Fragment Mass Analyzer (FMA)

The Fragment Mass Analyzer (FMA) is a recoil mass spectrometer located at the ATLAS accelerator at Argonne National Laboratory. It is used to separate nuclear reaction products from the primary heavy ion beam and to disperse them by mass/charge (M/q) at the focal plane. The FMA features wide acceptances in particle energy and M/q, as well as high mass resolution. Magnetic and electric fields (up to 500 kV across a 10 cm gap) are used to guide the desired particles and focus them onto detectors at the focal plane. The FMA rotates about a pivot directly under the target position, although for most experiments it is postioned at 0 degrees to the beam direction.

A wide variety of experiments can be performed at the FMA. High-spin spectroscopy can be studied by using an array of gamma-ray detectors around the target and tagging each event with the M/q of the recoil nucleus as determined at the focal plane. Both recoil-gamma and recoil-gamma-gamma coincidences have been measured in this way. Some nuclei recently studied include ^181,183Hg and ¹⁸⁶Pb. At the focal plane, studies of exotic alpha and proton decays can be carried out with the implantation facility using a double-sided silicon strip detector (DSSD). A number of heavy proton radioactivities have recently been discovered at the FMA, including ¹⁶⁷Ir and ¹⁷¹Au. In addition, a moving tape collector is available for measurements of beta radioactivity. Studies of microsecond isomers have been performed on rare-earth nuclei, using gamma and electron detectors at the focal plane.