Thirty years at the forefront: a perspective on the Bevatron/Bevalac Page: 4 of 9
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studies leading to clinical use of such beams for cancer radiotherapy and high
quality imaging, and the nuclear science efforts concentrated on fragmentation
reactions and on searching for hydrodynamic and other bulk-matter effects in central
collisions.
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W" a+aeFigure 3 - The Bevalac, 1974, formed by construction of the Transfer Line between
the SiuaerHILAC and the Bevatron. Note the expanded experimental area and the newly
completed Experimental Hall. (CBB 740-7911)
Beams available at the time were ions up to mass 40 at reasonable intensities,
around 109 ions per pulse for the lighter ions. Ion mass was limited to those
ions which could be fully stripped at the SuperHILAC exit energy. This limit
existed because of the poor vacuum in the Bevatron, and the problem of electron
pickup and loss of the circulating beam interacting with residual gas atons. (If an
ion changes its charge state, it no longer satisfies the proper relationship between
velocity, rigidity, and radius, and is rapidly lost from the beam.) Electron pickup
cross sections fall very rapidly with increasing velocity (d ta indicate &-6)
while electron loss has a much slower velocity dependence ( B -). Thus, an ion
not fully stripped will suffer charge-changing reactions much further into the
acceleration cycle than will an ion with no electrons.
At B.5 MeV/amu, argon is about 50% fully stripped by a 200-ug/cm2 carbon
foil. Iron, mass 5F, has at this energy a fully stripped fraction of less than 3%
and represented the heaviest beam that was attempted at the time. Cosmic-ray
physicists were keenly interested in such a beam to calibrate satellite instruments,
and with great effort several runs were made, at very low intensity. A technique
was employed for these runs which was also used during later uranium runs, that of
using a tracer ion of a lighter, more abundant species, but with the same q/A as the
desired ion, to tune up the accelerator. 15N7+ matches 56Fe26- to better
than 1%, and its use as a tracer allowed useful experiments to be conducted at
intensity levels below the sensitivity threshold of any of the existing beam
diagnostic instrumentation. The viability of the tracer technique points out one
of the problems inherent in heavy-ion acceleration, namely, unambiguous knowledge of
exactly what is being accelerated. We have had cases where incorrect beams have
been delivered; confusing lithium and carbon, nitrogen for silicon, even nitrogen
for neon. Experimenters have learned to always design apparatus that can measure
what they are getting.-4-
d . . araoarc
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Alonso, J. R. Thirty years at the forefront: a perspective on the Bevatron/Bevalac, article, September 1, 1984; [Berkeley,] California. (https://digital.library.unt.edu/ark:/67531/metadc1113765/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.