New Cold Source

A new liquid hydrogen cold source is being built for placement in the beam port BT-9 to provide an intense beam of long-wavelength neutrons for the Multi-Analyzer Crystal Spectrometer (MACS). Monte Carlo N-particle transport code (MCNP) calculations show that the heat load on the new source is sufficiently small that both the new and current cold sources can be handled by the existing refrigerator, and it is expected that they will be operated together from the same refrigerator for several years. Liquid hydrogen will be supplied by thermosiphon action through separate load lines to each condenser. A new, 0.4 m³ ballast tank will be installed so that the new system will be independent of the existing cold source. The new system will also have its own instrumentation.

Optimization of the new source must balance the requirements of MACS and engineering constraints arising from the small size of BT-9. MACS features a doubly focusing monochromator and it is designed to exploit a large-area beam. The source was designed to provide the largest possible number of neutrons from the maximum diameter geometry; there is no reentrant hole. Optimization was achieved by calculating the liquid hydrogen thickness that maximizes intensity for the anticipated void fraction of a 160 W heat load. Fortunately, the cold neutron beam intensity varies only by 3 % with the liquid hydrogenthickness over the range of 3.5 to 5.5 cm, so a thickness of 4.5 cm was chosen. The variation of performance as a function of void fraction at this thickness was also calculated; even if voids occupy 30 % of the liquid hydrogen volume, the intensity drops less than 10 %. Tests to determine the void fraction by gamma-ray transmission measurements are under development.

     

Extensive MCNP calculations have been used make comparisons of the properties that can be expected of the new source compared to the current cold source. The graph below shows the energy spectrum comparison in the range of interest as an unnormalized brightness versus neutron energy.

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A direct comparison of the expected brightness (intensity per unit wavelength per steradian) over the useful wavelength range is shown below.

Like the existing cold source, the BT-9 cold source will operate between 1 bar (cold) and 5 bars (warm), but will have at most 25 % of the hydrogen inventory of the existing could source. Hydrogen safety is assured by protecting components from physical hazards, minimizing gas handling, and having at least two monitored barriers preventing the mixing of air and hydrogen. The system is passively safe; the LH₂ simply expands into its ballast tank if the refrigerator fails.

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last modified 10 April 2008