Muon Telescopes Attack…Part IV

For reasons unknown, this article string has the largest number of pageviews for the last month, July 2011. My plan is to cash in on that kind of popular, 5 hits per day OR MORE, for my big comeback. 

I have updates. Mark H has been diligently working on this project in an effort to get some kind of degree in physics or something. The results are amazing. It turns out that if a person takes a series of muon detectors that may or may not have been used in a major neutrino detector experiment located at the south pole…it turns out, that guy sees a relationship between muon detections and elevation and detector orientation. There may also be a relationship between muon flux and the annual calendar.

Amazing...Thanks Mark. Your thesis is awesome.

This kind of measurement also takes a long time. The kind of timeframe that yields graduate-level results in muon detections is on the order of months. So, the next level of measurements is taking a stacked scintillating muon detector and rotate it relative to vertical. This should show the vector relationship between high energy events in the upper atmosphere and detection events at the ground level. The original experiment of separating the detector paddles and looking for a "field of view" relationship between the muon detectors and occurrence was also never completed. Why is that...Mark?  

Go ahead and do this particle physics types, I would appreciate it, and it looks like it will take a long time, so get on it.

There are people in Australia, Spain, Germany, the UK, Russia, and India who have been interested and at least glanced these posts. This is amazing and I hope to someday hear from you people.

Cheers,

John

Muon Telescopes...Attack! Part III

I got excited about Muon Telescope design because I think that the muon flux, and the neutrino-detection noise, will vary significantly at DUSEL. The variability is due to the topography and the geology of the ground above the 4850 level campus where the first muon telescopes could be installed. Today, I want to look at a couple more parameters that could be important.  

Here is a cross section of the mine showing geology. I pulled this from the DUSEL.org website which is a fantastic resource for DUSEL information, including full publications of the geology of the Northern Black Hills (shh, don’t tell anyone). http://homestake.sdsmt.edu/Resources.htm

Here we see the very complicated geology that was encountered during mining. The complicated folding and squeezing of the various layers occurs in all three dimensions with more than four large scale deformation events. Each layer has a different geology, and one would expect a different meters of water equivalent (M.W.E.) shielding capacity. In this figure the topography does change, I included a line that shows the shortest path to a hypothetical 4850 level campus is not vertical, but is in fact about fifteen degrees off vertical. However, the difference in length between those paths is small at <10%. I also drew another muon path at about 45% off vertical that illustrates the much longer path through the rock that it must travel. That additional travel time in rock will increase the odds of a muon interaction with a mineral particle, and thus the increased odds of attenuation for any particle coming in at an angle.  The little inset diagram shows a Gaussian curve that should roughly describe the distribution of muon flux from vertical (roughly maximum) to horizontal (minimum).  I might expect to detect muons even if the muon telescope was oriented horizontal, possibly due to the curvature of the earth or the persistence of the particle.

As we have discussed before, a muon telescope is two scintillators with photomultiplier tubes to count the photons coming off the scintillator when a particle interaction occurs.

The geometry of the telescope is simple. The two scintillators are separated by some distance. The size of the detectors and the distance between them determines the maximum angle off of vertical, theta, that an incoming muon could create a signal on both detectors. Another way of saying this is familiar to photographers, theta is proportional to the field of view of the telescope.

The further apart the scintillators are spaced, the smaller the field of view - primarily because the odds of an incoming muon will have the correct angle to hit both scintillators. Both scintillators may get the same number of hits, but by measuring the time interval of the hits, single muon particles can be traced and differentiated from decay of,  say,  thorium on the cavern walls that spews out random ions that could be detected on both scintillators at roughly the same time.

Lets look at it using a fancy Venn Diagram.

Both scintillators are triggered all the time by random ions, cosmic rays, and lab gremlins all the time, some we want to measure and some we don’t.   The ratio of good signal to bad signal is bad in both cases, but if the telescopes are separated, the magnitude of bad signal is small, but so is the magnitude of the good signal.

