The Physics of Spallation Processes

2.3. THE EUROPEAN SPALLATION NEUTRON SOURCE ESS 21Table 2.4: same as Tab. 2.3, but for current densities; norm. to solid angle of 4 ◦ opening.moderator type SPTS LPTSĴ ¯J Ĵ ¯Jpara H 2 [n/(cm 2 s sr)] 6.8 × 10 15 2.5 × 10 13 6.3 × 10 14 2.1 × 10 13amb. water [n/(cm 2 s sr)] 9.8 × 10 15 1.8 × 10 13 5.0 × 10 14 1.7 × 10 13current in (1/cm2*s*sr*Å)current in (1/cm2*s*sr*Å)x 1e144.543.532.521.510.50x 1e1310.90.80.70.60.50.40.30.20.104Åcoupleddecoupleddecoupled +poisoned0 100 200 300 400 500 600time in µs10Å0 100 200 300 400 500 600time in µspeak width FWHM in µs25022520017515012510075502500 2 4 6 8 10wavelength in ÅFigure 2.8: Neutron current density spectra (left panels) and neutron pulse widths(FWHM, right panel) as a function of wavelength for coupled (solid line), decoupled(dashed line) and decoupled+poisoned (dotted line) para-H 2 moderators.The influence of decoupling and poisoning 3 on the neutron pulse width for a 20 K liquidpara-H 2 moderator is shown in Fig. 2.8. When analysing the neutron current densityspectra (left panels of Fig. 2.8) for different wavelengths, an increase of the peak width isobserved with increasing wavelength (right panel), simply because for larger wavelengthsor smaller energies the thermalization of neutrons lasts longer. Irrespective of the selectedwavelength the pulse width can be reduced to less than 40% when decoupling and poisoningthe para-H 2 moderator. Unfortunately at the same time the neutron current densityis also decreasing by up to a factor of three as shown in the left panel of Fig. 2.8.The burn-up of the poisoning layer inside the moderator due to nuclear reactionshas a strong impact on its expected life time. Assuming e.g. a thermal neutron flux ofΦ th = 5 × 10 13 n/cm 2 s at the poisoning surface a life time of 106 days results for a 0.5 mmthick Gd layer. The neutron absorbing isotopes with cross sections of several 10 5 barn3 The purpose and realization of decoupling and poisoning is also illustrated in sec. 6.4.2 on page 86.