- Email: [email protected]
Atmospheric muon measurements using the OKAYAMA telescope
Nuclear Physics B (Proc. Suppl.) 151 (2006) 465–468 www.elsevierphysics.com
Atmospheric muon measurements using the OKAYAMA telescope T. Wadaa∗ , A.Iyonob , T.Katayamaa, S.Lana , T.Nakatsukac , K.Okeia , M.Tokiwaa , S.Tsujid , Y.Yamashitaa , I.Yamamotoe a
Department of Physics, Okayama University, Okayama 700-8530, Japan
b
Department of Computer Simulation, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan
c
Okayama Shoka University, Okayama 700-8601, Japan
d
Department of Information Sciences, Kawasaki Medical School, Kurashiki 701-0192, Japan
e
Department of Information and Computer Engineering, Okayama University of Science, Okayama 700-0005, Japan The atmospheric muon spectrum has been measured at sea level using a magnet spectrometer called the OKAYAMA telescope. The OKAYAMA telescope is able to point at any azimuth and zenith angle and to measure the muon energy and charge sign. We report the absolute atmospheric muon differential energy spectrum, charge ratio and azimuthal angular dependences.
1. Introduction Atmospheric muons produced by π/K decay at the top of atmosphere pass through the atmosphere or decay. They have a close relation to atmospheric neutrinos, because neutrinos are produced in π, K and μ decay. The atmospheric neutrino flux calculations [1–3] take into account the terrestrial magnetism and environment of the atmosphere. Therefore, information on propagation through the atmosphere of secondary cosmic rays is necessary. Terrestrial magnetism affects the muon because of its charge. Coming from the east a positive muon has a long path, and the positive muon fluxes are thus decreased in this direction by geomagnetic effects and vice versa. We have measured the azimuthal angular dependence of atmospheric muon fluxes using the Okayama telescope. It is able to move by a servo-motor mechanism to any azimuthal and zenith angle and measure the incoming direction, the momentum and charge sign of incident muons. ∗ [email protected]
0920-5632/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2005.07.080
Figure 1. Schematic drawing of the OKAYAMA telescope. The central grey box is an iron core magnet. Scintillation counters and Drift chambers are located above and below the magnet.
466
T. Wada et al. / Nuclear Physics B (Proc. Suppl.) 151 (2006) 465–468
2. The OKAYAMA telescope The Okayama telescope is an atmospheric muon detector with a solid iron magnet spectrometer as shown in Fig.1 and was improved in 2001 [4]. It was installed in the building of Okayama University in Japan at 34◦ 40’ N latitude, 133◦ 56’ E longitude, 28 m above sea level . The Rigidity cutoff is about 12 GV. The telescope is described in Table 1.
Table 2 Observation periods Vertical direction Jan 2001 ∼ Jun 2003 Oct 2003 ∼ May 2004 zenith angles 20◦ Mar 2003 ∼ Aug 2003 zenith angles 40◦ Sep 2003 ∼ Jan 2004
4. Analysis Table 1 Properties of the telescope • Detector scintillation counters drift chambers geometrical acceptance (distance between the drift chambers) (distance between the scintillation counters) • Solid iron magnet data useful magnetic Volume current, coil Magnetic flux density • Resolution position incident angle deflection angle detectable momentum
40 × 50 × 1 cm3 2 plates 31 × 40 × 2 cm3 20 layers(x=12)(y=8) 24.4 cm2 sr 132.2 cm2 sr
32 × 32 × 32 cm3 350A, 15 turns 17.6 ± 0.2 kgauss ∼ 0.5 mm for 1 layer 0.8 mrad (X axis) 1.2 mrad (Y axis) 1.2 mrad (X axis) 1.8 mrad (Y axis) 0.9 - 160 GeV/c
3. Measurements Measurements were made from 2002 to 2004 at 8 azimuthal angles in the vertical direction and at zenith angles 20◦ and 40◦ (Table 2). The number of events is about 16 million. The average trigger rate is ∼ 1.5 Hz in the vertical. The integral intensity is ∼ 0.7 × 10 −3 cm−2 sr−1 sec−1 above 0.7 GeV/c.
