Friday, October 29, 2010

The Coming Antarctic Season

Preparation for the coming Antarctic field season are in full swing, in both IceCube and ARIANNA.

On ARIANNA, Steve Barwick and Jordan Hanson visited LBNL last week, to learn about the station hardware and software. They will be going out to Moore's Bay in early December, to recondition the station, and perform a number of calibration measurements. These calibrations should allow us to set a limit on the flux of ultra-high energy neutrinos. Although the limit from one station will not be very stringent, carrying out the analysis through to the end is a good way to convince ourselves (and peer reviewers) that we understand the system thoroughly.

Unfortunately, even though the sun is now above the horizon 24 hours/day, the station has not yet 'woken up' from the Antarctic winter hiatus. The most likely possibility (also the most optimistic) is that the battery cracked during the winter. There are some good reasons to believe this - the charge controller left it in a discharged state, and batteries are known to be a problem at low temperatures. Alternately, maybe the solar panels are still coated with snow, or, more likely, the Iridium antenna or some other part broke. The worst case scenario would be if the station failed mechanically, and that the solar panels, etc. will be strewn across the Ice Shelf, buried under a winters accumulation of snow.

The first IceCube personnel are already in New Zealand, waiting for flights to McMurdo station. The plan for IceCube is to drill the final seven holes and deploy strings in them, and then prepare the drilling system for hibernation. In addition, two of the strings will include sodium-iodide crystals which will be used to look for dark matter. Although the IceCube project included funding to disassemble the drilling system and move it North (to avoid leaving any junk in Antarctica), the drill has many future applications. In particular, if the dark matter prototype works well, then additional crystals may be deployed, in special background-free pressure vessels.

Wednesday, August 25, 2010

Public Lecture

I gave a public "Sciece@Cal" lecture last Saturday, on "Neutrino Astronomy in Antarctica. I talked about why we want to do neutrino astronomy, and about IceCube and ARIANNA.

Science@Cal is a monthly series of Saturday lectures, intended for anybody who is interested, on various subjects. It was an interesting experience, attracting attendees with a wide range of backgrounds, and, clearly, based on the questions, a wide range of background knowledge.

A video of the talk is available , and the a pdf file with the slides is posted here.

Wednesday, August 18, 2010

Cosmic Rays and their effect on earth

One interesting 'practical application' of cosmic-rays is in how they may affect life on Earth.

The obvious effect is that cosmic-rays are responsible for a good chunk of the background radiation that we experience on earth (the Earth itself is responsible for most of the rest, in the form of natural radon gas, etc.).

Of course, a dramatic short-term increase in the background radiation is dangerous. An overly nearby supernovae or other high-energy astrophysical events would be deadly (but very, very rare); it has been speculated that they may be responsible for occasional mass extinctions.

The long-term effect of lower doses of radiation, as from cosmic-rays is less clear. Cells contain repair mechanisms which can repair chromosome damage if it occurs slowly enough, and experimental studies of low levels of radiation have not found an increase in mutations. But, increased radiation level could cause increased mutation rates, and it has even been speculated that moderately nearby supernovae could have aided human evolution. Of course, in the shorter term, most mutations are not beneficial. It has been argued that the increase in ground-level radiation during reversals of the Earth's magnetic field could lead to increased mutation, and possibly, even to extinctions.

These supernovae leave geochemical 'footprints' on earth. These footprints can be seen in the form of thin layers of otherwise rare isotopes and an increase in carbon-14 abundance. The changing production rate greatly complicates the use of carbon-14 for dating, and elaborate calibration curves have been needed to relate the measured carbon-14 abundance in artifacts with the actual age.

Of course, this is mostly speculation. Interesting and important speculation, but still speculation.

Thursday, June 24, 2010

Neutrino 2010 - conference report

Neutrino 2010 was an interesting conference. There were no earthshaking new results, but there was steady progress on many fronts.

The most interesting new results came from the MINOS and MiniBoone experiments. These are both detectors that observe neutrinos produced by an accelerator at Fermilab, near Chicago. Both experiments are studying neutrino oscillations, whereby a neutrino produced with one flavor (electron, muon or tau neutrino) oscillates as it travels from the accelerator to the detector.

