Neutron Sciences Directorate

The FNPB is operated as a user facility with all beam time allocated on the basis of independent peer reviews. It is not an instrument, rather it provides the infrastructure for a series of experiments geared towards answering questions about the nature and existence of matter in the Universe that are important to nuclear and particle physics and cosmology. Thus, it was designed according to a five-point strategy:

1. Maximize total intensity
2. Cold neutron intensity is the highest priority.
3. The unique properties of a spallation source should not be compromised.
4. Accommodate the different demands of different classes of proposed experiments consistent with 1-3.
5. Leave as much floor space as possible.

The FnPB views the bottom downstream liquid hydrogen moderator (20K, fully coupled, unpoisoned). The beam line begins 1m from the face of the moderator with a straight, rectangular shaped “core” guide 10 x 12 cm2 in are and 1.275 m long. The supermirror-coated guide has an aluminum to minimize radiation damage. Following the core guide is a 4.5 m long bender (radius of curvature = 117 m, beam-left) in order to minimize background from fast neutrons and gammas. Loss of direct line-of-sight to the moderator occurs 7.5 m from the moderator. The remaining guide is straight and extends from the end of the bender guide to the experimental area, terminating 15 m from the moderator face. Space is provided for four frame-overlap choppers; two are instrumented. The beam line is split shortly after exiting the target shielding monolith in order to deliver beams to two different experimental areas. 

Beam line 13A uses a double-crystal monochromator system to delivers 8.9 Å neutrons to an external building where they will be converted to ultra-cold neutrons (typical energy ~ 100 neV) by a scattering in superfluid helium. The 8.9Å beam line continues downstream of the monochromator assembly with an 8m section of “ballistic” guide in order to optimize neutron transport over long distances. The guide starts with a 12 x 14 cm2 cross section and expands to 20 x 30 cm2. The external building begins at the outside wall of the SNS target building. Its volume is 2789.2 m3 with a useful floor area of 220.18 m2 and a crane (10-ton capacity) hook height of 7.931 m. An isolation pad (1.22 m thick, 4.62 x 7.28 m2) begins 21.90 m from the end of the 13A guide.

Beam line13B is polychromatic and completely enclosed in a cave in the SNS target building. The useful space is 2.84 m across on the upstream end of the cave and 5.38 m on the upstream, with 12.65 m along the direction of the beam. The experimental area has a pit in the floor (4.88 x 2.13 m2 in area, 2.44 m deep, beginning 16.02 m downstream of the moderator face) to accommodate a large magnetic spectrometer.

For more information about BL13:

N. Fomin et al., "Fundamental neutron physics beamline at the spallation neutron source at ORNL." Nuclear Instruments and Methods. A773, 45 (2015).