So, is there anything we can do to optimize the design of the telescope so that it minimizes random noise and loss of signal because the muons decay between scintillators? There are a number of parameters we can plot up to look at this question. Using the muon telescope cartoon, we can come up with the math to solve for the muon flight time between detectors and also look at the angular field of view. The angular field of view should be proportional to the muon flux with some influence by that flux distribution that we showed before.

On the left axis, the travel time of a muon is plotted. For the case where a muon is coming down vertically, the wider the distance between scintillator paddles, the greater the travel time – it is a linear relationship. However, consider the maximum diagonal travel distance a particle could travel and still hit both scintillators. This differential distance increases as the paddles are brought closer together and as the paddles are increased in size. In this figure, the largest paddles placed one paddle-width from each other on the low end to find the maximum differential. The maximum travel time has a variance of more than 100% and it is actually larger than the 5 ns detection limit Mark told me about a while back. In that case, a muon travelling diagonal would not have been counted. However, the effect quickly drops off with smaller paddles separated by larger distance, maybe 4x the paddle diameter could be negligible depending on the required accuracy.

On the right axis, the field of view (FOV)  in degrees is shown. The closer the paddles are together, the larger the FOV. As the paddles are separated, the FOV quickly drops away and approaches zero as the paddles are separated.  The paddle separation should be selected to accommodate the shortest muon travel path. Since the FOV is proportional to flux, the farther the paddles are separated, the smaller the muon detection and the greater integration time required.

Finally, the decay time for a muon is about 2 µs = 2000 ns. The longest travel time between scintillators shown on this figure is about 35 ns. The odds of a decay occurring between detections is not zero, but the travel time is small compared to the decay time. It is a bit unclear to me if the 2 µs value is at earth frame or if it is particle frame dependent. The effects of relativity will have the effect of increasing the apparent decay time from the earth frame and, thus, overestimating the number of muons that would be counted.

Rocking the Neutrino Laboratory like its 1999 - an early tour of the first Homestake Neutrino Lab

One of the highlights of 1999 for me was a tour of the Ray Davis Neutrino laboratory at the 4850 level of the Homestake gold mine at Lead, SD.  It was a special tour set up by one of the miners and a physics professor at the South Dakota School of Mines & Technology. My good friend and fellow student, Mark was also there.

At that point, the mine was still operating, although it was late days and everyone knew that it was going to be shut down soon. Mark and I had just graduated from high school and were starting the first year of undergraduate education at Tech.

The mine has several access points. We started at the Ross headframe in Lead. There was a safety briefing at the beginning and everyone was assigned a helmet, eye protection, and steel toed boots. Mark's father, Jim worked at the mine and had previously outfitted us with old timey head protection. Once outfitted and organized, we caught a ride to the 4850 level on the hoist.

John and Mark ready to go

Accessing the lab involves hiking from the hoist access through a large tunnel for something like a half mile. The ventilation system and ore carts meant that the hike was warm or cold and we were constantly hugging the wall to avoid getting squished by miniature ore trains. There was no light, except for the headlamps and lights on the front of the passing ore carts. The tunnels were mostly dry, with occasional wet spots dripping water and making puddles. Ventilation pipes, water pipes, electrical wires and other infrastructure lined the ceiling. All the blowing air and moving equipment fill the tunnels with a roar of sound.

The lab is accessed by a smaller tunnel that shoots off the side of the main tunnel. There are, of course, threatening signs warning off miners without permission and, apparently other trespassers.

The first thing you notice when entering the lab is that it is mercifully quiet and racks of fluorescent lines buzz away, lighting up the facility in ghostly green light. The first room is a laboratory space with chemistry apparatus lining a wall and stacks of lead bricks. Old computers with orange and black screens cast a strange glow.

The strange and wonderful chemistry setup at the 4850 level.