MINOS has observed a possible difference between how neutrinos and antineutrinos oscillate. If correct, this would be very surprising, signalling a big difference between matter and antimatter. Although this result got significant publicity, apparently due to a Fermilab press release, the difference was not statistically large, and almost everyone at the conference was happy to treat it as a likely statistical fluctuation, pending more data. The other anomaly, from MiniBoone is harder to characterize, but is also likely a statistical fluctuation.

Two other popular topics were searches for neutrinoless double beta decay, and progress toward enormous (100,000-500,000 ton detector) next generation detectors.

In neutrinoless double beta decay, a nucleus changes it's atomic number by two (i.e. germanium decays to selenium, or xenon to barium), emitting two electrons and no neutrinos. This is only possible if a neutrino can act as it's own antiparticle, so this would be a major discovery. If this process occurs, it is very rare, with a half live of well over 10**20 (10 to the 20th power) years. So, these experiments must monitor large quantities (typically 100 pounds to 1 ton) of material for long periods, with a sensitivity to observe even a handful of decays. This is not easy. We heard 6 talks on neutrinoless double beta decay, discussing a wide variety of possible methods.

Over the past two years, there has been considerable progress toward a very large detector to make precision measurements of neutrino oscillations. The U.S. version would be located in DUSEL, the Deep Underground Science and Engineering Laboratory, which is proposed to be built in an old gold mine in South Dakota. The Japanese are also pursuing a similar project on an island between Japan and South Korea (the location is chosen to be the optimal distance from the Japan Hadron Facility accelerator), and the Europeans are considering several projects at diverse sites.

My talk, on radiodetection of neutrinos, went well, and seemed well received. It was a tough talk to prepare, since I had to introduce the concept, and also cover experiments looking for neutrino interactions in the moon, and two types of experiments looking for neutrino interactions in Antarctic ice (including, of course, ARIANNA). I also had a chance to talk to a number of people who are interested in ARIANNA.

Although Athens is a very interesting city, June is not the optimal time for a visit. They were having a heat wave during the conference, and temperatures were in the high 90's or low 100's (depending on which source you looked at), and it was also fairly humid. Worse, there was a 3-day metro (subway) strike during the conference. This was quite disruptive, since many of us were taking the metro between our hotels and the conference center. Of course, during the strike, the busses were jammed past capacity, and taxis were hard to get.

This strikes was not an isolated incident; more strikes are planned to protest government cutbacks due to the budget deficit and the economic conditions imposed by the European/IMF bailout. The threat of strikes has trimmed the tourist trade (it is down about 15% according to what I've read), and Athens seemed less crowded than usual. My hotel was not overly full, and a fair fraction of the residents were neutrino physicists. My flight to Greece was half empty, and there seemed to be a number of parked Olympic Air planes at the Athens airport.

Friday, June 11, 2010

Neutrino 2010

It is now June; school is getting out, and the summer conference season is starting. The big conference for neutrino physicists, Neutrino 2010 (it's held every 2 years) is next week, in Athens, Greece. About 530 neutrino physicists will gather for a week, to hear the latest results on everything neutrinos. Talks will cover a results from accelerators (Fermilab, CERN...) and non-accelerator experiments, along with the latest theory.

One hot topics is neutrino oscillations, whereby a neutrino from one flavor (like an electron neutrino) oscillates, over time turning into another flavor, like a muon neutrino. There are three different flavors, connected by three different mixing angles, which give a neutrinos propensity to turn into a different flavor. The three flavors have slightly different masses; the mass differences control how long the conversion takes. There is also a phase angle which, if non-zero, would allow charge-parity (CP) violation in neutrinos. This might help explain why the universe is all matter, with no visible antimatter. One way to study this is to shoot
a beam of neutrinos from an accelerator to a distant detector, and measure the oscillation probability. Another way to study oscillations is to use naturally occurring neutrinos. Neutrinos produced by nuclear reactions in the sun have plenty of time to oscillate before arriving at the earth; this is how neutrino oscillations were initially discovered. Or, one can use neutrinos produced in cosmic-ray air showers, which may oscillate as they pass through the earth on their way to a detector like IceCube.

