Commissioning of laser electron gamma beamline SLEGS at SSRF

The Shanghai Laser Electron Gamma Source (SLEGS) is a powerful gamma source that provides MeV gamma-ray beams for nuclear science and technology. It was developed as one of the 16 beamline stations in the Phase II Project of the Shanghai Synchrotron Radiation Facility. The slant-scattering mode is for the first time systematically employed in laser Compton scattering at SLEGS to produce energy-tunable quasi-monoenergetic gamma-ray beams. The SLEGS officially completed its commissioning from July to December 2021. Gamma rays in the energy range of 0.25–21.7 MeV with a flux of 2.1 × 104–1.2 × 107 photons/s and an energy spread of 2–15% were produced during the test. This paper reports the results from commissioning the SLEGS beamline.

HI  S are in operation as a major facility. The laser electron gamma source VEGA system in the Extreme Light Infrastructure-Nuclear Physics (ELI-NP) was redesigned in 2019, currently under construction, and is scheduled to run in 2023 [11].
Shanghai Laser Electron Gamma Source (SLEGS) was first proposed in 1998 at the Shanghai Synchrotron Radiation Facility (SSRF). After nearly 20 years of the feasibility study and proto-type development [12][13][14][15][16][17][18][19], it was approved in 2016 as one of the sixteen beamlines in the Phase II project of the SSRF by the National Development and Reform Commission, People's Republic of China. The construction of the SLEGS beamline began in October 2019 and ended in December 2021. The beamline has passed the commissioning test in 2021 and is open to users by the end of 2022 [20].
Using a CO2 laser with the wavelength of 10.64 m and the maximum power of 100 W, highintensity gamma-ray beams are generated in collisions with 3.5 GeV electrons in the SSRF storage ring. SLEGS is designed to operate in two operating modes: One is the laser Compton backscattering mode at 180˚, which is commonly used worldwide to achieve the highest energy and polarization of gamma-ray beams; The other is the laser Compton slant-scattering mode which enables one to tune the energy of gamma-ray beams by changing the collision angle between the laser and electron beams in the wide range from 160 o to 20 o .
The slant-scattering is a key technology to produce energy-tunable gamma-ray beams at synchrotron radiation facilities operated at a fixed electron-beam energy. In the past decade, the SINAP-II [15][16][17][18][19] and UVSOR-II have played the pioneering role in the technical development of the slant-scattering. While the UVSOR-II has succeeded in producing MeV gamma-rays at the limited slant-scattering angles of 70 o -110 o [21,22], the gamma-ray beam production at SLEGS is systematically carried out in an innovative manner with the dedicated interaction chamber [23] and double collimator system [24,25].

Research in SLEGS
SLEGS is designed to generate gamma-ray beams at energies ranging from 0.25 MeV to 21.1 MeV in the slant-scattering at 20°-160°and at the maximum energy of 21.7 MeV in the backscattering at 180°. The full-spectrum flux ranges from~10 5 photons/s at 20°to~10 7 at 180 o . The 2 % energy resolution is achieved with the 1 -2 mm collimator which confines the emission angle of the gamma-ray beam less than 0.5 mrad.
The SLEGS serves as a multi-functional experimental platform for research in nuclear science and technology [20]. The research spectrum in basic research, on the one hand, ranges from collective excitation of nuclei, nuclear equation of state, origin of elements, to ultra-high energy cosmic rays, while that in the application research, on the other hand, ranges from atomic energy, nuclear transmutation, gamma imaging, to national security.

SLEGS beamline
The SLEGS beamline is equipped with the following four main components as illustrated in Fig. 1. SLEGS is located in BL03SSID of the SSRF experimental hall. A 3D drawing is shown in Fig. 2.
(2) Interaction chamber for the slant-scattering and multi-function chamber for the backscattering .
(3) Two-stage coarse and fine collimator and gamma flux attenuator.
(4) Experimental hutch, experimental detector, gamma absorber and data acquisition system  (g) in the SLEGS beamline)

Laser transport and gamma production
The continuous-wave CO2 laser of 10.64  m in wave length and 100 W in laser power is placed in the laser hutch located adjacent to the SSRF vault. The hutch is designed as a clean room for a stable operation of the laser. The CO2 laser can also be operated in a pulse mode with adjustable laser interval and pulse width, for example, 1 ms laser Interval and 100 s pulse width at 10 W with a 10 % duty cycle.
The collision between the 10.64 m laser photons and 3.5 GeV electrons occurs inside the interaction chamber shown in Fig.3 in two collision modes: laser Compton back-scattering and slant-scattering. The CO2 laser beam goes through the multi-function chamber with the ZnSe window, bends by 90 through a water-cooled mirror and undergoes the head-on collision in the back-scattering mode. In contrast, the laser beam vertically enters into the interaction chamber, passes through the optical elements (the mirrors m4, m3, m2, and m1 and convex lens f1 in Fig.3), and horizontally collides with the electron beam in the slant-scattering. The CO2 laser is finally refocused and led to the beam absorber G through the mirror m0 and convex lens f0. The optical elements are mounted on the rotating bracket so that the collision angle between the laser and electron beams are changed from 20 o to 160 o through the safety stop angle at 90 o [23].

