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Det User Thin

  1. 1. QuarkNet/Walta/CROP Cosmic Ray Detectors User’s Manual Jeff Rylander and Tom Jordan, Fermilab R. J. Wilkes, Hans-Gerd Berns and Richard Gran, U. Washington August 2004
  2. 2. Classroom Cosmic Ray Detector User’s Manual II
  3. 3. Classroom Cosmic Ray Detector User’s Manual QuarkNet/Walta/CROP Cosmic Ray Detectors User’s Manual This manual provides information for setting up and using a cosmic ray detector with the QuarkNet Version 2 Data Acquisition (DAQ) board and ancillary hardware. It serves both beginning and advanced users. Much of the technical information was originally compiled in a user’s manual written by R. J. Wilkes. Hans Berns, Rik Gran, Sten Hansen, and Terry Kiper contributed some of the technical documentation with input from the practical experience of many beta testers.1 Although some of the components of your cosmic ray detector may have come from various sources, the heart of the detector is the QuarkNet DAQ board. The development of this board is a collaborative effort involving Fermilab, the University of Nebraska and the University of Washington. Appendix A has a description of the history of this project. Project Development Team Fermilab: Sten Hansen, Tom Jordan, Terry Kiper Univeristy of Nebraska: Dan Claes, Jared Kite, Victoria Mariupolskaya, Greg Snow University of Washington: Hans Berns, Toby Burnett, Paul Edmon, Rik Gran, Ben Laughlin, Jeremy Sandler, Graham Wheel, Jeffrey Wilkes 1 For more technical documentation on the DAQ board, see i
  4. 4. Classroom Cosmic Ray Detector User’s Manual Table of Contents Chapter 1 In the Classroom Cosmic Ray Experiments within the Classroom How Can I Use Cosmic Ray Detectors in My Classroom? What Experiments Can My Students Perform? Chapter 2 Hardware Introduction Hardware Overview The DAQ Readout Board Power Supply GPS Receiver Scintillation Counters Cables for Connecting to PC Chapter 3 Data Display Data Display on a PC Keyboard Commands Chapter 4 The DAQ Card GPS Observation Singles and Coincidence Rate Measurement Data Collection Chapter 5 Data Upload and Analysis QuarkNet Website Overview Data Upload to the Server Analysis Tools Chapter 6 Advanced Topics Counter Plateauing Data Words Coincidence Counting Variations On-board Barometer Calibration Appendices Appendix A: History of Card Development Appendix B: Schematic Diagrams Appendix C: Terminal Emulator Setup Appendix D: Precise Event Time Calculation Algorithm Appendix E: Details of Time Coincidence and Data Handling Appendix F: Acronym and Jargon Dictionary ii
  5. 5. Classroom Cosmic Ray Detector User’s Manual
  6. 6. Classroom Cosmic Ray Detector User’s Manual Chapter 1: In the Classroom Cosmic Ray Experiments very short -ived and generally decay into For most of today’s particle physics experi- daughter particles before reaching the ments, large accelerators accelerate particles earth’s surface. The charged pions, for in- to very high energies. However, it is possi- stance, will decay into a muon and a neu- ble to do high-energy physics in your school trino: without a particle accelerator! Nature serves as an accelerator for cosmic rays—particles π-  µ- + ν µ that are of astrophysical origin (It is not completely understood!) and seem to be eve- or rywhere throughout the universe. These “primary” cosmic rays, which are mainly π+  µ+ + ν µ. protons (and a few heavier nuclei), interact with nucleons in the earth’s upper atmos- Although these reactions are not the only phere in much the same way that fixed target possibilities—they are examples of common collisions occur at particle physics laborato- reactions that produce secondary particles ries. When these primary particles interact and their daughters. Counting all secondary with nucleons in the atmosphere, they pro- particles detected at sea level, 70% are duce mainly pions and kaons. In the colli- muons, 29% are electrons and positrons, and sions, if most of the incoming momentum is 1% are heavier particles. transferred to an atmospheric proton, the following reactions are common: If these secondary particles have sufficient energy, they may in turn interact with other p + p  p + p + π+ + π° + π- particles in the atmosphere producing a “shower” of secondary particles. The parti- and cles in this shower will strike a wide area on the earth’s surface (perhaps several m2 or p + p  p + n + π+ + π+ + π-. even km2) nearly simultaneously. Figure 1 illustrates one of these showers. If most of the momentum is transferred to a neutron, then these reactions are common: Although your students can do several cos- mic ray experiments with a single classroom p + n  p + p + π+ + π- + π- setup, the QuarkNet website <> provides and student access to data from multiple cosmic ray detectors to detect and study larger p + n  p + n + π+ + π° + π-. showers. Several professional versions of this same experiment are taking data that may help determine the origin of some of The products of such interactions are called these highly energetic primary cosmic rays. “secondary” particles or “secondary” cosmic rays. Some of these products, however, are
  7. 7. Classroom Cosmic Ray Detector User’s Manual experiments in parallel with your “standard” curriculum with days set aside periodically for cosmic ray detector work. After an introduction to cosmic rays, the equipment, and the research questions that physicists ask in this field, have student groups write up a proposal for the experi- ment that they want to perform. These ex- periments might include calibration and per- formance studies, muon lifetime experi- ments, shower studies, or flux studies as a function of one of many variables, e.g., time of day, solar activity, east/west asymmetry, angle from vertical, barometric pressure, etc. Your students can also look for particles that are strongly correlated in time and may be part of a wide-area shower. Figure 1. A cosmic ray shower produced by Once proposals are accepted, give each stu- a primary cosmic ray entering the earth’s dent “collaboration” a few days to a week to atmosphere. run their experiment. In some cases, two groups might pool their time and share the How Can I Use a Cosmic Ray same data but for different physics goals. This is not unlike how real high-energy Detector In My Classroom? physics experiments are done! If time per- The very nature of cosmic ray experiments mits, students can do a second run with the is quite different from traditional labs that student-suggested modifications. can be completed in a lab period or a couple of lab periods. Cosmic ray experiments typi- While one group is using the hardware, cally require a great deal of run time to col- other groups can design their setup, analyze lect data. Extra class time, however, is not their data (or other data), visit the “Poster” something most teachers have in an already section of the QuarkNet cosmic ray website full course. You may also wonder how to to research the results of other student provide each student time to do an experi- groups, etc. Having your students post their ment when you have only one setup. work here and/or having them give oral These are realistic questions. As a possible presentations (sometimes with guest physi- solution, the following approach allows you cists!) may make for a great summary to the not only to incorporate these experiments project. into your course but also to help your stu- dents discover how high-energy physics ex- periments are really done. What Experiments Can My Organize the cosmic ray experiments as a Student Perform? long-term project that spans a quarter or There are four categories of experiments that even more. If students work on this project students can do with the comic ray detec- intermittently, you can run these cosmic ray tors:
  8. 8. Classroom Cosmic Ray Detector User’s Manual 1. Calibrations and performance studies these muons as they travel to earth should be 2. Flux experiments at least 100 µs: 3. Muon lifetime experiments 4. Shower studies v muon 3x10 8 m / s t flight = = = 100 µs . d 30 x10 3 m Calibrations and Performance This calculation assumes that muons are Studies traveling at the speed of light—anything Before students can “trust” the cosmic ray slower would require even more time. If a equipment, they can and should do some student can determine the muon lifetime (as calibrations to study the response of the described in Chapter 5) and show that it is counters and the board. Calibration studies significantly less than this time, they are include plateauing the counters, threshold presented with the wonderful dilemma that selection and barometer calibration. These the muon’s time of flight is longer than its are discussed in Chapter 5. In addition, the lifetime! QuarkNet online analysis tools include a “system performance” study for uploaded This time dilation “proof” assumes that all data. The details of this study are outlined in muons are created in the upper atmosphere. Chapter 5 and on the website. Although this is actually a good approxima- tion, your students cannot test it. However, by using the mean lifetime value and by Flux Experiments measuring flux rates at two significantly dif- Your students can do a variety of flux ex- ferent elevations, you can develop experi- periments investigating such things as cos- mental proof for time dilation. This experi- mic ray flux as a function of time of day, ment requires you to have access to a solar activity, east/west asymmetry (show- mountain, an airplane, or collaboration with ing the µ+/µ- ratio by assuming these a team from another school that is at a sig- charged particles will bend in the earth’s nificantly different altitude! Here is a won- magnetic field), angle from vertical, baro- derful opportunity for schools to work to- metric pressure, altitude. The list goes on. gether proving time dilation. A very thor- This can be an exciting set of experiments as ough explanation of this experiment is out- students study the factors that they want to lined in the 1962 classroom movie titled, test. Chapter 5 discusses flux studies in more “Time Dilation: An Experiment with Mu detail. Mesons.” 2 This movie helps you (and your students) understand how the muon lifetime measurement (along with flux measurements Muon Lifetime Experiment to Ver- at two different altitudes) can be used to verify time dilation. ify Time Dilation A classic modern physics experiment to verify time dilation is the measurement of the muon mean lifetime. Since nearly all of Shower Studies With the GPS device connected to your the cosmic ray muons are created in the up- DAQ board, the absolute time stamp allows per part of the atmosphere (≈30 km above the earth’s surface), the time of flight for 2 This 30 minute movie can be ordered on CD for $10 from
  9. 9. Classroom Cosmic Ray Detector User’s Manual a network of detectors (at the same site or at different schools) to study cosmic ray show- ers. Your students can look for small show- ers or collaborate with other schools in your area to look for larger showers. The QuarkNet online analysis tools allow students to not only look for showers but to make predictions about the direction from which the shower (and thus the primary cosmic ray) originated. Details for doing shower studies can be found in Chapter 5 and on the QuarkNet website.
