- Email: [email protected]
Energy determination at BEPC-II
Nuclear Physics B (Proc. Suppl.) 189 (2009) 366–370 www.elsevierphysics.com
Energy determination at BEPC-II M. N. Achasova∗ , V. E. Blinova , A. V. Bogomyagkova, ChengDong Fub , F. A. Harrisc, V. V. Kaminskya , Q. Liuc , Xiaohu Mob , N. Yu. Muchnoia , S. A. Nikitina , I. B. Nikolaeva , Qing Qinb , Huamin Qub , S. L. Olsenc , E. E. Pyataa , A. G. Shamova , C. P. Shenc , G. S. Varnerc , Yifang Wangb , Jinqiang Xub , and V. N. Zhilicha a
Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences, 11 Lavrentyev, Novosibirsk, 630090, Russia b
Institute of High Energy Physics, Chinese Academy of Sciences, 100049, Beijing, China c
Dept. of Physics and Astronomy, University of Hawaii, Honolulu, Hawaii 96822, USA The BEPC-II collider beam energy calibration system is discussed. The system is based on the Compton backscattering method. The expected precision of the electron and positron beam energy ε determination is δε/ε ≈ 3 · 10−5 .
1. Introduction The experiment with BES3 detector at BEPCII collider (Beijing) has been started this July. The peak designed luminosity of BEPC-II is 1033 cm−2 s−1 at 1.89 GeV – the highest luminosity ever planned in the τ -charm region. The accurate beam energy calibration is important for particle mass measurements. In the c − τ energy region determination of the τ -lepton mass is of great importance, because τ is fundamental particle and its mass is a parameter of Standard Model. The masses of ψ and D mesons which contain c quark are also of interest. The nowadays value of τ -lepton mass mτ is 1776.76 ± 0.15 [1]. In BEPC-II – BES3 experiment the mass will be measured using threshold scan method. The accuracy of measurement was studied in Ref.[2,3]. It was shown that one week of data taking time will lead to a statistical uncertainty of less then 20 keV. Systematic uncer∗ Email:
[email protected]
0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.03.059
tainty (without accuracy of beam energy determination) is about 20 keV and include the following sources: luminosity, detection efficiency, branching fraction, background, energy spread and theoretical uncertainty. So the total error is about 30 keV. The most important source of uncertainty is the precision of the absolute beam energy determination. In some cases the energy scale of colliders can be calibrated with extremely high accuracy using the resonant depolarization technique [4]. But this approach is hardly applied for the e+ e− factories, where the great advance in the luminosity are possible due to creation of fast bunchto-bunch feedback systems that usually have a strong depolarization impact on the beam. There are two possible methods of the beam energy determination at BEPC-II. First is a calibration of the energy scale from scan of the J/ψ and ψ resonances [5]. The expected accuracy in this case is about 100 keV. Another possibility is the beam energy measurement using Compton backscatter-
367
M.N. Achasov et al. / Nuclear Physics B (Proc. Suppl.) 189 (2009) 366–370
ing of monochromatic laser radiation on the e± beams. The approach was already experimentally approved at the BESSY-I, BESSY-II [6] storage rings. In collider experiments it was applied at VEPP-4M [7,8]
scatt
ered
θ
phot
on,
ω
electron,
ε
2. The Compton Backscattering approach
• The maximal energy of the scattered photon ωmax is related with the electron energy ε by the kinematics of Compton scattering:
Figure 1. The Compton scattering process. ε, ω0 and ω are the particles energies.
