DESIGN OF AN IMPROVED ION CHAMBER FOR THE SNS. Page: 1 of 8
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10th Beam Instrumentation Workshop
Upton, NY, May 6-9, 2002 BNL-69185
Design of an Improved Ion Chamber for the SNS*
R. L. Witkovert and D. Gassner,
Brookhaven National Laboratory, f TechSource, Inc., Santa Fe, NM
Abstract. Ion chambers are in common use as beam loss monitors at many accelerators. A unit
designed and used at FNAL and later at BNL was proposed for the SNS. Concerns about the ion
collection times and low collection efficiency at high loss rates led to improvements to this unit
and the design of an alternate chamber with better characteristics. Prototypes have been tested
with pulsed beams. The design and test results for both detectors will be presented.
The ion chambers (IC's) designed for the FNAL Tevatron by Shafer in 19821 and
built by Troy-Onics2, have been used at both FNAL and BNL. These are simple in
design, consisting of a hollow nickel inner electrode and a nickel foil cylinder outer
electrode in a glass enclosure filled with argon. During testing of the chambers for use
in RHIC at BNL3 it was found that detector response varied unacceptably using the
preferred bias voltage polarity. Normally, from space charge considerations, electrons
are collected on the inner electrode. The IC's were tested with a Cs-137 source which
produced 1 Rad/hr at the chamber. The signal current was measured as the bias
voltage was varied. The resulting curve is characterized by a rapid increase in signal
which quickly rolls over ("knee") into a flat region ("plateau") as the bias becomes
high enough collect all charge produced. When the electric field is sufficient to cause
multiplication, the chamber enters the proportional region. Results for the first 40-50
RHIC IC's varied widely with many having extended "knees" and early onset of the
proportional region. Test results obtained from FNAL for the original detectors did not
this show behavior but it was stated that a "scrubbing" process had cured initial
problems. Unfortunately, the details of this process are no longer available. It is
possible that the original FNAL detectors did not have the same problem since they
were built 15 years earlier and subsequent vendor technicians may have unknowingly
changed details of the construction. However the detectors were not usable as
At BNL conditioning using a Tesla coil and "spot-knocking" showed some success
but required significant time per unit. It was suggested that the opposite bias polarity
would improve uniformity. Tests showed excellent unit-to-unit reproducibility when
ions, rather than electrons, were collected on the inner electrode, so it was decided to
use the "unconventional" field polarity for RHIC. The major consequence was lower
collection efficiency (saturation) for high dose rate losses, and slower signal risetimes,
which were not expected to be a problem in RHIC. For SNS, however, this drop in
efficiency could be significant for the worst-case 1% local loss.
*Work performed under the auspices of the U.S. Department of Energy.
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WITKOVER,R.L. & GASSNER,D. DESIGN OF AN IMPROVED ION CHAMBER FOR THE SNS., article, May 6, 2002; Upton, New York. (digital.library.unt.edu/ark:/67531/metadc733367/m1/1/: accessed November 12, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.