  10. 10. Classroom Cosmic Ray Detector User’s Manual Chapter 2: Hardware Introduction Hardware Overview For this setup, the DAQ board takes the sig- Figure 2 shows a typical cosmic ray detector nals from the counters and provides signal setup composed of: processing and logic basic to most nuclear 1. Counters—scintillators, light guides, and particle physics experiments. The DAQ photomultiplier tubes and bases (two board can anlyze signals from up to four shown). PMTs. (We show two in the figure.) The 2. QuarkNet DAQ board. board produces a record of output data 3. 5 VDC adapter. whenever the PMT signal meets a pre- 4. GPS receiver. defined trigger criterion (for example, when 5. GPS extension cable. two or more PMTs have signals above some 6. RS-232 cable (to link to computer serial predetermined threshold voltage, within a port). certain time window). The output data re- 7. Optional RS-232 to USB adapter (to link cord, which can be sent via a standard RS- to computer USB port instead of serial 232 serial interface to any PC, contains tem- port). poral information about the PMT signals. 8. Lemo signal cables. This information includes: how many chan- 9. Daisy-chained power cables. nels had above-threshold signals, their relative arrival times (precise to 0.75 ns), 1 9 4 3 9 8 2 5 7 6 Figure 2. A typical cosmic ray detector setup.
  11. 11. Classroom Cosmic Ray Detector User’s Manual and the starting and stopping times for each such as counting rates, auxiliary GPS data, detected pulse. In addition, an external GPS and environmental sensor data (temperature receiver module provides the absolute UTC and pressure). time of each trigger, accurate to about 50 ns. This allows counter arrays using separate This chapter includes an overview of each DAQ boards—such as different schools in a major component shown in Figure 2. Addi- wide-area array or two sets of counters at the tional details regarding some components same site—to correlate their timing data. are in the appendices. Keyboard commands allow you to define trigger criteria and retrieve additional data, Figure 3. The QuarkNet DAQ v2 board with major components labeled. DAQ Readout Board text lines readable by any terminal-emulator Figure 3 is a picture of the DAQ board with program. In addition, a GPS clock provides several major components labeled. This accurate event time data synchronized to board is the logic link between the scintilla- Universal Time (UTC), so widely separated tion counters and the PC. The board pro- sites can compare data. In datastream mode, vides discriminators and trigger logic for the DAQ board outputs a series of text lines four channels of PMTs. The board includes reporting event data: trigger time in UTC five built-in scalers, allowing simultaneous (with 24 ns resolution and absolute accuracy counts of singles on each channel, plus trig- about 100 ns), leading and trailing edge gers at whatever logic level you specify (2- times for each pulse recorded within the to 4-fold majority logic). coincidence time window (with 0.75 ns pre- cision), and data from the GPS and internal A standard RS-232 interface can be con- clocks. Additional keyboard commands al- nected to any PC (Windows, Linux or Mac). low reading of temperature, barometric pres- The datastream consists of simple ASCII sure and other sensors.
  12. 12. Classroom Cosmic Ray Detector User’s Manual Figure 4. Block diagram of the QuarkNet DAQ v2 board. Board Components mV threshold with NIM discriminators and Figure 4 is a block diagram of the QuarkNet have x10 amplification, your threshold level DAQ v2 board. (See Appendix B for a on the QuarkNet DAQ board should be set schematic diagram.) to -300 mV. Discriminators Complex Programmable Logic Device PMT signals are first pre-amplified by a (CPLD) factor determined from a set of changeable Trigger logic is implemented using a CPLD resistors (Amplification factor x10 is used chip. Software revisions for this chip must for QuarkNet, x2 by CROP, x1 by be prepared using special software but can WALTA.) Discriminators are implemented be downloaded via the serial port. This using voltage comparator chips with the ref- flexibility allows the engineers to distribute erence threshold voltage varied via screw- updates that alter the fast logic, if necessary. driver adjustable potentiometers (trimpots). Any trigger logic level from singles to 4- Threshold levels can be set from 0 to -750 fold can be set by keyboard commands to mV by adjusting each potentiometer while the board. Majority logic is used: any com- monitoring voltage at a nearby test point. bination of three active channels causes a It is important to remember that the voltage trigger at the 3-fold level, for example. comparators look at the amplifier output, so raw PMT signal levels are multiplied by any amplification factor present before being compared. For example, if you used a -30
  13. 13. Classroom Cosmic Ray Detector User’s Manual Time-to-Digital Converters (TDCs) reported whenever the board is started up. Discriminator output pulses are fed into While the board components are rated for TDCs3 which measure the arrival time of temperatures between –20°C and +80°C, the leading and trailing pulse edges. TDCs keep board temperature should not normally go track of their state (high or low) at 0.75 ns above +50°C. time intervals. If the trigger criterion is satis- fied, the TDC data are latched and read out, A second temperature sensor is located on giving leading and trailing edge times for the booster board built into the GPS cable’s each channel relative to the trigger time in “far end” DB9 connector. This sensor can be units of 0.75 ns. This allows you to calculate used to log outdoor temperature at the PMT pulse widths (time over threshold, or counter locations, and is read out with a TOT, see Figure 5) as a crude estimate of keyboard command. The GPS module may pulse area and thus energy. be damaged if its temperature goes below –40º C or above +85º C.4 A barometric pressure sensor is built into the board as well. It can also be calibrated and read out (in units of kPa) with a keyboard command. Board Functionality Figure 5. Typical PMT pulse from which the Time Over Threshold (TOT) can be Threshold Detection and Setting determined. After amplification, the PMT signals are fed to voltage comparators. The comparators set a HIGH logic level whenever the amplified Microcontroller (MCU) PMT signal exceeds their negative threshold The MCU is really just a special-purpose voltage setting as shown in Figure 6. This CPU that provides the onboard ‘slow’ logic logic level has 0.75 ns resolution. This (with a time scale of microseconds, not means that we can measure the time differ- nanoseconds) to interface the board to you ence between pulses on different channels at via a terminal window or equivalent on your the same site down to less than a nanosec- PC. At present, the MCU can be repro- ond! We depend on the GPS clocks, with 24 grammed to redefine functionality only by ns resolution, for timing between school using special software and burn-in hardware. sites. Auxiliary Sensors There is a temperature sensor built into the microcontroller chip. This sensor is present so that CPU temperature can be measured. This temperature and the supply voltage are 4 At DAQ card power-up or reset, the temperature on the DAQ card (actually, inside the MCU chip) is reported. Note that this is 3 not a good indicator of air temperature at the card site, even if the “TDC” is a generic term in particle physics technology. The card is located outdoors, since the chip generates considerable heat specific chips used on the board are TMCs (“Time Measurement while operating. It is a useful way to make sure the card is healthy, Chips”). You may see this designation in some documentation. however! Temperatures over +40º C may be dangerous to the card’s chips.
