Vol.12 No.2 August 28, 1999
Beijing Electron-Positron Collider National Laboratory Institute of High Energy Physics, CAS
Three tasks were defined for 99' annual operation of BEPC( Beijing Electron Positron Collider), BES (BEijing Spectrometer) and BSRF (Beijing Synchrotron Radiation Facility), the first, upgrading BEPC's injection energy and luminosity; the second, R scan in the center-of-mass energy of 2-5GeV with BES; the third, multidiscipline research with 221 user subjects arranged in BSRF. Now, all of the three tasks have been accomplished well.
The linac began its start-up at the end of October 1998, while the storage ring started on November 7. Synchrotron Radiation(SR) experiments were arranged from November 17 to December 20, followed by a 40-day machine study aiming at commissioning for BEPC upgrade. Through February 11 to June 7, R-value measurement was carried out with BES. The 98-99 operation was ended with SR experiments from June 7 to July 10. In the 8 months, the total operation time is 5752 hours, among them, BES took 1609 hours (29% of the total), BSRF took 1363 hours (23%), machine study 1161 hours (20%), injection 916 hours (16%), start-up 244 hours (4%) and failure time 490 hours (8%).
In machine study, with the full energy injected beams from the linac, the luminosity of the collider reached 5 x 10 30cm-2s-1 at 1.55GeV. The performance of BES-II is better than BES- I and the noise of the detector was significantly restrained. At BSRF, the construction of two new beamlines and the relevant experiment stations have been completed, and other beamlines and stations are improved. On February 7, the BEPC / BES / BSRF upgrading project passed the CAS appraisal.
The upgraded BEPC lays a foundation for the success of the R scan. The efforts to decrease the uncertainties in R measurement, particularly in the energy region of below 5GeV, have drawn great attention of high energy physics community. After the first R scan done in the spring of 1998, BEPC / BES collaboration performed more finer R scans at the energies between 2-5 GeV. The scanned energy points totalled up to 85. Some results of R measurement will be presented in the conference this fall.
In the aspect of BSRF, 221 user experiments are arranged, finally 251 have been performed, total user time for all experimental stations up to 7347 hours. The users came from 17 institutes of CAS, 4 institutes of other state departments, 20 universities throughout the country and a research institute of Romania. The experiments covered the fields of material physics, molecular biology, condensed physics, semiconductors, chemistry, chemical engineering, medicine, lithography, etc. The storage ring operated in multi-bunch mode, among the typical beam paraments, intensity is 80-100mA, lifetime is 20-30 hours at 2.2GeV.
Now BEPC / BES / BSRF are under summer maintenance. The machine will be resumed by the end of September.(From Zhang Chuang)
As is well known, the precise measurement of the R value, the ratio of the cross section for production of hadron to µ pair in e+ e - annihilation at low energies has drawn a great attention in high energy physics community, because it tightly relates to the theoretical calculation precision of the two important parameters for testing the Standard Model, the QED fine structure constant at the Z pole ¦Á(Mz2) and the anomalous magnetic moment of the muon, aµ. Just for such a purpose, BEPC / BES performed the finer R scan measurement in their available energy region of 2-5 GeV since last year.
After the first R scan at 6 energy points done in the spring of 1998, the BES collaboration performed a finer R scan recently in 2-5GeV energy region, almost the extremes of energy region that the BEPC can cover. The scan began in February and finished in early June of 1999, data were taken at 85 energy points. To subtract the beam associated background, the separated beam runs at 26 energy points and the single beams runs both for e- and e+ at 7 energy points were carried out, respectively. In addition, special runs at the J / ¦× energy were taken to determine the trigger efficiencies and to calibrate the detector, and both J /¦× and ¦×'(2S) resonances were scanned at the beginning and the end of the R experiment for energy calibration.
During the whole period of R scan experiment, the operations of BEPC and BES had been stable. The results from the J/ ¦× calibration runs show that the vertex chamber has a spatial resolution of about 100¦Ìm; the new main drift chamber gives a momentum resolution of 1.8%, (1+p2)1/2 and a dE/dx resolution of 8%; the barrel TOF provides 180ps time resolution; and the energy resolution and the spatial resolution of the barrel shower counter are 21% /E1/2 and 2.5cm, respectively.
So far all the R scan data have been reconstructed. Preliminary R values measured from the new scan experiment will be reported in the coming international conference this autumn.(From Xue Shengtian)
Progress in High Resolution Powder Diffraction
Image plate is a kind of 2-dimensional X-ray detector with high sensitivity. Using the image plate in the powder diffraction, the experiment efficiency can be remarkably raised since the data can be simultaneously collected in a large range of diffraction angle, while the measuring time can be reduced from 7 to 8 hours to several tens minutes.
In recent beamtime, we made a new plane image plate support system with a precision higher than the previous one used in last year. When the two image plates were used together in experiment, the measurable diffraction angle range extended to 135o . The distance from the sample to the image plate is 300mm and the spatial resolution is 0.01o With the new support system used, the job for adjustment, data processing, data analysis and fitting becomes much easier.