The detector room itself is a large cavernous space almost completely filled with a large steel tank and numerous gray cask boxes about 1 foot by 1 foot by about four feet long. The steel tank is the main neutrino detector and is filled with perchloroethylene, dry cleaning fluid. The casks were scintillator detectors filled with a mineral oil.

Mark, safe from eye injury, but vulnerable to the rocks we were trying to drop on his head the entire trip. The big grey tank under his hand is the main detector.

Scintillators stacked above the main detector in the Davis cavern. 

The way it works is, neutrinos travel through the earth down to the 4850 level and a few of them, maybe one per month, interacts with the chlorine in the perchloroethylene tank. When that interaction occurs the chlorine ion is transformed to an argon isotope and a flash of light is emitted (I think). I think the light is detected by the scintillators. But the most interesting things is how the argon isotope is detected. Periodically, maybe once a month or once a quarter, they filter the entire huge tank of perchloroethylene for argon isotopes. The exact method is unimportant and a mystery to me, but Ray Davis somehow managed to develop a method to distill and collect just a few atoms from a tank that holds one hundred thousand gallons of fluid. The individual atoms of argon isotopes are counted up and used to determine how many interactions occurred in that period.

It is a truly remarkable feat that Davis managed to do this experiment in an active mining environment over many years with a precision good enough to tell that there were not enough neutrinos detected within a factor of three. This result first proved that the sun was the source of neutrinos, the first direct proof of fusion in the sun (as opposed to coal, an actual scientific hypothesis that survived into the 20th century). However, his work, specifically the smaller number of neutrinos they detected turned out to be one of the lines of evidence supporting the idea that there are multiple types of neutrinos and that they oscillate, or change flavor as they travel, and also that they have a small but non-negligible mass. This complete hypothesis for neutrino oscillation came about with a bunch of other work at Japan's Super-Kamiokande and Canada's Sudbury Neutrino labs. Ray Davis deservedly shared the Nobel Prize for this work in 2002.

Left to right, Mark, Prof. Bob Corey from SDSM&T, Jim Hanhardt, and (I think this was Ken Lande from U of Penn., but his current picture looks nothing like this guy. He was really interesting to talk to and the entire experience was inspirational about physics. Too bad I don't remember his name for sure). 

To finish the story, it was announced that the mine would be shut down on September 11, 2000, about a year after this tour. In 2002, Barrick purchased the Homestake mining company and agreed in principal to donate the facility to the state for use as a National Underground Science Laboratory (NUSEL). At this time, the mine went into care and maintenance mode, high grade mining and milling, and environmental cleanup. In June, 2003 Barrick closed the mine by shutting down the pumps and the lower levels were allowed to flood with natural ground water. The Ross and Yates shafts were sealed and access to the mine was effectively shut down. In 2004, the state of South Dakota made funding available to begin the process of converting the mine to a lab. This work required refurbishing the access and pumping out water. In March 2008, pumping began at the 4600 level with a push of a button by the Lab Director, Jose Alonso. The water level dropped down to the 4850 level in May 2009, thus opening the Davis Cavern up to access for eventual construction of the Interim Laboratory.

For more information on the current laboratory efforts, follow this link to the Sanford Underground Laboratory at Homestake, DUSEL.

Muon Telescopes...Attack! Part II

Cosmic rays flying through space with a tremendous amount of energy enter the earth's atmosphere and collide with the sparse matter in the upper atmosphere. A shower of particles emerge from the collision, and even more particles are made from subsequent high energy collisions. These particles travel through the atmosphere in a fraction of a second retaining a large amount of their original velocity. Below the ground surface, daughter products from these collisions called muons penetrate deep into the earth. And there they can interfere with sensitive measurement devices intended to detect neutrinos. Here, I want to look at the way that particle physicists observe this muon flux in order to subtract that effect out from their neutrino detection efforts.

