Choice of High Voltage Bias Polarity for the SNS BLM Ion Chambers

R. L. Witkover and D. Gassner

March 5, 2001

# Background

After the FNAL style ion chambers were selected for the RHIC BLM
detectors, acceptance tests were made on the first units received. The
units were tested with the high voltage bias polarity preferred by their
designer, Bob Shafer[^1]. He recommended that the positive voltage be
applied to the center electrode of the cylindrical geometry chamber to
minimize the effects of space charge and stray fields.

Figure 1 shows the RHIC version of the FNAL ion chamber. Changes
included a G-10 plate to insulate the signal connector and break ground
loops, and multiple connectors for both signal and HV to allow easy
daisy-chaining. Another difference was that the signal was taken from
the center electrode and the HV was applied to the outer electrode,
opposite to FNAL. This was done to shield the signal (center) electrode
for improved low signal characteristics, and to provide a printed
circuit board on which to mount resistors to isolate the HV. A board
already existed in the FNAL design but it was only used to provide the
mount for the chamber via the pins to the outer electrode. Making the
outer cylinder the HV electrode allowed a single PC board to serve both
needs.

![](media/image1.wmf){width="5.998611111111111in"
height="3.6222222222222222in"}

Figure 1. RHIC version of the FNAL Ion Chamber

In the FNAL application, positive voltage was applied to the center
electrode, allowing the electrons to move towards the region of
increasing electric field. With negative high voltage on the outer
electrode, the same electric field orientation could be achieved in the
BNL design.

Tests by Mike Plum[^2] at the REX pulsed X-ray source Facility at LANL,
which provided a dose of up to 2 Rad in a 50 nsec pulse, indicated
that + HV on the center electrode reduced saturation effects in high
dose rate situations. His data is reproduced in Figure 2.

![](media/image2.png){width="6.0in" height="3.439583333333333in"}

Figure 2. Response of LANL ion chamber to high dose rate radiation. (M.
Plum, et al.)

While the LANL chamber was filled with nitrogen and the RHIC/FNAL
chamber is filled with argon, the conclusion could be expected to be the
same. That is, the use of "positive polarity" will minimize saturation
effects under high dose rates.

# Tests of the First RHIC Ion Chambers

The RHIC ion chambers were purchased using the same specifications and
from the same vendor as FNAL and packaged as shown in Figure 1.
Acceptance testing of the first batch indicated that the signal versus
bias voltage was very different from the FNAL data and varied
considerably from unit to unit. Typical test data supplied by FNAL
showed a signal which leveled off at about 100 V bias and stayed flat
until 2500-2700 V, after which it rose by 10-20% at 3000V. The tests of
the RHIC detectors showed an extended, rounded "knee" which didn't level
off until 500-700 V and began rising again at 1500-2000V depending on
individual detectors. Some increased by 50% at 3000V. Clearly this was
not the response of the FNAL chambers and was unacceptable.

Discussions with Bob Shafer (now at LANL) and Alan Hahn (FNAL) indicated
that the FNAL chambers were "baptized" before being tested. No clear
statement of this "holy rite" could be obtained but it was essentially
an HV "cleaning" of the detector. To simulate this at BNL a Tesla coil
was used on the detector for 30 minutes, followed by pulse cleaning with
a "Spot Knocker". The "Spot Knocker" consisted of a HV power supply
charging a 0.5 F capacitor through a string of resistors totaling 100
k. The HV was raised until breakdown occurred, forming a relaxation
oscillator. The results of this cleaning are shown in Figure 3. "Neg HV"
is with the FNAL field direction but with negative HV on the outer
electrode rather than positive on the inner electrode. "Pos HV" is with
the opposite field direction but the curve has been inverted for ease of
plotting.

![](media/image3.wmf)

Figure 3. Effects of High Voltage Cleaning on a RHIC Ion Chamber

This was a particularly poor chamber, as can be seen from the upper most
curve. There is virtually no flat region in which to work. The curve
immediately below this is the same chamber after cleaning with the Tesla
coil. The third curve down is the same chamber after cleaning with the
Spot Knocker. A definite plateau is now observed.

It was clear that with nearly 500 chambers to be installed in RHIC, it
would not be practical to spend more than 1 hour cleaning each one. The
measurements with the opposite HV polarity showed that chambers tested
as poor with --HV on the outer electrode, all showed acceptable flat
operating regions in the opposite polarity even before the cleaning.
This can be seen from Figure 3. Note that even this polarity showed some
improvement after cleaning. At this point it was decided to use the ion
chambers biased so that the electrons would be collected on the outer,
rather than the inner, electrode. Referring to Figure 2, it was expected
that the chambers would stay within the linear range for either polarity
for the RHIC application. No further study of this unresolved question
was made at that time.