6
4
2
ε , ε + m2e /4ω0
If one measures ωmax , then the electron energy can be calculated: m2e ωmax 1+ 1+ . ε= 2 ω0 ωmax • The very high energy resolution (∼ 10−3 ) of commercially available High Purity Germanium (HPGe) detectors allow the statistical accuracy in the beam energy measurement to be at the level of δε/ε 10−5 . • The systematical accuracy is mostly defined by absolute calibration of the detector energy scale. Accurate calibration can be performed in the photon energy range up to 10 MeV by using γ-active radionuclides. The measurement procedure is as follows. As a source of initial photons the monochromatic laser radiation with ω0 ≈ 0.12eV is used. The laser light is put in collision with the electron or positron beams, and the energy of the backscattered photons is precisely measured using HPGe
2
0
0
0.02
0.04
0.06
0.08
0.1
θ, degree
Figure 2. The dependence of the scattered photon energy ω from angle θ between initial electron and final photon in the Compton scattering process.
(ds/dω)/σtot
ωmax =
α=π
laser photon, ω 0
ω, MeV
Let us consider the Compton scattering process in a case when the angle between initial particles is equal to π and their energies are ω0 me ε (Fig.1). Here ω0 and ε are the energies of initial photon and electron respectively. The backscattered photons have the maximal energy (Fig.2) and the energy spectrum of the scattered photons has sharp edge at the maximal energy (Fig.3) The general idea is based on the following:
0.3
0.2
0.1
0
0
2
4
6
ω, MeV
Figure 3. Energy spectrum of scattered Compton photons.
368
M.N. Achasov et al. / Nuclear Physics B (Proc. Suppl.) 189 (2009) 366–370 Calibration Difference
χ2 / ndf = 9.321 / 10 Prob
14
Mean
12
20071230 | 120017 | beam N Counts
200 180
140 120
Edge spectrum 5889672
χ2 / ndf
287.4 / 293 0.5812
edge place
8377 ± 0.4
edge width
7.059 ± 0.371
edge height
146.4 ± 2.5
background
6.154 ± 0.460
left slope
100
-0.009334 ± 0.027103
right slope
-0.0238 ± 0.0046
80 60
0.003313 ± 0.005359 0.04171 ± 0.00563
8 6 4 2 0
-0.2
-0.1
0
0.1 0.2 E_RDP - E_CBS, MeV
Figure 6. Accuracy check with resonant depolarization method at beam energy equal to 1777 MeV. The line is the result of Gaussian fit.
40 20 0 8150 8200 8250
Sigma
12.82 ± 2.28
10
Entries
Prob
160
0.5019
Constant
8300 8350 8400 8450 8500 8550 Channels
Figure 4. The measured edge of the scattered photons energy spectrum. The line is the fit result. 2200 electron beam energy, MeV
2000 1800 1600 1400 1200
λ = 10.6 μm
1000
Na24 Cs137 Co60 Pu238/C13
800 600 400 200 0
1 2 3 4 5 6 7 compton spectrum edge energy, MeV
8
Figure 5. Relation between ωmax and ε (solid line). Dots are the energies of γ-active radionuclide (reference lines for the HPGe detector calibration).
detector. The maximal energy of the scattered photons is determined by fitting of the abrupt edge in the energy spectrum by the erfc-like function (Fig.4). Fig.5 shows the relation between the measured ωmax and the beam energy ε. The detector energy scale can be accurately calibrated by using well-known radiative sources of γ-radiation (Fig.5). The method was tested using the resonant depolarization technique in experiments at VEPP4M collider [7,8]. The comparison of the results of beam energy determination in both approaches is shown in Fig.6. The accuracy of the resonant depolarization is about 1keV. The r.m.s. spread of distribution in Fig.6 is about 40 keV was taken as estimation of accuracy of the Compton backscattering method. Then the expected accuracy of the τ -mass determination is about 50 keV. The real accuracy of the beam energy determination at BEPC-II will depend on experimental conditions. It should be tested by measuring precisely known masses of J/ψ and ψ resonances.
