The study of nuclear matter under extreme
conditions is constantly pressing at the limitations of current experimental
techniques and equipment. With the development of new Radioactive
Ion Beam facilities world wide, there soon promises to be an important
step forward in spectroscopic resolution. The Notre Dame Nuclear Science
Laboratory has been the site of pioneering work in the development
of low-energy RIBS of sufficient intensity to investigate their effect
in nuclear reactions [1-3]. An experiment has been performed to study
gamma-ray spectroscopy of fusion evaporation residues produced by
a radioactive beam of 6He on a natural Copper (63% 63Cu)
target. Only one other radioactive beam spectroscopy experiment has
been published to date [4], making this a state of the art apparatus.
The tandem and Twinsol duel superconducting magnet system were used
to produce the beam, as described in references [1-3] . A photograph
of the twinsol magnets is shown here.
The twinsol magnets were run in a new mode, namely, with a parallel beam through the mid point, rather than the traditional crossover mode. This reduced the size of the final focus of the recoils of interest, and allowed the un-reacted primary 7Li beam particles which were brought to a focus at the pre-existing crossover point between the magnets, to be stopped by a 2" diameter 12" long heavimet rod. This rod, also served to capture any neutrons passing through the beam pipe, having first been thermalised by a 24" long high density polyethylene rod, placed in the bore of the first solenoid.
The existing beam line was extended to a new target chamber situated in the doorway of the sliding shield at the back of the west target room, which formed a neutron shielded `cave' surrounded on all but one sides by concrete at least 1.2m thick. A schematic of the experiment is shown to the left, with the calculated trajectory of the radioactive beam shown in red dashed lines. A moveable wall of borated de-ionized water, 0.5m thick, was constructed across the face of the cave and around the beam pipe. A photograph is shown here.
Two 55% efficient HPGe detectors were placed around the target chamber in the neutron shielded cave at 90o and 280o to the beam line. These detectors had a calculated detection efficiency for a 1MeV gamma ray of ~10%. A photograph of the detectors is shown here, taken from the back, looking towards the primary target. The water wall can be seen with all the holes around the beam line plugged to protect the germanium detectors from fast neutrons.
A spectrum of coincident gamma rays showed
lines at the energies of the low-lying transitions in the reaction
product 66Zn. A gate was placed on the 1039 keV 2
0+, and the resulting spectrum, given below, shows the
1412 keV 4
2+ and the 834 keV 2
2+ in coincidence, proving that these are gamma rays from
66Zn reaction residues. This setup will now be used to
study unknown low-lying non-yrast levels in other nuclei, which are
not accessible using stable beams.
References:
W. Catford et al., Nucl. Instr. Meth A371 (1996)p 449
J.J. Kolata et al., Nucl. Instr. Meth. Phys.Rev. Sect.
B40/41 (1989), p 503
F.D. Becchetti and J.J. Kolata, Application of accelerators in
Research and Industry, edited by J.L. Duggan and I.L. Morgan,
AIP Conf. Proc. No 392 (AIP Press, New York, 1997) p 369-375
M.Y. Lee et al., ucl. Instr. Meth. Phys. Rev. Sect.
A422 (1999) p 536
S.M. Vincent, A. Aprahamian, J.J. Kolata,
L.O. Lamm, V. Guimaraes, R.C. de Haan, D. Peterson, P. Santi, A. Teymurazyan,
F.D. Becchetti, T.W. O'Donnell, M. Lee, D.A. Robertson, J.A. Zimmerman,
and J.A. Brown,"Gamma-Ray Spectroscopy with a low-energy 6He radioactive
ion beam", Nucl. Instr. Methods in Phys. Res. A491,
426-436 (2002).