  14. 14. Classroom Cosmic Ray Detector User’s Manual Reference ground Signal after User determined amplification threshold Figure 6. A typical negative PMT pulse showing the user-defined threshold value compared to reference ground. To measure the current threshold voltage value, compare the test points, to ground. The test points for each threshold are labeled “TPVTH” and the ground labeled “TPGND” TPVTH in Figure 7. TPGND Figure 7. Close-up picture of the DAQ board Setting the threshold level too high will miss showing threshold test points used to measure all but those events that correspond to a threshold voltage for each channel. large amount of energy deposited in the de- tector; setting the threshold too low will re- sult in background noise within the elec- The plot shows the singles rate in that chan- tronics being mistaken as event counts. nel as a function of threshold value. You can see that the counting rate decreases as the So how do you determine the appropriate threshold setting increases. Once the thresh- threshold setting? Consider the graph Figure old value became as high as 0.5 V, the 8 on the next page as a ‘ballpark’ determi- counting rate decreased more linearly. The nation of the optimum threshold value to use “kink” in the graph may be used to suggest for a channel. For the data shown in this that noise contaminated the counting rate graph, the PMT voltage was set to that de- much more significantly when the threshold termined by plateauing the counter (see sec- was set below 0.5 V. Operating slightly tion 6.1 for help on how this is done) and above this value is probably the optimum kept constant throughout the experiment. threshold value to choose.
  15. 15. Classroom Cosmic Ray Detector User’s Manual Threshold Determination 900 800 700 (counts/minute) Frequency 600 500 400 300 200 100 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Threshold (V) Figure 80. A graph of frequency vs. threshold for a counter used to de- termine threshold setting. Coincidence Logic Rate Counters The DAQ board can also determine whether The card has five built-in counters, num- signals in separate channels are coincident in bered 0 through 4: counters 0 through 3 re- time. For example, if the trigger criterion is cord the singles count for each channel, and set to 2-fold, then as soon as any channel counter 4 records the trigger count for what- goes above threshold, a time window is ever coincidence level you have set. By ze- opened. (Window time-width is adjustable.) roing these counters with a software com- If any other channel goes above threshold mand, then running the board for a fixed during this time window, all event data are length of time and reading out the counters latched and outputted for the overlap time at the end, you can obtain singles rates for interval when both are active. Notice that each channel as well as the coincidence pulse data are reported for a time interval rates. that is not of fixed length but just covers the overlap period when two or more channels are active. Leading and trailing edge times Power Supply are reported for any active channels (not just The board requires a stable +5 VDC power for the two channels that launched the trig- supply, with 800 mA or greater output cur- ger), with empty data entries for channels rent. A 110VAC to 5VDC/2.4A modular that remained inactive during the trigger switching power supply, of the type used for window. For a single event trigger, the DAQ many small electronic devices, is supplied board may need to output several lines of with the board. Replacements are readily data. The first line has an “event flag” for available at Radio Shack or a similar elec- identification. Any following lines without tronics store. this flag are simply additional data for the same event. PMT bases provided by QuarkNet can be powered by the same +5 VDC power supply
  16. 16. Classroom Cosmic Ray Detector User’s Manual that is plugged into the DAQ board. These if necessary. Although the GPS module’s bases are daisy-chained from the board us- RS-232 D plug appears conventional, it has ing a common male-male phono jack cable been modified with a special connector, which is also readily available at Radio which contains the circuitry required to de- Shack or similar store. liver DC power to the module and interface the 1PPS (1 Pulse Per Second) signal to the Note: It is important to distinguish these board. This connector should not be inserted DAQ power supply modules from other directly into your PC’s serial port. units used for computers or other electronic devices requiring DV voltage. Connection of You may wish to use the GPS module alone the wrong power supply to your DAQ board to observe satellite data without operating will probably fry it. the DAQ card. For stand-alone operation of the GPS module, you need to construct a special adapter to allow connection of a GPS Receiver +5VDC power supply directly to the mod- The external LeadTek GPS module shown ule—you can use the same power supply in Figure 9 is connected through a special used for the DAQ card. The wiring cable which has a seemingly-standard D diagram for the adapter is shown in Figure connector (serial port connector) on the far 10 on the next page. Parts shown in the end. This connector mates with the commer- wiring diagram can be obtained at most cial GPS module’s serial connector. How- electronics supply stores. The LED is op- ever, the special cable’s D connector actu- tional and just blinks to display the 1PPS ally contains a tiny circuit board with com- signal, indicating valid data. ponents needed to ensure good performance over 100’ or longer cables. The GPS clock The GPS module itself is weatherproof, but will not work if the special cable is not used. the connectors should be protected. Also, the screws on the cover of the GPS module tend to rust, so they should be covered with plas- tic paint or nail polish before deployment Interfacing the GPS with the DAQ outdoors. Board The external GPS module is interfaced to the Quarknet DAQ board with a special ex- GPS Startup tension cable, which can be over 100’ long Once powered-up, the GPS module should quickly “find itself” if it is in a location with a clear view of at least half the sky. Usually it does not work well through windows and should be physically outdoors. The receiver will lock onto four or more satellites, download the data needed to operate accu- rately and start averaging its position and clock settings at one-second intervals. Within a few minutes after startup, you Figure 9. The LeadTek GPS receiver and should obtain accurate GPS data. The unit’s cable. The cable shown here can be con- LED display blinks red when first powered- nected to a GPS extension cable. up and searching, and changes to steady-
  17. 17. Classroom Cosmic Ray Detector User’s Manual green when it has acquired enough satellite commercial GPS module transmits the 1PPS data to locate itself accurately. Time data are signal about 100’ or more without excessive not accurate until then. timing degradation. The GPS module provides several kinds of data. The commercial GPS module directly How to Calculate Event Times supplies the date and “coarse” time (in UTC, The DAQ board has onboard a 41.667 MHz not local time) down to milliseconds, or oscillator, which “ticks” every 24 nanosec- three decimal places after seconds, and re- onds (1/41.667x106 sec). Such a device does ports its geographic location in latitude and not maintain its frequency very accurately, longitude down to the equivalent of a few and our oscillator’s frequency may drift by 10s of meters. 