LEPS - Laser Electron Photon Experiment at SPring-8

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The BL31LEP has been constructed as the second laser-electron photon (LEP) beamline. We have obtained the first beam in January, 2013 and started the commissioning. The photon energies between 1.4 GeV and 3.0 GeV are determined by tagging the electrons which are recoiled in the Compton scattering.  The tagging detector is placed at the downstream of the BM1 bending magnet.  n the LEPS2 experiment at BL31LEP, two upgrades are the key ingredients, the intensity upgrade about one order and the construction of the large acceptance and high resolution detectors. The former is achieved by the simultaneous injection of multi-number of high power lasers. For the latter, we have constructed a large experimental building outside the storage ring building. By using this high intensity beam and large detectors, we aim to make greater progress in the study of quark-nuclear physics.

A schematic view of the LEPS2 beamline


Beam conditions

Energy range (Tagged Photon)

1.3 - 2.4 GeV (with 355-nm UV Solid-state laser)

1.3 - 2.9 GeV (with 266-nm DUV Solid-state laser)

Energy Resolution < 15 MeV
Beam intensity

< 2 x 107 photons/s (355-nm lasers)

< 2 x 106 photons/s (266-nm lasers)



Detectors for the LEPS2 large charged particle spectrometer will be placed inside the 1 T solenoid magnet with 5-m diameter and 400-t weight. The magnet, which had been used in the kaon rare decay experiment (E949) at the Brookhaven National Laboratory, has a large bore of 2.22-m length and 2.96-m diameter. We intend to start operation of the LEPS2 spectrometer in 2015. Large acceptance tracking detectors, high resolution particle identification detectors, etc., are under development and construction.
In the space upstream of the solenoid magnet, an electromagnetic calorimeter covered a large solid angle, called BGOegg, is placed, which has been constructed in the ELPH, Tohoku University. The BGOegg consists of 1320 BGO crystals with 20 radiation length assembled to an egg shape and has the highest energy resolution in the world. The experiment using the BGOegg starts from 2013B, together with a forward TOF counter composed of the array of resistive plate chambers (RPC).

 Large solenoid (former BNL-E949) magnet (left) & BGOegg detector (right)


BGOegg Experiment (ELPH page)

Laser Injection system

In the laser injection room, two solid tables are placed for 355-nm lasers and 266-nm lasers, respectively. Four lasers are put on each table and can be injected simultaneously through the prism mirrors. Using 355-nm lasers, the maximum energy is 2.4 GeV and the intensity is high, while the maximum energy is 2.9 GeV and the intensity is one order low by using 266-nm lasers. Depending on the experimental purpose, we can choose preferable one.


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"Interference effect between ϕ and Λ(1520) production channels in the  γpK+K reaction near threshold", S.Y. Ryu, J.K. Ahn, T. Nakano et alPhys.Rev.Lett. 116 (2016) no.23, 232001

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