MeV- and Sub-MeV-photon Sources Based on Compton Backscattering at Spring-8 and KPSI-JAEA

Recently we have constructed two facilities for generating photon beams in the MeV and sub-MeV energy regions by means of the Compton backscattering with a laser and an electron beam at SPring-8 and at Kansai Photon Science Institute of Japan Atomic Energy Agency(KPSIJAEA).The MeV-photon source at SPring-8 consists of a continuous-wave optically-pumped far infrared laser with a wavelength of 118.8 μm and an 8 GeV stored electron beam.Present MeV-photon flux is estimated to be 1.3×10~3 photons/s.On the other hand,the sub-MeV-photon source at KPSI-JAEA consists of a pulse Nd:YAG laser with a wavelength of 1 064 nm and a 150 MeV electron beam accelerated by microtron.In the first trial of the photon production in this source,backscattered photon flux is estimated to be 20 photons/pulse.Both the Compton backscattered photon sources have possibilities to be used for new tools in various fields such as nuclear physics,materials science,and astronomy.     

Experiment studies on the polarized gamma-rays generation at KEK-ATF

Polarized positrons can be created through electron-positron pair creation from circularly polarized gamma-rays. Laser-Compton scattering is an efficient method to generate circularly polarized gamma-rays. A high finesse 2-mirror optical stacking cavity had been installed on the straight section of the electron storage ring at KEK-ATF. A 1064 nm circularly polarized pulsed laser beam was stacked in the cavity. Polarized gamma-rays with a maximum energy of 28.3 MeV were produced via inverse Compton scattering of the enhanced laser pulse off an electron beam of 1.28 GeV. The number of generated gamma photons per collision was estimated by a photon detector. It was found that the experimental result was in agreement with the simulated value.     

On possibility of measurement of the electron beam energy using absorption of radiation by electrons in magnetic field

A possibility of precise measurement of the electron beam energy using absorption of radiation by electrons in a homogeneous magnetic field for electrons of high energy in the range up to a few hundred GeV, was considered earlier. In this paper, with the purpose of experimental checking of this method in the range of several tens MeV of electrons energies, a possibility of measurement of absolute energy of the electron beam with a relative accuracy up to 10^?4, is considered. We take into account influence of the laser beam diffraction, of the spread of electrons over energies, and of the length of formation of radiation absorption in the process of electron beam energy measurement. The laser wavelength and the length of the magnet are chosen depending on the length of photon absorption formation. It is found that the kinematical restrictions on the photon absorption process lead to the selection in angles of propagation of photons, which can be absorbed by the beam electrons. It is shown that parameters of the electron beam will noticeably not vary during the measurement of the energy.     

Energie- und Winkelverteilung sekundärer Photonen bei der Wechselwirkung eines 6,2 GeV-Gammastrahls mit komplexen Kernen

The energy spectra of secondary photons from the interaction of a 6.2 GeV bremsstrahlung beam with targets of H₂, Be, C, Al and Cu are measured for laboratory angles from 35° to 120°. All spectra exhibit an exponential decrease of the photon yield with increasing momentum. Difference measurements with energies of the bremsstrahlung beam from 3.5 to 6.2 GeV for a center of mass angle of 90° are compared with the thermodynamical model of Fermi and the reaction temperature is determined to be 83MeV±8%. The results agree approximately with the measurements of p-Be-collisions at small laboratory angles.     

First Measurement of the Gerasimov-Drell-Hearn Sum Rule for 1H from 0.7 to 1.8 GeV at ELSA

To verify the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the first time experimentally, we measured the helicity dependent total photoabsorption cross section with circularly polarized real photons and longitudinally polarized nucleons in the photon energy range 0.68-1.82 GeV with the tagged photon facility at ELSA. The experiment was carried out with a 4pi detection system, a circularly polarized tagged photon beam, and a frozen spin polarized proton target. The contribution to the GDH sum rule in this photon energy range is [49.9+/-2.4(stat)+/-2.2(syst)] microb.     

Tagging system for almost-real photons at VEPP-3 storage ring

At BINP the construction of the tagging system for almost-real photons (TS) is in progress. The energy of tagging photons can be up to 1.5 GeV. The projected energy resolution of TS is better then 1%. For at least a half of photons the linear polarization can be determined. The tagging system will extend the possibilities for photoreaction studying at VEPP-3 significantly. TS would allow to perform a complete kinematics reconstruction, thus permitting a reliable rejection of the background processes; to extend the measurements to higher photon energy; to enabling Σ-asymmetry measurements and double polarization experiments.     

Bremsstrahlung in intermediate-energy nucleon reactions within an effective one-boson exchange model

Within a covariant effective one-boson exchange model for the T matrix of NN interactions we present detailed calculations of bremsstrahlung cross sections for proton-proton and proton-neutron reactions at beam energies in the 1 GeV region. Besides pure bremsstrahlung processes we consider photons from Λ decays and contributions from the η → γγ process. At beam energies above 700 MeV the Λ decay channel dominates the spectra at large photon energies, where the interference between non-resonance processes and the Λ decay channel becomes also important. Low energy photons stem from pure bremsstrahlung processes. The available experimental data at 730 MeV beam energy is well described. We extrapolate the model down to 280 MeV, where more detailed experimental data exist, and find agreement with angular distributions.     

