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Frequently Asked Tour Questions
- How many people work in the lab?
There are 7 full time University of Minnesota employees at the lab. There are also several physicists or graduate students that are working here at any given time. During the summer months we have had high school teachers here working on outreach, tours and learning how the MINOS detector operates. We also employ several undergraduate students to help during the summer.
- How much did the labs and experiments cost to construct?
The Soudan2 project excavation and construction total cost was about $13 million. The MINOS project total cost was $53 million. CDMS uses part of the Soudan2 cavern so there was no excavation cost but the clean room and detector cost $18.5 million
- How long will the various experiments last?
MINOS was scheduled to run until spring of 2012, and will then be called "MINOS+" and take data in the higher-energy NuMI beam for NOvA through 2015. CDMS is scheduled to run until 2013. The Low Background Counting Facility and associated smaller experiments continue to grow.
- What is the history of U of M and Soudan? What projects have been
conducted here and when did they start?
Soudan1 was started in 1980 by Dr. Marvin Marshak and ran until 1990
Soudan2 excavation was started in 1984 and started taking data in 1987. It ran until 2001
Double Beta experiment came here in 1989 and operated for 18 months studying enriched molybdenum 100. It is now being used as a part of the Low Background Counting Facility
MINOS excavation started in 1998. The detector was completed in July of 2003
CDMS II installation started in 1999. Data acquisition started in 2003 with 2 towers
1. Why use steel for the MINOS detector?
The chance of a neutrino hitting any one proton or neutron (a “nucleon”) is really tiny. However, if we can pile a lot of nucleons in one place, then a neutrino has that many more chances to hit something so we can see what happens. Iron is heavy (it has a lot of nucleons in a small space) so makes a good target. It's also not very expensive per pound, and is strong enough to support its own weight, so we could build a large heavy detector as cheaply as possible.
2. How long will it take for the beam to go from Fermilab to Soudan?
The neutrinos are very, very light and traveling at very high energies, so are going almost the speed of light. That's 186,000 miles/second, and it is 457 miles from Fermilab to Soudan. It takes them 2.5 thousandths of a second to make the trip.
3. How do you know that you got a neutrino from Fermilab and not a stray neutrino?
We know what direction Fermilab is, so a beam neutrino must be coming straight out of the south wall of the cavern to have come from Fermilab. Also, the beam is only on for 12 millionths of a second every 2.2 seconds, so only neutrinos arriving in that short burst of time could be coming from the beam. That's a small enough slice of space and time that we estimate there is only a small chance that one of our hundreds of observed beam neutrinos might have actually been a stray neutrino in the wrong place at the wrong time.
4. How did the Soudan 2 experiment lead to the MINOS experiment?
Soudan 2 was looking for decaying protons, which would be a very rare thing (no one has yet seen this happen!). A proton decay would look like a sudden burst of activity in the detector, with nothing having come zipping in to create it. Since neutrinos are so slippery, an incoming neutrino would not be seen, but if it clobbered a proton, it could make a spray of stuff that looks a lot like the proton decaying all by itself. So, proton decay experiments had to study the neutrinos very carefully to understand the chances of a neutrino faking a proton decay. The neutrinos were not behaving as advertised – up to half of them seemed to have gone missing on their trip from their birthplace in the upper atmosphere in collisions between cosmic rays and the air to the lab. It turned out that they were “oscillating” or changing flavor to an even harder to seen flavor of neutrino, so MINOS was built to study this disappearing act using a before and after measurement on a man-made beam of neutrinos.
5. What happens if one of the fiber optics cables stops working? How do you fix it?
The fibers themselves only stop working if you smash or cut them. Since they're armored by a few thousand tons of steel, they're pretty safe and don't break once they're installed (and we were careful to check that they all worked before they got installed). However, sometimes the electronics that watches for and computerizes the flashes of light from the scintillator do break down. When that happens, the mine crew notices the resulting odd pattern on their monitoring software, and replaces the faulty electronics. The electronics is on the edges of the detector so can be easily maintained.
6. Why is the MINOS detector magnetized?
A charged particle bends as it travels through a magnetic field. It will bend one way if it's a positively charged particle, and the other way if it's negative. So, which way it bends tells us what the particle's charge is, and thus if it was made by a neutrino or an anti-neutrino. Also, the degree to which it bends tells us its momentum. A more energetic, faster-moving particle has a larger turning radius, just like a car on the freeway compared to a car puttering around a parking lot. So if we see a particle turn tightly, we know it's not as energetic as one which curves less.
7. How wide is the beam when it leaves Fermilab? When it arrives in Soudan?
The neutrino beam is a foot or so across when it passes through the Near Detector at Fermilab. It has spread out to a mile wide by the time it gets to Soudan, much like the beam of a flashlight.
