Shielding assessment of the Helmholtz-Beamline at the european XFEL


Shielding assessment of the Helmholtz-Beamline at the european XFEL

Ferrari, A.; Cowan, T.; Schlengvoigt, H.-P.; Schramm, U.

The Helmholtz-Beamline will operate as user facility at the European XFEL, providing a high-power and ultra-intense (PW class) optical laser “end-station”. Unique combination of high-power/high-intensity lasers with high-brilliance X-ray sources, it has the goal to extend the strong scientific potential of the XFEL project.
The laser beams will be transported to the experimental area dedicated to the investigation of matter under high energy density conditions, the HED (High Energy Density) Instrument. Here they will largely be used for the generation of secondary particles for pumping and probing and to create strong field for QED experiments. Aim of this work is the shielding assessment of the HED hutch, including an analysis of the possible activation problems.
As first step the effective radiation source term has been evaluated and verified with the present short-pulse laser operations. The possible radiation source terms, above that of the XFEL beam itself, occur only for the ultra-intense laser beams focused to above 1017 W/cm2. This holds for all of the applications in which the laser is used to accelerate protons or ions for heating or probing, creating additional x-ray backlighters, or generating intense surface harmonics, betatron radiation or electron beams in underdense targets. The hard penetrating radiation comes primarily from energetic electron beams of up to 1 nC charge escaping from underdense targets. At the available laser intensities, energies and target conditions, the effective electron source terms are up to a few-10 nC of bunch charge, in a Mawellian-like distribution with an average up to the relativistic ponderomotive limit of 10 MeV.
This representative radiation field has been then characterized in a full simulation with the Monte Carlo code FLUKA, with the goal to find a reasonable shielding optimization. In addition to the Bremsstrahlung, the most important component of the radiation is given by the neutrons that are produced via photonuclear reactions in the interaction of the electromagnetic shower with the aluminum chamber around the target and the shielding itself. The optimization has been investigated in a standard-condition, high repetition rate experiment (10 Hz) and includes an analysis of both the prompt and the residual radiation, to guarantee safe activities around the chamber after the irradiations and to avoid the eventual accumulation of long-lived radionuclides in the whole hutch area.
All the results of the shielding and activation analysis are here presented and discussed. An excellent solution is obtained with the use of moderate thicknesses of heavy concrete for the shielding walls, in combination with a thin lead layer and a local system of shielding panels of suitable materials. The local shielding is put close to the chamber to suppress the forward directed dose distribution with an efficient structure energy degrader/absorber, with the additional advantage of a large flexibility in case of non standard operation modes, where the use of thicker targets induces intense Bremsstrahlung environments and then more energetic electron beams. These cases will be handled with a case-by-case basis, with additional panels or electron beam transport into dedicated beam dumps.

Keywords: Laser-particle acceleration; Monte Carlo; Shielding Design; Residual Dose Distributions

  • Poster
    ICRS12 - 12th International Conference on Radiation Shielding, 02.-07.09.2012, Nara, Japan

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Publ.-Id: 18036