369
M.N. Achasov et al. / Nuclear Physics B (Proc. Suppl.) 189 (2009) 366–370
electrons
R2IAMB
R1IAMB
HPGe
1.5m
Laser
0.4m
6.0m
1.8m
3.75m
positrons
Lenses
Figure 7. Simplified schematic of the energy measurement system. The positron and electron beams are indicated. R1IAMB and R2IAMB are accelerator magnets, and the HPGe detector is represented by the dot at the center. The shielding wall of the beam tunnel is shown cross-hatched, and the laser will be located outside the tunnel. 3. The beam energy measurement system for BEPC-II. The beam energy determination system for BEPC-II collider based on Compton backscattering method was proposed in Ref.[9]. In order to calibrate both electron and positron beam energies using the same HPGe detector the system should be placed at the opposite to the beams interaction region side of the collider. The system layout is shown in Fig.7. As a source of photons a continuous operation (CW), high power, and single-line narrow-width laser is required. An excellent candidate is the GEM Selected 50TM CO2 laser from Coherent, Inc. It provides 25 – 50 W of CW power at the wavelength 10.591 μm. The average energy stability is about 0.1 ppm. The laser of this type is used in the similar system at the VEPP-4M collider in Novosibirsk since 2005 [7,8]. During this period no degradation of its parameters were noticed. The purpose of a HPGe detector is to convert gamma rays into electrical impulses which can be
used with suitable signal processing, to determine their energy and intensity. A HPGe detector is a large germanium diode of the p-n or p-i-n type operated in the reverse bias mode. At a suitable operating temperature (normally 85 K), the barrier created at the junction reduces the leakage current to acceptably low values. Thus an electric field can be applied that is sufficient to collect the charge carriers liberated by the ionizing radiation. The most suitable HPGe detectors for the 1–10 MeV photons are Coaxial type detectors. For the BEPC-II energy calibration system the coaxial HPGe detector manufactured by ORTEC (model GEM25P4-70) was chosen. It has diameter of 57.8 mm and height of 52.7 mm with 31.2% relative efficiency2 . at 1.33 MeV. The energy resolution for the 1.33 MeV line of 60 Co is 1.74 keV. The detector is connected to the spectrometric 2 The efficiency of each detector is usually specified by a parameter called relative detection efficiency. The relative detection efficiency of coaxial germanium detectors is defined at 1.33 MeV relative to that of a standard 3-in.diameter, 3-in.-long N aI(T l) scintillator.
370
M.N. Achasov et al. / Nuclear Physics B (Proc. Suppl.) 189 (2009) 366–370
station, which transfers data using the USB port of the computer. The laser beam is focused using a doublet of lenses, reflected through an angle 90◦ using a 45◦ mirror, and then reflected either to the right or left by means of a movable prism (Fig.7). The beam is again reflected by 90◦ through a hole in the wall of the beam tunnel and is then incident on a window in a vacuum pipe extension of the beam pipe. The insertion of the laser beam into the vacuum chamber is performed using the laser-to-vacuum insertion system. The system is the special vacuum chamber with an GaAs entrance window. In the vacuum chamber the laser beam is reflected through an angle of 90◦ by a copper mirror which is mounted to the support. This support can be turned by bending the vacuum flexible bellow, so the angle between the mirror and the laser can be adjusted as necessary. After backscattering the photons return to the mirror, pass through it, leave the vacuum chamber, and are detected by the HPGe detector. Synchrotron radiation (SR) photons also fall on the mirror and heat it. In order to reduce the heating of the mirror, it is placed 1.8 m from the BEPC-II vacuum chamber flange. The SR power absorbed by the mirror approximately equal to 200 W.The extraction of heat will be provided by the water cooling system. The laser and e+ /e− beams will interact in the straight sections of BEPC-II behind the R1IAMB dipole for e+ (R2IAMB for e− ). The laser beam is focused at the BEPC-II vacuum chamber entrance flange, where the geometrical aperture is minimal (vertical size is 14 mm). The total yield of scattered photons is about 17000 ph/s/mA/W. The optical system was deployed at BEPCII in May 2008, the HPGe detector is now under study in BINP (Novosibirsk). The laser-tovacuum insertion system is under construction in BINP also. The gamma sources for the HPGe detector calibration are available in IHEP (Beijing). The USA collaborators from Hawaii University will supply the system with CO2 laser.