8. Is the detector in line with Fermilab? How did they line it up?
The detector is lined up along the direction of the beam to within a few inches. While it's easy for surveyors to use GPS systems to line things up anywhere on the surface of the Earth, underground you can't see the GPS satellites. So, the surveying team took an inertial navigation device up and down the shaft. This device kept track of where it was during this trip using gyroscopes, and after averaging many trips from the surface (where the GPS coordinates work) to the 27th level, was able to connect the lab coordinate system to the rest of the world.
9. What is the size comparison between an atom and a neutrino?
An atom is a hundred-millionth of a centimeter across (10-10m). The nucleus of an atom, where all the protons and neutrons are, is one hundred thousandth the size of the whole atom (10-15m). A neutrino must hit a nucleon dead on center to interact with it, where “bang on” means one thousandth the size of the nucleus (10-18m). So, the neutrino could be said to be one hundred millionth the size of an atom (as much smaller than an atom as the atom is smaller than your fingertip!), or a billionth of a billionth of a centimeter.
10. How does MINOS differ from the other neutrino experiments around the world? Have other sites found evidence of oscillation?
MINOS is a magnetized iron detector in an especially intense neutrino beam. Other experiments are much larger. Super-Kamiokande in Japan is made of water and ten times as heavy, so it sees many more natural neutrinos and found the first solid evidence of neutrino oscillations in neutrinos from cosmic rays. Super-K can also see neutrinos from the Sun, and when combined with the smaller SNO detector in Sudbury, Ontario (a tank of heavy water that can make precise measurements of a different aspect of the neutrinos), these solar neutrinos were also seen to be oscillating. A faint beam of neutrinos was shot across Japan from the KEK accelerator to Super-K, confirming the neutrino oscillations with an experiment called “K2K”. Also, tanks of mineral oil with liquid scintillator in Japan (the KAMLand experiment) and France (the CHOOZ experiment) have observed neutrinos from nuclear reactors and the core of the Earth. All of these experiments are underground just like MINOS. Many other earlier experiments saw neutrinos using similar methods, but were smaller and couldn't definitively find oscillations alone. Their data, howecer, does match the more precise modern numbers.
1. Why use germanium and silicon for the CDMS II detector?
These materials are both semi-conductors, used in the making of integrated circuits for electronics. Very pure crystals of these materials can be grown and the semi-conductor industry has developed techniques for placing tiny electrodes on their surfaces. We use these techniques to install sensitive, superconducting thermometers on our detectors in order to sense the tiny pulse of heat liberated by a WIMP.
2. How does the detector work, i.e. once a particle hits the detector, how is the data transmitted?
Sensors on the surface of the detectors pick up both the charge and heat liberated by a particle interacting in the detector. Tiny superconducting wires transmit these signals up the sides of the detector tower to electronics mounted at the top. After conversion to voltages, these signals are sent on stripline cables to room-temperature electronics for further amplification and conversion to digital signals, which are then recorded by computers and written to hard disks and magnetic tape.
3. How does the electric “cable” work in the CDMS II experiment?
They contain copper traces on a flexible Kapton (plastic) strip. Essentially they are like tiny wires embedded in plastic. These copper wires transmit signals from the detectors to room-temperature amplifiers and recorders, Since copper conducts heat very well, and since these striplines connect room temperature to the cold detector towers, we have to minimize the amount of copper and intercept the heat at several points along their length.
4. How close to absolute zero does the CDMS II experiment get?
On the Kelvin temperature scale, absolute zero is 0K (-293 Celsius, -460 Fahrenheit). At absolute zero, all molecular motion ceases. The coldest part of the CDMS experiment reaches 0.02 K, or 2/100ths of a degree above absolute zero. At that temperature, molecules are moving at an average speed of only about 9 miles/hour, comparable to the running speed of a human. At room temperature, molecular speeds average about 1100 miles/hour, about twice as fast as a jet airplane.
5. What process is used to get the detector that cold, i.e. how are gases used?
One must employ several methods in combination to reach such cold temperatures. First, two layers of vacuum ensure that heat from the outside is not transmitted inwards. Second, a layer of liquid nitrogen (77K, -216 Celsius, -383 Fahrenheit) surrounds the inner parts of the refrigerator. Third, a bath of liquid Helium (4K, -289 Celsius, -456 Fahrenheit) maintains the inner parts of the refrigerator and the tops of the detector towers at 4K. [These cryogens are liquefied by compressors and delivered to Soudan by a vendor in Virginia, MN.] The remainder of the temperature decrease is achieved using the dilution refrigerator, which pumps one Helium isotope (3He) gas from another (4He) as a liquid and then condenses the 3He back into the 4He. This is very similar to the process used in normal refrigerators except the refrigerant is Helium instead of Freon (or one of its modern replacements).
