchers create simulated

reactions at higher speeds, then use math toextrapolate the results to actual stars.

While LENAs equipment is low energy

Hugon Karwowski and graduate student Doug Leonard with TUNLs tandem Vande Graaff accelerator. Wolfgang

Weinzierl, an

undergradu-

ate exchangestudent from

Berlin, with

a target ofspin-polarized

helium-3 nuclei.

It produces agas jet, which

scientists blast

with an ion

beam, scat-tering the gas

particles. The

scattering pat-tern reveals

clues about

forces that holdthese particles

together.



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but high intensity, the people here are highenergy and high intensity. Explaining whythey spend many of their days in this win-

dowless basement lab, Champagne, profes-sor of physics and astronomy, says that ourearth is a mere side effect of a reaction insidesome star billions of years ago. Were livingon the debris, he says. The oxygen yourebreathing right now was made inside explod-ing stars. Such reactions happen constantly,and scientists dont understand much of whatdrives them. The universe is a zoo, saysIliadis, associate professor of physics andastronomy. Understanding these reactionsbetter, they say, would mean getting a clearerpicture of where our universe came from and

where it may be going.

Dark energy

Lately, scientists find that the answers tothose big questions have a lot to do with stuff we cant even see. Dark energya forcethat we know nothing aboutappears todominate the universe, according to studiespublished in the journal Science earlier thisyear. Scientists have suspected the existenceof dark energy because of how radiation andother energy fluctuate at different places inthe cosmos. The newest findings support

the idea that most of the universe73 per-centis dark energy.

Dark energy seems to act against grav-ityit pushes things apart rather thanpulling things together, Champagne says.Because of this anti-gravity effect, some sci-entists think that the universe is expandingat an ever-increasing rate; galaxies that arefar, far away will only move farther, and theywill do it faster. And, what we traditionallythink of as matterstuff that is made out of

atomsoccupies only about 4 percent of theuniverse. Essentially, we dont know whatour universe is, Iliadis says.

Champagne, Iliadis, and Ph.D. studentBob Runkle may add a clue later this yearwhen they publish work in which they mea-sure the rate of one tiny reactionand endup revising the age of our galaxy by about600 million years. The experiments, whichRunkle has been conducting for his doc-toral dissertation, are complicated, but likeother projects atTUNL they involve usingan accelerator to hurl beams of protons ata target. In this case the target is nitrogen14; they shoot protons at it, over and over,for about 1,200 hours. Since its my experi-

ment, I get to work the midnight to eighta.m. shift, Runkle says. Which is why we

have this couch over here.The researchers want to find out how

often the proton fuses with nitrogen 14

To determine that probability, they usmachines that detect gamma rays, whichare high-energy radiation created by thereaction. To home in on true by-productof the reaction and filter out background(the natural radiation given off by ordinaryobjects such as concrete), LENAis equippedwith seventeen detectors. As many as wcan afford, Iliadis says.

What does this experiment inside machine have to do with the age of thegalaxy? Collisions of nucleireactionsimilar to those that go on inside LENA

machinesprovide the fuel for stars. Tinyparticles like the ones in our accelerato

Some of the LENA guys: Ph.D. student Bob Runkle with facultyArt Champagne and Christian Iliadis.

In the LENA lab, a vacuum tubeused to monitor pressure sitsatop metal pipes that make uppart of the beam line. The airinside the pipes is put under

high vacuum to keep the ion-ized particles going.



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control these stars that are thirty times asbig as our sun, Runkle says. Isnt thatridiculous?

As Runkle explains, some stars are com-pact cars (they burn their fuel slowly), whileothers are SUVs (they burn it faster). Theslower the fuel burns, the longer the star

lives. The rate at which some stars burn their

fuel is determined largely by the nitrogen 14reaction, which the LENAteam measuredmore directly than had been done before.They found that the reaction is less probablethan previous measurements had indicated,which also means that the reaction happensslower than scientists had thought. Using

these data, the team can calculate how fast

the same reaction happens inside groups ostars called globular clusters. That reactionrate tells us how fast the globular clusterare burning their fuel.

Revising the age of the galaxy

The LENAcalculations show that thoseclusters are burning fuel slower thanexpected, which means that the clusters arolder than scientists had believed. Howlong would that star have to have evolvedto reach this point? Champagne says. Wlook at the star right now, and we look at itcharacteristics, and we can work our wayback. The galaxy cant be younger than group of stars in it, so revising the age oclusters revises the age of the galaxy.

The LENA team hopes that once theypublish this work, scientists will use thenew rate for this reaction to more accurately age-date globular clusters. This newthinking about the galaxys age could changehow we think about dark energy, becausedark energy affects how the galaxy (and thuniverse) evolves.

This result is one part of a big picture,Champagne says. But its something thatgetting more and more clear. I think overthe next ten years, there are actually goingto be some hard answers about past, present

and future.

