HELIOS and the radioactive beam program at Argonne

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