Coarse and fine Collimators
The energy spread and spot size of gamma-ray beams at the target position are determined by the two-stage coarse and fine collimator system, as shown in Fig.4(a) and 4(b). The coarse collimator of a revolver type is made of 300 mm thick lead(Pb), located at 18 m away from the gamma production point inside the interaction chamber and separated from the high-vacuum SSRF with a 150 m beryllium(Be) window. It has 10 fixed holes with apertures of 0, 1, 2, 3, 4, 5, 8, 10, 20 and 30 mm. The fine collimator of a camera-shutter type, which is located at 36 m away from the gamma production point in the optical hutch, consists of four sets of shielding shutters, each made of 50 mm tungsten-copper alloy (W 80 % + Cu 20 %). The first and second two sets of the shutters are relatively rotated around the beam axis by 30 , shaping the beam spot into a regular dodecagon [24,25]. The aperture of the fine collimator is changed from 0 mm to 40 mm continuously. chinaXiv:202206.00139v1

Flux attenuator
The flux of gamma-ray beams is regulated by the attenuator consisting of 8 pieces oxygenfree copper blocks of the diameter of 50 mm, the thickness of 200 mm, 100 mm (3 pieces), 50 mm (2 pieces), 25 mm and 15 mm, and total thickness of 640 mm, as shown in Fig.4(c). Localized shielding around the fine collimator and flux attenuator are shown in Fig 4(d).   Figure 5(d) shows the control display of the laser system to be tuned for the laser Compton back and slant-scatterings. The operation condition of the CO2 laser in the pulse modes is shown in the monitor display, including the pulse interval and width.

Commissioning of the SLEGS beamline
Measurements of gamma-ray beams in the commissioning of the SLEGS beamline were performed in the experimental hutch. At the end of the beamline, the unreacted gamma rays enter the gamma absorber made of thick concrete, lead, and high-density polyethylene, as shown in Fig. 1.

Collimator alignment
The collimators need to be aligned with respect to the gamma-ray beam axis. The collimator alignment was carried out horizontally and vertically by maximizing the intensity of gamma-rays that passed through the collimator. Figures 6(a) and 6(b) show the gamma-ray intensity as a function of vertical and horizontal position of the coarse collimator, respectively. In this collimator position scan, the coarse collimator with the aperture of 2 mm was used along with the 20 mm fine collimator and the 100 mm flux attenuator. A  3" x 4"( 76.2 mm x 101.6 mm) LaBr3(Ce) detector [27] was used to measure gamma rays. The optimized position of the coarse collimator was found at (51.97  0.04) mm vertically and (34.54  0.03) mm horizontally. The laser -electron collision position and fine collimator also have the same adjustment process.

Gamma-ray beam spot
The SLEGS beamline is equipped with two beam spot monitors in the experimental hutch (Fig.7), an X-ray camera called MiniPIX [28] shown in Fig. 7(b) and the Gamma Spot Monitor (GSM) shown in Fig. 7(d). The MiniPIX consists of 256 x 256 silicon pixels with each pixel size of 55 m x 55 m and the silicon wafer thickness of 500 m. The GSM involves an LYSO scintillator of 70 mm in diameter and 2.5 mm in thickness, and collects the scintillation light generated by X and gamma-rays with a CCD/CMOS camera through a 45-degree plane mirror, also designed a 135 optical path as a reserved monitoring channel.

Energy-profile and flux of gamma-ray beams
A Saint-Gobain lanthanum bromide (LaBr3(Ce)) detector of 3"(76.2 mm) in diameter and 4" (101.6 mm) in length with the better energy resolution about 3 % at 661 keV [27], was used as the energy-profile and flux monitor detector in the commissioning run. The BGO detector of 76.2 chinaXiv:202206.00139v1 mm in diameter and 200 mm in length from SICCAS [29] located in the gamma absorber was used to monitor the gamma flux. The flux value was fed back to the control system of the collimator and laser for online recording and display, and be used as automatic correction of laser power output in future. Figure 8(a) shows a typical response function of the LaBr3(Ce) detector to the gamma-ray beam, where the 3 mm coarse collimator, 3 mm fine collimator, and 1 mm additional lead external collimator were used, but without the flux attenuator. The background gamma spectrum is shown by the green line, where the lower-energy synchrotron radiation and higherenergy bremsstrahlung are seen as well as the internal radiation from 138 La in the LaBr3(Ce) detector. Figure 8(b) shows background-subtracted spectra measured at 20 o , 90 o and 180 o .
We have more systematically measured response functions of the LaBr3(Ce) detector to gamma-ray beams produced in the slant-scattering. The background-subtracted spectra are shown in Fig.9. The gamma-ray energy follows the kinematics of the laser Compton slant scattering. The production of gamma-ray beams be confirmed in the energy range of 0.

