Hee-Seock Lee

Top-up operation at Pohang Light Source-II

After three years of upgrading work, PLS-II (S. Shin, Commissioning of the PLS-II, JINST, January 2013) is now successfully operating. The top-up operation of the 3 GeV linear accelerator had to be delayed because of some challenges encountered, and PLS-II was run in decay mode at the beginning in March 2012. The main difficulties encountered in the top-up operation of PLS-II are different levels between the linear accelerator and the storage ring, the 14 narrow gap in-vacuum undulators in operation, and the full energy injection by 3 GeV linear accelerator. Large vertical emittance and energy jitter of the linac were the major obstacles that called for careful control of injected beam to reduce beam loss in the storage ring during injection. The following measures were taken to resolve these problems: (1) The high resolution Libera BPM (see http://www.i-tech.si) was implemented to measure the beam trajectory and energy. (2) Three slit systems were installed to filter the beam edge. (3) De-Qing circuit was applied to the modulator system to improve the energy stability of injected beam. As a result, the radiation by beam loss during injection is reduced drastically, and the top-up mode has been successfully operating since 19th March 2013. In this paper, we describe the experimental results of the PLS-II top-up operation and the improvement plan.

Measurement of the neutron spectrum by the irradiation of a 2.04-GeV electron beam into thick targets

Abstract Inclusive neutron spectra produced by the irradiation of a 2.04-GeV electron beam into thick Al, Cu, Sn and Pb targets were measured by the TOF method. For such a high-energy region, it was the first experiment to measure the inclusive neutron spectra. A lead shield was placed at the middle point of the flight path to suppress any strong photon or electron background. The effects of the shield on the neutron spectra were calculated by the LAHET2.7 code with a small modification. The measured neutron energy in the experiments was from 10 to 200 MeV . The measured spectra were used for a benchmark test of our modified PICA95. It was found that the neutron spectra predicted by a combination of EGS4, our modified PICA95 and LAHET2.7 codes, tend to underestimate the measured ones.

Systematics of Differential Photoneutron Yields Produced from Al, Ti, Cu, Sn, W, and Pb Targets by Irradiation of 2.04 GeV Electrons

Differential photoneutron yields produced from Al, Ti, Cu, Sn, W, and Pb targets by an irradiation of 2.04 GeV electrons were measured at the angle of 90° relative to the incident beam. The neutron energy range measured in this experiments was between 10 and 400 MeV. The systematics of the yields was studied for two target conditions : a target element and a target thickness. The neutron productions by a photonuclear reaction and by a hadron cascade due to secondary particles were considered to develop semi-empirical formula for the application of shielding calculation.

Measurement of the neutron spectra inside and outside the target room of the 65 MeV electron LINAC using an extended Bonner sphere

An extended Bonner Sphere (BS) set consisting of four polyethylene (PE) spheres and three PE spheres with an inserted lead shell to increase the detection efficiency for high energy neutron was used to measure the neutron spectra from the thick Pb target bombarded by 65 MeV electron at the Pohang Accelerator Laboratory (PAL). The response function of the BS was calculated using the MCNPX code and calibrated using a 252Cf neutron source. Active BS using the LiI(Eu) scintillator for detecting of the thermal neutron was implemented outside the target room and passive BS with gold foils was used as the thermal neutron detector due to a high intense field of gamma radiation inside the room. With the results of the spectral measurement, the design and calculation of the response function of a newly modified BS of the Korea Atomic Energy Research Institute (KAERI) are also presented.

Demagnetization of Nd-Fe-B Permanent Magnet at 2.5 GeV Electron Accelerator

Permanent magnets Nd2Fe14B are the main component of the insertion devices of the synchrotron radiation facilities and used at other accelerators. The radiation induced demagnetization of permanent magnets, becomes one of the most important issues of the next generation light sources. To investigate the radiation damage the magnetic field change of Nd2Fe14B magnet exposed to a 2.5 GeV electron beam has been measured. The radiation field near the magnet and the distribution of energy deposition in the magnet are studied with Monte Carlo code FLUKA. The radiation induced demagnetization is analyzed and a fitted formula is proposed to estimate the relative reduction of remanence.

Damage analysis in PFC materials from the D-T nuclear fusion reaction

The interactions of fusion reaction products such as 2.45 MeV and 14.06 MeV neutrons and 3.05 MeV protons with PFC(plasma facing component) are analyzed using Monte Carlo codes. The dpa(displacement per atom) values in the three-layered PFC of graphite, Cu, and SS316L are calculated and the depth profile was analyzed for different-type secondary particles. The PHITS and MCNPX code were used for this study. The products of hydrogen, helium gas were also calculated for analysis of nuclear damage effect. The serious irradiation condition of the ITER parameter, a neutron flux of 3.5×1013 neutrons/cm2·sec, was applied. The dpa values during one year operation were estimated as 0.40-0.75 dpa for graphite, 2.43-4.57 dpa for Cu and 1.78-2.69 dpa for SS316L. The production rate of H, He atom for SS316L is estimated to 316 appm, 42 appm, respectively.

Monte Carlo Study in Radiation Damage Experiments of Permanent Magnets by High Energy Electrons

The demagnetization of permanent magnets used in the insertion device, becomes one of the most important issues of the next generation light sources. The demagnetization experiment has been carried out at the electron linac of Pohang Light Source. To find out the mechanism of radiation damage to the magnet, the radiation field around and inside the target and magnet in the experiment is investigated with Monte Carlo code FLUKA (2005 version). The energy spectra of photons and neutrons around the target and magnet are derived and analyzed The 3‐dimensional dose distributions in the target and magnet due to 2 GeV incident electrons and secondary particles are also calculated. The calculated results under different target conditions (Cu, Ta target or no target) are compared. Some important parameters such as dose and 1 MeV equivalent fluence are summarized. Finally the demagnetization of the magnet due to radiation at high energy accelerator is discussed.

Secondary Charged Particle Measurement from 2‐GeV Electron‐Induced Reactions with a Current Time‐of‐Flight Technique

The energy spectra of secondary charged particles from 2.04 and 2.5 GeV electron‐induced reactions were measured using a current time‐of‐flight technique at the Pohang accelerator Laboratory. A long flight path and the current time of flight method were adopted to measure the pulse height of each event that reached the detectors in a short period. Secondary proton and deuteron energy spectra were obtained at 90 degrees with respect to the electron incident angle. The results were compared with those calculated by the MCNPX‐2.5e code.

Radioactivity in Aluminum, Water and Carbon Beam Dumps by 2-2.5 GeV Electrons

Thick beam stops were bombarded by 2-25 GeV electrons. The Al foils and liquid containers were placed at the several depth in 10-radiation-length thick beam stop made of Cu plates. The containers were filled with the water or xylene (C8H10). The residual radioactivity was measured using activation gamma-ray spectroscopy. Tritium production yields in Al, O and C were measured using liquid scintillation counters. Depth distribution of the residual activity in the beam dumps were calculated using EGS4 and PICA3 Monte Carlo code.

PICA3, an Updated Code of Photo-nuclear Cascade Evaporation Code PICA95, and Its Benchmark Experiments

PICA95, a calculation code for high-energy photo-nuclear reactions is updated as PICA3. PIC A3 can predict experimental total photo-nuclear reaction cross section for incident photon energies from the reaction threshold to 3.5 GeV very well. It also can predict other experimental data such as yields of residual nuclides and emitted particle spectra from mono-energetic photo-nuclear reaction, but underestimates neutron yields produced from electron bombarded thick targets.