The Okayama telescope is installed on the top floor of the building, therefore we considered the energy loss due to the roof and wall. The main material is concrete and the thickness changes from 72 to 200 g/cm2 in the 8 azimuthal directions at 40◦ zenith angle. We chose events in the incident angle range −5◦ ∼ +5◦ . We also considered the acceptance for bending muons in the magnet. The effect appears below 3 GeV/c. To estimate the momentum, we made a maximum likelihood estimate [5] that used not only the bending angle in the magnet but also some information of multiple coulomb scattering. Using the maximum likelihood estimate, the scattering angle to bending angle ratio decreased from 0.37 to 0.27.
5. Results Figs. 2 and 3 and Table 3 show the muon fluxes at zenith angles 20◦ , 40◦ in 8 azimuthal directions in the momentum range 1 - 2 GeV/c. The azimuthal dependences have a tendency to increase in the westerly direction and to decrease in the easterly direction, and the dependence at 40◦ zenith angle is bigger than at 20◦ zenith angle. Fig. 4 shows the muon charge ratio in 8 azimuthal directions at zenith angles 20◦ , 40◦ in the momentum range 1 - 2 GeV/c. Figs. 5 and 6 show the positive and negative vertical momentum spectra, and in the East and West directions at 20◦ and 40◦ zenith angles. At 40◦ zenith angle, the difference of intensities in the East and West directions appear below 50 GeV/c for positive muons and 30 GeV/c for negative muons.
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T. Wada et al. / Nuclear Physics B (Proc. Suppl.) 151 (2006) 465–468
Table 3 The measured muon fluxes in 8 azimuthal directions at 20◦ and 40◦ zenith angles in the momentum range 1 - 2 GeV/c. Unit is [104 × cm−1 sr−1 sec−1 ]. Errors are only statistical. Number of events is shown in brackets. Zenith angle 20◦ Zenith angle 40◦ Azimuth μ+ μ− μ+ μ− South 7.96±0.13 (3850) 6.99±0.12 (3343) 3.88±0.14 (789) 3.65±0.14 (717) South-East 7.06±0.12 (3736) 6.64±0.11 (3461) 3.32±0.12 (815) 3.56±0.12 (814) East 6.94±0.11 (4117) 6.25±0.10 (3723) 2.85±0.11 (657) 2.83±0.11 (676) North-East 6.63±0.11 (3512) 6.47±0.11 (3421) 3.30±0.11 (935) 3.46±0.11 (960) North 7.13±0.12 (3527) 6.63±0.12 (3258) 3.66±0.13 (814) 3.28±0.12 (778) North-West 7.63±0.12 (3935) 6.43±0.11 (3325) 4.42±0.14 (1064) 3.56±0.12 (859) West 8.27±0.13 (4017) 6.83±0.12 (3311) 4.92±0.15 (1149) 3.69±0.13 (846) South-West 7.78±0.13 (3815) 6.75±0.12 (3276) 4.42±0.14 (1059) 3.67±0.13 (861)
0.002
0.0012
zenith angle 20 ± 5 deg 1 ∼ 2 GeV/c
zenith angle 40 ± 5 deg
0.001
1 ∼ 2 GeV/c
-1 -1
0.001
0.0008
-2
Flux [cm sr s ]
-2
-1 -1
Flux [cm sr s ]
0.0015
0.0006
0.0004
0.0005 +
-
μ +μ μ+ μ
0 South
East
North
West
+
-
μ +μ + μ μ
0.0002
0 South
East
North
West
Figure 2. The azimuthal angular dependence of the muon flux in 8 azimuthal directions at zenith angle 20◦ in the momentum range 1 - 2 GeV/c.
Figure 3. The azimuthal angular dependence of the muon flux in 8 azimuthal directions at zenith angle 40◦ in the momentum range 1 - 2 GeV/c.
6. Conclusion
from the east decreases and the negative muon flux increases due to geomagnetic effects and vice versa. Variations of the atmospheric muon flux affect the flux of neutrinos produced in muon decay. Thus, geomagnetic effects should be taken into account in calculating the propagation of secondary cosmic rays in the atmosphere.