The new frozen spin target at MAMI

The new frozen spin polarized target for experiments at the polarized beam of the real photon facility A2 of the MAMI accelerator is described. The A2-collaboration at the Mainz Microtron MAMI is measuring photon absorption cross section using circularly and linearly polarized photons up to the energy of 1.5 GeV. The photons are produced in the’ Bremsstrahlungs’ process. In the years 2005/2006 the Crystal Ball detector with its unique capability to cope with multi photon final states was set up in Mainz. Since 2010 the experimental apparatus has been completed by a polarized target. The horizontal dilution refrigerator of the Frozen-Spin Target has been constructed and is operated in close cooperation with the Joint Institute for Nuclear Research in Dubna, Russia. The system offers the opportunity to provide longitudinally and transversely polarized protons and deuteron. In this paper the operation experience of this new Frozen-Spin Target and first results from the runs in 2010 and 2011 are presented.     

Mapping gluonic excitations and confinement

One of the outstanding and fundamental questions in physics is the quantitative understanding of the confinement of quarks and gluons in quantum chromodynamics (QCD). Confinement is a unique feature of QCD. Exotic hybrid mesons manifest gluonic degrees of freedom and their spectroscopy will provide the crucial data needed to test assumptions in lattice QCD and phenomenology leading to confinement. Photoproduction is expected to be particularly effective in producing exotic hybrids but data using photon probes are sparse. At Jefferson Lab, plans are underway to use the coherent bremsstrahlung technique to produce a linearly polarized photon beam. A solenoid-based hermetic detector will be used to collect data on meson production and decays with statistics that will exceed the current photoproduction data in hand by several orders of magnitude after the first year of running. In order to reach the ideal photon energy of 9 GeV/c for this mapping of the exotic spectra, the energy of the Jefferson Lab electron accelerator, CEBAF, will be doubled from its current maximum of 6 GeV to 12 GeV. The physics and project are described.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.39.Mk Glueball and nonstandard multi-quark/gluon states - 13.60.Le Meson productionA.R. Dzierba: Present address: Department of Physics, Indiana University, Bloomington, IN 47405 USA     

Photon discrimination simulation with the PCA method

The shower shape of n,n,p,p,K+,π+ and photons,generated by JPCIAE code for 5.5 TeV/A 208Pb+208Pb collisions,incident on the ALICE photon spectrometer(PHOS),is analyzed with the principal component analysis(PCA) method.The efficiency dependence of purity for the photon discrimination is simulated for the deposited energy ranges 0.5-2 GeV,2-10 GeV,10-50 GeV and 50-100 GeV.The result shows that in the energy range of 0.5 to 100 GeV,the efficiency of the photon identification can reach 90% with purity of 90%.     

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用KMR模型研究了等离子体中多光子非线性Compton散射下电子相轨道的演化,发现电子与光子进行能量交换后,只有未被俘获电子的相轨道才能从周期性向非周期性、随机型演化,其演化剧烈程度随电子吸收光子数的增大而加剧,随电子与光子碰撞非弹性成分和碰撞前初始速度的增大而迅速减弱,但低于强激光场中的剧烈程度。当电子被光场俘获时,这种演化过程结束并做稳定的运动,光子不再为电子提供能量。     

Compton散射下等离子体中电子相轨道的演化

用KMR模型研究了等离子体中多光子非线性Compton散射下电子相轨道的演化,发现电子与光子进行能量交换后,只有未被俘获电子的相轨道才能从周期性向非周期性、随机型演化,其演化剧烈程度随电子吸收光子数的增大而加剧,随电子与光子碰撞非弹性成分和碰撞前初始速度的增大而迅速减弱,但低于强激光场中的剧烈程度。当电子被光场俘获时,这种演化过程结束并做稳定的运动,光子不再为电子提供能量。     

利用康普顿后向散射产生X光源的研究

 利用高能电子对电磁波的康普顿后向散射可以获得波长短、方向性好的X光辐射,这种具有亮度高、发散角小、光子能量可调等特点的辐射,有很好的应用前景。由于电子对电磁场的散射截面很小,要获得较高强度的辐射,不仅要求很强的电磁场作为散射场,而且要求电子束的亮度高;利用光阴极RF腔注入器及30MeV射频加速器产生的30MeV电子束同Nd:YAG激光光腔中的强光场作用,可以获得较高强度的康普顿后向散射,其波长为0.07~0.14nm,发光本领可以达到2.5×10¹¹光子/s。     

Photon and positron generation by ultrahigh intensity laser interaction with electron beams

This study investigates the generation of high energy photons and positrons using focused ultrahigh intensity femtosecond laser pulses on a relativistic electron beam with a set of two-dimensional particle-in-cell simulations. We consider circularly and linearly polarized, single and spatially separated double laser pulses. We model both 500 MeV and 1 GeV electron beams. Higher positron production is obtained using circularly polarized laser pulses. Using double pulses, the focusing effect of the ponderomotive force confines the electrons to a small volume, generating additional energetic photons and positrons. The positron spectral distributions are effectively modified by these variations. When the electron beam energy is doubled, the number of positrons increased, while the cutoff energy remained nearly constant.     

