The Fragment Mass Analyzer (FMA) is a triple-focusing recoil mass spectrometer designed for use with beams from the ATLAS heavy-ion accelerator. Its function is to separate nuclear reaction recoils from the primary beam and then to disperse the reaction products by mass/charge (m/q) at the focal plane. When the FMA is positioned at 0 deg., the primary beam stops on the anode of the first electric dipole. The FMA support structure enables rotation about the target position to cover angles between -5 deg. and +45 deg. with respect to the incident beam direction. It also allows for radial motion of up to 60 cm in order to accommodate large detector arrays at the target. Figure 1 shows a schematic layout of the FMA, and figure 2 is a photograph of the FMA. Also see the FMA web page at https://www.phy.anl.gov/fma/index.html.
The ion optics of the FMA are quite flexible. Focusing modes such as point-to-point and point-to-parllel are available, as well as variable dispersion. Users with special requirements should contact the FMA manager in advance of the scheduled experiment.
The FMA is controlled and monitored by a dedicated computer. It also monitors vacuum conditions, support structure parameters, and target chamber parameters. Targets can be positioned by computer as well.
Users can easily set up the FMA for their individual experiments. An FMA setup program is available on the world wide web at the following site: (https://www.phy.anl.gov/fma/fmarecoil_form.html). This program calculates the properties of the particle which will follow the central FMA trajectory, using the beam and target information entered by the experimenter. The User then enters the energy, mass and charge of the central ion into the control computer.
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A number of different experimental systems are available for use with the FMA. Some of these have been constructed by University users. More information on the available FMA apparatus can be found at https://www.phy.anl.gov/fma/apparatus.html.
A 38-cm diameter sliding seal scattering chamber is available for use at the target position. It has provision for a target ladder as well as a rotating target wheel used under high beam current conditions. There are two independently controlled rings for mounting detectors, and one side has a 30.5 cm by 30.5 cm opening to accommodate an extension for housing large gas detectors. All variable parameters are equipped with stepping motors, controllable manually or by computer. With this scattering chamber in place, the normal distance between the target and the entrance quadrupole is 30 cm.
For in-beam gamma ray spectroscopy experiments, ten Compton-suppressed Ge detectors can be mounted around the FMA target position. In this case the 38 cm scattering chamber is removed, and a small special-purpose chamber substituted in its place. A permanent setup of all the electronics needed for these experiments except for ADCs is located near the FMA target. A 16-segment neutron detector array can also be placed near the FMA target, normally used in conjunction with the Compton-suppressed Ge detectors. It occupies the region between the small target chamber and the entrance to the FMA, so that neutrons evaporated in the forward direction can be detected. In coincidence measurements the neutron array can be used to provide additional information on the Z of the recoils. The detectors utilize pulse-shape to discriminate between neutrons and gamma rays at neutron energies above 1 to 2 MeV, and time-of-flight for lower energies.
At the focal plane, all experiments utilize the position, time, and energy-loss measurements provided by a thin parallel-grid avalanche counter. Position resolution is about 1.2 mm, and time resolution is better than 1 ns. This detector operates at a pressure of 3 Torr of isobutane gas. A moving tape collector is available for studies of delayed activities, including isomers with half-lives down to several hundred nanoseconds. Also available is an implantation system, using a completely instrumented double-sided Si strip detector. Experiments on alpha- and proton radioactivity can be performed using this system. A facility to study nuclear moments is under construction behind the FMA.
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