SEPARATOR INTRODUCTION
Separators are used in nuclear structure experiments to select a particlar range of reaction prodicts for study.
The "workhorse" of any separator is the dipole; electric dipoles separate by energy/charge ratios while magnetic dipoles separate by momentum/charge ratios. Magnetic quadrupoles are also commonly used for focussing. Quadrupoles may be either vertically or horzontally foscussing; these elements are often used in pairs, or doublets, to provide foscussing in both directions.
Sextupoles may also be used to correct for second-order aberrations.
Characteristics may also be mixed, as they are in SASSYER, where curved
dipole magnets are used to provide vertical focussing
in addition to momentum selection.
The excellent mass selection of the FMA and RMS allows structure studies of nuclei produced with very low cross-sections relative to their neighbors. For example, The Argonne FMA was used to select only A=109 nuclei following a reaction which largely produces A=108 isobars;
The FMA tunes A=109 nuclei to the center of the focal plane, where
physical slits may then be used to block other nuclei from being implanted
into the recoil detector.
However, only one charge state may be selected. As nuclear reaction
products exit the target following production, they will have a range of
charge states due to electron pickup/stripping in the target. Mass
separators are typically tuned to select the most prominent charge state.
These devices have low transmission efficiencies as they may select
only one (or two if they are lucky), charge states for separation.
The Recoil Ion Transport Unit (RITU) at the University of Jyvaskyla in Finland and SASSYER are recoil separators. These spectromters use a magnetic dipole for separation;
Because electric elements are not used, the interior of the magnetic chambers are filled with a low density, light gas. The use of the gas allows the fragmented charge states of the reaction products to coalesce into a single state, optimizing the transmission efficiency of the separator. However, mass selectivity is lost as all reaction products will be tuned to the focal plane together. For example, SASSYER was used to separate medium-mass nuclei in the Hf region from fission products;
The transition from vacuum to gas was nicely illustrated in a paper by
M. Paul at Argonne, where a Ni-58 beam was separated and studied in various pressure
ranges using a spectrograph filled with nitrogen gas;
(Paul et al., NIM A227, 418 (98).)
To focus recoils with SASSYER, the average charge state in the gas must be calculated.
We use
qave = (1.8x10-7)vZ1/3 + 1.65
or
qave = Z(1 - 1.04e((4.15x10-7)vZ-2/3) )
where the second equation is used for recoil velocities
v > 8.2x106 m/s, while the greater calculated charge
of either equation is used for slower velocities.
SASSYER is similar to other gas-filled devices. RITU is the only other spectrometer dedicated to nuclear structure.
For structure experiments, we primarily focus on the recoil tagging method.
Prompt gamma rays directly following heavy-ion fusion evaporation reactions are detected in an array of Ge clover detectors surrounding the target;
Reaction recoils are subsequently separated by SASSYER and implanted into a solar cell array near the exit of the second dipole. The array we presently use consists of 30 cells of 1 cm 2 each. They are arranged in a 3x10 oblong rectangle.
The signals from the Ge detectors are delayed and coincide with signals from the solar cells. This allows us to only observe gamma-rays for events which produced fusion recoil products.