4. Conclusion The BEPC-II calibration system based on the Compton backscattering method is in progress now. We hope to finish its installation and testing in 2009. Then the preliminary physics results can be obtained in the end of 2010. The authors are grateful to A.E. Bondar, E.B. Levichev, Yu.A. Tikhonov for discussions and support of the work. The work is supported in part by grants RFBR 08-02-00328-a, 08-02-00251a, 08-02-92200-NSFC-a, Research and Development Project of Important Scientific Equipment of CAS (H7292330S7). REFERENCES 1. A. Shamov for KEDR/VEPP4M, this proceedings 2. Y.K. Wang, X.H. Mo, C.Z.Yuan, J.P. Liu, Nucl. Instr. and Meth A 583 (2007) 479. 3. X.H. Mo, In Proceedings of 9-th Intirnational Workshop on tau lepton physics, Pisa, Italy, September 19-22, 2006, Nucl. Phys. Proc. Suppl. 169 (2007) 132 4. A.N. Skrinsky and Yu.M. Shatunov, Sov. Phys. Uspekhi 32 (1989) 548 5. J.Z. Bai et al., Phys. Rev. D 53 (1996) 20 6. R. Klein et al., Nucl. Instr. Meth. A 384 (1997) 293; J. Synchrotron Rad. 5 (1998) 392 7. N. Muchnoi et al., In Proc. of the EPAC, Scotland, Eidenburgh,June 26-30, 2006, EPAC 1181 8. V.E. Blinov et al, in Proc. of International Conference on instrumentation for colliding beam physics, Novosibirsk, Russia February 28 - March 5, 2008, Nucl. Instr. and Meth. A 598 (2009) 23 9. M.N. Achasov et al., BINP Preprint 2008-4 (2008); ArXiv:0804.0159 (2008)
Energy determination at BEPC-II M. N. Achasova∗ , V. E. Blinova , A. V. Bogomyagkova, ChengDong Fub , F. A. Harrisc, V. V. Kaminskya , Q. Liuc , Xiaohu Mob , N. Yu. Muchnoia , S. A. Nikitina , I. B. Nikolaeva , Qing Qinb , Huamin Qub , S. L. Olsenc , E. E. Pyataa , A. G. Shamova , C. P. Shenc , G. S. Varnerc , Yifang Wangb , Jinqiang Xub , and V. N. Zhilicha a
Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences, 11 Lavrentyev, Novosibirsk, 630090, Russia b
Institute of High Energy Physics, Chinese Academy of Sciences, 100049, Beijing, China c
Dept. of Physics and Astronomy, University of Hawaii, Honolulu, Hawaii 96822, USA The BEPC-II collider beam energy calibration system is discussed. The system is based on the Compton backscattering method. The expected precision of the electron and positron beam energy ε determination is δε/ε ≈ 3 · 10−5 .