10-100 Hz from its nominal value (perhaps even more under extreme temperature changes). A counter (scaler) keeps counting The “1PPS” Signal the clock ticks. Whenever this 32-bit counter The stock LeadTek GPS module was modi- reaches its maximum capacity of 4.3 billion, fied slightly for our application to allow the approximately every 100 seconds, it just more precise timing we need down to 10s of “rolls over” back to zero, like an old car’s nanoseconds (eight decimal places following odometer, and continues counting. Clock integer seconds). The GPS receiver outputs calibration occurs the internal clock counter a logic pulse at the beginning of each UTC every time a 1PPS signal arrives. second, called the 1PPS (1 Pulse Per Sec- ond) signal. Find the precise event time by getting the date and time down to the nearest second The GPS receiver sends two types of from the GPS module’s coarse time (with datastreams to the board. The first is RS-232 one correction described below), finding the ASCII data telling what time it is, at what number of clock ticks between the last 1PPS latitude, longitude and altitude the receiver and the event trigger, and dividing by the is, and information about the satellites the number of clock ticks between the last two receiver is using. The other data is a 5V 1PPS pulses to get the fraction of a second 100ms pulse telling exactly when the data is down to 24 ns precision. Of course, you true. Each stream of 5V 100ms pulses ar- have to know whether the counter rolled rives every second, thus the 5V pulse is over during the last second when doing the named 1 pulse per second (1PPS). The mi- arithmetic. crocontroller on the board records the counter value during which the pulse is re- There is a caveat to the previous paragraph: ceived. The time is according to a counter If the data rate is very slow—less than about running at ~41.67 MHz. one event per 100 sec—the internal counter may roll over more than once between In principle, the leading edges of 1PPS sig- readings. In this case, all bets are off, and nals from GPS receivers anywhere in the the seconds cannot be reliably calculated. world are all in synch, to within the accu- The next firmware update will include racy of the non-military GPS system (about commands to produce an output line at each 100 ns.) This feature allows accurate time 1PPS, and/or automatic DG commands at synchronization between school sites. The regular intervals, ensuring that the necessary special connector and cable attached to the
  18. 18. Classroom Cosmic Ray Detector User’s Manual information is logged between counter Using the GPS without the DAQ rollovers. Board To use the GPS module via the serial port of One minor correction is also needed: the a PC (without the DAQ board) requires the GPS module reports the time down to the construction of a simple adapter circuit millisecond, but there is a delay of a fraction shown in Figure 10. The LED is optional of a second relative to the 1PPS edge. For and simply blinks to display the 1PPS signal example, when the 1PPS signal says it is and indicates when the GPS module has exactly 12:01:25.000000000, the GPS time locked onto a position and time fix. 5VDC field in the data record may say it is power can come from a 110VAC adapter or 12:01:24.850. This delay (reported as a PC’s PS/2 mouse jack. Power drawn is +0.150 sec in this example) will cause you equivalent to what a mouse would consume, to associate the wrong integer second with and will not affect your computer. You can the event time if you are not careful. (The substitute a connector to mate with a stan- GPS time delay may be negative, i.e., a for- dard 5VDC power adapter, available from ward shift, with the ASCII time data ahead any electronics store. of the 1PPS signal.) The DAQ board is pro- grammed to list the delay in milliseconds for each data record so the reported time in sec- onds can be corrected to match the 1PPS data. The algorithm described in Appendix D implements these calculations. For people who like really deep-down de- tails, the delay mentioned above, which you can determine and correct, is not the whole story. There is an additional delay of 150- 250 ms, which is unresolvable, since it is buried in the manufacturer’s firmware. Luckily, it is too small to cause the rounding procedure described above to go wrong. Figure 10. The adapter circuit wiring dia- Remember, all this is just about determining gram required when the GPS is connected the integer seconds part of the time. Nano- directly to the computer (without connection second-level timing is not affected. Also, we to the DAQ board). have not discussed corrections for the an- tenna cable delay—the event time calculated is when coincidence occurs at the DAQ Scintillation Counters board, not at the counters. This is of no con- A typical counter used in this experiment is sequence as long as all school sites in an ar- shown in Figure 11. A counter is comprised ray use approximately the same cable of a plastic scintillator, a light guide, a PMT, lengths, within a few 10s of meters. The al- and a PMT base which provides the neces- gorithm showing how to get the precise sary operating voltage for the PMT and event time is shown in Appendix D. sends an electrical signal to the DAQ board when a light pulse is “seen” by the PMT. For the counter shown in Figure 11, only the square portion of the paddle actively scin- tillates. The trapezoidal shape is merely a
  19. 19. Classroom Cosmic Ray Detector User’s Manual light guide which directs the light toward the nomena, however, is an important one in PMT using total internal reflection. When helping your students understand the opera- calculating the surface area of the active tion of the detectors. portion of your detector, use only the square portion. PMT/Base Along with the production of the DAQ board, QuarkNet and Fermilab have pro- duced a Cockcroft-Walton base that can be used with an RCA 6342 PMT or similar 2” 10-stage tube. This base does not require the Figure11. A typical cosmic ray counter con- use of a high voltage power supply but is sisting of scintillator and light guide (the flat powered by the same +5VDC supply that is paddle region to the right), the PMT (black cylinder), and the PMT base (light-colored used for the DAQ board. The base is housed cylinder on the left end.) in an aluminum can (see Figure 12). Appen- dix B includes a schematic for this PMT base. If a separate tube and base are used in It will be necessary to “plateau” your your experiment, you may need to make counter before the counting rate can be slight resistor modifications to the board in trusted. Small fluctuations in the high volt- order to achieve the proper signal size after age can offer significant variations in amplification. counter response if the operating voltage is not near the voltage plateau. Details for how The PMT voltage across the first and last to plateau a counter are in Chapter 6. dynode—the operating tube voltage—ranges from 0–1600 VDC for the RCA 6342 PMT and the corresponding base provided by Scintillators QuarkNet. A potentiometer at the end of the Since muon flux at sea level is ~1 x 10-2 base can be used for adjusting this voltage. muons/cm2/sec, and since these comprise The “pot” is marked off from 0 to 10 which about 70% of the cosmic rays at sea level, a correspond linearly to the voltages from 0 – detector of significant surface area will help 1600 V. This means that increasing the pot to increase the counting rate to 10s or even by “1 unit” corresponds to an increase in 100s of events each minute. If shower ex- 160 V across the first and last dynode. periments or flux studies are of interest, then increasing surface area is the primary way to If you want to check the value of the tube increase the counting rate. If muon lifetime voltage, two test points on the end of the experiments are of interest, however, then base are provided to probe with a standard increasing the thickness of the scintillator DC low voltage voltmeter (see Figure 12). will also increase the counting rate since This probe voltage measurement must be only the muons that have stopped and decay multiplied by 385 to obtain the actual tube within the detector can be used for lifetime voltage. studies. An important question to ask is, “What The process by which a scintillator scintil- should be the voltage setting for my lates light when a changed particle traverses counter?” Because the response of each is not discussed in this manual. This phe- counter is unique, the answer to this ques-
  20. 20. Classroom Cosmic Ray Detector User’s Manual tion is different for each counter. To deter- Lemo cables, which are thin coaxial cables mine the operating voltage for a particular with “lemo” connectors on the ends, carry counter, you must “plateau the counter.” the PMT signal to the DAQ board. A toggle Details can be found in Chapter 6. switch on the end of the base allows you to turn on/off the tube without disconnecting the power cable. An amber LED near the test points tells when the base is “on.” Cables for Connecting to PC The DAQ board connects to your PC’s serial port with a standard serial cable. If your PC does not have a standard RS-232 serial port, you can use a USB-to-serial converter, available commercially. Test Points Figure12. The end of a PMT base showing test points that may be probed with a low voltage DC voltmeter to measure PMT oper- ating voltage. A multiplication factor of 385 is used to determine actual tube voltage.
