QuarkNet Cosmic Ray Muon Detector User’s Manual Series .

Classroom Cosmic Ray Muon Detector User’s ManualThis calculation assumes that muons aretraveling at the speed of light—anythingslower would require even more time. If astudent can determine the muon lifetime (asdescribed in Chapter 5) and show that it issignificantly less than this time, they arepresented with the wonderful dilemma thatthe muon’s time of flight is longer than itslifetime!tools of the e-lab can check for multipledetectors firing in coincidence.The QuarkNet online analysis tools allowstudents to not only look for showers but tomake predictions about the direction fromwhich the shower (and thus the primarycosmic ray) originated. Details for doingshower studies can be found in Chapter 5and in the QuarkNet Cosmic Ray shower/tutorial.jspThis time dilation “proof” assumes that allmuons are created in the upper atmosphere.Although this is actually a goodapproximation, your students cannot test it.However, by using the mean lifetime valueand by measuring flux rates at twosignificantly different elevations, you candevelop experimental proof for timedilation. This experiment requires you tohave access to a mountain, an airplane, orcollaboration with a team from anotherschool that is at a significantly differentaltitude! Here is a wonderful opportunity forschools to work together proving timedilation. A very thorough explanation of thisexperiment is outlined in the 1962 classroommovie titled, “Time Dilation: AnExperiment with Mu Mesons.”2 This moviehelps you (and your students) understandhow the muon lifetime measurement (alongwith flux measurements at two differentaltitudes) can be used to verify time dilation.Shower StudiesWith the GPS device connected to yourDAQ board, the absolute time stamp allowsa network of detectors (at the same site or atdifferent schools) to study cosmic rayshowers. Your students can look for smallshowers over their own detectors, orcollaborate with other schools in the area tolook for larger showers. The online analysis2This 30-minute movie can be ordered on CD for 10from http://www.physics2000.com/.5

Classroom Cosmic Ray Muon Detector User’s ManualChapter 2: Hardware Introduction5. GPS module.6. GPS antenna.7. Temperature sensor.8. 5 VDC power supply.9. PDU power cable.10. Power distribution unit, PDU.11. Power extension cables for PMTs.12. USB cable.13. Personal Computer.Hardware OverviewSee Appendix G for alternate setup.Figure 7 shows QuarkNet’s most recentcosmic ray muon detector (CRMD) setupcomposed of:1. Counters – Scintillators,photomultiplier tubes and PVChousing.2. BNC signal extension cables.3. QuarkNet DAQ data acquisitionboard.4. CAT-5 network cable.91381275103461112Figure 7. The components of the 6000 series QuarkNet cosmic ray muondetector.6

Classroom Cosmic Ray Muon Detector User’s ManualFor this setup, the DAQ board takes thesignals from the counters and providessignal processing and logic basic to mostnuclear and particle physics experiments.The DAQ board can analyze signals from upto four PMTs. (We show two in the figure.)The board produces a record of output datawhenever the PMT signal meets a predefined trigger criterion (for example, whentwo or more PMTs have signals above somepredetermined threshold voltage, within acertain time window).and environmental sensor data (temperatureand pressure).This chapter includes an overview of eachmajor component shown in Figures 7 and 8.Additional details regarding somecomponents are in the appendices.DAQ Readout BoardFigure 8 is a picture of the DAQ board withseveral major components numbered.1. GPS input2. GPS fanout to another DAQ board3. Board reset button4. Coincidence counter display5. Inputs for 4 counters (channels 0-3)6. CPLD (programmable fast logic)7. Time-to-digital converter (TMC)8. USB port (output to PC)9. 5 VDC input10. 5 VDC output to power distributionunitThis board is the logic link between thescintillation counters and the PC. The boardprovides discriminators and trigger logic forfour channels of PMTs. The board includesfive built-in scalers, allowing simultaneouscounts of singles on each channel, plustriggers at whatever logic level you specify(2- to 4-fold majority logic).The output data record, which can be sentvia a standard USB cable to any PC,contains temporal information about thePMT signals. This information includes:how many channels had above-thresholdsignals, theirrelative arrival times (precise to 0.75 ns),detected pulse. In addition, an external GPSreceiver module provides the absolute UTCtime of each trigger, accurate to about 50 ns.This allows counter arrays using separateDAQ boards—such as different schools in awide-area array or two sets of counters at thesame site—to correlate their timing data.Keyboard commands allow you to definetrigger criteria and retrieve additional data,such as counting rates, auxiliary GPS data,95610874213Figure 8. Close-up view of the DAQ: dataacquisition card.7