6. What evidence do you have that WIMPS exist? How did you know to be looking for them in the first place?
We know that dark matter exists, because we can directly see its gravity affecting galaxies and clusters of galaxies. We also know that dark matter cannot be made up of known particles, because these would produce other detectable effects that have not been seen. However, particle physics has produced a theory, called Supersymmetry, that predicts the existence of a new class of particles that are massive, neutral, and stable. These WIMPS would have been produced in sufficient quantity during the Big Bang to explain the current abundance of dark matter.
7. How will you know when you a see a WIMP, if they are hypothetical particles?
Since WIMPS have masses at least as large as atomic nuclei, but have no electric charge, they can only interact with normal matter by scattering off nuclei like billiard balls (elastic scattering). The signature of WIMP elastic scattering in CDMS is the release of heat in the detectors, without very much charge. Only neutrons, among normal matter particles, can mimic this signature. We are deep underground to avoid neutrons, but can also distinguish them from WIMPS because they scatter much more frequently in our detectors. For example, about 50% of neutrons will leave energy in at least two of our detectors but WIMPS interact so rarely that they will never cause more than one detector signal.
8. How many towers are in the CDMS detector? How many do you plan to put in the detector? How many “pucks” are in each tower?
We are currently operating 5 towers of detectors in the CDMS II experiment, each of which contains 6 detector ‘pucks’. We will be building seven new towers of larger ‘pucks’ for the next version of CDMS.
9. Where is CDMS I located? Is there a CDMS III or CDMS IV? What other similar experiments are happening around the world?
CDMS I was run at the Stanford Underground Facility, on the Stanford University campus in California, from 1996-2002. This facility was a shallow tunnel and the experiment was limited by backgrounds due to cosmic ray particles. That is why CDMS II was constructed in the Soudan Underground Laboratory at a depth of 2341 feet. After 2-3 more years at Soudan, we will likely again be limited by the small rate of cosmic ray particles penetrating to this depth. So the next phase of the experiment, which we call SuperCDMS, is planned for the Sudbury Inco nickel mine in Ontario, Canada, at a depth of approximately 6800 feet.
10. Can you provide a timeline for the development of CDMS research – when first start, where first start, advancements, who were the first physicist involved in the research, etc.
The concepts for using cryogenic detectors to detect dark matter originated in an experiment conducted by UC Berkeley, UC Santa Barbara and Lawrence Berkeley Laboratory in the 1980’s. The Berkeley team was led by Bernard Sadoulet and the UCSB team by David Caldwell. The CDMS collaboration formed in the early 1990’s with the addition of Stanford University, led by Blas Cabrera. Development of the first detectors and the complicated cryogenics system resulted in the first CDMS data run in 1996. Although data taken by CDMS I did not lead to discovery of WIMPs, it became clear by 1998 that backgrounds would limit the first experiment and that the detectors could be considerably improved. This formed the basis of the CDMS II experiment, which was funded in 1999, and includes 12 university groups and two national laboratories . Construction began at Soudan in that same year and continued until the first data were collected in 2003-2004 with two towers of detectors. We are continuing to collect data with five towers of detectors. As of December 2009, data showed two possible WIMPs.
11. What happens if you do not detect a WIMP?
Although we would be disappointed, scientific research progresses with both positive and negative results. If we do not detect WIMPS, we will conclude that the current explanations for dark matter are not correct and new theories are needed. Accelerators are also searching for WIMPS, including the collider at Fermilab and the Large Hadron Collider (LHC) at CERN (Geneva, Switzerland).
12. What is the enclosure called in which the detector sits? Why use this type of enclosure?
It is technically called a class-10000 RF-shielded clean room. Even small amounts of radioactivity can compromise our search for WIMPS and all dust contains traces of radioactive elements. So the clean room is needed to keep such dust away from our detectors. For the same reason, we must wear special clothing inside this room. Class-10000 just means that there are fewer than 10000 dust particles of a certain size within a cubic foot of air in the room. Electrical noise can also adversely affect our detectors, so the room has a metal skin and special grounding to reduce high-frequency noise.
13. How (if at all) is CDMS related to the string theory?
Not directly. String theory postulates that all particles are really made up of tiny strings of energy and requires something like Supersymmetry to explain the particles seen in nature. However, if CDMS does detect WIMPS, it will not be able to determine whether they are made of strings because the size of these strings is much too small. Accelerator experiments may be able to determine whether WIMPS seen by CDMS are the same as particles predicted by Supersymmetry.