  21. 21. Classroom Cosmic Ray Detector User’s Manual Chapter 3: Data Display Data Display on a PC a single event may result in more than one View the data with a terminal emulation line of data! In this section, we discuss the program such as Zterm (Mac or Windows), meaning of some of the keyboard commands Hyperterminal (Windows) or a “homemade” that you can type in order to modify/read Unix-based program you can download various values to/from the board that are not from the QuarkNet website. See Appendix C displayed on the screen until requested. for details on setting up these terminal emulation programs. Your PC is capable of sending commands to the DAQ board in addition to receiving the datastream from the board. Figure 14 shows an example of three keyboard commands Keyboard Commands that you type in order to measure the tem- Once the terminal emulator is initialized and perature at the GPS sensor, the value of the the scintillation counters and board are pow- scalers, and to display GPS data. ered, you should begin to see data on the screen that looks something like that shown The “HE” (help) command sends a summary in Figure 13. of all keyboard commands. Figure 15 shows this command summary. The meaning of these “words” will be dis- cussed in great detail in Chapter 6. Note that Figure 130. Sample screen of datastream from DAQ board when viewed with a terminal emulation program.
  22. 22. Classroom Cosmic Ray Detector User’s Manual Figure14. Example of keyboard commands that may be sent to the DAQ board with the appropriate board responses. Figure15. A list of all keyboard commands viewable by typing “HE.”
  23. 23. Classroom Cosmic Ray Detector User’s Manual Some of the most commonly used com- Chapters 4 and 6 for more detailed descrip- mands are discussed in Table 1. See tions of how and when to use these com- mands. Keyboard Description Command CD Counters Disabled. Does not write event lines on the PC monitor even though the scalers will still increment. This is useful when you do not want to “see” all the data coming in. CE Counters Enabled. Writes event lines on the PC monitor. This com- mand has the opposite effect of the CD command. DS Display Scalers. Displays values of the scalers in hexadecimal. Scal- ers 0-3 store the singles rates for channels 0-3, and counter 4 records the number of n-fold coincidences, where “n” is whatever level you set. These counts are running totals since the last time the counters were zeroed. RB Reset Board. Resets scalers and TMC only. RE Reset Everything. Resets everything (including card setup parameters that you may have modified); will zero all scalers. TH Thermometer. Reports reading of temperature sensor on the GPS module connector, about 3 feet from the GPS module itself. GP GPS Data. Displays current date, time, GPS position, and number of satellites visible. Table 1. Some of the most common keyboard commands with descriptions.
  24. 24. Classroom Cosmic Ray Detector User’s Manual Chapter 4: The DAQ Card GPS Observation To read out the current GPS time, latitude, If you connect the GPS module directly to longitude, altitude and number of satellites your PC’s serial port, as discussed before, in view, type “GP” as shown in the previous you can also use a software package pro- section. You can, however, directly monitor vided by LeadTek to display sky maps of the GPS module’s serial datastream by satellites and other interesting data. This can sending the command “NM 1.” This com- be found online. mand will pass all data from the GPS mod- < ule directly to your PC. Normally, the chive/Leadtek/GMonitor.exe> board’s MCU reads, interprets and distills the raw GPS information to produce the DG command output. Although this will con- Singles and Coincidence Rate tinue to be true even after you send the “NM 1” command, you will get many more lines Measurement of detailed data (in the GPS industry’s stan- Important measurements you want to make dard NMEA format). Refer to the link to are the rate at which an individual counter is understand this data format. hit (“singles rate”) as well as the rate at < which a 2- to 4-fold coincidence occurs chive/Leadtek/GPS_Protocol_ReferenceMa (“coincidence rate”). To determine these, nual.pdf> type the following commands shown in Ta- ble 2: To disable NMEA data-forwarding and re- turn to normal operation, send the command NM (i.e., no “1”). CD Counter Disabled. Prevents the datastream from off your screen. RB Reset Scalers. But immediately after type . . . DG Display GPS Data. Gives the initial time—when you began counting. (You could use a stopwatch to measure the elapsed time instead.) DS Display Scaler. After an adequate amount of time has passed (say 5 min- utes or so) take the final reading of the scalers and then immediate type . . . DG Display GPS Data. Give the final time. Table 2. The keyboard command sequence performed for determining counting rate. The counting rates for any particular scaler elapsed time is recorded in scaler 4. Divid- (0-3) can be determined by dividing the hex ing the number of coincidences by the valued scaler by the difference of the begin- elapsed time will yield the coincidence rate. ning and ending times. The number of n-fold Figure 16 illustrates this command sequence coincidences that occurred within the and the corresponding board outputs.
  25. 25. Classroom Cosmic Ray Detector User’s Manual As an example of the singles rate, consider 3023counts counts Rsin gles = = 607 the data in which the counting rate for chan- 4.98 min min nel 0 was BCF = 3023 . The elapsed run H 10 time was 21:35:45 – 21:30:46 = 4:59 or 4.98 minutes. Thus, the singles rate is Initial GPS time Scaler values after elapsed time Ending GPS time Figure16. An example of keyboard commands and board responses used to de- termine the singles counting rate in a particular channel. Data Collection the datastream appears on your monitor, se- Once you have setup and tested your equip- lect the command that will allow you to be- ment and have plateaued the counters, you gin capturing text. You will be prompted to are ready to collect and save data for a run. type a name and a folder into which these Although the commands for starting and data will be saved. Your emulator will save stopping a run vary slightly with each termi- these data as a text file that can be analyzed nal emulation program (see Appendix C for later. The data will be written to this file these commands), the idea is the same: Once continuously. For example, opening this text
  26. 26. Classroom Cosmic Ray Detector User’s Manual file while data is being saved will display all the stop command for your particular emu- the data collected up to that moment. lator. (See Appendix C for this command.) When you want to stop a run (which could This file can be uploaded to the server and be minutes, hours or even weeks later!), use the data analyzed.
  27. 27. Classroom Cosmic Ray Detector User’s Manual Chapter 5: Data Upload and Analysis Figure 17. The QuarkNet Cosmic Ray Collaboration webpage serves as the studenthomepage for the cosmic ray project. QuarkNet Website Overview from their detector. A link to the student homepage is also located here. Teacher Pages The QuarkNet teacher activities database is Student Pages found at The student homepage for this project is < <> (Figure 17). ivities/view.lasso>. Among many resources, students will find: By selecting the “Grid Cosmic Ray Collabo  Studies – Interesting questions along ration” link, you can find out more about with ideas to answer these questions this project and use online tools for imple- through experiment. menting this project in your classroom. You  Resources – Links to related projects, will find a page to register their school as physicists and other student group con- part of the cosmic ray investigations net- tacts. work so that your students may upload data  Upload Data – Links to upload data and geometry files.