Classroom Cosmic Ray Muon Detector User’s ManualA standard USB interface can be connectedto any PC (Windows, Linux or Mac). Thedatastream consists of simple ASCII textlines readable by any terminal emulationprogram. In addition, a GPS clock providesaccurate event time data synchronized toUniversal Time (UTC), so widely separatedsites can compare data. In datastream mode,the DAQ board outputs a series of text linesreporting event data: trigger time in UTC(with 10 ns resolution and absolute accuracyabout 100 ns), leading and trailing edgetimes for each pulse recorded within thecoincidence time window (with 1.25 nsprecision), and data from the GPS andinternal clocks. Additional keyboardcommands allow reading of temperature,barometric pressure and other sensors.Board ComponentsFigure 9 is a block diagram of the QuarkNetDAQ v2.5 board. (See Appendix B for aschematic diagram.)Figure 9. Block diagram of the QuarkNet DAQ v2.5 board.8

Classroom Cosmic Ray Muon Detector User’s ManualDiscriminatorsPMT signals are first pre-amplified by afactor determined from a set of changeableresistors (Amplification factor x10 is usedfor QuarkNet DAQ.) Discriminators areimplemented using voltage comparator chipswith the reference threshold voltage variedthrough the terminal emulator program andthe TL command. Default settings on theDAQ card are -300-mV threshold. It isimportant to remember that the voltagecomparators look at the amplifier output, soraw PMT signal levels are multiplied by anyamplification factor present before beingcompared. For example, if you used a-30 mV threshold with NIM discriminatorsand have x10 amplification, your thresholdlevel on the QuarkNet DAQ board should beset to -300 mV.time intervals (pre-6000 DAQ: 0.75 ns). Ifthe trigger criterion is satisfied, the TDCdata are latched and read out, giving leadingand trailing edge times for each channelrelative to the trigger time in units of 1.25ns. This allows you to calculate PMT pulsewidths (time over threshold, or ToT; seeFigure 10) as a crude estimate of pulse areaand thus energy.Figure 10. Typical PMT pulse from whichthe time over threshold (ToT) can bedetermined.Complex Programmable Logic Device(CPLD)Trigger logic is implemented using a CPLDchip. Software revisions for this chip mustbe prepared using special software but canbe downloaded via the serial port. Thisflexibility allows the engineers to distributeupdates that alter the fast logic, if necessary.Any trigger logic level from singles to 4fold can be set by keyboard commands tothe board. Majority logic is used: anycombination of three active channels causesa trigger at the 3-fold level, for example.Time-to-Digital Converters (TDCs)Discriminator output pulses are fed intoTDCs3 which measure the arrival time ofleading and trailing pulse edges. TDCs keeptrack of their state (high or low) at 1.25 ns3“TDC” is a generic term in particle physics technology. Thespecific chips used on the board are TMCs (“Time MeasurementChips”). You may see this designation in some documentation.9

Classroom Cosmic Ray Muon Detector User’s ManualMicrocontroller (MCU)The MCU is really just a special-purposeCPU that provides the onboard “slow” logic(with a time scale of microseconds, notnanoseconds) to interface the board to youvia a terminal window or equivalent on yourPC. At present, the MCU can bereprogrammed to redefine functionality onlyby using special software and burn-inhardware.Board FunctionalityThreshold Detection and SettingAfter amplification, the PMT signals are fedto voltage comparators. The comparators seta HIGH logic level whenever the amplifiedPMT signal exceeds their negative thresholdvoltage setting as shown in Figure 11. Thislogic level has 1.25 ns resolution. Thismeans that we can measure the timedifference between pulses on differentchannels at the same site down to less than ananosecond! We depend on the GPS clocks,with 10 ns resolution (pre-6000 DAQ: 24ns), for timing between school sites.Auxiliary SensorsThere is a temperature sensor built into themicrocontroller chip. This sensor is presentso that CPU temperature can be measured.This temperature and the supply voltage arereported whenever the board is started up.While the board components are rated fortemperatures between –20 C and 80 C,the board temperature should not normallygo above 50 C.ReferencegroundSignal afteramplificationA second temperature sensor is located onthe booster board built into the GPS cable’s“far-end” DB9 connector. This sensor canbe used to log outdoor temperature at thecounter locations, and is read out with akeyboard command. The GPS module maybe damaged if its temperature goes below–40 ºC or above 85 ºC.4User determinedthresholdFigure 11. A typical negative PMT pulseshowing the user-defined threshold valuecompared to reference ground.A barometric pressure sensor is built into theboard as well. It can also be calibrated andread out (in units of millibars) with akeyboard command.4At DAQ card power-up or reset, the temperature on the DAQ card(actually, inside the MCU chip) is reported. Note that this is not agood indicator of air temperature at the card site, even if the card islocated outdoors, since the chip generates considerable heat whileoperating. It is a useful way to make sure the card is healthy,however! Temperatures over 40 ºC may be dangerous to thecard’s chips.10