In researching this topic, I had a few specific questions that I wanted to answer while thinking about muon "telescopes".
-What is the velocity of muon particles, and how would the effect the size/configuration of the telescope?
-What is the approximate flux of muon particles that might be detected?
-Are there any other considerations that might influence a telescope design?

As an amateur scientist without access to expensive "Peer Reviewed Journals" I am forced to use random undergraduate term papers and wikipedia to determine my facts. In this case, I found a great looking MIT undergrad physics paper. "The Speed and Lifetime of Cosmic Ray Muons" by Lulu Liu, an MIT Undergrad, dated November 17, 2007 is worth the read if you have any interest in this stuff. (http://web.mit.edu/lululiu/Public/pixx/not-pixx/muons.pdf Warning, PDF)

The velocity of muon particles is relativistic, at about 99% the speed of light. The relativistic speed of the particles change the apparent decay rate and so the flux is actually higher at the ground surface than one would expect. Does the increased mass of the particle change how the system works?

The flux they reported was 20 muon counts per second at sea level. This flux would likely be decreased by placing a detector in a deep lab with significant geological shielding above. My next question is what factor of decrease that shielding will provide? Also, is this true flux, or is it only the flux that was counted? What percentage of the muons pass through the scintilators without an interaction?

The relativistic effects on expected flux is significant. Because the muon is travelling at a speed greater than 99% the speed of light, relativistic effects have an impact on the apparent decay rate. If we just had some muons lying around they would have a certain decay rate. But because the very high velocity of cosmic ray daughter particle muons relative to our frame, the muon experiences less time in a trip through the atmosphere than we do measuring time from the ground. The muons would decay less frequently as measured from our frame and we would measure a larger flux at sea level.

Another scenario where relativistic decay could impact muon measurements is during the measurement itself. You would expect a certain number of muons to decay between the two scintillators, which will reduce the number of interactions that would be counted as a muon. But, because of those relativistic effects, there will be a higher count of muons that will be detected.

So, how quickly do we need to measure pulses in order to determine if a pulse is a muon? Using the number for muon velocity provided by Liu, ~30 cm/nanoseconds, we can estimate that scintilators separated by 10 m will require a time resolution of 33 ns. The frequency of a measurement that can time resolve a 33 ns event is about 33 Gigahertz. Agilent, the most expensive oscilloscope make I know lists a maximum frequency measurement of 1 GHz, "The World's fastest uncompromised update rate: up to 100,000 waveforms/sec" making this measurement impossible. There must be some kind of trick that they use to make the measurement or they have really good oscilloscopes at these labs.

This is a super-interesting, fiddly measurement that only the most dedicated and downtrodden interns and graduate students can be successful at. I look forward to seeing this work advance at DUSEL over the next few years.

PS I love how all of the papers I found had the same complaints about bad PMTs as Mark. Apparently they are very flaky, personality ridden devices.

Muon Telescopes...Attack! Part I

The last time I was in Rapid City I got a chance to visit my friend's physics lab at SDSM&T. Mark Hanhardt showed me his work detecting incoming particles generated from cosmic ray interactions with the atmosphere. He does this with an uber-sensitive photomultiplier tube that can measure individual photons. When I was there, I noted the suspicious absense of duct tape. That is simultaneously comforting and disturbing as his work will eventually become part of the research lab at the Deep Underground Science and Engineering Lab at the Sanford Homestake Laboratory in Lead, SD.

Mark's Lab - Note how time and space are visibly distorted there. Two photomultiplier tubes are visible on the left, middle - they are the gold colored glass bulbs with grey bases in the box. 

