A1900 A1900 Fragment Separator NSCL

Planning an NSCL Rare-Isotope Beam Experiment

A basic understanding of what is feasible in terms of fragment beam development and delivery is the key to planning a successful rare-isotope beam experiment The user is the one who is in the best postion to optimize his or her experiment based on the trade-offs that are inherrent in establishing a fragment setting and based on what is possible in terms of the experimental setup. The user is obliged to show that his or her experiment is feasible from the point of view of the rare-isotope setting used. The NSCL provides users the tools (see below) to develop the basic understanding of rare-isotope settings needed for planning strong experiments. The role of the A1900 group is to be available for consultation during the planning phases of an experiment and to provide expertise and experience in executing the fragment beam development and delivery during the experiment. Users with questions regarding rare-isotope settings in the context of planning an experiment can contact the A1900 group through the contact person for the A1900 posted here.

The NSCL program advisory committee (PAC) allocates beam time not only on the basis of the scientific merrit a proposed experiment, but also in terms of efficient use of NSCL resources required to support the work. It is therefore important for users to plan their experiments in a way that minimizes the beam delivery time while optimizing the scientific goals. The current call for proposal provides the latest guidelines for estimating the beam delivery time needed for various primary or secondary beam settings.

In cases where the rare-isotope beam development is very difficult, users should explicitly address the issues associated with beam development as part of the proposal and, if appropriate, request beam time specifically for proof-of-principle studies.

Planning a rare-isotope beam experiment starts with the choice of the appropriate primary beam(s) from those available on the CCF Primary Beam List. The best primary beam for producing a particular fragment is usually the one that lies closest to the fragment on the chart of nuclides but which is higher in both Z and A than the fragment. The next step is to use the program LISE++ to make a more detailed analysis of what is feasible and to determine the setup needed. A list of the commonly used wedges and targets is availible in the A1900 Service Level Description (PDF). The A1900 group strongly reccommends that LISE++ be configured with the A1900 setup files provided to ensure that the many program options are set properly. A PDF file is available to guide users through calculations with LISE++.

Rate estimates from LISE++ can be over-optimistic; in some cases they can be off by more than an order of magnitude – especially in regions of nuclei where there is little or no experimental data. Users are encouraged to contact the A1900 group to find out about our experience in particular cases. Acceptance of rare-isotopes at the A1900 focal plane is higher than the acceptance at end stations downstream from the A1900; this fact impacts not only the rate of ions delivered, but also the composition of fragment cocktails.

Many properties of a rare-isotope beam tune can be optimized depending on the requirements of a particular experiment. Examples of these properties include:

Examples of factors that can be varied to achieve an optimum setting include:

Since it is not possible to optimize for all secondary beam properties simultaneously, it makes sense to optimize those properties that are most critical for the success of the experiment at the expense of the properties that are not significantly detrimental.

An example of a trade-off that could benefit an experiment that requires two or more different secondary beams is to produce all of the secondary beams with a single primary beam isotope and energy even though the fragment rates might be optimized using different primary beams. This approach avoids the extra facility usage time required for making an additional primary beam tune for the experiment. This approach gives the experiment more flexibility in scheduling the switch from one sedondary beam setting to another since the secondary beam change would not be coupled to a new primary beam tune. This approach makes it feasible for the experiment to switch back-and-forth between fragment settings.during the experiment. This approach may avoid the need to break the experiment into parts which could entail (depending on scheduling) the dismantling and re-assembly of the experimental setup. The cost of not changing the primary beam is that the fragment production rates must be compromised to accommodate all of the secondary beams needed for the experiment.

An example of a subtler trade-off that could benefit an experiment that requires two or more different secondary beams is to have all of the secondary beams delivered to the experimental setup with the same rigidity. This arrangement eliminates the need to retune the beamline between the A1900 and an experiment downstream from the A1900 with each secondary beam change. The cost of this arrangement, however, is that the rate can be maximized usually for only one of the secondary beams. This trade-off is not a problem, for example, if the fragments that are not rate-optimized have enough intensity to be above upper limits defined by detector or data-acquisition capabilities. Of course, this approach only makes sense if the experiment does not have energy requirements for the different fragment settings that are incompatible with a single rigidity.

A common pitfall in LISE++ calculations is to end up with a secondary beam having too low of an energy. Fragment settings with an energy below approximately 25 MeV/nucleon have high losses.to experiments downstream of the A1900.

Members of the A1900 group are available to help with questions that arise during the planning of an experiment – the group can be reached via the contact person posted here. As users finalize their planning, A1900 group members can help optimize the beam setup as well as steer users away from common pitfalls. Once an experiment is approved and scheduled, the A1900 group will work with the user group to complete detailed planning of the beam delivery. The A1900 group will be in the best position to optimize the beam delivery if the experimenters do a good job of communicating both factors that are important to the experiment as well as factors that are not. Users are encouraged to include members of the A1900 group as collaborators on experiments in cases where the beam development and identification represent the bulk of the experimental effort,

Experiments that require changes to the standard A1900 hardware/detector configuration must also include a request for any time necessary to modify and restore the A1900 setup since the NSCL will be unable to deliver beam to other experiments while the changes are taking place. A description of the standard A1900 configuration is contained in the A1900 Service Level Description (PDF). Members of the A1900 group can provide an estimate of the time required for the changing and restoring the standard setup.

© A1900 Group, 2018-02-26