The East-West Cosmic Muon Assymmetry - McGill Physicsgriffins/muon_poster.pdf · THE EAST-WEST...
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T HE E AST -W EST C OSMIC M UON A SYMMETRY S EAN C. G RIFFIN S UPERVISOR :DAVID H ANNA D EPARTMENT OF P HYSICS M C G ILL U NIVERSITY ,M ONTREAL ,Q C M OTIVATION The constant bombardment of the Earth’s atmosphere by cosmic rays cre- ates a constant shower of secondary particles, one of which is the π meson (also known as the pion). Having a lifetime of 2.6 × 10 -9 s, pions are ex- tremely unstable and rapidly decay into secondary particles. The primary decay modes of both charged and neutral pions are given by π 0 → 2γ, π + → μ + + ν μ , π - → μ - + ν μ . Charged pion decays are of particular interest. Due to the fact that there are two different charged particles in question (μ + and μ - ) and the fact that the Earth possesses a magnetic field, it is simple to see that positively charged particles will be deflected in a direction opposite to the negatively charged ones. Positive muons will be deflected east (referred to as “westerly” muons) and negative muons will be deflected west (referred to as “easterly” muons). Given that most cosmic rays are positively charged, there should be slightly more positive muons than negative muons. One would thus expect to see an east-west asymmetry. This effect has previously been measured to be on the order of 15 - 20%[1]. This measurement was, however, made at lower latitudes where the rigidity cutoff (that is, the minimum energy required by a particle to be deflected by a magnetic field) is 17GeV/c. The objective of this experiment is to measure this asymmetry in Montreal, where the rigidity cutoff is an order of magnitude less (1.45 GeV/c). A PPARATUS The apparatus used to measure the east-west asymmetry consists of three muon counters arranged in the configuration seen in Fig.1. A muon counter consists of a photomultiplier tube (PMT) in direct contact (with an opti- cal grease intermediary) with a large piece of scintillator wrapped in alu- minium and sealed with electrical tape to prevent light leaks. 0 1 2 2 1 0 Counters Delay Trigger Electronics PC interface QDC TDC High Voltage Supply Figure 1: Left: Photo of apparatus with major components identified. Right: Apparatus block diagram. Red: PMTs. Blue: Scintillator. The arrows correspond to either trajectory that muons must take in order to trigger an event. The numbering convention remains the same in subsequent references to the muon counters. The concept behind this design is simple: an event is registered when a muon is detected by both the top counter and one of the lower ones (i.e. such that an event constitutes a muon travelling in one direction or the other). The electronics used to register each event consist of a charge-to-digital converter (QDC) and time-to-digital converter (TDC). A QDC functions by means of a gate input and one or more signal inputs (referred to as channels). While the gate is open, any signal will be integrated and the value read out to a computer. In the event that there is no signal on a given channel, what is known as the “pedestal” (that is, the residual current on the input) will still be integrated yielding a non-zero result. It is thus important to remove the pedestal in the data analysis by cutting off data on the lower range. A TDC also requires two (or more) inputs, one constituting the timer start signal and the stop signals. The TDC converts the time difference between the start signal (which is common to all channels) and the stop signal. This value is then converted to arbitrary digital units which can be used to compare, for example, differences in arrival times between different counters. The passing of a muon between the top and either of the two bottom coun- ters opens the gate on the QDC and starts the TDC. The outputs of the counter PMTs are connected to both a QDC and a TDC. The complete setup is depicted in Fig.2. 0 1 TDC Delay QDC 1 2 Figure 2: Data acquisition setup. The delay (which allows time for the start and gate signals to be produced) was taken to be 200ns. Note that the purple triangles on the TDC and QDC are the common start and gate inputs, respectively. Any inputs requiring digital logic are first discriminated. It must be noted that due to the inherent differences in the efficiency of each muon counter, the rates calculated for both lower counters cannot be directly compared. Instead, to calculate an asymmetry, the apparatus must be rotated 180° and another data set must be taken. C ALIBRATION In order for the flux to be correctly measured, that is, for a muon counter not to miss any events, a high voltage must be chosen for the PMTs such that they react to all incoming photons released by the scintillator, but that the amount of “dark” current is minimal. The operational voltages for the PMTs were determined using two smaller muon counters (henceforth referred to as “calibrators”) with voltages set far into their expected operating ranges (≈ 1800V). The counter to be calibrated is sandwiched between the calibrators. The output of each counter is connected to a discriminator (which turns analogue signals into digital logic levels) such that digital logic can then be applied to the signals. The counter outputs are sent through the circuit as seen in Fig.3. Doubles correspond