Mark, the FLOEAS resident particle physicist, had a response to the DUSEL muon flux shielding questions that came up a while back:


I have two notes about the work you did to calculate the mwe: First of all, I may have just been really tired when I read one of your calculation emails, but I don't think that comparing the relative densities of different types of rock to water will work.  I'm probably overthinking this, but when we are talking about muons, we are talking about charged particles, so it's not just the mass of the barrier, but also the charge distribution, structural lattice, and other atomic considerations. Then again, as I type that out, I think those things may smooth out enough over large distances that maybe my objection is moot.  Anyway, my second note would be this: I think that the muon rate is so easy to measure empirically that it's entirely possible that when people do mwe estimates, they simply let it be equal to some close, round value of the depth of rock (adjusted by experience) until someone actually measures it with a muon telescope or other apparatus to update the estimate. 


Yes, muon flux can be measured directionally.  I have been so focused on trying to finish my PMT work, that I have had no time to study the muon telescope I will be installing at Sanford Lab in January, but as I understand the proposed design it will primarily consist of two separate scintillation/PMT devices placed tens of meters apart in the line in which we expect the muons to travel.  (The underlying concept is that a muon is confirmed detected when we get a coincident signal in both detectors.)  As such, I expect that by placing these two devices along different lines we can measure muons incident from a different direction.  Given good enough time resolution, I also believe I can tell if the muon is traveling parallel or antiparallel to my device-line, although it's something I don't think my professor has considered since we will probably just assume that the bulk of the muons are traveling downward.


Mark is a great guy. I hope he finishes his physics degree, because he is my friend and not because of the monster it will make him. Only getting a PhD and becoming a tenured professor could make his ego any bigger, but his belligerence is boundless.

So, scintillators and photomultiplier tubes are arranged to detect incoming muons. The scintillators are pieces of plastic that emit a flash of light when struck by ionized particles, such as muons. The flash of light is detected by the  photomultiplier tube, converted to an electrical pulse, and amplified hugely to a voltage that can be measured by an oscilloscope. There are two sets of these scintillator/PMTs separated by some distance, tens of meters as Mark says. When an incoming particle strikes both scintillators (nearly simultaneously), scientists know it is a muon as opposed to some other type of radiation - say, decaying thorium in the nearby rocks which would only strike one scintillator. This apparatus is called...wait for it...a muon telescope!

Mark is saying that if one were to place two sets of these detectors at right angles, you could tell the difference between cosmic rays incoming from the atmosphere and random flux. This would help answer my question about heterogeneous muon flux because of changes in topography or overlying geology. The only catch is you would have to rotate this big, 30+ foot long apparatus inside a relatively small underground cavern long enough to gather enough muon measurements. I would even argue that the perpendicular apparatus is not necessary to measure flux.

That's it for this one. I have a bunch of detailed questions about how this stuff works and attempt to answer it in the next post, Part II.

Bacteria in Deep Rocks, but not really Deep Rocks

Life is found in deepest layer of Earth's crust - environment - 18 November 2010 - New Scientist

www.newscientist.com

An expedition to the deepest layer of the Earth's oceanic crust has revealed an ecosystem living over a kilometre beneath our feet

Bacteria were found in a deep drill hole where life was not expected. Now Sam sent this to me and I think I read the abstract that day too. This guy Sam does some kind of computery work and he rides a bike, but he definately has a sensitive science antenna because it clearly quivered enough for him to recognize that this is a potentially important article.

The idea of microscopic life surviving, and even thriving, in pore spaces or tiny fractures deep underground is not new. Dr. Tom Kieft has been doing this for some time (http://infohost.nmt.edu/~biology/people/faculty/t_kieft/research.html) and his group has been going much deeper than the New Scientist story. The difference is that Dr. Kieft has started at the bottom of deep mines in South Africa (with plans to do similar studies at DUSEL in South Dakota) and drilling down to access even deeper depths. The depths his group could theoretically reach with the infrastructure of DUSEL make the New Scientist study look like they just scratched the surface...and they expect life as far as they can drill. 

