1. Detectors First the physical processes of how the particles interact with matter and then the details of how we get electrical signals from the process. We start by asking how can particles penetrate solid matter and what happens when they do?

2. What is a detector? Anything which gives us information about the particles we wish to study. Electroscopes such as Wolf and Kohlhorster took up the Eifel Tower or up in balloons The multitude of devices which make up CMS surrounding an intersection point of the LHC The huge vats of cleaning fluid, along with a system for measuring a few atoms of radioactive Argon, used to detect neutrinos from the sun. Lots of ice and photomultipliers as used in Ice Cube. The ice is part of the detector too.

3. A common thread: How do particles get through matter and what do they do to it as they pass What does a film only one atomic layer thick look like to an incident particle? Hitting an electron is generally no big deal but hitting a nucleus may be serious. (Demo, different size balls) The electrons contribute a sort of friction and gradually slow the particle, but also make side effects revealing the passage of the particle. Nuclear collisions can be dramatic as in the cloud chamber pictures yesterday. So is a particle incident on our single atomic layer likely to hit an electron or a nucleus? The radius of an atom is larger by a factor of 2 to 5 x10^4 than that of the nucleus. Let’s see what that means

5. Pretend atoms and nuclei are cubes so we can make simple pictures The projected area of an Al atom is (2.4e-8)^2 = 6.2 e-16 sq cm. The projected area of the nucleus is (6.92e-13)^2 = 4.8 e-25 sq cm. Fraction of area covered by nuclei is 7.7e-10, and a stack of 1.3 e9 layers = 31 cm of aluminum, arranged with no overlap, would make the nuclei just cover the slide so that a particle perpendicular to this plane would have to hit a nucleus to get through. If layers are arranged at random, 1/e is NOT covered. Is this consistent with results from our counters?

6. Not hit a nucleus is dotted line

7. Extending our straight line gives

8. So even our simple counter shows that muons can pass right through a nucleus! The elements in a brick have about the same atomic and nuclear sizes as Al, so our 5 bricks should let only 1/e = 0.37 pass through without interacting. 295/314 = 0.94 Mines or deep water provide much more than 5 bricks of cover. Are there cosmic rays down 4 km into the ocean? I worked on one such measurement and as preparation we needed to measure bioluminescence at depth.

9. Picture from deck of Ship

10. Second DUMAND Ship

11. DUMAND Beamline

12. Lowering Phototube String

13. Conclusion: The chance that a muon can penetrate to 4 km WITHOUT passing through a nucleus is 10^-3500 We measured at 2 km and 4 km depth. 4 km is 8000 of those 1/e distances, so the fraction which could get down there without a collision is e^-8000 or about 10^-3500 Our measurements are consistent with others and compared to sea level: Sea level 1 km 2 km 4 km 10 km 7e-3 1e-6 1e-7 5e-9 5e-13  -- 10^-6 observed --  vs 10^-3500

14. Why are there fewer muons deep in the ocean than at the surface? Those electrons we brushed aside are to blame: “Friction” loss of energy is 2 MeV per cm. A kilometer of water is 10^5 cm and so muons with energies of less than 2x10^5 MeV = 200 GeV will slow down and stop in a kilometer of water. The fact that muons are observed at 10 km means that there are 2 TeV muons at the surface!

15. A piece of history left out yesterday. The remarkable penetrating power of cosmic rays and rough mass measurements were known early on in our saga. The need for a strong, short range (strong) force was obvious. The quantum theory of the electromagnetic field (QED) involved photons and was a phenomenal success. Yukawa generalized QED to the nuclear force and predicted a mass about the same as the mass from cosmic rays – but it interacted stongly with the nucleus. A non-interacting particle should not be made in nuclear collisions. The pion was discovered a bit later. It is made in nuclear reactions and it decays into a muon and neutrino! Most cosmic rays at sea level and an even larger fraction under a kilometer of water are decay products of the pions made in the nuclear showers such as you saw in the cloud chamber pictures yesterday.

16. An idea for a detector! Put a few meters of water in the particles path. If it gets through, it can pass right through the nucleus – It does not “feel” the nuclear force. (1 meter of steel is equivalent to about 5 m of water. That steel can often do double duty also as part of a magnet, as in CMS.)