We have measured the sea level atmospheric muon flux at 8 azimuthal directions at zenith angles of 0◦ , 20◦ and 40◦ . Muon measurements in west and east at zenith angles 20◦ and 40◦ indicate a geomagnetic effect. The positive muon flux
468
T. Wada et al. / Nuclear Physics B (Proc. Suppl.) 151 (2006) 465–468 -3
10 1.6
μ+
1 ∼ 2 GeV/c
+
Charge Ratio μ / μ
-
1.4
1.3
1.2
1.1
1
0.9 zenith angle 20 ± 5 deg zenith angle 40 ± 5 deg vertical
0.8
0.7 South
East
North
West
Intensity [cm-2sr-1s-1(GeV/c)-1]
1.5
-6
G. D. Barr et al., Phys.Rev.D70, 23006(2004) M. Honda et al., Phys.Rev.D70, 43008(2004) J. Wentz et al., Phys.Rev.D67, 73020(2003) S. Tsuji et al., J.Phys.G24, p1805-1822(1999) K. Okei et al., Proc. 28th Int. Cosmic Ray Conf. (Tsukuba) HE2:1, 1155 (2003)
vertical z20 west z20 east z40 west z40 east
100 101 momentum at sea level [GeV/c]
Figure 5. Positive muon momentum spectrum at sea level in the vertical, and in the East and West directions at zenith angles 20◦ and 40◦ . 10-3 μ-
Intensity [cm-2sr-1s-1(GeV/c)-1]
1. 2. 3. 4. 5.
10-5
10
Figure 4. The azimuthal angular dependence of the muon charge ratio in 8 azimuthal directions at zenith angles 20◦ and 40◦ in the momentum range 1 - 2 GeV/c.
REFERENCES
10-4
-4
10
-5
10
-6
10
vertical z20 west z20 east z40 west z40 east
100 101 momentum at sea level [GeV/c]
Figure 6. Negative muon momentum spectrum at sea level in the vertical, and in the East and West directions at zenith angles 20◦ and 40◦ .
Atmospheric muon measurements using the OKAYAMA telescope T. Wadaa∗ , A.Iyonob , T.Katayamaa, S.Lana , T.Nakatsukac , K.Okeia , M.Tokiwaa , S.Tsujid , Y.Yamashitaa , I.Yamamotoe a
Department of Physics, Okayama University, Okayama 700-8530, Japan
b
Department of Computer Simulation, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan
c
Okayama Shoka University, Okayama 700-8601, Japan
d
Department of Information Sciences, Kawasaki Medical School, Kurashiki 701-0192, Japan
e
Department of Information and Computer Engineering, Okayama University of Science, Okayama 700-0005, Japan The atmospheric muon spectrum has been measured at sea level using a magnet spectrometer called the OKAYAMA telescope. The OKAYAMA telescope is able to point at any azimuth and zenith angle and to measure the muon energy and charge sign. We report the absolute atmospheric muon differential energy spectrum, charge ratio and azimuthal angular dependences.
1. Introduction Atmospheric muons produced by π/K decay at the top of atmosphere pass through the atmosphere or decay. They have a close relation to atmospheric neutrinos, because neutrinos are produced in π, K and μ decay. The atmospheric neutrino flux calculations [1–3] take into account the terrestrial magnetism and environment of the atmosphere. Therefore, information on propagation through the atmosphere of secondary cosmic rays is necessary. Terrestrial magnetism affects the muon because of its charge. Coming from the east a positive muon has a long path, and the positive muon fluxes are thus decreased in this direction by geomagnetic effects and vice versa. We have measured the azimuthal angular dependence of atmospheric muon fluxes using the Okayama telescope. It is able to move by a servo-motor mechanism to any azimuthal and zenith angle and measure the incoming direction, the momentum and charge sign of incident muons. ∗ [email protected]
0920-5632/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2005.07.080
Figure 1. Schematic drawing of the OKAYAMA telescope. The central grey box is an iron core magnet. Scintillation counters and Drift chambers are located above and below the magnet.