This research is funded by the U.S. Departmenof Energy. Look for a future story in Endeavors about Carolinas growing commitment tastronomy and astrophysics, which includes thcompletion of the SOAR telescope in Chile andconstruction of the SALT telescope in SouthAfrica. Art Champagne is associate directoofTUNL, along with N.C. State Universityprofessor Gary Mitchell.

An acceleration tube that runs between the ionsource and the system that transports the ion

beam to the target. The acceleration voltage isapplied in steps along the length of the tube.

Tom Clegg (top), postdoctoral researcher Tatsuya Katabuchi (bottom left)and graduate student Melissa Boswell try out a new experimentputtingsomething together quick and dirty to see if it works.

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second, anti-neutrino? This machine actu-ally detects anti-neutrinos, which are createdin the decay of products of fission reactionssuch as those that drive nuclear powerplants. Neutrinos are created during fusionreactions, which occur inside the sun. Suntravel not being practical, researchers builta detector in Japan near numerous nuclearreactors. And in the world of physics, as far

as scientists know, anti-neutrinos behave likeneutrinos. Anti-matter is the mirror imageof matter. So, you study one, you study theother.

So lets just call everything neutrinos.Because the nuclear reactors surroundingKamLAND are a relatively well-controlledsource, researchers could predict how manyneutrinos theyd expect to find. The phys-ics of nuclear reactors is very well known,Karwowski says. So if you know the poweroutput of the reactor and you know the fuel,then you can predict to very good accuracy

which fission products are going to beproduced, and therefore you know exactlythe neutrino flux. Taking into account thedistance from the detector to each reactor,the scientists expected to find eighty-sixneutrinos in six months. But the detectorfound significantly fewerfifty-four. Theneutrinos seemed to disappear.

How does that disappearance tell themanything about mass? Neutrinos, it seems,come in three different flavorselectron,muon, and taudetermined by the way inwhich theyre made. The KamLAND detec-

tor records only electron neutrinos. Theresearchers conclude that the only way theneutrinos could have vanished is if theywere oscillatingchanging from electronflavor to some other flavor. The only waythe neutrinos could change flavor is if theyhave a nonzero mass.

What? If your mass is zero, Karwowskisays, your mass cannot change. Zero is alwayszero. It cannot change flavor unless it hasa mass, Karwowski says. If its a massless

particle, then it will always remain whateveit is. This explanation makes common senseas long as you dont think about it too muchTo truly understand, Karwowski says, youhave to delve into quantum mechanics.

The detector consists of a steel casing sur-rounding a forty-three-foot diameter bal-loon filled with 1,000 tons of liquid scintilla-tor, a chemical mixture that converts energy

lost by ionizing radiation into pulses of lightAt KamLAND the scintillator emits flashesof light in sequence when certain neutrinoevents happen. The flashes cant be seenby the naked eye but are detected by almos2,000 photomultiplier light sensors.

On the outside of the balloon, a tankof ultrapure water detects and filters ounonneutrino events. Karwowski, graduatestudent Doug Leonard, and other TUNLscientists traveled to Japan to help build thisouter water detector. Leonard helped fill itwhich had to be done at a controlled rate

He describes his work there as valves andgauges and running around from the topof the detector to the bottom to make surenothing went wrong. Data collection beganin January 2002, and Karwowski has takenhis share of shifts in the control room.

The confirmation that neutrinos havemass gives them the title of lightest par-ticle in the universe with nonzero mass. Italso means that neutrinos could have beeninvolved in density fluctuations that helpedcreate galaxies. Karwowski says, There area lot of processes that are dependent on the

presence of neutrinosstellar evolutionsuper nova explosions. Even more sofor him, neutrinos are another tiny pieceof a puzzle worth solving. Because theyare here.

This study was published in the December2002 issue of Physical Review Letters andwas funded by the U.S. Department of Energyand the Japanese Ministry of Education andScience. Adjunct associate professor Ryan Rohmalso participated in KamLAND.

You cant see them, but they are here. These so-called ghost particles haveno charge, so they rarely interact withothers; they can pass through lead. Theirnameneutrino, Italian for little neutraloneexplains it well.

Though neutrinos are tiny, a recent studyconcludes that, yes, they do have mass.Hugon Karwowski, professor of physics

and astronomy, collaborated with an inter-national team of ninety-two researchers tostudy neutrinos using KamLAND, a hugedetector built underground in a mine inJapan. While other studies have suggestedthat neutrinos have mass, the KamLANDstudy puts the nail in the coffin, Kar-wowski says.

KamLAND stands for Kamioka LiquidScintillator Anti-Neutrino Detector. Wait a

An artists conception of the maindetector at KamLAND, which useschemicals and light sensors to detectneutrinos.

Courtesy of RCNS, Tohoku University

going underground to capture neutrinos

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Endeavors- Dark Energy, Neutrinos, and Other Strangeness