to muons passing through the counters where a signal is only registered by the calibrator PMTs whereas triples correspond to a signal received from all three. A B T D B C Figure 3: Muon counter calibration circuit. A and C correspond to calibrator counters and B is the counter being calibrated. D is the doubles output, T the triples output. The black arrow represents a muon passing through the calibration circuit. In order to pass through both callibrators, it must also pass through the counter requiring calibration. The ratio of triples to doubles is plotted in Fig.4. One sees that for high voltages, the curve saturates. This is the region where we are assured that the counter is picking up all traversing muons. We must, however, recall that an operational voltage must be chosen low enough to avoid excessive dark current so voltages are taken close to the inflection point but still well within the plateau. V 1200 1300 1400 1500 1600 1700 1800 Triples/Doubles 0 0.2 0.4 0.6 0.8 1 Figure 4: Calibration curve for PMT2. The chosen voltage was 1450V The charge data were found to be very clean (i.e. no noise at the high end or low end), as is seen in Fig.5. Each charge distribution has the form of a Landau distribution, which is expected as this distribution describes the fluctuations in the energy loss of a charged particle passing through a layer of matter. Each bin is filled with the integrated current from a muon pulse (an example of which is seen in Fig.6). 0 500 1000 1500 2000 2500 3000 3500 4000 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 Counter 0 0 500 1000 1500 2000 2500 3000 3500 4000 0 2000 4000 6000 8000 10000 12000 14000 16000 Counter 1 QDC Channels 0 500 1000 1500 2000 2500 3000 3500 4000 0 2000 4000 6000 8000 10000 Counter 2 Figure 5: The charge distributions caused by muons. This plot corresponds to the inte- grated charge values as measured by the QDC. Cuts on the data have been made such that all events lie between 400 and 3700 QDC units such that the QDC pedestal and overflow values do not appear. Figure 6: An oscilliscope trace containing the gate signals and counter signal as a muon passes through the apparatus. Trace 1: The gate pulse (width 200ns, 1V/div). Trace 2: The counter signal (100mV/div). P RELIMINARY R ESULTS The muon flux rate can be calculated two ways. The obvious and simple method of doing so is to divide the total number of valid events in a data set by the duration of the run (i.e the pure average). This is a reasonable method of calculating it because over long periods of time, the muon flux is constant as is shown in Fig.7. s 300 400 500 600 700 800 900 3 10 × 0 2000 4000 6000 8000 10000 12000 s 300 400 500 600 700 800 900 3 10 × 11800 11900 12000 12100 12200 12300 12400 12500 12600 12700 12800 Figure 7: An example of an unmodified data set (i.e. no cuts) taken over roughly 4 days. Top: Bin sizes correspond to one hour. The number of events per unit time is seen to be fairly stable. Bottom: The same data set (zero-suppressed) with error bars corresponding to the Poissonian error. The second is to generate a plot corresponding to the time delay between two events. The curve will be an exponential with a slope whose absolute value is equal to the muon flux rate. Such a curve is depicted below in Fig.8. s 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Frequency 10 2 10 3 10 4 10 Figure 8: Example histogram of event delay times for muons passing through counter 1. The fit curve is of the form exp(αt + β) with α =-1.487 ± 0.003s -1 and β = 9.506 ± 0.002. For the above data, the muon flux rate through a given counter was found to be 1.437 ± .004s -1 and 1.487 ± .003s -1 for the direct average and ex- ponential methods, respectively. While not strictly consistent, they differ only by ≈ 3%. Given that the lower value originates from the direct aver- age method, one can attribute this discrepancy to dead time (time when the equipment is busy and cannot accept more events) between events on very short timescales within the apparatus. The fit method, however, yielding a function that can describe the expected number of events for very short times between events, automatically compensates for this. C ONCLUSIONS The east-west muon asymmetry can be measured using a fairly simple apparatus consisting of three muon counters, a QDC and TDC. Due to the fact that this effect is very small (expected to be on the order of a few %), long data runs (on the order of several hours to days) must be taken to ensure that the flux rates are statistically significant enough for the ratio to be measured without having to worry about error (which is proportional to 1 √ t , where t is the length of the data run). The main issue with measuring the asymmetry lies in the systematic error, given that the statistical error is known to less than 1%. Therefore, for the next several weeks, measurements will continue in an effort to detect the asymmetry. R EFERENCES [1] P. N. Dinh, N. T. Dung, B. D. Hieu, N. Phuc, P. T. Phuong, P. Darriulat, D. Q. Thieu, and V. V. Thuan. Measurement of the vertical cosmic muon flux in a region of large rigidity cutoff. Nuclear Physics B, 627(1-2):29 – 42, 2002. http://www.physics.mcgill.ca/ :griffins griffi[email protected]
Transcript of The East-West Cosmic Muon Assymmetry - McGill Physicsgriffins/muon_poster.pdf · THE EAST-WEST...