# The SNS BLM System Beam Loss Range

The SNS BLM System must cover a wide range of beam loss. The low end is
set by the condition that the operators must be notified when the loss
rate exceeds 1 W/m, or about 0.01% beam loss. The high end is not
clearly defined. A limit of 1% beam loss at a single location has been
taken. This represents a range of approximately 2.5 x 10^4^. Now, the
loss of 1 W/m roughly corresponds to 100 mRem/hr activation. A "rule of
thumb" indicates that there is a factor of 500-1000 between the beam-off
activation and the beam-on dose rate. Thus 100 mRem/hr implies 100
Rem/hr beam-on dose rate for 0.01% loss. Scaling this by the 2.5x10^4^
range gives 2.5 x 10^6^ Rem/hr (average) or 194 Rem/pulse (peak). Since
the electrons are collected in a time the order of a microsecond, pulse
irradiation tests on ion chambers normally consider this the time of
interest. For a SNS pulse length of 1 msec, this amounts to 0.194
Rem/microsecond, which is within the linear range of Figure 2. However,
there has been a lot of hand waving to arrive at this number, and, since
the figure of 1 % loss at a single point is completely arbitrary, it was
considered important to again consider the question of HV bias polarity.

# Further Tests of the FNAL and RHIC Ion Chambers

The only FNAL ion chambers available at BNL had an unknown history and
showed evidence of heavy irradiation so recently purchased glass
detectors bottles were placed in an FNAL housing. This unit was tested
with both positive and negative voltage on the inner conductor.
Similarly, a typical RHIC unit was tested with both positive and
negative voltage on the outer conductor. The four configurations tested
are shown in Figure 4. Test results are shown in Figure 5. The curves
marked FNAL(+) and BNL(+) are with positive HV on the inner electrode
(the preferred polarity), and negative on the outer electrode,
respectively. The curves marked FNAL(-) and BNL(-) are with the opposite
electric field. Again, all curves have been plotted positive regardless
of sign.

![](media/image4.wmf){width="5.684722222222222in"
height="3.432638888888889in"}

Figure 4. BLM Test Configurations

![](media/image5.wmf)Figure 5. Test results for FNAL and BNL BLM
electrode configurations.

The normal configurations are FNAL(+) for the Tevatron and BNL(-) for
RHIC, which, ignoring measurement noise excursions, show a plateau
across nearly the range from several hundred volts to 3 kV. The other
two configurations show much smaller plateau regions. In the case of the
BNL(+) data, the curve starts rising at about 2kV. This rise marks the
start of the proportional region for the chamber in which the some
electron gain enough energy between collisions to cause further
ionization. The BNL(+) configuration is the one which will be less
susceptible to saturation under high dose rate, however, since it cannot
be used above 2 kV this advantage compromised.

# Conclusions

Clearly the FNAL(+) configuration offers the best response as well as
being less susceptible to saturation under high radiation. While the
BNL(+) configuration should have the same good high-dose rate saturation
resistance, since the E-field is in the same direction, it clearly does
not have as good plateau characteristics as either the FNAL(+) or the
BNL(-) configuration. The cause of the poor plateau is not understood
but might be due to local high gradient points resulting from mounting
the outer electrode.

While the BNL(-) configuration BLMs should work linearly up to about a
1% local beam loss, modification of the design to use the FNAL(+)
configuration is being studied. This would entail the use of 2 printed
circuit boards rather than the present single one. It should be noted
that the FNAL design only uses one PC board, to support the tube at the
outer electrode end, with the HV isolation resistor string free-standing
in the air. This is not good practice and a second PC board must be used
to mount the resistors if this configuration is to be used.

# Acknowledgements

Paul Ziminski of BNL did all the BLM testing and made many helpful
suggestions. Discussions with Bob Shafer and Mike Plum of LANL and
I-Hung Chang of BNL were of great value both during the RHIC development
and in understanding the present polarity problem.

[^1]: Shafer, Robert, "Doubler Beam Loss Monitor Detector", 8/19/82.
    Private Communication

[^2]: Plum, M., et al, "Fail-Safe Ion Chamber Errant Beam Detector
    Tailored for Personnel Protection", PAC 1989, p1556