466
T. Wada et al. / Nuclear Physics B (Proc. Suppl.) 151 (2006) 465–468
2. The OKAYAMA telescope The Okayama telescope is an atmospheric muon detector with a solid iron magnet spectrometer as shown in Fig.1 and was improved in 2001 [4]. It was installed in the building of Okayama University in Japan at 34◦ 40’ N latitude, 133◦ 56’ E longitude, 28 m above sea level . The Rigidity cutoff is about 12 GV. The telescope is described in Table 1.
Table 2 Observation periods Vertical direction Jan 2001 ∼ Jun 2003 Oct 2003 ∼ May 2004 zenith angles 20◦ Mar 2003 ∼ Aug 2003 zenith angles 40◦ Sep 2003 ∼ Jan 2004
4. Analysis Table 1 Properties of the telescope • Detector scintillation counters drift chambers geometrical acceptance (distance between the drift chambers) (distance between the scintillation counters) • Solid iron magnet data useful magnetic Volume current, coil Magnetic flux density • Resolution position incident angle deflection angle detectable momentum
40 × 50 × 1 cm3 2 plates 31 × 40 × 2 cm3 20 layers(x=12)(y=8) 24.4 cm2 sr 132.2 cm2 sr
32 × 32 × 32 cm3 350A, 15 turns 17.6 ± 0.2 kgauss ∼ 0.5 mm for 1 layer 0.8 mrad (X axis) 1.2 mrad (Y axis) 1.2 mrad (X axis) 1.8 mrad (Y axis) 0.9 - 160 GeV/c
3. Measurements Measurements were made from 2002 to 2004 at 8 azimuthal angles in the vertical direction and at zenith angles 20◦ and 40◦ (Table 2). The number of events is about 16 million. The average trigger rate is ∼ 1.5 Hz in the vertical. The integral intensity is ∼ 0.7 × 10 −3 cm−2 sr−1 sec−1 above 0.7 GeV/c.
The Okayama telescope is installed on the top floor of the building, therefore we considered the energy loss due to the roof and wall. The main material is concrete and the thickness changes from 72 to 200 g/cm2 in the 8 azimuthal directions at 40◦ zenith angle. We chose events in the incident angle range −5◦ ∼ +5◦ . We also considered the acceptance for bending muons in the magnet. The effect appears below 3 GeV/c. To estimate the momentum, we made a maximum likelihood estimate [5] that used not only the bending angle in the magnet but also some information of multiple coulomb scattering. Using the maximum likelihood estimate, the scattering angle to bending angle ratio decreased from 0.37 to 0.27.
5. Results Figs. 2 and 3 and Table 3 show the muon fluxes at zenith angles 20◦ , 40◦ in 8 azimuthal directions in the momentum range 1 - 2 GeV/c. The azimuthal dependences have a tendency to increase in the westerly direction and to decrease in the easterly direction, and the dependence at 40◦ zenith angle is bigger than at 20◦ zenith angle. Fig. 4 shows the muon charge ratio in 8 azimuthal directions at zenith angles 20◦ , 40◦ in the momentum range 1 - 2 GeV/c. Figs. 5 and 6 show the positive and negative vertical momentum spectra, and in the East and West directions at 20◦ and 40◦ zenith angles. At 40◦ zenith angle, the difference of intensities in the East and West directions appear below 50 GeV/c for positive muons and 30 GeV/c for negative muons.