The high resolution diffraction patterns of standard samples and specimens have been collected in the experiment. (From Liu Peng)
Located on the end of 3W1A Beam Line of BSRF, a new high pressure research station has been built. It is equipped with HT-HP diamond anvil cell (DAC) apparatus, external heating system, energy dispersive X-ray diffractometer, ruby fluorescence pressure calibration system and micro-beam alignment. During the three commissioning runs, about 615 hours of dedicated beam time were offered to 12 EDXD experiments under high pressure conditions. The phase transitions on some nano-crystals, such as Fe2O3, GaN, CuxS, were observed.
The modified Mao-Bell diamond anvil cell (type PTD-01) was designed to generate and to keep high pressure. A motorized loading apparatus with worm gearing can finely control the force applied to the anvils. A novel technical consideration in the design of this equipment is sticking strain sensors on the loading lever arm. During the pressure increasing, the strain-pressure curves calibrated previously depending on different gaskets can be used to estimate the next pressure value needed. This method can ensure applied pressure against skip interesting experiment points. It should prevent diamonds from damage due to irregular operation as well as. Both of simulated loading test and the EDXD measuring results indicate that the new apparatus possesses ability to provide pressure as high as lOOGpa or more.
High temperature can be produced by means of external heating with two resistance heaters. An automatic control looping can keep the temperature constant with fewer errors. The DAC is enclosed in a chamber filled with argon to prevent diamonds from oxidation. The micro-beam scanning system replaced traditional beam alignment with burning paper. It has been achieved that a tiny X-ray beam is excellently adjusted to hit the sample in small gasket hole and saves beam time effectively. The ruby fluorescence pressure calibration system has been completed its setup and will be used to in situ determine the pressure in near future.(From Liu Jing)
In 1972, an exotic event was found in the clound chamber cosmic ray experiment in Yunnan, China, indicating the possible existence of an unknown particle 10 times heavier than proton. Chinese particle physicists have been paying attention to this event and hoping a magnetic spectrometer could be built to search and verify such a WIMP and other events found in Kola, LVD and other experiments. Meanwhile, some physicists at CERN suggested to make full use of the L3's large acceptance and high momentum resolution to measure cosmic ray spectrum with high precision. Thus started a collaboration between the two sides since 1997. The experiment needs to read signals independently from all 8 octants of L3's ¦Ì chamber and put 2.2m2 TO scintillation counters on the top of L3 magent.
This is the first international collaboration project launched overseas with China as the main partner. More than 30 scientists from IHEP and Institute of Theoretical Physics, the Chinese Academy of Sciences(CAS) join the collaboration. The Department of Science and Technology, CAS and the Committee of Chinese Natural Science Foundation gave strong support to this project.
In order to install the L3 Cosmic experiment before LEP tunnel closed for another physics run, 154m2 TO scintillation counter, 1/4 of the read electronics and a PC farm, etc, must be built in two months, which was an unexpected tight schedule. But Chinese scientists did it, which won widespread acclaim in CERN. President Jiang Zemin inspected the experiment during his visit in Switzerland in March 1999.
L3Cosmic experiment was successfully commissioned in May. Data have been taken since then. The event rate is about 500Hz, and the data rate is 50GByte / day. 2x 109 events have been collected so far. The experiment will be running until LEP shutdown in late 2000. (From Guo Yanan)
The Beijing Free Electron Laser(BFEL), driven by a 30MeV rf-linac, is a Compton-type middle IR FEL oscillator . The development of BFEL started in 1986, its preliminary experimental result of spontaneous emission was obtained in August 1992, its stimulated emission and saturation oscillations were carried out in May and at the end of 1993, respectively. It is the first lasing middle IR FEL in Asia.
Since 1997, BFEL Laboratory has been dedicating itself to improving the BFEL performance and expanding its range of radiation wavelength, including refix of the beam energy, wiggler field, and output wavelength; readjustment of the electron and optical beamlines; use of the newly designed anti-radiation SmCo wiggler of 4 cm period and 1.5-2.5cm variable gap. At the end of April of this year, the ideal spontaneous emission was observed, its output pulse width is larger than 3¦Ìs; the restored spontaneous radiation in the optical cavity more than 10 times was obtained in the middle of May, the stimulated emission and saturation were successfully realized at the end of this month. Using the ZnSe cavity mirrors, the output laser energy is up to 5 millijoule per macropulse was measured, the laser pulsewidth is about 2¦Ìs, the output radiation wavelength is adjustable within 9-12¦Ìm. With use of the copper cavity mirrors of small hole and newly designed small period and anti-radiation wiggler, the range of radiation wavelength will be expanded to 5-20¦Ìm.
Recently mid-IR FEL applications in the fields of material and biomedicine science are carried out. Mid-IR absorption spectra for three kinds of doped GaN, four kinds of InGaN and eight kinds of dentine samples were measured from 500 to 2000cm-1 with a Perkin-Elmer Model FT-IR spectrometer. These samples were radiated with different power densities and different characteristic absorption wave length such as 10.08, 10.74, 11.10 and 9.66¦Ìm based on selectable properties of FEL wavelength, radiation power density or energy. Some interesting results have been observed.(From Zhu Junbiao)