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Update on SuperK Detector Failure
This note came in from Director Totsuka to the press and other scientists. Hemos and I felt it deserved more than just a regular SlashBack reference, as we feel that this is an important project. (I belive this comes form a translation from japanese, so forgive the errors) this is an update to the original post on the Super-K malfunction.
As a director of the Kamioka Observatory, which owns and is responsible to operate and maintain the Super-Kamiokande detector, it is really sad that I have to announce the severe accident that occurred on November 12 and damaged the significant part of the detector. The cause and how to deal with the lo ss in future will be discussed by newly found committees. However, even before discussing with my colleagues of the Super-K and K2K collaborations, I have decided to express my intension on behalf of the staff of the Kamioka Observatory.
We will rebuild the detector. There is no question. The strategy may be the following two steps, which will be proposed and discussed by my colleagues.
-
1. Quick restart of the K2K experiment.
- (1) We will clear the safety measures which may be suggested by the committees.
- (2) reduce the number density of the photomultiplier tubes by about a half.
- (3) use the existing resources.
- (4) resume the K2K experiment as soon as possible; the goal may be within one year.
- (1) Restore the full Super-Kamiokande detector armed with the state-of-the-art techniques.
- (2) The detector will be ready by the time of the commissioning of the JHF machine.
Best regards,
Yoji Totsuka
director, Kamioka Observatory
On behalf of the Kamioka Observatory staff
A detector for neutrinos. Have a look at their web page.
I attended a talk last night by one of the scientists from the Sudbury neutrino detector. One of their Big Issues at the moment is figuring out why all the best neutrino detectors only pick up a fraction of the neutrinos predicted by all the best theories on the innards of stars.
...laura
Using double the desity of Photomultiplier Tubes allows them to get a better resolution picture of the energy released when the nutrino passes through. They won't get half the pictures, but they'll see them half as well.
It's a good solution for the time being because at least they can take pictures. If they waited until longer to get all the PMTs replaced, then they'd have less pictures overall instead of less resolution for a short period of time.
No, the same number are emitted, but if they have to travel through the bulk of the earth before reaching the detector, it will effect how many you detect. That's true of photons too (you see a lot more of them durring the day, even though the sun emits at a ~constant rate), but here it is even more interesting; the neutrinos aren't being absorbed by the earth, they are being converted between two forms, one of which is easier for a particular detector to detect. So you can wind up detecting more at night!
--MarkusQ
Super-K is basically a huge underground cylindrical tank, about 40 meters wide and 40 meters deep, containing 50,000 tons of very nearly pure water. The sides, top, and bottom of the tank are covered with PMT's, the photomultiplier tubes which serve as detectors in the telescope. They are all pointed inward toward the mass of water, ready to detect the slightest Cherenkov light. (And slight it is—the Cherenkov light generated by the shockwave of a single muon is about as bright to the detector as a single candle seen from the Moon.)
Fortunately, each PMT is sensitive enough to detect a single photon of Cherenkov light. How does it do this? The same way you eat an elephant—one bite at a time. First, the photon hits a photo-cathode on the inner surface of the PMT's glass bulb, and the photo-cathode, in turn, releases an electron. The electron is attracted to a dynode, which carries a high-voltage positive charge, and accelerates toward it. When it hits, its great kinetic energy causes the dynode to emit several electrons, which are attracted to a second dynode with an even higher positive charge. The process repeats once for every dynode in the detector, until the final dynode is deluged with electrons, and sends a signal indicating that it has detected a photon. Neat, eh?
As you can imagine, PMT's are expensive ($3000 each, in this case), delicate, precision instruments, and you don't move them around like lightbulbs on a Christmas tree. Especially if you've recently gone from having 11,242 of them to having only 4,000 or so in one horrific oops.
Yes they will detect it. What you lose by reducing sensors is resolution as to direction and energy.
It's actually rather unlike that they'll miss nuetrino events because of such a change. I've had the oppurtunity to look at individual event plots and raw data, and the Cerenkov light from a single event actually registers in a considerable fraction of the tank. IIRC, typically 5-30% of detectors see each event.
They use the timing of when each detector becomes active to reconstruct the path and speed of the particle generating the light. So fewer PMT tubes means less accuracy in determining the direction and energy of the nuetrino that produced the event. I would guess that it's not the case that half as many tubes means half the accuracy. If I were to make an estimate I'd say you're probably increasing the error on individual measurements by around 30-60% (as opposed to 100%, if it were doubled). This is most important on electron nuetrino events which were somewhat hard to accurately determine to begin with, compared to their muonic cousins.
With only half the amount of sensors - wont these sensors each have more pressure placed on them?
No. Hydrodynamics doesn't work that way.