467
T. Wada et al. / Nuclear Physics B (Proc. Suppl.) 151 (2006) 465–468
Table 3 The measured muon fluxes in 8 azimuthal directions at 20◦ and 40◦ zenith angles in the momentum range 1 - 2 GeV/c. Unit is [104 × cm−1 sr−1 sec−1 ]. Errors are only statistical. Number of events is shown in brackets. Zenith angle 20◦ Zenith angle 40◦ Azimuth μ+ μ− μ+ μ− South 7.96±0.13 (3850) 6.99±0.12 (3343) 3.88±0.14 (789) 3.65±0.14 (717) South-East 7.06±0.12 (3736) 6.64±0.11 (3461) 3.32±0.12 (815) 3.56±0.12 (814) East 6.94±0.11 (4117) 6.25±0.10 (3723) 2.85±0.11 (657) 2.83±0.11 (676) North-East 6.63±0.11 (3512) 6.47±0.11 (3421) 3.30±0.11 (935) 3.46±0.11 (960) North 7.13±0.12 (3527) 6.63±0.12 (3258) 3.66±0.13 (814) 3.28±0.12 (778) North-West 7.63±0.12 (3935) 6.43±0.11 (3325) 4.42±0.14 (1064) 3.56±0.12 (859) West 8.27±0.13 (4017) 6.83±0.12 (3311) 4.92±0.15 (1149) 3.69±0.13 (846) South-West 7.78±0.13 (3815) 6.75±0.12 (3276) 4.42±0.14 (1059) 3.67±0.13 (861)
0.002
0.0012
zenith angle 20 ± 5 deg 1 ∼ 2 GeV/c
zenith angle 40 ± 5 deg
0.001
1 ∼ 2 GeV/c
-1 -1
0.001
0.0008
-2
Flux [cm sr s ]
-2
-1 -1
Flux [cm sr s ]
0.0015
0.0006
0.0004
0.0005 +
-
μ +μ μ+ μ
0 South
East
North
West
+
-
μ +μ + μ μ
0.0002
0 South
East
North
West
Figure 2. The azimuthal angular dependence of the muon flux in 8 azimuthal directions at zenith angle 20◦ in the momentum range 1 - 2 GeV/c.
Figure 3. The azimuthal angular dependence of the muon flux in 8 azimuthal directions at zenith angle 40◦ in the momentum range 1 - 2 GeV/c.
6. Conclusion
from the east decreases and the negative muon flux increases due to geomagnetic effects and vice versa. Variations of the atmospheric muon flux affect the flux of neutrinos produced in muon decay. Thus, geomagnetic effects should be taken into account in calculating the propagation of secondary cosmic rays in the atmosphere.
We have measured the sea level atmospheric muon flux at 8 azimuthal directions at zenith angles of 0◦ , 20◦ and 40◦ . Muon measurements in west and east at zenith angles 20◦ and 40◦ indicate a geomagnetic effect. The positive muon flux
468
T. Wada et al. / Nuclear Physics B (Proc. Suppl.) 151 (2006) 465–468 -3
10 1.6
μ+
1 ∼ 2 GeV/c
+
Charge Ratio μ / μ
-
1.4
1.3
1.2
1.1
1
0.9 zenith angle 20 ± 5 deg zenith angle 40 ± 5 deg vertical
0.8
0.7 South
East
North
West
Intensity [cm-2sr-1s-1(GeV/c)-1]
1.5
-6
G. D. Barr et al., Phys.Rev.D70, 23006(2004) M. Honda et al., Phys.Rev.D70, 43008(2004) J. Wentz et al., Phys.Rev.D67, 73020(2003) S. Tsuji et al., J.Phys.G24, p1805-1822(1999) K. Okei et al., Proc. 28th Int. Cosmic Ray Conf. (Tsukuba) HE2:1, 1155 (2003)
vertical z20 west z20 east z40 west z40 east
100 101 momentum at sea level [GeV/c]
Figure 5. Positive muon momentum spectrum at sea level in the vertical, and in the East and West directions at zenith angles 20◦ and 40◦ . 10-3 μ-
Intensity [cm-2sr-1s-1(GeV/c)-1]
1. 2. 3. 4. 5.
10-5
10
Figure 4. The azimuthal angular dependence of the muon charge ratio in 8 azimuthal directions at zenith angles 20◦ and 40◦ in the momentum range 1 - 2 GeV/c.
REFERENCES
10-4
-4
10
-5
10
-6
10
vertical z20 west z20 east z40 west z40 east
100 101 momentum at sea level [GeV/c]
Figure 6. Negative muon momentum spectrum at sea level in the vertical, and in the East and West directions at zenith angles 20◦ and 40◦ .