Classroom Cosmic Ray Muon Detector User’s ManualSetting the threshold level too high will missall but those events that correspond to alarge amount of energy deposited in thedetector; setting the threshold too low willresult in background noise within theelectronics being mistaken as event counts.The plot shows the singles rate in thatchannel as a function of threshold value.You can see that the counting rate decreasesas the threshold setting increases. Once thethreshold value became as high as 0.5 V, thecounting rate decreased much more slowly.The “kink” in the graph suggests that noisecontaminated the counting rate when thethreshold was set below 0.5 V. Operatingslightly above this value is probably theoptimum threshold value to choose.So how do you determine the appropriatethreshold setting? Consider the graph inFigure 12 below as a “ballpark”determination of the optimum thresholdvalue to use for a channel. For the datashown in this graph, the PMT voltage wasset to that determined by plateauing thecounter (see section 6.1 for help on how thisis done) and kept constant throughout theexperiment.Change the threshold by using the TLcommand from hyperterm. For instance,TL 2 500 will set the threshold of channel 2to -500 mV. The command TL 4 800 will setall four channels to a threshold of -800 mV.The default threshold equals -300mV.Frequency(counts/minute)Threshold 30.40.50.6Threshold (V)Figure 12. A graph of frequency vs. threshold for a counter used todetermine threshold setting.110.70.8

Classroom Cosmic Ray Muon Detector User’s Manualmodular switching power supply, of the typeused for many small electronic devices, issupplied with the board. Replacements arereadily available at Radio Shack or a similarelectronics store.Coincidence LogicThe DAQ board can also determine whethersignals in separate channels are coincident intime. For example, if the trigger criterion isset to 2-fold, then as soon as any channelgoes above threshold, a time window isopened. (Window time width is adjustable.)If any other channel goes above thresholdduring this time window, all event data arelatched and outputted for the overlap timeinterval when both are active. Notice thatpulse data are reported for a time intervalthat is not of fixed length but just covers theoverlap period when two or more channelsare active. Leading and trailing edge timesare reported for any active channels (not justfor the two channels that launched thetrigger), with empty data entries forchannels that remained inactive during thetrigger window. For a single event trigger,the DAQ board may need to output severallines of data. The first line has an “eventflag” for identification. Any following lineswithout this flag are simply additional datafor the same event.PMT bases provided by QuarkNet can bepowered by the same 5 VDC power supplythat is plugged into the DAQ board. ThePMTs are connected to the board through apower distribution unit (PDU: item #10 inFigure 7). This simple box of fourpotentiometers allows one to easily changethe voltage to each independent PMT foroptimal settings.Note: It is important to distinguish theseDAQ power supply modules from otherunits used for computers or other electronicdevices requiring DC voltage. Connection ofthe wrong power supply to your DAQ boardwill probably damage it with too muchcurrent or voltage.GPS ReceiverRate CountersThe card has five built-in counters,numbered 0 through 4: counters 0 through 3record the singles count for each channel,and counter 4 records the trigger count forwhatever coincidence level you have set. Byzeroing these counters with a softwarecommand (RB), then running the board for afixed length of time and reading out thecounters at the end, you can obtain singlesrates for each channel as well as thecoincidence rate.Figure 13. The GPS unit consists of a100-ft. CAT-5 network cable (greencable), a GPS module (gray box), anantenna (black cable), and atemperature probe (gray cable, redend).Power SupplyThe board requires a stable 5 VDC powersupply, with 800 mA or greater outputcurrent. A 110 VAC to 5 VDC/2.4A12

