HELIOS and the radioactive beam program at Argonne

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6 B.B.Back et al. Fig. 4. Cut-through of the HELIOS spectrometer showing the end-flanges, the angle-adjustable supports for the Si-detector array, and the target-fan mounted off the rotatable rail at the bottom of the magnet bore. 6. First results The spectrometer was commissioned in August 2008 using a stable 28 Si beam incident upon an 84 µg/cm 2 CD 2 target to measure single neutron states in 28 Si using the d( 28 Si,p) reaction. The magnetic field was 1.915 T. Fig. 5 shows a 2-dimensional spectrum of the proton energy, E p , versus the longitudinal position z p (the distance from the target) for several longitudinal target positions. The top diagonal trace corresponds to the ground state of 29 Si. Several excited states are visible at lower energies. Note that these traces are parallel, as expected on the basis of Eq. 2. The horizontal lines correspond to α contamination from a 228 Th source used for an energy calibration of the detectors at the beginning of the experiment. Fig. 6 shows the excitation spectrum of 29 Si integrated over a wide range of angles. The energy resolution is clearly demonstrated by the fact that the E x =6.19 and 6.38 MeV states are well separated. This commissioning experiment clearly validates the basic operating principle of the HELIOS spectrometer. The preservation of good energy resolution for inverse kinematic reactions will be of central importance for exploring charged particle
HELIOS ... 7 Fig. 5. The energy of protons, E p, is plotted vs. z p for the d( 28 Si,p) reaction. The diagonal traces represent states in 29 Si, whereas horizontal lines arise from a shortlived α-contamination in the chamber. Horizontal gaps of about 1 cm between the detectors are evident, and a proton background from fusion-evaporation on the carbon in the CD 2 target is seen. reactions with heavy radioactive beams, such as those becoming available from the CARIBU injector at ATLAS in the near future. 7. Conclusions In this paper we have given a historical perspective on the radioactive beam program at ATLAS. This work has been ongoing for over a decade, starting with beams obtained from relatively long-lived activities produced at other accelerators, and continuing with a very productive program with in-flight produced radioactive, but relatively light, beams that have been used mostly for nuclear astrophysics studies. With the new CARIBU injector, which will be commissioned in the summer/fall of 2009, the range of radioactive beams will be expanded to include very neutron-rich fragments from spontaneous fission of a strong, 1 Ci, 252 Cf source. In order to use these beams for transfer reaction studies, a new type of spectrometer has been built, which is ideally suited for the study of such reactions in inverse kinematics. It is expected that the CARIBU injector and the HELIOS spectrometer will allow for a wide range of detailed studies of the nuclear structure in the neutron-rich region of heavy nuclei from CARIBU as well as nuclear astrophysics studies with beams produced via the in-flight method.
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HELIOS and the radioactive beam program at Argonne

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