1. Introduction The experiment with BES3 detector at BEPCII collider (Beijing) has been started this July. The peak designed luminosity of BEPC-II is 1033 cm−2 s−1 at 1.89 GeV – the highest luminosity ever planned in the τ -charm region. The accurate beam energy calibration is important for particle mass measurements. In the c − τ energy region determination of the τ -lepton mass is of great importance, because τ is fundamental particle and its mass is a parameter of Standard Model. The masses of ψ and D mesons which contain c quark are also of interest. The nowadays value of τ -lepton mass mτ is 1776.76 ± 0.15 [1]. In BEPC-II – BES3 experiment the mass will be measured using threshold scan method. The accuracy of measurement was studied in Ref.[2,3]. It was shown that one week of data taking time will lead to a statistical uncertainty of less then 20 keV. Systematic uncer∗ Email:
[email protected]
0920-5632/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2009.03.059
tainty (without accuracy of beam energy determination) is about 20 keV and include the following sources: luminosity, detection efficiency, branching fraction, background, energy spread and theoretical uncertainty. So the total error is about 30 keV. The most important source of uncertainty is the precision of the absolute beam energy determination. In some cases the energy scale of colliders can be calibrated with extremely high accuracy using the resonant depolarization technique [4]. But this approach is hardly applied for the e+ e− factories, where the great advance in the luminosity are possible due to creation of fast bunchto-bunch feedback systems that usually have a strong depolarization impact on the beam. There are two possible methods of the beam energy determination at BEPC-II. First is a calibration of the energy scale from scan of the J/ψ and ψ resonances [5]. The expected accuracy in this case is about 100 keV. Another possibility is the beam energy measurement using Compton backscatter-
367
M.N. Achasov et al. / Nuclear Physics B (Proc. Suppl.) 189 (2009) 366–370
ing of monochromatic laser radiation on the e± beams. The approach was already experimentally approved at the BESSY-I, BESSY-II [6] storage rings. In collider experiments it was applied at VEPP-4M [7,8]
scatt
ered
θ
phot
on,
ω
electron,
ε
2. The Compton Backscattering approach
• The maximal energy of the scattered photon ωmax is related with the electron energy ε by the kinematics of Compton scattering:
Figure 1. The Compton scattering process. ε, ω0 and ω are the particles energies.
6
4
2
ε , ε + m2e /4ω0
If one measures ωmax , then the electron energy can be calculated: m2e ωmax 1+ 1+ . ε= 2 ω0 ωmax • The very high energy resolution (∼ 10−3 ) of commercially available High Purity Germanium (HPGe) detectors allow the statistical accuracy in the beam energy measurement to be at the level of δε/ε 10−5 . • The systematical accuracy is mostly defined by absolute calibration of the detector energy scale. Accurate calibration can be performed in the photon energy range up to 10 MeV by using γ-active radionuclides. The measurement procedure is as follows. As a source of initial photons the monochromatic laser radiation with ω0 ≈ 0.12eV is used. The laser light is put in collision with the electron or positron beams, and the energy of the backscattered photons is precisely measured using HPGe
2
0
0
0.02
0.04
0.06
0.08
0.1
θ, degree
Figure 2. The dependence of the scattered photon energy ω from angle θ between initial electron and final photon in the Compton scattering process.
(ds/dω)/σtot
ωmax =
α=π
laser photon, ω 0
ω, MeV
Let us consider the Compton scattering process in a case when the angle between initial particles is equal to π and their energies are ω0 me ε (Fig.1). Here ω0 and ε are the energies of initial photon and electron respectively. The backscattered photons have the maximal energy (Fig.2) and the energy spectrum of the scattered photons has sharp edge at the maximal energy (Fig.3) The general idea is based on the following:
0.3
0.2
0.1
0
0
2
4
6
ω, MeV
Figure 3. Energy spectrum of scattered Compton photons.
368
M.N. Achasov et al. / Nuclear Physics B (Proc. Suppl.) 189 (2009) 366–370 Calibration Difference
χ2 / ndf = 9.321 / 10 Prob
14
Mean
12
20071230 | 120017 | beam N Counts
200 180
140 120
Edge spectrum 5889672
χ2 / ndf
287.4 / 293 0.5812
edge place
8377 ± 0.4
edge width
7.059 ± 0.371
edge height
146.4 ± 2.5
background
6.154 ± 0.460
left slope
100
-0.009334 ± 0.027103
right slope
-0.0238 ± 0.0046
80 60
0.003313 ± 0.005359 0.04171 ± 0.00563
8 6 4 2 0
-0.2
-0.1
0
0.1 0.2 E_RDP - E_CBS, MeV
Figure 6. Accuracy check with resonant depolarization method at beam energy equal to 1777 MeV. The line is the result of Gaussian fit.
