APS News

MuCap Results, Nucleon Spin Structure Highlight 2006 DNP Meeting

To a non-scientist, nuclear physics calls to mind atomic bombs and power plants. However, physicists know that the field encompasses a much wider range of topics, from fundamental research on muon capture reaction rates and nucleon spin structure, to more practical applications such as particle radiotherapy, using gas-electron multipliers to inspect cargo containers, and non-destructive elemental analysis of paintings and metal artifacts.

Those were just a few of the highlights during the 2006 annual meeting of the APS Division of Nuclear Physics held October 26-28 in Nashville, Tennessee. In addition to the technical program, Friday evening’s banquet included a dash of local color with a performance by the acclaimed Nashville band Brazilbilly. Meeting program notes described their music as “an eclectic blend of traditional country roots with a unique Latin flair.”

MuCap First Results. Peter Kammel (University of Illinois, Urbana-Champaign) reported on the first results obtained from data analysis of the MuCap experiment, designed to measure specific muon capture reaction rates. This has been a controversial area for the last 30 years, since prior experimental results were subject to large uncertainties in the data interpretation. By combining novel detector techniques, the MuCap experiment managed to overcome many of the problems that plagued earlier efforts. According to Kammel, the data “surpass all previous experiments both in statistics and in reduction of systematic uncertainties.”

Nucleon Spin Structure. The exploration of the spin of the proton and its relation to the angular momenta of the quark and gluon constituents is still a classic topic of nuclear physics, according to Marco Stratmann (RIKEN), who gave an overview of our present understanding, theoretical concepts, and recent developments of the field. Those developments include new results on single-spin asymmetries obtained with semi-inclusive deep inelastic scattering and polarized pp-scattering tools, as well as results from new experiments conducted at Jefferson Lab enabling physicists to study various aspects of nucleon spin structure. Furthermore, experiments by HERMES, COMPASS and at RHIC are underway to determine the polarization of gluons within spin-polarized protons.

Gas Electron Multipliers. The latest innovation in micropattern gaseous detectors is the gas electron multiplier (GEM), developed at CERN by Fabio Sauli and introduced a few years ago. GEMs have proved valuable for fast tracking at numerous high energy physics experiments because of their excellent high rate capabilities. According to Sauli, they’ve been used as end-cap detectors in time projection chambers, and also as a hadron-blind detector at Brookhaven. Enhanced with a photosensitive layer, GEMS can detect and localize single photons, and they have also found use in neutron detection, and for measuring x-ray polarization in astrophysics.

On a more practical level, Tony Forest of Idaho State University has been developing a system to image cargo containers in order to detect small shielded radioactive cargo. Current cargo container inspection systems use gamma rays or x-rays with resolutions designed to detect contraband, but these may suffer from false alarms due to naturally radioactive cargo. It is even more difficult to detect small shielded radioactive elements. GEM-based imaging systems could address those challenges.

Proton Radiotherapy. Radiation has been used in medical applications since it was first discovered, and the advent of higher energy particle accelerators in the latter half of the 20th century made it possible to use mega-voltage (MV) X-rays to deliver high doses of radiation to treat malignant cancers. Those doses must be limited, however, because they also severely damage surrounding tissue. In many cases, according to Jonathan Farr (Indiana University), “heavy” particle radiotherapy using protons and light ions can be a desirable alternative, allowing either a higher dose of radiation, or maintaining the same dose with less collateral damage to healthy cells. The latest advances in such systems are making treatment with fast protons more widespread. More than 20 facilities now operate worldwide, and several more are under construction.

PIXE Artifact Analysis. It can be difficult to do sample analyses of precious art objects because most techniques can cause some damage. In fact, sampling is often prohibited, because of the uniqueness and fragility of such valuable items. At the DNP meeting, Andrea Denker (Hahn-Meitner-Institut) discussed her work non-destructively analyzing ancient paintings and Bronze Age and medieval metal artifacts using proton- induced X-ray emission (PIXE). High energy protons with energies of around 60 MeV have a large range in the material being investigated, and can thus provide useful information from deep inside the object, complementing data obtained through other analytical methods, such as neutron autoradiography

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Editor: Alan Chodos
Contributing Editor: Jennifer Ouellette
Staff Writer: Ernie Tretkoff