Energy distribution of gamma-ray beams
The electron beam energy of the SSRF storage ring is 3.5 GeV. Because of this high electronbeam energy, the energy of gamma-ray beams produced in the laser Compton scattering strongly depends on the emission angle. Thus, the energy spread (resolution) of the gamma-ray beam is highly sensitive to the angular acceptance defined by the collimator aperture. The black line of Fig. 11 shows the experimental response function of the LaBr3(Ce) detector to gamma rays produced in the back-scattering at 180 o . Assuming that the energy distribution of the gamma-ray beam incident on the detector is given by a truncated Breit-Wigner function which is shown by the blue line, we made a Geant4 simulation of the response function of the LaBr3(Ce) detector to the gamma-ray beam. The result is shown by the red line in Fig. 11. One can see that the Geant4 simulation well reproduces the experimental response function (black line). Phenomenologically, as the scattering angle decreases in the slant-scattering, incident gamma-ray spectra of Gaussian function better fit the experimental response function in the Geant4 [30] simulation. In this phenomenological study, it was found that the energy resolution of 2 -15 % can be obtained in the collimator aperture of 1 -3 mm.
A thorough study of the energy resolution of the gamma-ray beam requires more realistic Geant4 simulations in which gamma-ray beams are kinematically produced in the laser Compton scattering, transported through the collimator system, and incident on the LaBr3(Ce) detector. The energy distribution of the gamma-ray beam produced in the laser Compton scattering depends on the electron beam emittance, which is an important parameter to reproduce the experimental response function. A thorough analysis by means of more realistic Geant4 simulations will be done in the near future. Fig. 11 Experimental response function of the LaBr3(Ce) detector to a gamma-ray beam produce in the back-scattering at 180 (black line), a truncated Breit-Wigner function (blue line) assumed for the energy distribution of the gamma-ray beam, and a Geant4 simulation of the LaBr3(Ce) response function (red line) to the gamma-ray beam (blue line).

Experimental spectrometers
We have developed four types of experimental spectrometers (sometime called as detector array) shown in Fig. 12, which are gamma nuclear resonance fluorescence (NRF) spectrometer(a), neutron flat-efficiency detector(FED) spectrometer (b), neutron time-of-flight (TOF) spectrometer(c) , and light charged-particles (LCP) spectrometer(d) . The four experimental spectrometers are used for the photon elastic/ inelastic scattering reaction, the photo-neutron reaction and the photo-light charged particle reaction, respectively. The spectrometer for photofission reaction is not involved in the current construction stage. The photonuclear reaction experiment can also employ a combination of two spectrometers, such as FED combined with NRF spectrometer will be used in (,n) experiment [31].

Results of the commissioning of the SLEGS beamline
The SLEGS beamline started the first test run at a small electron-beam current 10 mA in July 2021 and finished the debugging of the gamma-beam generation in back-and slanting-scattering modes at 200 mA. The performance of the SLEGS beamline in generating gamma-ray beams and the experimental spectrometer was approved by the Chinese Academy of Sciences (CAS) [38] in December 2021 and the construction of the SLEGS beamline station is completed. Results obtained in the commissioning run are summarized in Table 1.

Summary and prospects
The completion of the SLEGS beamline station has been approved by the Chinese Academy of Sciences (CAS) in December 2021 through the successful commissioning run. Gamma-ray beams are produced in the energy range from 0.25 to 21.1 MeV in the laser Compton slant-scattering with 10.64 m CO2 laser photons at scattering angles from 20 o to 160 o , and at the maximum energy of 21.7 MeV in the laser Compton back-scattering at 180 o . The slant-scattering mode is employed in the SLEGS beamline to systematically produce gamma-ray beams.
The two-stage coarse and fine collimators are used to produce fine pencil-like quasimonochromatic gamma-ray beams. The gamma-ray integrated flux is 2.1 x 10 4 photons/s at 20, 2.5 x 10 6 photons/s at 90 and 1.2 x 10 7 photons/s at 180. The energy spread of the gamma-ray beam is 2 -15 % when the gamma-ray emission angle is confined less than 0.45 mrad at 38m with the collimator system. Four types of spectrometers, which may meet the versatility of nuclear physics experiments [39][40][41][42][43][44], are available in the SLEGS experimental station. The SLEGS beam line will be open to users by the end of 2022. In recent years, some large nuclear scientific facilities have started construction and put into operation, such as Back-streaming neutron(Backn) source [45,46] at China Spallation Neutron Source(CSNS), High Intensity Heavy-ion Accelerator Facility (HIAF) [47][48][49] and CSR External-target Experiment(CEE) [50], Collinear Laser Spectroscopy (CLS) [51,52] and the new Beijing Radioactive Ion-beam Facility (BRIF) [53] etc., The completion of these facilities or devices will surely push China's nuclear physics basic and applied research to a new stage.