  28. 28. Classroom Cosmic Ray Detector User’s Manual  Data Analysis – Powerful analysis tools data run spans more than one day, the data to analyze data for a variety of experi- will be broken into separate files labeled ments. Even students without a detector with the appropriate date. can analyze data.  Poster Session – A method for students to share their results, search the work of Analysis Tools other students and collaborate with stu- Once your students have collected and up- dents at other schools. loaded the data, there are four studies they can do. Analyze data from your detector as well any other detector that has uploaded Data Upload to the Server data. You can even look for coincidences While many of the cosmic ray detector experi- between your school and other schools if run ments that your students can do require only the times overlap! This section will explain the equipment at your local site, analyzing the data is nature of each study/analysis and provide a cumbersome with software such as Excel. framework from which you can get started. Working locally discourages student collaboration like that found in real high-energy experiments. Accordingly, QuarkNet is developing a website System Performance that allows data uploads and analysis. This The main objective of the “system performance” simplifies the analysis of these complex data and study is to understand the quality of the data that supports student collaboration. you have collected or want to analyze. Before students can upload data to the server, you The primary technique used to evaluate data in the will need to register your detector as part of the performance study is a histogram of the number cosmic ray investigations network. This is easily of events as a function of time over threshold done on the teacher website. Registration will (TOT, see Chapter 2). In a very loose sense, TOT require you to enter the serial number of the is a measure of the amount of energy deposited in detector found on the bottom of the DAQ card the scintillator for a given event. Ideally, the DAQ itself, or by typing the keyboard command “SN” board would have been equipped with an ADC in the terminal emulator. (Analog-to-Digital Converter) to measure pulse height and thus the energy deposited in the By selecting the “Upload” link from the scintillator. To keep the cost down, however, a <> page, you will TDC (Time to Digital Converter) was used. (See seee instructions and links for uploading data. Chapter 2 for more on the TDC.). Since taller (You do not need to worry about uploading data pulses (pulses with more energy) are generally that may be “contaminated” by keyboard wider, there is a correlation between the time that command lines typed during the operation of this the pulse exceeded the threshold value and the run as the analysis software will create a new file amount of energy deposited in the scintillator. for this data using only data lines that are the complete 16 words across.) Once you have uploaded data, to see that it has been loaded successfully select the “Data” link. You can “View” your data in your school’s folder. The link to view this data lists the date the data was collected. If a
  29. 29. Classroom Cosmic Ray Detector User’s Manual Number of Events Number of Events Time over Threshold (ns) (ns) Time over Threshold (ns) a b Figure 18. (a) The Gaussian distribution expected from an ‘ideal’ system performance graph, (b) an actual system performance grph. Figure 18(a)5 shows the ideal, Gaussian- traversing the counter. No real data histogram will shaped distribution about a mean value for ever look perfectly Gaussian, but a shape the time over threshold; Figure 18(b) shows somewhat resembling that of Figure 18(a) can be an actual data run plot. a reasonable indicator that much of your data is “good” and represents real muons. The actual data run histogram peak does oc- cur around 12 ns (which is typical for the One factor you can modify in plotting a histogram 1/2” thick scintillator, the PMT efficiency, is the number of bins into which the data are and threshold settings used in our experi- divided. Choosing a different number of bins for ment). This actual data, however, appears the same data set can significantly change the more skewed than Gaussian. This histogram shape of the graph and may help evaluate the may tell us that there is “good” data in our quality of the data. actual run data, but that it is contaminated with short pulses (a.k.a. noise) that were just above threshold. Increasing the threshold Flux Experiments value may “clean-up” this channel in the Cosmic ray flux, Φ CR , can be defined as future. There is probably a large portion of the actual run events data in Figure 18(b) that represents actual muons Φ CR = . (time )(area ) Since the area of your detector will probably 5 Figure is used with permission. Eric W. Weisstein. not be varied in during an experiment, flux quot;Gaussian Distribution.quot; From MathWorld--A Wolf- studies ask the question, “How does the rate ram Web Resource. at which cosmic rays pass through my de- html tector depend on . . . ?” Since there are many ways to complete this question, this study is
  30. 30. Classroom Cosmic Ray Detector User’s Manual 3 a fairly straightforward yet excellent open- time dilation. Chapter 1 discusses how the ended research question for your students. lifetime measurement can be used to verify time dilation. Here we develop the muon Flux experiments may include several geo- lifetime measurement itself. metries of your detectors such as (1) a single counter, (2) multiple counters placed in the same plane to increase detector area, (3) closely stacked counters that require coinci- dence, and (4) separated, stacked counters that require coincidence used to look at flux from only a particular direction of the sky. 1 There are several interesting variables to test 2 in flux studies. Have your students generate the questions that they want to study. In analyzing flux study data, students select the data set and channel(s) to be analyzed. Figure20. A three counter muon lifetime de- The analysis tools produce a histogram of tector system. flux vs. time similar to the one shown in Figure 19. Students can juxtapose data re- Consider Figure 20. When a muon enters a garding their independent variable as a counter, a signal is generated in that counter. function of time to look for correlations. If the muon traveled through counter 1 and into counter 2, both of these counters should have a signal at approximately the same Muon Lifetime Experiments time. If the muon has stopped within counter 2, however, it will “wait around” until it de- cays and generate a second signal in counter 2 upon decay. The time tDECAY, the time between the two signals in counter 2, can be extrapolated from the data file. Since no sig- nal occurred in counter 3, this is a strong indication that a muon did indeed decay in counter 2. Theory suggests that the muon lifetime ex- hibits the characteristic exponential decay that is common to other physical phenom- ena, such as radioactive decay. In the case of Figure19. Cosmic ray flux (events/detector area/sec) vs. time graph radioactive materials, the decay expression over a period of two hours. is given by t − A classic modern physics experiment is the N (t ) = N 0 e τ measurement of the muon mean lifetime. This experiment has historical significance as the very first experimental evidence of
  31. 31. Classroom Cosmic Ray Detector User’s Manual where N0 is the initial population, N(t) is the level should be set locally. Instead, coinci- population after time t, and τ is the mean dence levels may be requested in the web lifetime. The fact that muons in our experi- analysis. Students select the data file to be ment do not coexist, whereas they do in ra- analyzed, the coincidence level, and the time dioactive materials, does not change the de- interval between which coincidences must cay expression. This does, however, give a have occurred. The lifetime can be deter- slightly different meaning to N(t) and N0. In mined by fitting this decay plot with an ex- this experiment, N0 is not the number of ponential decay function of the form de- muons present at t = 0 but represents the to- scribed above. It is also possible for students tal number of muons that were captured and to estimate the background value that should decayed within the detector. N(t) is then the be subtracted from each bin before fitting. number of these muons with tDECAY > t. This is also shown in Figure 21. It should also be mentioned here that the muon’s time of flight before it entered the Shower Studies detector is of no concern. Though tDECAY is With the absolute time stamp that GPS of- not its entire lifetime, plotting the number of fers, a network of detectors (at the same site, decays as a function of decay time will yield or at different schools) can study cosmic ray the same exponential decay shape from showers. The web analysis tools allow stu- which lifetimes can be found. The common dents to make predictions about which di- “zero time” that we use here is when each rection in the sky the shower (and thus the muon entered the detector. If a large number primary cosmic ray) came from. of decays are collected in the manner de- scribed above, a plot can be produced using Students select the specific data sets to ana- the web analysis tool similar to the one lyze (these may be from different locations), shown in Figure 21. the coincidence level required, and the time interval between which coincidences must While doing a lifetime study, no coincidence have occurred. A 3-D plot of position on the earth (xy) is graphed as a function of signal time (z). This graph allows them to ‘see’ the Muon Lifetime Decay Plot direction from which the shower came. May 28, 2004 (ns) Figure21. A lifetime graph showing number Figure22. Shower study plot showing coinci- of number of decays as a function decay dences within a 70 ns time window. from when the muon entered the scintilla- tor. Fitting this data to an exponential de- cay function can yield the lifetime.