Wasnt a collapse because of water pressure what caused the initial sensor implosion chain reaction?
Well the machine worked successfully for several years at the same amount of pressure, so this shouldn't be the initial cause of the accident. However it is entirely likely that the pressure facillitated the disasterous chain reaction once some faulty equipment or human error got it started.
This is an exotic size of tube and most of the replacements will have to be manufactured (which takes time), so this is probably the best solution we can expect in the near term.
The real kicker is cost. Solid-state devices cost on the order of $1,000,000 per square meter of active area! PMTs are on the order of $100,000 per square meter. If you want hundreds of square meters of active area -- like in a neutrino observatory -- PMTs are the only way to go.
-- ;-)
Kuro5hin.org: where the good times never end.
If you want to count each individual photon, photomultiplier tubes are the only choice.
In a PMT, a photon hitting the first plate releases an electron. The first plate (cathode) is negatively charged, so the electron flies off towards the less-negative 2nd plate, picking up enough energy to knock several electrons loose. These hit the third plate, knocking out more electrons, and so on. After many plates, the pulse of electrons is large enough to be easily measured, so they are collected and output on a wire at the back of the tube (anode). You can either measure the average current to determine photons/seconds, or detect each pulse to determine when each photon arrived. The super-K uses the latter method, since it has to compare photon arrival times to find the position of the event which created a burst of photons.
The PMT has very high gain and a remarkably good signal to noise ratio. "Gain" is the number of electrons out for one freed electron in, and you just add plates (and increase the overall voltage) until you get what you need. "Noise" would be an electron spontaneously flying off from the cathode, and this is pretty rare.
Solid-state detectors also start with a photon energizing one electron to jump somewhere it wouldn't normally go. Then you need an amplifier. It's possible to build solid-state circuits that will amplify a single electron to a measurable pulse, but to make it that sensitive you must also make it possible for electrons to just tunnel through the first amplifier stage on their own, and this is indistinguishable from detected photons. So it's hard to sort out the signal from the noise.
The main page, www-sk.icrr.u-tokyo.ac.jp has somei gh .html
"press" sized pics, last I checked. Yes, here:
http://www-sk.icrr.u-tokyo.ac.jp/doc/sk/photo/h
It's actually rather unlike that they'll miss nuetrino events because of such a change. I've had the oppurtunity to look at individual event plots and raw data, and the Cerenkov light from a single event actually registers in a considerable fraction of the tank. IIRC, typically 5-30% of detectors see each event.
This isn't entirely true. It depends a lot on the type of event. The pictures you probably saw were either of an atmospheric neutrino event, a cosmic ray background event, or one of the K2K events. In all of these cases, the particle in the detector will have somewhere between 100 and 5000 MeV of energy (and in some cases more).
As a particle travels through matter, it loses energy as it goes. The more energy it has to start with, the longer it will go before it stops. A general rule of thumb is that a muon travelling through water loses 2MeV for every centimeter travelled. So a 100MeV muon produced by a neutrino interaction would travel for 50cm. The higher the energy, the longer the track, the longer the track, the more light produced, the more light produced, the easier it is to see.
Very high energy cosmic ray muons will produce so much light that every tube in the detector will register a hit. A typical high energy event picture is here. Would you see the pattern with half as many pixels? Of course.
The problem is that "solar neutrinos", neutrinos which come from the sun, typically have much lower energies. (Only 1-10 MeV) So low, that even before the accident, SK would miss most of them (anything below 5MeV) because they just didn't produce enough light to be distinguishable from random noise in the dector or the decay of stray radon particles. If you look at pictures of these events, normally you can't see anything by eye. There's just a few photons (5-10) which are recorded which can only be identified as a real event by their timing because you can triangulate back to a single point based on their arrival time at the PMTs. Solar neutrino physicists rarely post event display photos because there's so little to see in them. Even then, it's hard to distinguish solar neutrino events from noise. In fact, it's not possible to identify an individual event as a solar neutrino event and not a radon event that looks like a solar neutrino event. It can only be done statistically. (Radon events don't point in any particular direction, while solar events all come from the sun, so you can compare the number of events coming from the sun and the number of events apparently coming from other directions and do a background subtraction.)
It's these types of events which will be hurt most by the loss of the extra tubes.
If you believe that news is accurate...
BUT - in this case it was... I just read the Official Mishap Investigation Board Phase I Report... turns out that the problem was a small program called SM_FORCES that was to read a table of pound-second figures, while the table provided for the flight was in newton-second figures. Read the whole thing here.
--
Evan "Not too proud to admit when he's wrong"
"$30 for the One True Ring. $10 each additional ring!" -- JRR "Bob" Tolkien