Classroom Cosmic Ray Muon Detector User’s ManualThe GPS unit shown in Figure 13 isconnected to the DAQ with a standardEthernet cable. An external temperaturesensor and the GPS antenna plug into oneside of the GPS module (a circuit board),while the Ethernet cable plugs into the otherside. The GPS antenna is weatherproof withmagnetic backing, but the module should beprotected.and longitude down to the equivalent of afew 10 s of meters.The “1PPS” SignalThe stock NavSync GPS module wasmodified slightly for our application toallow the more precise timing we need downto 10 s of nanoseconds (eight decimal placesfollowing integer seconds). The GPSreceiver outputs a logic pulse at thebeginning of each UTC second, called the1PPS (1 Pulse Per Second) signal.GPS StartupOnce powered-up, the GPS module shouldquickly “find itself” if it is in a location witha clear view of at least half the sky. Usuallyit does not work well through windows andshould be physically outdoors. The receiverwill lock onto four or more satellites,download the data needed to operateaccurately and start averaging its positionand clock settings at one-second intervals.Within a few minutes after startup, youshould obtain accurate GPS data. The unit’sLED display blinks red when first poweredup and searching, and changes to long greenfollowed by short greens for number ofsatellites when it has acquired enoughsatellite data to locate itself accurately. Timedata are not accurate until then.The GPS receiver sends two types ofdatastreams to the board. The first is RS-232ASCII data telling what time it is, at whatlatitude, longitude and altitude the receiveris, and information about the satellites thereceiver is using. The other data is a 5 V,100-ms pulse telling exactly when the datais true. Each stream of 5 V, 100-ms pulsesarrives every second, thus the 5 V pulse isnamed 1 pulse per second (1PPS). Themicrocontroller on the board records thecounter value during which the pulse isreceived. The time is according to a counterrunning at 25 MHz.In principle, the leading edges of 1PPSsignals from GPS receivers anywhere in theworld are all in synch, to within theaccuracy of the non-military GPS system(about 100 ns.) This feature allows accuratetime synchronization between school sites.The special connector and cable attached tothe commercial GPS module transmits the1PPS signal about 100 ft. or more withoutexcessive timing degradation.The GPS module provides several kinds ofdata. The commercial GPS module directlysupplies the date and “coarse” time (in UTC,not local time) down to milliseconds, orthree decimal places after seconds, andreports its geographic location in latitude13

Classroom Cosmic Ray Muon Detector User’s Manualmillisecond, but there is a delay of a fractionof a second relative to the 1PPS edge. Forexample, when the 1PPS signal says it isexactly 12:01:25.000000000, the GPS timefield in the data record may say it is12:01:24.850. This delay (reported as 0.150 sec in this example) will cause youto associate the wrong integer second withthe event time if you are not careful. (TheGPS time delay may be negative, i.e., aforward shift, with the ASCII time dataahead of the 1PPS signal.) The DAQ boardis programmed to list the delay inmilliseconds for each data record so thereported time in seconds can be corrected tomatch the 1PPS data. The algorithmdescribed in Appendix D implements thesecalculations.How to Calculate Event TimesThe DAQ board has onboard a 25 MHzoscillator, which “ticks” every 40nanoseconds (1/25 x 106 sec). Such a devicedoes not maintain its frequency veryaccurately, and our oscillator’s frequencymay drift by 10-100 Hz from its nominalvalue (perhaps even more under extremetemperature changes). A counter (scaler)keeps counting the clock ticks. Wheneverthis 32-bit counter reaches its maximumcapacity of 4.3 billion, approximately every100 seconds, it just “rolls over” back to zero,like an old car’s odometer, and continuescounting. Clock calibration occurs in theinternal clock counter every time a 1PPSsignal arrives.Find the precise event time by getting thedate and time down to the nearest secondfrom the GPS module’s coarse time (withone correction described below), finding thenumber of clock ticks between the last 1PPSand the event trigger, and dividing by thenumber of clock ticks between the last two1PPS pulses to get the fraction of a seconddown to 40 ns precision. Of course, youhave to know whether the counter rolledover during the last second when doing thearithmetic.For people who like really deep-downdetails, the delay mentioned above, whichyou can determine and correct, is not thewhole story. There is an additional delay of150-250 ms, which is unresolvable, since itis buried in the manufacturer’s firmware.Luckily, it is too small to cause the roundingprocedure described above to go wrong.Remember, all this is just about determiningthe integer seconds part of the time.Nanosecond-level timing is not affected.Also, we have not discussed corrections forthe antenna cable delay—the event timecalculated is when coincidence occurs at theDAQ board, not at the counters. This is ofno consequence as long as all school sites inan array use approximately the same cablelengths, within a few 10 s of meters. Thealgorithm showing how to get the preciseevent time is shown in Appendix D.There i

Cosmic Ray Experiments within the Classroom . 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 . you can run these cosmic ray experiments in parallel with your standard curriculum, setting aside a day or two .