  32. 32. Classroom Cosmic Ray Detector User’s Manual Chapter 6: Advanced Topics Plateauing Counters In order to determine the operating voltage Noting the potentiometer reading on the of each of your counters, it is necessary to bottom of the PMT base, perform a simple perform a simple procedure in which you experiment to determine the singles rate for “plateau the counter.” This step is necessary the channel that the counter is connected to because counting variations due to drifts in using the method described on page 19. the tube gain or voltage can occur during an Continue the process for many different experiment. By using an operating voltage voltages (potentiometer settings) to produce near the center of this plateau, these effects a graph similar to the one shown in Figure are minimized. 23. Remember that the scalers increment in hex format. Also, note that the channels are numbered 1, 2, 3, and 4 on the board, num- bered 0, 1, 2, and 3 respectively on the How to Plateau a counter monitor. This numbering scheme (starting There are two methods to plateau a counter: from 0 instead of 1) is consistent with engi- 1) Plateauing using a single counter neering and computer programming prac- 2) Plateauing using multiple counters tice. The first technique is simpler, however, it may make “seeing” the plateau more chal- Counter Plateau Plot lenging. 100000 Singles Frequency (counts/minute) 10000 1000 100 Plateauing Using a Single Counter 10 To plateau a single counter, choose a thresh- 1 4 4.5 5 5.5 6 6.5 7 7.5 old value for the discriminator and hold this Potentiometer Value constant for your experiment. (Chapter 2 Figure23. Example of counter plateau graph discusses how to choose a threshold value.) used to determine operating voltage of PMT. Turn on the DAQ board and connect it to your computer so that you can view the The optimum operating voltage is where the datastream on your monitor using a terminal semi-log graph looks the most horizontal. emulation program. (See Appendix C for For the graph shown, this occurs around 5.5 help setting up a terminal emulation pro- V. Sometimes it is difficult to “see” the pla- gram.) Once you see the datastream, con- teau with this technique. If multiple counters vince yourself that lower potentiometer dial are available, the technique outlined below values result in a lower counting rate while make the plateau more apparent. higher settings produce a very high datastream rate on your monitor.
  33. 33. Classroom Cosmic Ray Detector User’s Manual Plateauing Using Multiple Counters Multiple Counter Plateau Plot (Counter 2 is being plateaued) Consider the three-counter setup in Figure 24. Let’s plateau counter 2. (This can occur 100 (1 & 2 / 1 & 3) Coincidence 10 even if counters 1 and 3 have not been pla- Frequency 1 teaued.) You can setup an experiment to in- 0.1 vestigate the coincidence frequency between 0.01 4 5 6 7 counters 1 & 2, 2 & 3, and 1 & 3 as a func- Potentiometer Value (for counter 2) tion of the potentiometer setting on the base of counter 2. (You may need to collect data Figure 25. Example of multiple counter for these three coincidence sets during sepa- plateau graph used to determine operating voltage of the PMT for counter 2. rate runs unless you have a splitter for the signal cables and a second DAQ board.) Figure 25 shows that the “flattest” part of the graph is near the potentiometer setting of 5.5. This value will serve as a reasonable value at which to operate this counter. 1 Data Words 2 This section explains the meaning of each of 3 the data “words” that appear as a result of a single event. It is not imperative for you to understand these words in order to collect and analyze data, but understanding these words can help as you attempt to evaluate Figure24. Shown here is a three-counter setup used in plateauing a counter in order to deter- the quality of your data. mine the optimum operating tube voltage. The datastream from the DAQ board is in Since coincidences between 1 & 3 should be ASCII format. Each data line contains 16 relatively constant over the experiment, this words as shown are a sample set of data on can serve as a baseline from which to com- the nest page in Figure 26. Words 1-9 are in pare the 1 & 2 and/or 2 & 3 coincidences. A hex format. The data shown below is for a single event—a single event can span sev- 1& 2 plot of vs. counter 2’s potentiometer eral lines of data! 1& 3 2&3 reading or a plot of vs. counter 2’s 1& 3 potentiometer reading produces a plateau plot similar to that shown in Figure 25. You can see that this plateau region is somewhat “flatter” than was the case with the single counter technique discussed above.
  34. 34. Classroom Cosmic Ray Detector User’s Manual 80EE0049 80 01 00 01 38 01 3C 01 7EB7491F 202133.242 080803 A 04 2 -0389 80EE004A 24 3D 25 01 00 01 00 01 7EB7491F 202133.242 080803 A 04 2 -0389 80EE004B 21 01 00 23 00 01 00 01 7EB7491F 202133.242 080803 A 04 2 -0389 80EE004C 01 2A 00 01 00 01 00 01 7EB7491F 202133.242 080803 A 04 2 -0389 80EE004D 00 01 00 01 00 39 32 2F 81331170 202133.242 080803 A 04 2 +0610 Figure26. A sample event containing five lines of data. Each line is comprised of the same 16-word format with each word separated by a space. Below is a brief description of each of the a new event; 0 = follow-up data of a trigger “words.” event) Word 1: A 32-bit trigger count of the 41.66 Word 3: TMC count of Falling Edge at in- MHz CPLD clock mounted on the DAQ put 0 (FE0). The format used is shown be- board with a range from low: 00000000...FFFFFFFF. Its resolution (1 bits 0-4 = TMC count of falling edge LSB increment) is 24 ns.6,7 A trigger count bit 5 = channel edge tag (1 = valid falling of 00000000 means that the DAQ card is edge, 0 = no falling edge) still in the initialization phase, i.e., the GPS bits 6-7 = not used, always 0. receiver has not started to generate 1PPS pulses yet. Do not use the initial data until Word 4: TMC count of rising edge at input the trigger count becomes non-zero! 1 (RE1); same format as RE0, except bit 7 is always 0. Word 2: TMC count of the Rising Edge at input 0 (RE0). It is also the trigger tag. The Word 5: TMC count of falling edge at input format used is shown below: 1 (FE1); same format as FE0. bits 0-4 = TMC count of rising edge, reso- lution = 0.75 ns (=24 ns/32) Word 6: TMC count of rising edge at input bit 5 = channel edge tag (1 = valid rising 2 (RE2); same format as RE1. edge, 0 = no rising edge) bit 6 = not used, always 0 Word 7: TMC count of falling edge at input bit 7 = trigger tag (1 = new trigger, start of 2 (FE2); same format as FE1. 6 Word 8: TMC count of rising edge at input 24.00 ns if the CPLD clock is exactly 41666666.67 Hz. The actual CPLD frequency of each card is usually tuned within ±30 3 (RE3); same format as RE1. Hz of the target frequency of 41666667 Hz. Generally, an Hz drift from the target CPLD frequency result in accuracy errors of up to n*24 ns. For example, a 30 Hz drift means that the accuracy time Word 9: TMC count of falling edge at input error has a range of ±720 ns if exactly 24.00 ns are assumed for a 3 (FE3); same format as FE1. CPLD clock tick time. It is reasonable to assume that an error within ±1000 ns is acceptable for a school network if the schools are more than 1 mile apart from each other. Word 10: A 32-bit CPLD count of the most 7 recent 1PPS (1 pulse per second) time mark The CPLD clock frequency fluctuates slightly over time, de- from the GPS receiver. This hex word pending on temperature changes and oscillator ageing drifts. Therefore, in order to achieve high accuracy (±50 ns) in computing ranges from 00000000...FFFFFFFF and has the absolute trigger times, you need to poll the current CPLD fre- a resolution of 24 ns just like word 1.8 quency at a regular basis (say once every 5 minutes) with com- mand DG (Display GPS data). If the event rate is high enough (at least 1 event per 100 seconds), the CPLD frequency can be com- puted from the 1PPS counter numbers of consecutive events. 8 The same uncertainty comments apply here as in word 1.