40 20 0 8150 8200 8250
Sigma
12.82 ± 2.28
10
Entries
Prob
160
0.5019
Constant
8300 8350 8400 8450 8500 8550 Channels
Figure 4. The measured edge of the scattered photons energy spectrum. The line is the fit result. 2200 electron beam energy, MeV
2000 1800 1600 1400 1200
λ = 10.6 μm
1000
Na24 Cs137 Co60 Pu238/C13
800 600 400 200 0
1 2 3 4 5 6 7 compton spectrum edge energy, MeV
8
Figure 5. Relation between ωmax and ε (solid line). Dots are the energies of γ-active radionuclide (reference lines for the HPGe detector calibration).
detector. The maximal energy of the scattered photons is determined by fitting of the abrupt edge in the energy spectrum by the erfc-like function (Fig.4). Fig.5 shows the relation between the measured ωmax and the beam energy ε. The detector energy scale can be accurately calibrated by using well-known radiative sources of γ-radiation (Fig.5). The method was tested using the resonant depolarization technique in experiments at VEPP4M collider [7,8]. The comparison of the results of beam energy determination in both approaches is shown in Fig.6. The accuracy of the resonant depolarization is about 1keV. The r.m.s. spread of distribution in Fig.6 is about 40 keV was taken as estimation of accuracy of the Compton backscattering method. Then the expected accuracy of the τ -mass determination is about 50 keV. The real accuracy of the beam energy determination at BEPC-II will depend on experimental conditions. It should be tested by measuring precisely known masses of J/ψ and ψ resonances.
369
M.N. Achasov et al. / Nuclear Physics B (Proc. Suppl.) 189 (2009) 366–370
electrons
R2IAMB
R1IAMB
HPGe
1.5m
Laser
0.4m
6.0m
1.8m
3.75m
positrons
Lenses
Figure 7. Simplified schematic of the energy measurement system. The positron and electron beams are indicated. R1IAMB and R2IAMB are accelerator magnets, and the HPGe detector is represented by the dot at the center. The shielding wall of the beam tunnel is shown cross-hatched, and the laser will be located outside the tunnel. 3. The beam energy measurement system for BEPC-II. The beam energy determination system for BEPC-II collider based on Compton backscattering method was proposed in Ref.[9]. In order to calibrate both electron and positron beam energies using the same HPGe detector the system should be placed at the opposite to the beams interaction region side of the collider. The system layout is shown in Fig.7. As a source of photons a continuous operation (CW), high power, and single-line narrow-width laser is required. An excellent candidate is the GEM Selected 50TM CO2 laser from Coherent, Inc. It provides 25 – 50 W of CW power at the wavelength 10.591 μm. The average energy stability is about 0.1 ppm. The laser of this type is used in the similar system at the VEPP-4M collider in Novosibirsk since 2005 [7,8]. During this period no degradation of its parameters were noticed. The purpose of a HPGe detector is to convert gamma rays into electrical impulses which can be
used with suitable signal processing, to determine their energy and intensity. A HPGe detector is a large germanium diode of the p-n or p-i-n type operated in the reverse bias mode. At a suitable operating temperature (normally 85 K), the barrier created at the junction reduces the leakage current to acceptably low values. Thus an electric field can be applied that is sufficient to collect the charge carriers liberated by the ionizing radiation. The most suitable HPGe detectors for the 1–10 MeV photons are Coaxial type detectors. For the BEPC-II energy calibration system the coaxial HPGe detector manufactured by ORTEC (model GEM25P4-70) was chosen. It has diameter of 57.8 mm and height of 52.7 mm with 31.2% relative efficiency2 . at 1.33 MeV. The energy resolution for the 1.33 MeV line of 60 Co is 1.74 keV. The detector is connected to the spectrometric 2 The efficiency of each detector is usually specified by a parameter called relative detection efficiency. The relative detection efficiency of coaxial germanium detectors is defined at 1.33 MeV relative to that of a standard 3-in.diameter, 3-in.-long N aI(T l) scintillator.