A number of non-accelerator experiment are looking for a process called neutrinoless double beta decay, whereby an atomic nucleus decays, producing two electrons; the nuclear charge changes by two. For example, ^36Germanium decays into ^36Selenium, plus two neutrinos. This process can only happen if a neutrino is something called a "Majorana particle" which means that it is it's own antiparticle. In any case, the half-life for this process must be very long, well over 10^{22} years, so, one need a very large chunk of germanium to study this.

Neutrino astrophysics is also represented at the conference, with a couple of sessions including talks on high-energy astrophysical neutrinos. I will be giving an overview talk on radio-detection of neutrinos, covering ~ half a dozen experiments, including ARIANNA. It was a challenge to squeeze this all into a 15 + 5 minute (15 to speak, 5 for questions) talk.

I'm not looking forward to the long plane-flight to Athens; this will occupy a good chunk of the weekend.

I will try to post more frequently during the conference, both on conference life, and on new results.

Wednesday, June 2, 2010

A paper!

It's been a while since I've posted. The main reason is that I've been busy. One of the things that I've been doing is writing a scientific paper describing the prototype hardware, and the results of our season.

The paper is finally done, and submitted to "Nuclear Instruments and Methods," a journal that publishes papers on instrumentation for nuclear, particle and astro-physics. There it will be peer reviewed by anonymous referees (most likely 2 reviewers), who will recommend whether it should be published or not. Most likely, they will recommend acceptance, but they will also most likely suggest some ways to improve the paper. At the same time, the preprint version was posted to the Cornell preprint server, as number 1005.5193; click on 'PDF' on the upper right to get the paper text.

It may not be obvious, but writing any scientific paper is a lot of work, even a relatively 'simple' paper like this one. Besides the text, there are figures (graphs, plots, etc) which can take a lot of work to make. There are also many numbers to be checked. When you actually sit down to write a paper, you are forcibly confronted with all of the pesky details that you've successfully avoided over the past few months.

This was true even though we made some specific decisions to speed things up. For example, we do not discuss our on-going data analyses. These are interesting, but there is a lot to do before we're ready to publish these analyses.

The major 'result' in this paper is a measurement of the ice thickness at our site: 572 +/- 6 m. In principle, this is a simple calculation - take the round-trip travel time for the radio waves we bounced off the ice-water interface, and divide by twice (for travel in both directions) the speed of light. Unfortunately, nothing is that simple. In any material, the radio waves travel slower than Einstein's famous unchanging speed of light - that invariance only holds in a vacuum. The reason is that the radio waves interact with the medium. One way to think about this is to imagine the radio waves scattering off the atoms in the ice, so they don't travel in a direct, straight line. The speed of the waves depends on the snow/ice density throughout the trip. The top 75 meters of snow/ice (the 'firn') changes gradually from snow (density 40% of that of ice) at the top, to pure ice about 75 m down. After trying to model this myself, I consulted with colleagues and poked around in the library (now mostly on the internet), and finally found a review written by two real professionals who bounce radio waves through Antarctic ice sheets for a living. Suddenly, it was simple. Their article even included different density profiles for different places in Antarctica, along with estimates of the consequent uncertainty.

Another thing that took some time was getting comments from the other authors, and responding to them. Our paper has 7 authors from 3 institutions; everyone who contributed to the experiment and wanted to be an author. We also got some comments from non-authors, mostly people who were involved less directly.

Most of the comments were good, and strengthened the paper. Of course, a good suggestion (e.g. improve this figure by...) takes longer to implement than a bad idea that can be rejected. And, there is always discussion about expanding the scope of the paper.

With larger groups,dealing with comments can get sticky, especially if people disagree on what should or should not go into the paper (that wasn't the case for us). Large collaborations, like IceCube have formal policies and procedures for internally reviewing papers, collecting and mediating over comments from collaborators, etc.

In the end, I think that the paper came out pretty well. But I may be biased.