  35. 35. Classroom Cosmic Ray Detector User’s Manual Word 11: UTC time of most recent GPS re- 1 = GPS data possibly corrupted ceiver data update. Although one update is while DAQ uC was/is busy. sent each second, it is asynchronous with the bit 3: 0 = OK 1PPS pulse. The format used is shown be- 1 = Current or last 1PPS rate is not low: within 41666666 ±50 CPLD HHMMSS.mmm clock tick.s (This is a result of a where: HH = hour [00...23] GPS glitch, the DAQ uC being MM = minute [00...59] busy, or the CPLD oscillator not SS = second [00...59] tuned correctly.) mmm = millisecond [000...999] Word 16: The time delay in milliseconds Word 12: UTC date of most recent GPS re- between the 1PPS pulse and the GPS data ceiver data update. The format used is interrupt. A positive number means 1PPS shown below: pulse is ahead of GPS data, and negative ddmmyy number means GPS data is ahead of 1PPS. where: dd = day of month [01...31] To get the actual GPS time to the nearest mm = month [01...12] second, round (word 11 + word 16/1000) to yy = year [00...99] nearest full second. This gives the actual e.g. 03=2003] GPS time at the last 1PPS pulse. Word 13: A GPS valid/invalid flag. See Appendix F for the interpretation of the A = valid (GPS data OK), data for the previous example. V = invalid (insufficient satellite lock for 3- D positioning, or GPS receiver is in ini- tializing phase); time data might be OK Coincidence Counting Varia- if number of GPS satellites is 3 or more and previous GPS status was quot;Aquot; (valid) tions within the last minute. Setting with Keyboard Commands Word 14: The number of GPS satellites Chapter 4 discusses how to use some key- visible for time and position information. board commands to read scaler values over a This is a decimal number between 00…12. known period of time in an effort to deter- mine basic counting rate. That simple cal- Word 15: This hex word is a DAQ status culation does not discuss two parame- flag. The format used is shown below: ters—gate window and TMC delay (d and w bit 0: 0 = OK respectively). The gate window, d, refers to 1 = 1PPS interrupt pending (Warn- how close in time pulses must be to cause a ing flag: If DAQ card is busy, trigger. The TMC delay, w, more difficult to then 1PPS count might lag be- define, determines which pulse edges get hind or get mismatched.) read out into the datastream when a trigger bit 1: 0 = OK occurs, by delaying this information until 1 = trigger interrupt pending (pos- the trigger actually happens. (See Appendix sibly high trigger rate; if con- E for a full explanation of each quantity and tinues, then data might be cor- the details of time coincidence and data rupted.) handling.) bit 2: 0 = OK
  36. 36. Classroom Cosmic Ray Detector User’s Manual Consider the actual physical quantities and Muon Lifetime Measurements relate them to the two parameters, d and w. Set d = 2, and w = 401.  TTRG: This quantity is the maximum If you want to look for muons that stop and time window during which real physics decay (2.2 µs mean lifetime) in a counter, processes might cause a trigger. In other you look for the decay electron pulses sev- words, if you set the card for 2-fold co- eral microseconds after the trigger, so you incidences, and one channel receives a have TWIDTH = 400 (9600 ns, or about 10 signal at t = 0, at least one other channel µs). must show a signal before t = TTRG or you will not receive any information about that pair of pulses. The examples Cosmic Ray Showers in Appendix E may help. Set d = 50 (= 1.2 µs) and w = 100.  TWIDTH: This quantity is the duration In extensive air showers, the main “pan- after the first pulse in a trigger during cake” of arriving particles should be be- which you want to record all rising and tween 10 m and 200 m thick. The earliest falling edge times. The channel in which and latest particles might arrive at the count- these extra pulses occur does not matter; ers about 0.6 µs apart, for a vertical shower. all of them will be saved. Thus you want TTRG to e about 1 µs. TWIDTH can be about the same. The physical quantities TTRG and TWIDTH are the values for d and w you set on the board. However, you should not set huge time win- You set TTRG and TWIDTH based on the type dows “just to be safe.” Doing this causes the of experiment that you want to perform. card to be busy all the time, and its effective Below is a list of several types of experi- dead time will become a serious problem. ments and typical parameter values for each. The values of d and w should be as small as possible for the given physics goals. Muon Telescope Flux Experiments Default values, effective when the card is Set d = 2, and w = 401. powered up, are d = 6 (= 144 ns) and w = If you want to build a muon telescope with 10 (= 240 ns). Modifying these values is counters less than a meter apart, TTRG is the probably unnecessary for muon telescope time it takes a muon to go from the top experiments. However, for air shower de- counter to the bottom counter. Since these tectors or muon lifetime experiments, you muons travel at essentially the speed of light should set them at larger values. These val- (≈ 1 ft/ns), the TTRG is only a few ns so you ues serve as a good guide. If you want to use set d to the minimum value of 2 clock ticks, the values of d = 50 and w = 100, Table 3 or 48 ns. Similarly, to simply count muons, shows how to set the DAQ card to these you are only interested in a short time win- values: dow for edges, so you could take TWIDTH = 48 ns as well. RE Resets the card by setting all counters to zero and return everything to de- faults CD Disables the counters and TMC
  37. 37. Classroom Cosmic Ray Detector User’s Manual WT 01 00 Sets the time register with read pointer (register 01) = 00 and the write WT 02 32 pointer (register 02) to d, in hexadecimal. The TMC delay d will be the dif- ference of these values (so in fact you could set 01 to some value nn, and 02 to nn + d). This example sets d = 50. We have set register 02 to 32 = 50 . H 10 The maximum is 7F = 127 . H 10 WC 00 1F Sets the coincidence level and enable desired channels. Set control reg- isters with the WC (Write Control register) command. Register 0 sets the coincidence level, using its first (left) hex character. Here, 0 means singles, 1 means 2-fold, and n-1 means n-fold coincidence level is to be required. This is different from how coincidence is described in the data analysis software found online. The second (right) hex character represents the de- sired pattern of enabled/disabled channels in its bits, with 0s for disabled channels and 1s for enabled channels. In this example, 1F means set 2-fold coincidences with all 4 channels enabled: F = 1111. If you wanted to dis- H able channel 2 you would use 1101 = D . If you wanted to enable three-fold H coincidences with channel 2 turned off, you would use WC 00 2D. WC 02 64 Sets the value of w (gate width). Registers 2 and 3 hold the gate width set- WC 03 00 ting in integer clock ticks (units of 24 ns). The lower 8 bits of the binary number representing w go in register 2, and the higher 8 bits in 3. Thus, the desired gate width w is given as abcd and is typed as WC 02 cd WC 03 ab As an example, to get w = 100 clock ticks (100 = 0064 = 0000 0000 0110 10 H 0100 ) you would type what is shown to the left. 2 CE Re-enables the onboard counters DS Displays the scaler values Table 3. A keyboard command sequence that allows for coincidence counting variations of the gate window and TMC delay. Interpreting Coincidence Data from has past, you type “DS,” the card will return the DAQ Board the scaler values for each channel as dis- This example shows how to interpret the cussed previously. In addition, the ‘@04’ coincidence data from the board. Assume line returns the value of the 2-fold coinci- you are looking for 2-fold coincidence be- dences. This number is also in hexadecimal. tween any two of the inputs. As discussed in The screen capture on the next page (Figure the previous section, “WC 00 1F” will set up 27) illustrates these commands and outputs. this 2-fold coincidence. If, after some time
  38. 38. Classroom Cosmic Ray Detector User’s Manual RB—resets the scalers and TMC WC 00 1F—sets up two fold coincidence between any of the four channels Board responds that coincidence is set as specified DS—displays the scalers for channels 0-3 (@00 through @03), displays the number of coincidences (@04) present for above defined criteria, displays the 1PPS signal (@5) Figure27. A sample terminal emulation window displaying how to set coincidence levels and how to read this coincidence data. On-board Barometer Calibra- should be recorded every time the command “BA” is typed on the keyboard. The ‘BA’ tion command displays a number that represents You can measure air pressure with a ba- the ADC (analog-to-digital converter) count rometer on the DAQ board. The barometer readout of the on-board barometer. Figure sensor must be calibrated before it can yield 28 illustrates this command. Notice that the pressure in units of kPa. To calibrate the value changes each time the “BA” command sensor, you will need a real barometer (or is typed due to fluctuations in local air pres- some way to retrieve actual pressure in the sure. local area). The actual barometer reading If you take actual barometer readings each sure), a graph similar to the one shown in time the ADC count is recorded (over a pe- Figure 29 can be produced. riod of significant fluctuation in air pres- Figure28. The ADC count is displayed each time the ‘BA’ command is typed.