THE EAST-WEST COSMIC MUON ASYMMETRYSEAN C. GRIFFIN
SUPERVISOR: DAVID HANNADEPARTMENT OF PHYSICS
MCGILL UNIVERSITY, MONTREAL, QC
MOTIVATIONThe constant bombardment of the Earth’s atmosphere by cosmic rays cre-ates a constant shower of secondary particles, one of which is the π meson(also known as the pion). Having a lifetime of 2.6 × 10−9s, pions are ex-tremely unstable and rapidly decay into secondary particles. The primarydecay modes of both charged and neutral pions are given by
π0 → 2γ,π+ → µ+ + νµ,π− → µ− + νµ.
Charged pion decays are of particular interest. Due to the fact that thereare two different charged particles in question (µ+ and µ−) and thefact that the Earth possesses a magnetic field, it is simple to see thatpositively charged particles will be deflected in a direction opposite to thenegatively charged ones. Positive muons will be deflected east (referred toas “westerly” muons) and negative muons will be deflected west (referredto as “easterly” muons).
Given that most cosmic rays are positively charged, there should be slightlymore positive muons than negative muons. One would thus expect to seean east-west asymmetry. This effect has previously been measured to be onthe order of 15 − 20%[1]. This measurement was, however, made at lowerlatitudes where the rigidity cutoff (that is, the minimum energy required bya particle to be deflected by a magnetic field) is 17GeV/c. The objectiveof this experiment is to measure this asymmetry in Montreal, where therigidity cutoff is an order of magnitude less (1.45 GeV/c).
APPARATUSThe apparatus used to measure the east-west asymmetry consists of threemuon counters arranged in the configuration seen in Fig.1. A muon counterconsists of a photomultiplier tube (PMT) in direct contact (with an opti-cal grease intermediary) with a large piece of scintillator wrapped in alu-minium and sealed with electrical tape to prevent light leaks.
0
12 2 1
0Counters
Delay
Trigger
Electronics
PC interface
QDC
TDCHigh
Voltage
Supply
Figure 1: Left: Photo of apparatus with major components identified. Right: Apparatusblock diagram. Red: PMTs. Blue: Scintillator. The arrows correspond to either trajectorythat muons must take in order to trigger an event. The numbering convention remains thesame in subsequent references to the muon counters.
The concept behind this design is simple: an event is registered whena muon is detected by both the top counter and one of the lower ones(i.e. such that an event constitutes a muon travelling in one directionor the other). The electronics used to register each event consist of acharge-to-digital converter (QDC) and time-to-digital converter (TDC).
A QDC functions by means of a gate input and one or more signal inputs(referred to as channels). While the gate is open, any signal will beintegrated and the value read out to a computer. In the event that there isno signal on a given channel, what is known as the “pedestal” (that is, theresidual current on the input) will still be integrated yielding a non-zeroresult. It is thus important to remove the pedestal in the data analysis bycutting off data on the lower range.
A TDC also requires two (or more) inputs, one constituting the timer startsignal and the stop signals. The TDC converts the time difference betweenthe start signal (which is common to all channels) and the stop signal.This value is then converted to arbitrary digital units which can be usedto compare, for example, differences in arrival times between differentcounters.
The passing of a muon between the top and either of the two bottom coun-ters opens the gate on the QDC and starts the TDC. The outputs of thecounter PMTs are connected to both a QDC and a TDC. The completesetup is depicted in Fig.2.
0
1
TDC
Delay
QDC
1
2
Figure 2: Data acquisition setup. The delay (which allows time for the start and gatesignals to be produced) was taken to be 200ns. Note that the purple triangles on the TDCand QDC are the common start and gate inputs, respectively. Any inputs requiring digitallogic are first discriminated.
It must be noted that due to the inherent differences in the efficiency ofeach muon counter, the rates calculated for both lower counters cannot bedirectly compared. Instead, to calculate an asymmetry, the apparatus mustbe rotated 180° and another data set must be taken.
CALIBRATION
In order for the flux to be correctly measured, that is, for a muon counternot to miss any events, a high voltage must be chosen for the PMTs suchthat they react to all incoming photons released by the scintillator, but thatthe amount of “dark” current is minimal.
The operational voltages for the PMTs were determined using two smallermuon counters (henceforth referred to as “calibrators”) with voltages setfar into their expected operating ranges (≈ 1800V). The counter to becalibrated is sandwiched between the calibrators. The output of eachcounter is connected to a discriminator (which turns analogue signals intodigital logic levels) such that digital logic can then be applied to the signals.
The counter outputs are sent through the circuit as seen in Fig.3. Doublescorrespond to muons passing through the counters where a signal is onlyregistered by the calibrator PMTs whereas triples correspond to a signalreceived from all three.