So, what's the big deal? They say in the article that they didn't expect to find life in this Gabbro layer, which is right above the mantle. However, careful study of the little cartoon shows that they just hit the very top of the gabbro layer. The substantive difference between the basalt and the gabbro is minimal. There is a relatively small geochemical difference and the crystals are bigger. Both will possess very similar porosities and minerals that the bugs can use to survive. I think as long as the temperatures are low enough to allow proteins to function, there is sufficient chemistry for microbiota to process, and water, you will find life in earth. And life seems to find ways to push the limits of temperature, chemistry, and water that we think will allow them to survive. 


There was one little gem that they mentioned but did not elaborate. They only found bacteria, but no Archaea. Now Archaea are a type of single celled microscopic bug that look, smell and taste just like bacteria, which is why they were not included in the textbooks until recently. For example, Archaea are not to be found in my high school text books. But recent advances in cell culturing, genetic sampling of ground up DNA, and a phylogenetic analysis indicate that Archea are distinct from bacteria and our branch of the tree, eukarotes. Archaea are typically known as the extremophiles that have the ability to live in super-high temperatures or chemical environments that kill off all the other types of life. They also do a lot of interesting geochemistry that allows them to not only survive, but use poisonous gasses and chemicals to gain energy. 

Why, why, why are Archaea not present in the basalt layer? What kind of genetic advantage do bacteria have that allow them to live so deep? Is it possible that the Archaea simply were not detected? The authors do include a hypothesis that the bacteria migrated from oil reserves. But is it reasonable that bacteria would go from a very high hydrocarbon environment to a very sparse hydrocarbon environment? Why would Archaea not be capable of making the same evolutionary step? 

Understanding the evolution of microscopic single celled life on earth is important because it holds the spot at the root of our tree of life. You could also imagine that better understanding the conditions that these critters live and evolve under will be helpful when we investigate other planets, like Mars and Europa, for life. And that is exciting. I think there are some big holes in this particular story that are open to the researcher community. It will be interesting following this story over the next few years.

Deep Underground Science and Engineering Lab - Topography Shielding Part III

The best way to calculate permutations for the endmember cases of muon flux at the DUSEL 8000 ft level campus. 

Best case: Say the entire depth of the lab is a full 8000 feet below the lowest ground surface. Also, assume that the rock is very dense Iron Formation with a specific gravity of 6.0. The calculation is simple, 8000ft of rock * 6.0 g/cc / 1g/cc water / 3.28 ft/m = 14,600 MWE, meters of water equivalent. This is more than double the estimated MWE.

Worst Case: Assume that the 8000 ft campus is actually 7000 feet below the ground surface because of topography. Also assume that mining methods of removed about 75% of the in-place rock and backfilled it with sand. This sand would be largely dewatered by gravity drain effects and have an air void volume of about 30%. Any water that remains trapped in the pores of the rock will increase the effective density, but we ignore that here. Finally, we could assume that the rock would be a granite-like composition with a specific gravity of about 2.85. The in-place rock would be 1,750 ft thick * 2.85 g/cc Qtz / 1 g/cc water / 3.28 ft/m = 1,520 MWE. The sand would have a thickness (7000 ft total depth-1750 ft in-situ depth)*(100%-30% sand volume) = 3675 rock in-place equivalent. The rock-in-place equivalent should have a similar specific gravity so: 3675 ft * 2.85 g/cc Qtz / 1 g/cc / 3.28 ft/m = 3190 MWE. The two values are additive so 3675 + 3190 = 6865 MWE. This is only slightly below the estimated 7000 MWE for the 8000 ft level. 

The best case scenario is more than double the published estimate for the MWE. This estimate is also likely a significant overestimate because the iron formation is not largely extensive around Homestake, it is some fraction of the total thickness of the rock over the 8000 ft level campus. 

The worst case scenario is close to the estimated value, suggesting that the DUSEL folks were being conservative. I can not imagine a case where the ground below Lead would not be actively caving in with a lower MWE (although it is subsiding). In the muon flux shielding world, more is always better, so I can confidently conclude that the the MWE shielding estimate is a good one and likely to prove more effective when the lower campus is built.