A new beam asymmetry measurement from pion photoproduction on the neutron using CLAS

We present a preliminary analysis of the photon beam asymmetry observable (Σ) from the pho-toproduction reaction channel γn→pπ~- in the invariant mass range 1.6-2.3 GeV. The measurement was obtained using the near-4π CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory, USA, employing a linearly polarised photon beam with an energy range 1.1-2.3 GeV, incident on a liquid deuterium target. The measurement will provide new data to address the poorly established neutron excitation spectrum and will greatly expand the sparse world data-set both in energy and angle.     

Measurement of prompt photons with associated jets in photoproduction at HERA

The photoproduction of prompt photons, together with an accompanying jet, has been studied in ep collisions at a centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 77 pb^-1. Cross sections were measured for the transverse energy of the photon and the jet larger than 5 and 6 GeV, respectively. The differential γ+jet cross sections were reconstructed as functions of the transverse energy, pseudorapidity and x_γ ^obs, the fraction of the incoming photon momentum taken by the photon-jet system. Predictions based on leading-logarithm parton-shower Monte Carlo models and next-to-leading-order (NLO) QCD generally underestimate the cross sections for the transverse energies of prompt photons below 7 GeV, while the k_T-factorisation QCD calculation agrees with the data better. When the minimum transverse energy of prompt photons is increased to 7 GeV, both NLO QCD and the k_T-factorisation calculations are in good agreement with the data.     

多光子非线性Compton散射的能量转换

研究了多光子非线性Compton散射中电子与光子的能量转换及其转换效率.结果表明:散射光子频率随电子吸收光子数n的增大而增大,随碰撞非弹性成分ξ的增大而迅速减小.在超强激光场中,当极端相对论性电子与光子发生多光子非线性Compton散射且被光场俘获时,能量转换效率趋于无限大,即电子可以从超强激光场中获得巨大的加速能量.用高速电子束入射并与光子发生多光子非线性Compton散射,是提高非线性Compton散射能量转换效率的重要途径.     

Barrel calorimeter of the CMD-3 detector

The structure of the barrel calorimeter of the CMD-3 detector is presented in this work. The procedure of energy calibration of the calorimeter and the method of photon energy restoration are described. The distinctive feature of this barrel calorimeter is its combined structure; it is composed of two coaxial subsystems: a liquid xenon calorimeter and a crystalline CsI calorimeter. The calorimeter spatial resolution of the photon conversion point is about 2 mm, which corresponds to an angular resolution of ～6 mrad. The energy resolution of the calorimeter is about 8% for photons with energy of 200 MeV and 4% for photons with energy of 1 GeV.     

上海激光电子γ源

激光具有高强度、 高极化度等优异的性能。 用激光束轰击高能电子束就可以产生高强度、 高极化度的γ射线束。 上海激光电子γ源就是上海同步辐射装置上的这样一条束线站。 预计可以获得能量范围为1—22 MeV的准单色、 高强度（109—1011 s-1）和高极化度（线极化或圆极化）的γ射线束。 介绍了这条束线站目前的进展情况。 Shanghai Laser Electron Gamma Source (SLEGS) is a high intensity,short pulse and compact γ ray source which is based on inverse Compton scattering via interaction between pulsed high power laser beams and picosecond relativistic electron bunches. One of the attractive features of the laser Compton scattering is the easy control of polarization of the produced high energy photons that duplicates polarization of the applied laser beam. The γ ray with energy up to 22 MeV and intensity of 109—1011s 1 are expected to be produced by Compton backscattering of CO2 laser photons on the 3.5 GeV electrons bunches in the Shanghai Synchrotron Radiation Facility (SSRF). In this communication, we report same simulation results and the progressing status of SLEGS.     

Measurements of the QED structure of the photon

The structure of both quasi-real and highly virtual photons is investigated using the reaction , proceeding via the exchange of two photons. The results are based on the complete OPAL dataset taken at centre-of-mass energies close to the mass of the Z boson. The QED structure function and the differential cross-section for quasi-real photons are obtained as functions of the fractional momentum x from the muon momentum which is carried by the struck muon in the quasi-real photon for values of ranging from 1.5 to 400 GeV. The differential cross-section for highly virtual photons is measured for GeV and GeV, where and are the negative values of the four-momentum squared of the two photons such that . Based on azimuthal correlations the QED structure functions and for quasi-real photons are determined for an average of 5.4 GeV. Received: 8 February 1999 / Published online: 28 September 1999