Important Issues in the Design of the FNPSNS Beamline

Geoff Greene

In anticipation of next week's workshop, we wanted to call to your attention to some important issues regarding the design of the beamline(s) for fundamental physics at the SNS. We hope that the advanced notice of these issues will help make the workshop even more productive. The important questions to be addressed include:

  1. Should we request a dual or single beamline from the SNS? As the name implies, a dual beamline comprises two beamlines in a single shutter. Each experiment would have its own neutron guide, choppers, and etc. The main advantage of this approach is that it can make possible two independent experiments in slightly more floor space than a single experiment. The main drawbacks are that there is less room for each experiment, the flux may be less than on a single beamline, and the backgrounds could be higher. SNS scientists plan to construct Magnetism and Liquids Reflectometers on a dual beamline. A good deal of design work has been done for these beamlines and it is felt that the approach will work for these instruments despite the fact that their background requirements are rather stringent (see for a brief description of the reflectometers and the layouts of other instruments). Hence, it is likely that a similar design may work for fundamental physics experiments.
  2. Should the neutron guide be extended into the "core vessel insert"? This is the region between the moderator and the beginning of the main shutter (1 2.2 m from the moderator). Access to this area is more limited than for the shutter, and the radiation is higher. However, our simulations show that the cold neutron flux at the end of a reasonable guide is significantly lower if the guide starts at 2.2 m instead of 1 m. Designing a guide for this region is more difficult for the dual beamline, but our current understanding of the relevant dimensions indicates that it is possible to extend both guides to within 1 m of the source and not have them overlap. Drawings of possible dual and single beamline scenarios are available on the workshop web site (
  3. What are the optimum types and lengths of the neutron guides? Presumably, the guides will be curved to reduce backgrounds from high-energy neutrons and gamma rays, and hence eliminate the need for a t-zero chopper to perform this task. However, a curved guide would result in some loss of flux at shorter wavelengths. The experiments appear to separate into two classes; those requiring maximum flux across a range of wavelengths, and those requiring 9 A neutrons for UCN production in LHe. The minimum flight path length is probably on the order of 15 m. The maximum flight path length to keep the experiment inside the experimental hall is about 25 m. Should tapered guides be considered for increasing the flux over a smaller area while increasing the divergence?
  4. How many and what type of choppers are needed? Following the example of the Reflectometer beamlines, 3 choppers probably will be needed to define the proper wavelength frame(s). Should we consider other approaches (e.g. mirror wedges) to eliminate long wavelength neutrons? What approach should be used to select 9 A neutrons?

In addition to addressing the technical issues noted above the workshop will also serve to initiate activities leading to the establishment of fundamental neutron physics "Instrument Development Team" (IDT). A brief introduction to the IDT process at the SNS can be found at:
Roughly speaking, the IDT concept is based upon a model that has become the norm at DOE Basic Energy Sciences funded user facilities (especially synchrotron light sources). Examples of this model are the PRT's (Participating research Teams) at the NSLS, and the CAT's (Collaborative Access Teams) at the APS. In the BES world, IDT-like collaborations secure funding for instruments that are not included in the scope of the initial facility construction project. To a greater or lesser extent, the IDT is expected to design a user instrument and to take responsibility for its construction. In return for their efforts in raising money, and for their time devoted to the construction of the instrument, the IDT receives a fraction of the beam time for their own research. The remaining beam time is distributed to other users through a facility run competitive proposal process. The fraction of time given to the collaboration, and the operational expectations for the collaboration vary from facility to facility. The allocation of beam time is usually for a fixed period of time (3-5 years) with possibility for renewal following a review. For the SNS, these details are in the process of being finalized.

It is important to note that the anticipated scope of the IDT's activities relate to the construction of the beamline(s) and appropriate user support for experiments that are carried out at the SNS. We anticipate that capital equipment funding for the construction of individual experiments will be approved and allocated separately. Of course one anticipates that IDT members will be heavily involved in these experiments, but that their role in the development and execution of the experimental program is outside their role as IDT members.

The character of fundamental neutron physics experiments is rather different than those carried out at the neutron scattering spectrometers (either part of the SNS project of built by other IDT's). Our challenge is to put together an organizational structure that satisfies the strictures of the SNS project while accommodating the special needs of this type of research. In doing so, we must bear in mind that our management of this project must satisfy the needs of several constituencies including the SNS project, the Physics Division at ORNL, our sponsoring organizations (DOE NP and NSF Nuclear Physics), The DOE office of Basic Energy Sciences, and, of course, our user community (in and out of the IDT). In the following, we list a few of the concerns that should be addressed.

The SNS management recognizes that the character of the Fundamental Neutron Physics effort will be rather different than that at the other beamlines at SNS. They appear to be prepared to give us rather broad latitude in our management of our project as long as they are convinced that we are making productive use of our beamline. Given that the SNS project does not at the moment include expertise in our fields of research we will need to work with the SNS to establish appropriate external panels that can provide appropriate review of our efforts. In particular, the SNS program advisory committee will not necessarily include a nuclear physics panel. Both the SNS management and the funding agency responsible for the SNS construction and operation (BES) must see a transparent process that ensures the quality of the science at their facility.

It is anticipated that support for construction and operations of the fundamental neutron physics beamline will be funded through the Physics Division at ORNL (with subcontracts to other institutions as appropriate). The IDT must therefore be responsive to line management at ORNL with respect to fiscal and project management.

The selection and scheduling process for experiments at the SNS must be acceptable to the agencies (most likely DOE NP and NSF) that will provide funds for each experiment. An appropriate level of coordination must be established to ensure that scarce funding dollars and scarce beam time are coordinated and used effectively.

Finally, and perhaps most critically, the operation of the beamline must be carried out in a way that meets the needs of the individuals and teams contemplating experiments at the SNS. This implies adequate technical support as well appropriate facilities and amenities at the SNS. It also implies that there be an open and clearly defined proposal, review, and scheduling process that allows all users to make productive use of both their own time and the beam time at the SNS. we should, as well, take cognizance of the fact that it may prove very difficult to make a distinction between the IDT and the "outside" user community. Indeed it may be that this distinction is not appropriate for our beam line.