Thursday, March 18, 2010

Results from ANITA

The Antarctic Impulsive Transient Antenna (ANITA) experiment has just published their results from their second flight. Like ARIANNA, ANITA searches for radio waves from extra-terrestrial neutrino interactions. However, instead of embedding their antennae in the ice, the antennae are mounted on a long-duration high-altitude balloon. These balloons are launched from McMurdo Station, where the prevailing winds push them in a circle around Antarctica; the data they report is from a 31 day flight in December, 2008, at an altitude of about 35 km (113,000 feet). The picture shows ANITAs 32 antennae, which look for radio waves from neutrino interactions in the ice out to the horizon (up to 650 km away).

After a series of cuts to remove background events, they are left with two events, against an estimated background of 1 +/- 0.4 events - no statistically significant signal is observed. They set a limit on the possible flux of extra-terrestrial neutrinos which is considerably more restrictive than previous studies. Their limit is just starting to constrain models of GZK neutrino production. Clearly, a more sensitive experiment is needed.

ANITAs ability to observe a large chunk of Antarctica in one shot is a big plus. However, balloons have a limited observation time. And, the up-to-650 km separation between the source and the detector inevitably means that ANITA can only see the most energetic neutrino interactions. By placing our antennae directly in the Antarctic ice, ARIANNA will be able to see a wider range of neutrino energies, covering most of the GZK energy range.

Tuesday, February 16, 2010

The IceCube Construction Season

ARIANNA is not the only experiment with a successful construction season. IceCube also had a great season. They deployed 20 strings into the South Pole ice (beating their goal), and finished a week early.

This is a very impressive achievement. 2500 meter deep (1 1/2 miles), 60 cm (about 2 feet) diameter holes are drilled in the ice, and strings (cables) of 60 optical modules are lowered into the hole. The photo above shows the drill head, which is moved from hole to hole. The holes are drilled with a water jet that shoots 200 gallons/minute of water at 88 degrees (Centrigrade) through a 1.8 cm (about 3/4 inch) diameter nozzle at a pressure of 200 pounds per square inch.
Drilling each hole takes about 40 hours, plus another 12 hours for deploying the strings. This photo shows a DOM being lowered down the hole.

LBNL postdoc Lisa Gerhardt has just returned from the South Pole, where she worked on testing the deployed strings. She kept a nice blog. Laura Gladstone (U Wisconsin) also kept a blog of her trip. There are accounts of past trips (with tons of pictures) elsewhere. In view of all of these other accounts, I will be brief and end here.

[n.b. These two photos are from my 2006 trip]

Monday, February 8, 2010

Global Warming in Antarctica?

Several people have asked me about the 'warm' weather that I experienced, and whether this was due to global warming. Others have asked about my photos of liquid water (e. g. above). Is this due to global warming? Did I see global warming in Antarctica?

I've hesitated to answer these questions here, because these are not simple ones. However, with the caveat that I am NOT an expert on climatology, let me tell you what I know.

The evidence for global warming is very clear.

The carbon dioxide levels in the atmosphere have risen to levels far beyond anything seen at least since the last Ice Age, and very probably for the last 400,00 years. For the purposes of considering the future of humanity, I don't think that we need to go any further back. This increase is clearly due to human activity.

Global temperatures are rising, and the last 50 years have been the warmest in centuries. There are many many places where one can compare photos of glaciers taken 50-100 years ago with recent photos; the differences are dramatic. A quick google search found examples from Glacier National Park, elsewhere in North America , and even from Africa's Mt. Kilamanjaro. One can debate temperature trends, the effect of heat islands, etc., but it is clear that these glaciers have formed over long periods, and are now rapidly disappearing. Further, they are far from cities or other local heat sources.

These examples are all from the Northern hemisphere or equatorial region (Mt. Kilamanjaro is 3 degrees south of the Equator). In Antarctica, the evidence is less dramatic, but there are clear signs of thinning glaciers.

Of course, one can find contrary evidence. In McMurdo, I heard a talk about exploring the dry valleys; the speaker pointed out one location where there is now apparently much more ice than in 1904, when Scott visited the site. On balance, however, there is much more evidence for global warming than against it.

That said, what I observed was not evidence of glacial warming. The temperatures during my visit were well within the expected range. The water pools were not anomalies; the snow around McMurdo always melts on hot summer days, forming small freshwater pools. Global warming is real, but one needs careful, long-term study to observe the effects - one visit to an area is not enough.