Deep Underground Science and Engineering Lab - Topography Shielding Part II

Meters of water equivalent, m.w.e. is the unit of shielding used to describe how much muon flux can be filtered by placing an underground lab at a certain depth. I did a calculation looking at the mwe reported for the DUSEL. DUSEL's main cavern will be placed at approximately 8,000 feet below ground surface and the mwe is reported as 7,200. With a little unit conversion mathemagic, the specific gravity that the scientists used was 2.95. The density of water is 1 gram per cubic centimeter and specific gravity is the factor of times more dense than water. So, the scientists that estimated the mwe for Homestake used some kind of logic that assumed the rock above the lab was 2.95 times the density of water. 

This is interesting for a few reasons. 1) The 2.95 is close to the average density of continental material, the kind of thing that a physicist would look up in a book. 2) The Homestake formation and the surrounding units are highly metamorphosed greenschists, quartzites, and iron formations. The specific gravity of these units, in place, is about 2.85 on the low end for relatively light quartzite up to 5 or 6 for heavy iron formation. The estimate for specific gravity may be low by a bit under a factor of 2. 3) The 8,000 foot depth is based on a mine coordinate system that has a zero elevation somewhere near the top of the open pit (the zero point was mined out when the Homestake company opened up the pit in the 1980s). Anyone who has been to Lead knows that those hills are as much as thousand feet high. The actual depth from the collar of the shaft is much different than the depth from the Kirk trailhead to the 8000 foot level. And 4) the mining method was vertical stope retreat where large blocks of material were mined away and then refilled with a sand slurry. This fill material could be a relatively large fraction of the material between the lab and the ground surface. All of these factors are sure to introduce uncertainty in the back of the envelope mwe calculation. 

Instead, I would suggest using the block model that Barrick trusted to the state to estimate mwe. The computer model includes all the geologic formations, known rock density from drill core, and accurate 3d locations of the drifts and ground surface. These parameters could be used to estimate the mwe for any location that was sampled with diamond drill holes or mined during Homestake's operational period. The alternative would be to characterize the muon flux by accessing the opened caverns and directly measuring the flux. A final option would be to measure the gravity at each level and use the results to determine the effective density of the ground between levels - this method would provide a better understanding of the backfill and dewatering which could change the density after geologic modelling was completed by Homestake. 

I think tomorrow I want to look at the permutations of the best and worst case scenario for specific gravity of the ground above the DUSEL caverns.

Deep Underground Science and Engineering Lab - Topography Shielding

There is a figure that the Deep Underground Science and Engineering Lab (DUSEL) guys were bandying about back when the NSF was making its initial decision on which underground facility would best suit an underground neutrino detection lab, maybe seven years ago. The figure showed Homestake mine was the deepest candidate with operational facilities at 8,000 feet below the ground surface. This was considered favorable because the deeper the lab was buried the more cosmic rays would be filtered out. This was good because those pesky cosmic rays interfered with the neutrino detectors (the neutrinos were not affected by the earth above). 

My question is how the topography of the ground and the density of the ground change the shielding properties of the earth above the neutrino lab? I know this work has been done already to some extent. I had a conversation with Bill Roggenthen who told me Wick Haxton at U. of Cal Berkely did the theoretical work and Nicolai Tolich at U of Washington looked at the topography issue. But I still don't think anyone has looked in detail at the properties of the rock surrounding the lab, which could impart a directional shielding effect. 

Talking to my friend Mark, who is helping with a little experiment called LUX at the DUSEL, he says the important thing to look for is how many Muons can be shielded with depth. I told him this would be a problem since I did not believe in Muons, but he insists they really exist. So, if anyone is interested, I want to look into the question and you are welcome to join me. It is work that is likely already done or could be tested in a few years when the lower levels of the lab are developed and the muon background is directly measured. But it is a fantastic question and probably will impart lots of edumacation in the process.