370
M.N. Achasov et al. / Nuclear Physics B (Proc. Suppl.) 189 (2009) 366–370
station, which transfers data using the USB port of the computer. The laser beam is focused using a doublet of lenses, reflected through an angle 90◦ using a 45◦ mirror, and then reflected either to the right or left by means of a movable prism (Fig.7). The beam is again reflected by 90◦ through a hole in the wall of the beam tunnel and is then incident on a window in a vacuum pipe extension of the beam pipe. The insertion of the laser beam into the vacuum chamber is performed using the laser-to-vacuum insertion system. The system is the special vacuum chamber with an GaAs entrance window. In the vacuum chamber the laser beam is reflected through an angle of 90◦ by a copper mirror which is mounted to the support. This support can be turned by bending the vacuum flexible bellow, so the angle between the mirror and the laser can be adjusted as necessary. After backscattering the photons return to the mirror, pass through it, leave the vacuum chamber, and are detected by the HPGe detector. Synchrotron radiation (SR) photons also fall on the mirror and heat it. In order to reduce the heating of the mirror, it is placed 1.8 m from the BEPC-II vacuum chamber flange. The SR power absorbed by the mirror approximately equal to 200 W.The extraction of heat will be provided by the water cooling system. The laser and e+ /e− beams will interact in the straight sections of BEPC-II behind the R1IAMB dipole for e+ (R2IAMB for e− ). The laser beam is focused at the BEPC-II vacuum chamber entrance flange, where the geometrical aperture is minimal (vertical size is 14 mm). The total yield of scattered photons is about 17000 ph/s/mA/W. The optical system was deployed at BEPCII in May 2008, the HPGe detector is now under study in BINP (Novosibirsk). The laser-tovacuum insertion system is under construction in BINP also. The gamma sources for the HPGe detector calibration are available in IHEP (Beijing). The USA collaborators from Hawaii University will supply the system with CO2 laser.
4. Conclusion The BEPC-II calibration system based on the Compton backscattering method is in progress now. We hope to finish its installation and testing in 2009. Then the preliminary physics results can be obtained in the end of 2010. The authors are grateful to A.E. Bondar, E.B. Levichev, Yu.A. Tikhonov for discussions and support of the work. The work is supported in part by grants RFBR 08-02-00328-a, 08-02-00251a, 08-02-92200-NSFC-a, Research and Development Project of Important Scientific Equipment of CAS (H7292330S7). REFERENCES 1. A. Shamov for KEDR/VEPP4M, this proceedings 2. Y.K. Wang, X.H. Mo, C.Z.Yuan, J.P. Liu, Nucl. Instr. and Meth A 583 (2007) 479. 3. X.H. Mo, In Proceedings of 9-th Intirnational Workshop on tau lepton physics, Pisa, Italy, September 19-22, 2006, Nucl. Phys. Proc. Suppl. 169 (2007) 132 4. A.N. Skrinsky and Yu.M. Shatunov, Sov. Phys. Uspekhi 32 (1989) 548 5. J.Z. Bai et al., Phys. Rev. D 53 (1996) 20 6. R. Klein et al., Nucl. Instr. Meth. A 384 (1997) 293; J. Synchrotron Rad. 5 (1998) 392 7. N. Muchnoi et al., In Proc. of the EPAC, Scotland, Eidenburgh,June 26-30, 2006, EPAC 1181 8. V.E. Blinov et al, in Proc. of International Conference on instrumentation for colliding beam physics, Novosibirsk, Russia February 28 - March 5, 2008, Nucl. Instr. and Meth. A 598 (2009) 23 9. M.N. Achasov et al., BINP Preprint 2008-4 (2008); ArXiv:0804.0159 (2008)