As most readers of this newsletter will already know, Fermi National Accelerator Lab (Fermilab) is a single-purpose U.S. Department of Energy (DOE) laboratory whose focus is high-energy elementary particle physics. Fermilab’s Tevatron is presently the highest-energy accelerator in the world, and it is now being used in Run II to exploit the discovery of the top quark in Run I and search for new physics besides. It’s an exciting time for Fermilab, but by the end of Run II, the energy frontier will have passed to the Large Hadron Collider (LHC) at CERN and the question that we at Fermilab must then answer is, where do we go from here? To have the answer then, we must ask the question now. With this in mind, Fermilab has recently held a series of long-range planning reviews to consider a wide variety of projects that might be undertaken. In one of these sessions, which addressed the application of Fermilab’s core abilities to projects outside the realm of traditional high-energy physics, Kamran Vaziri presented an overview of accelerator health physics studies. In this note, I suggest a specific project in the context of Kamran’s presentation.
What I suggest is an international muon dosimetry intercomparison. For those unfamiliar with the subject, a dosimetry intercomparison is an exercise in which participating institutions send in a set of dosimeters to the host institution, which exposes them to a measured amount of a particular type of radiation. The participants then receive the dosimeters and read them, without knowing the exposure. When the results are collected and summarized, everybody (in principle) has a better idea of how their dosimeters respond and perhaps how to process them for the field in which they were tested. There have been many dosimeter intercomparisons for photons and neutrons over the years, but as far as I am aware, there has not been a muon dosimetry intercomparison since 1987. Dosimetric technology and techniques have changed over time and a fresh study is overdue.
Muons may constitute a substantial portion of the beam-on dose received at a high-energy accelerator facility. They are also a significant part of the natural background radiation due to cosmic rays. Furthermore, dosimeters such as thermoluminescent dosimeters (TLDs) are frequently used to measure the dose to experimental instrumentation as an aid to gauging radiation damage, so understanding their response to muons is worthwhile. Finally, as we plan the next generation of research accelerators (NLC? Neutrino Factory? Muon Collider?), which may produce muons in unprecedented amounts, understanding dosimetric response to muons might be more important than ever.
One reason that muon dosimetry intercomparisons are rare is that it's hard to come by a good experimental area with a muon field that's free of other charged particles. However, Fermilab will soon have such an area, thanks to the Neutrinos at the Main Injector (NuMI) Project. As the reader might guess, aiming a particle beam at a detector 735 km away is a tricky business. To help ensure that the beam is behaving as it ought, three alcoves have been dug into the rock off of the main NuMI tunnel. These go into several regions where muons are passing through and will contain detectors to monitor the muon flux and distribution. The spacing of the alcoves also gives an upper bound on the muon energy.
Hence, a muon study could in principle be carried out parasitically in the NuMI beam. Neutron contamination from the beam absorber can be characterized with instrumentation that the radiation physics group has on hand. Since stable operation of the NuMI beam is a prerequisite for the intercomparison, mid-2005 or 2006 would seem a likely time frame for this project, pending laboratory approval.
Meanwhile, there would be a good deal of preparatory work to do, the first item of which would be to obtain approval from Fermilab management and answer annoying questions like "Who’s going to pay for this?" While this would not be an expensive project on the Fermilab scale of things, it would require an investment of manpower and resources and possibly the purchase of some instrumentation. We would appreciate comments from the section on the theory that a display of interest from the accelerator health physics community would demonstrate that this muon study is worth a modest investment by Fermilab. Please contact either Kamran Vaziri or myself, David Boehnlein.