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| NSCL / A1900 Home / Overview | |||
The responsibility of planning any NSCL experiment lies with the user. For experiments requiring rare-isotope beams, this responsibility means showing that the experiment is feasible from the point of view of the secondary beams used, including in terms of the needed count rate and purity. 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.
Since the program advisory committee (PAC) allocates beam time not only on the basis of a proposed experiment's scientific merrit, but also in terms of the strain on NSCL resources required to support the work, it is important for users to plan their experiments in a way that minimizes the beam delivery time while not compromising 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.
Planning a rare-isotope beam experiment starts with the choice of the appropriate primary beam(s) from those available on the NSCL Primary Beam List. A JAVA applet provides an easy way to choose an appropriate primary beam for producing a particular 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 target inventory and the wedge inventory. 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 as much as a factor of 10 – especially in regions of nuclei where there is little or no experimental data. Acceptance of rare-isotopes at the A1900 focal plane is higher than the acceptance at end stations located in the experimental vaults; 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 the secondary beams with a single primary beam isotope and energy even though the fragment rates can be optimized using different primary beams. This approach avoids the large investment of time and resources required for changing a primary beam in the middle of an experiment or 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 the experiment with each secondary beam change – a process that can sometimes be time-consuming. 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 require each fragment to have a particular energy.
A common pitfall in LISE++ calculations is to end up with a secondary beam having too low of an energy. Beams with an energy below approximately 25 MeV/nucleon do not tune well beyond the A1900 focal plane.
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 A1900 contact person, Tom Ginter. As users finalize their planning, A1900 group members can help optimize the beam setup as well as steer users away from the common pitfalls that experience has shown to cause trouble. 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 linked from here . Members of the A1900 group can provide an estimate of the time required for the changing and restoring the standard setup.