Anybody who wants to learn more about global warming should visit the website of the International Panel on Climate Change (IPCC). Their reports (available on the website) discuss an enormous volume of climatological data drawn from many sources. Notwithstanding an occasional gaffe, their carefully worded conclusions represent the views of the vast majority of climate scientists.

Thursday, February 4, 2010

Looking at the Data

It's been several weeks since I posted. The major reason for the gap is that my father passed away in early January, and it's been tough. I'm very glad that this didn't happen while I was in Antarctica.

In the past few weeks, we (the royal we - mostly, it has been Steve Barwick, Jordan Hanson, Lisa Gerhardt and Ryan Nichol) have been looking at the data, which is flowing smoothly through the internet link. Mostly, things look very good, and the station continues to work well. This data falls into two classes: housekeeping data, relating to station performance, and triggered data, collected when the antennae see something).

Lisa Gerhardt posted some of the early (through early January) housekeeping data on the web Please remember that not all of this data has been calibrated yet. The temperatures in particular seem low. That said, a few trends are clear. One is that the wind kicked up shortly after we left (starting around Dec. 26th), and again in early January. So, there may be hope for the wind generator. A second is that the diurnal (day-night) variations are present, but small.

The antenna data is harder to describe here, but we are also making progress. Some background might be helpful here. We collect data (and call it an event) whenever the signals on two of the four antennae are above an adjustable threshold. These signals could come from background noise, natural sources, or man-made sources. When a trigger occurs, we record the data for each antenna for 60 nanoseconds (billionths of a second). This may seem like a short interval, but a real neutrino event should produce a pulse that is less than a few nanoseconds long.

One thing that jumped out early was that the trigger rate was partly periodic. We saw triggers every 60 seconds, as expected, These are the 'heartbeat' pulses that we create to check the detector. But, we also see other, unexpected periodic signals. Sometimes, triggers are separated by almost exactly 6 seconds - pretty clearly, a man-made source. The rate of these pairs varies over 24 hours, and we suspect that it comes from the switching power supplies that power the internet hardware. If so, this will disappear when the internet equipment is removed. We also see other events not related to these periodic signals. These might be 'thermal noise' the irreducible background associated with random noise due to molecular motion, etc. This thermal noise provides the 'natural' limit to detector performance. There are ways to reduce it (better antenna, lower noise preamplifier, etc.), but this thermal noise limit is our immediate goal.

Thursday, January 7, 2010


I made it home in 2009, but only barely, arriving at SFO in the early afternoon of Dec. 31st.

The days of waiting for the C-17 were also days of snow, which was beautiful, but limited our outdoor opportunities. The picture above is actually in color! The snow also caused a 24 hour flight delay, from Dec. 29th to Dec. 30th.

The 30th was sunny, which was doubly fortunate because we had to wait outside while the plane landed and then while they unloaded the incoming cargo and loaded the outgoing. The flight was fairly quiet - there were only about 20 of us. The cargo load was also light - my seat had a view of a large hydraulic jack, used, per the stencil, to raise airplane noses. I'm not sure what it was doing in Antarctica, or why it rated air cargo back.

One nice thing about the wait was that we got to see both the C-17 and a Basler land. A Basler is a re-engined, updated DC-3 - a 60+ year old airframe from a 75 year old design! These are the mid-sized transports, between the smaller "Twin Otters" and the larger LC-130's.

We got back to Christchurch around 8 pm that evening, leaving just enough time for dinner and a nights sleep before catching an 8:40 am flight to Auckland. Unfortunately, there was no time for another visit to the botanical garden.

Auckland was a rough return to civilization. First, there was a 5-hour layover. This was followed by an introduction to post-Dec-25th airport security. After the metal detector and X-ray machines, we were frisked and our carry-on luggage was minutely inspected. Quite a contrast from the -17, where the security check was a reminder not to pack anything sharp in your carry-on luggage.

Now that the trip is over, I will post less frequently to the blog. I have a few more trip-related posts, but, in the longer term I want to talk about how our data analysis is going, what we've learned from ARIANNA, and how it fits into the wider world.