Summary of Recent Target Studies Page: 3 of 13
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intensity on target (M:TOR109) was about 1.6 x 1012 protons per
pulse, and the beam was tightly focused (v = .14 mm). The
debuncher gap monitor signal on the first turn was measured and
integrated for several bunches throughout the 82-bunch beam pulse.
Fig. 6 shows the integrated signal as a function of bunch number.
The points marked 'first' and 'last' were the first and last data
points taken in the sequence, and thus represent the level of
reproducibility of the data. Note the reduced signal in the late
bunches of the beam pulse. This reduction, when normalized
against the incoming bunch structure (see below), indicates a yield
reduction in the range of 8*4% over the final portion of the pulse.
The proton bunch structure was measured by integrating the current
in proton bunches passing the AP1 wall current monitor, upstream
of the target. The data, shown in Fig. 7, indicate a downward
slope in the signal as a function of bunch number. The final
bunches are about 8% smaller than the initial bunches. The droop
is attibuted to beam loading in the main ring. Finally, data taken
in a nickel target under conditions similar to those in the rhenium
target investiation (Fig. 8) show no drop in yield toward the end of
the pulse. This result indicates that no density depletion is taking
place in nickel, as expected.
Discussion of the density depletion study.
During the course of the investigation with rhenium, the airborne
radiation monitor in the building APO showed a rapidly rising count
rate. The experiment was halted after 2 hours, and the count rate
peaked at approximately 10 times the normal count rate.
Subsequent analysis showed the presence of radionuclides
(predominantly iodine, with smaller amounts of tellurium, sodium,
and potassium) in the airborne radiation monitor [Ref. 1]. The
total release was relatively small (about 5 mCi). A likely source
for these particles is the following. It is known that bombardment
of metals by a high-energy proton beam produces noble gases by
spallation reactions between the target nuclei and the incident
protons [Ref. 21. This fact is of general interest in determining the
rate of void formation, swelling, and radiation embrittlement of solid
target materials. In the case of rhenium, the predominant gas is
xenon. The numbers in the CERN paper would indicate a
production on the order of 1012 xenon atoms in the course of our
experiment. We hypothesize that the xenon, normally trapped in
the solid metal target, was released as the target material melted,
and subsequently escaped through the voids in the pressed-powder
target material and the seams of the titanium can. Radioactive
xenon nuclei would then have decayed to the daughter products
observed (iodine and tellurium). A similar, although much smaller,
release occurred in the earlier study at 1.2 x 1012 protons.
One would expect similar release of radioactive argon and daughter
products in the case of melting solid copper and nickel targets
under Main Injector conditions. However, the amount of release
may be smaller because of the smaller surface area available to the
escaping gases in a solid target. A beam-sweeping system that
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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/m1/3/: accessed November 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.