A
BT
D
B
C
Figure 3: Muon counter calibration circuit. A and C correspond to calibrator countersand B is the counter being calibrated. D is the doubles output, T the triples output. Theblack arrow represents a muon passing through the calibration circuit. In order to passthrough both callibrators, it must also pass through the counter requiring calibration.
The ratio of triples to doubles is plotted in Fig.4. One sees that for highvoltages, the curve saturates. This is the region where we are assured thatthe counter is picking up all traversing muons. We must, however, recallthat an operational voltage must be chosen low enough to avoid excessivedark current so voltages are taken close to the inflection point but still wellwithin the plateau.
V1200 1300 1400 1500 1600 1700 1800
Tri
ple
s/D
ou
ble
s
0
0.2
0.4
0.6
0.8
1
Figure 4: Calibration curve for PMT2. The chosen voltage was 1450V
The charge data were found to be very clean (i.e. no noise at the high endor low end), as is seen in Fig.5. Each charge distribution has the formof a Landau distribution, which is expected as this distribution describesthe fluctuations in the energy loss of a charged particle passing through alayer of matter. Each bin is filled with the integrated current from a muonpulse (an example of which is seen in Fig.6).
0 500 1000 1500 2000 2500 3000 3500 400002000400060008000
1000012000140001600018000200002200024000
Counter 0
0 500 1000 1500 2000 2500 3000 3500 40000
2000
4000
6000
8000
10000
12000
14000
16000
Counter 1
QDC Channels0 500 1000 1500 2000 2500 3000 3500 40000
2000
4000
6000
8000
10000
Counter 2
Figure 5: The charge distributions caused by muons. This plot corresponds to the inte-grated charge values as measured by the QDC. Cuts on the data have been made such thatall events lie between 400 and 3700 QDC units such that the QDC pedestal and overflowvalues do not appear.
Figure 6: An oscilliscope trace containing the gate signals and counter signal as a muonpasses through the apparatus. Trace 1: The gate pulse (width 200ns, 1V/div). Trace 2:The counter signal (100mV/div).
PRELIMINARY RESULTSThe muon flux rate can be calculated two ways. The obvious and simplemethod of doing so is to divide the total number of valid events in a dataset by the duration of the run (i.e the pure average). This is a reasonablemethod of calculating it because over long periods of time, the muon fluxis constant as is shown in Fig.7.
s300 400 500 600 700 800 900
310×0
2000
4000
6000
8000
10000
12000
s300 400 500 600 700 800 900
310×1180011900120001210012200123001240012500126001270012800
Figure 7: An example of an unmodified data set (i.e. no cuts) taken over roughly 4 days.Top: Bin sizes correspond to one hour. The number of events per unit time is seen to befairly stable. Bottom: The same data set (zero-suppressed) with error bars correspondingto the Poissonian error.
The second is to generate a plot corresponding to the time delay betweentwo events. The curve will be an exponential with a slope whose absolutevalue is equal to the muon flux rate. Such a curve is depicted below inFig.8.
s0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Fre
qu
ency
10
210
310
410
Figure 8: Example histogram of event delay times for muons passing through counter1. The fit curve is of the form exp(αt + β) with α = −1.487 ± 0.003s−1 and β =9.506± 0.002.
For the above data, the muon flux rate through a given counter was foundto be 1.437± .004s−1 and 1.487± .003s−1 for the direct average and ex-ponential methods, respectively. While not strictly consistent, they differonly by ≈ 3%. Given that the lower value originates from the direct aver-age method, one can attribute this discrepancy to dead time (time when theequipment is busy and cannot accept more events) between events on veryshort timescales within the apparatus. The fit method, however, yieldinga function that can describe the expected number of events for very shorttimes between events, automatically compensates for this.
CONCLUSIONS
The east-west muon asymmetry can be measured using a fairly simpleapparatus consisting of three muon counters, a QDC and TDC. Due tothe fact that this effect is very small (expected to be on the order of afew %), long data runs (on the order of several hours to days) must betaken to ensure that the flux rates are statistically significant enough forthe ratio to be measured without having to worry about error (which isproportional to 1√
t, where t is the length of the data run). The main issue
with measuring the asymmetry lies in the systematic error, given that thestatistical error is known to less than 1%. Therefore, for the next severalweeks, measurements will continue in an effort to detect the asymmetry.
REFERENCES
[1] P. N. Dinh, N. T. Dung, B. D. Hieu, N. Phuc, P. T. Phuong, P. Darriulat,D. Q. Thieu, and V. V. Thuan.Measurement of the vertical cosmic muon flux in a region of largerigidity cutoff.Nuclear Physics B, 627(1-2):29 – 42, 2002.
http://www.physics.mcgill.ca/:griffins [email protected]
