Summary of Recent Target Studies Page: 2 of 13
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Edge enhancement of yield. This study was designed to test a
prediction of enhanced yield at the interface between copper and
aluminum, under the appropriate conditions. The enhancement is
due to the reduced attenuation of antiprotons created in the dense
material (copper), but exiting the target through the light material
(aluminum). Normally, the target is positioned with its center near
the focal point of the lithium lens. No edge effect has been
observed in this case. However, if the target is moved such that
the lens is focused on the upstream end of the target, the total
pbar yield drops by about 30%, but the differential edge effects are
more easily observed because of the longer path lengths of the
antiprotons through the dissimilar materials. Figure 2 shows the
yield to IC728 as a function of vertical position of the target
(D:TRY) at the interface between a copper cooling disk and the
bottom of the aluminum target. The lens is at its normal position
(D:TRZ=19.5 cm). The edge effect is not observed. Figure 3
shows the yield at the top edge of the aluminum target, with
D:TRZ=15.43 cm, i.e. the lens is focused at the upstream edge of
the target. Larger values of D:TRY correspond to the titanium can
of the powdered rhenium target, and smaller values correspond to
the aluminum target disk. In this case the edge effect (an
enhancement in yield at the edge of the copper portions, and a
reduction at the edges of the aluminum disk) is clearly observed.
Figure 4 shows a similar scan of an identical cooling disk located
between a copper and a nickel disk. The edge effect is gone,
(lower trace, D:TRZ=15.43) except perhaps for a small effect at the
interface at the copper/air cooling channel. The upper trace is the
yield with the target at its normal position. Again, no edge effect
is observed. (Note the hysteresis in the readback D:TRY.)
Detailed measurements at the upper and lower interfaces were
performed with P87 to verify the effects on the pbar yield
D:YIELDI. These results are summarized in figure 5. Again, there
is an enhancement at the edge of copper, and a reduction at the
edge of aluminum. The measured effect is 4-5% in amplitude in
both cases. The result was expected, and is due to changes in
attenuation of the pbars as they leave the target.
The demonstration of edge enhancement indicates that it should be
possible to increase yield by designing a target that takes advantage
of the edge effect, specifically, a wire target with about a 1-mm
diameter. Detailed modelling of pbar production, including realistic
estimates of the rate of secondary production of pbars, will be
necessary to determine the expected amount of yield increase.
Density depletion study. Large energy density on target is expected
to melt target materials and hence cause density depletion,
accompanied by a reduction in yield. The potential exists for target
melting to occur routinely under Main Injector conditions, in the
absence of a beam-sweeping system. Therefore it is important to
understand the behavior of the target under the extreme conditions
expected with high-intensity Main Injector beams. Since rhenium
has a relatively low melting-point energy, we performed a study to
demonstrate density depletion in rhenium. In this study the beam
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Bieniosek, F. & O'Day, S. Summary of Recent Target Studies, report, February 4, 1993; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc1014573/m1/2/: accessed January 15, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.