17. All charged particles lose energy to electrons as they pass - friction A singly charged particle with speed greater than 0.95 the speed of light looses about 2 MeV energy to the electrons in passing through a cm of water. Slower particles lose energy more rapidly. This energy given to the electrons is the basis for our detectors. Most of the electrons are bound in atoms: excitation or ionization.

18. What about electrons? All charged particles lose energy gradually by this “friction” mechanism. Strongly interacting particles also lose energy dramatically by hitting nuclei. Electrons and photons have an additional way to lose energy: radiation and pair production. Lets explore these.

19. How does a radio (or cell phone) antenna work? A steady (DC) current makes a constant magnetic field. An alternating current (AC) also produces radio waves. Accelerated charge radiates. When a high energy electron goes past another electron or a nuleus, the electric force accelerates the electron and it radiates. At high energy the radiation is likely to be a single photon – which often has a significant fraction of the electron’s initial energy. *****Heavier particles are dramatically less likely to radiate.********

20. What does a high energy photon do? Pair production dominates above a few MeV. (Compton below a few MeV) A high energy photon passing near an electron or a nucleus turns into an electron and a positron both going very nearly along the direction of the photon. The probability this will happen is proportional to the square of the charge of the target particle. Much more likely near a uranium nucleus than near a proton – by a factor of 82^2. A relatively thin shell of uranium will make most electrons produce pairs while strongly interacting particles produce only a few nuclear interactions.

21. Summary of physical properties of detector materials All charged particles lose energy by “friction”. Muons don’t lose energy (significantly) any other way. Strongly interaction particles (hadrons - particles made of quarks) lose energy by nuclear interaction, often making more hadrons which interact to make a shower of hadrons. Electrons turn into photons which turn into pairs. Each electron or positron radiates photons – a shower is born. In material of high atomic number, the electron showers develop and die out very much more quickly than hadron showers.

22. What have I meant by “friction”? The interaction with atomic electrons causes a particle to lose energy – which is transferred to the electrons, and this disturbance of the atomic electrons is the source of the electrical signals we record and feed to our computers to analyze the event and to make the pictures you have seen.

23. Kick an electron and it may go to a higher shell or may escape entirely

24. Kicking an electron clear out of the atom is ionization. We have seen the droplets and bubbles which form on the ions. The ions can be “collected” as in Geiger tubes, proportional tubes, or silicon (transistor) detectors. An excited atom emits a photon and that is the basic process in the scintillator paddles of our muon counters. A third process is Cherenkov radiation, the production of a shock wave of light analogous to the sonic boom sound wave of a jet.

25. Ionization, pixels, prop tubes, our visual detector Pixel – a transistor which has an input signal of electrons produced by the passage of a charged particle in the silicon a few hundred electrons are produced and that can be turned into a useful signal by an amplifier of manageable cost – even for millions of pixels. But too expensive to use for large areas. Prop tube – filled with gas. A few tens of electrons. Much cheaper than layers of silicon, but too little charge to detect with a reasonable amplifier – gas amplification needed.

26. Excitation – The scintillator paddles in our muon counters The disturbed atom radiates low energy (a few eV) photons. Unfortunately most of these photons are ultraviolet (like those that make sunburn, skin cancer) and these are absorbed in a few mm of the plastic. Addition of a substance with large organic molecules captures the uv and reradiates it as visual photons, where the plastic is much more transparent. These second generation photons bounce around in the plastic and some hit the photomultiplier at the end.

27. What does the photomultiplier do? It is misnamed – it first converts photons to electrons and then multiplies the number of electrons. A photon hits the photocathode and simple Einstein photoeffect ejects an electron (about 30% of photons make photo electrons). A series of carefully placed and shaped plates and successively higher voltages direct that first electron to hit the first plate where it ejects 3 or 4 electrons which are directed to the next plate where each of these 3 makes 3 or 4, etc.

28. A phototube typically has 10 or 12 such stages and turns each initial photoelectron into a cascade of a few million electrons. That is a manageable electrical signal which is fed into logic cards. These cards look for instances when several of the attached scintillator paddles gave a signal at the same time, and those are the events of interest.