HELIOS and the radioactive beam program at Argonne

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4 B.B.Back et al. performed in inverse kinematics will play a central role in advancing our understanding of nuclei far from stability; first with relatively weak beams from HRIBF at ORNL, ISAC-2 at TRIUMF, Rex-Isolde at CERN, ReA-3 at NSCL, Spiral-2 at GANIL and CARIBU at ATLAS, but on a longer time scale abundant beams from FRIB will substantially expand the range of nuclear transfer reaction studies. The inverse kinematics method carries with it, however, substantial obstacles for obtaining sufficient energy resolution to be able to separate adjacent nuclear states when the ejectiles are measured as a function of emission angle. In many cases, the forward angles in the center-of-mass system correspond to very backward scattering angles in the laboratory, where kinematical compression of the nuclear states into a small energy interval can be quite severe as illustrated in Fig. 2. This effect limits the achievable Q-value resolution and the ability to identify particles using the standard ∆E-E technique. At first glance, these experimental limitations would appear to severely limit the utility of transfer reaction studies using radioactive beams. As explained in the following section, these problems can be overcome by employing a new concept in which the ejectiles are transported back to the beam axis in a homogeneous magnetic field aligned with the beam axis, a method that is realized in the recently commissioned HELIOS spectrometer at ATLAS. Fig. 3. Schematic illustration of the HELIOS concept. From the left, the beam enters the homogeneous, axial field region through the hollow Si detector array. Backward-going charged particles follow helical trajectories and are intercepted by the position-sensitive Si detector before returning to the beam axis. Coincidences with recoils detected in a forward detector may be used, if needed. 4. The HELIOS concept The basic principle of this spectrometer [6] is that in a sufficiently strong, homogeneous, longitudinal magnetic field B, charged particles with mass m and charge q, emerging from the beam-target interaction point, will follow helical trajectories
HELIOS ... 5 and return to the beam axis after the cyclotron period t cyc = 2π m B qe , (1) where e is the unit charge. By placing a hollow, position-sensitive Si-detector array (see Fig. 3) to intercept the particles before they reach the beam axis, their energy, E, flight-time, t, and position, z, can be measured. These measured quantities determine completely the kinematic parameters of 2-body reactions. The flight-time provides particle identification via the cyclotron period, Eq. 1, and the center-of-mass energy is given by E cm = E + m 2 V cm 2 − mV cmz , (2) t cyc where V cm and z are the velocity of the center-of-mass system, and the intercept position, respectively. V cm and z, along with the measured energy, E lab , determine the center-of-mass scattering angle, θ cm . The fact that the energy in the centerof-mass system equals the measured laboratory energy plus a constant offset, when measured at a fixed axial position, z, illustrates the second important property of HELIOS, namely that the kinematic compression is eliminated. This property allows for improved Q-value resolution in inverse kinematics measurements, which use heavy radioactive beams impinging on light H and He targets. Finally, the fact that particles return to the beam axis means that only a relatively small area needs to be covered with position sensitive Si detectors. A large acceptance can therefore be obtained at a moderate cost and complexity of the detection system. 5. Building HELIOS The large, homogeneous magnetic field volume needed for the HELIOS spectrometer is provided by a 3 T superconducting solenoid obtained from a de-commissioned Magnetic Resonance Imaging magnet. The solenoid is installed in the former general purpose area at ATLAS such that the magnetic field axis coincides with the beam line. The 90 cm diameter warm bore of the magnet is enclosed by large end-flanges, which provides a vacuum enclosure that serves as the target chamber. The beam enters via a 1 cm diameter hole through the Si detector array support rod, which is clad on the outside with twenty-four 1 x 5 cm 2 position sensitive Si detectors. It is planned to upgrade the Si detector array to forty 2 x 5 cm 2 wafers in order to achieve almost 100% azimuthal acceptance over a length of >60 cm. The position of a particle impact is obtained by charge division in the resistive front electrode of the detectors. A fan-like structure supports up to nine targets, which can be selected by rotation of the mounting rail located at the bottom of the magnet bore. The position of the target fan and the Si detector array can independently be adjusted longitudinally. Combined with magnetic field settings up to a maximum of 3 T, this allows for optimal coverage of excitation energy and scattering angle of interest. A three-dimensional engineering drawing cut-through of the setup is shown in Fig. 4.
Page 1 and 2: Proc. 25th Winter Workshop on Nucle
Page 3: HELIOS ... 3 isobar mass separator.
Page 7 and 8: HELIOS ... 7 Fig. 5. The energy of

HELIOS and the radioactive beam program at Argonne

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