Design of an Improved Ion Chamber for the SNS

R. L. Witkover† and D. Gassner

*Brookhaven National Laboratory, †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.

**BACKGROUND**

The ion chambers (IC's) designed for the FNAL Tevatron by Shafer in
1982[^1] and built by Troy-Onics[^2], 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 BNL[^3] 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, measured as the bias voltage is varied, is
characterized by a rapid increase which quickly rolls over ("knee") into
a flat region ("plateau") as the field becomes high enough that all
charge produced is collected. Ion chambers operate in this plateau
region. At higher voltage, the electric field is sufficient to cause
multiplication and the chamber enters the proportional region. Results
for the first 40-50 RHIC production 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 show this
behavior. It is possible that the original FNAL detectors, built 15
years earlier, did not exhibit this problem but subsequent vendor
technicians may have unknowingly changed details of the construction.
However the detectors were not usable as delivered.

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 consequences were
lower collection efficiency (saturation) for high dose rate losses, and
slower signal risetimes, which were not expected to be problems in RHIC.
For SNS, however, this drop in efficiency would be significant for the
worst-case 1% local loss, and a rise time comparable to the pulse width
would be unacceptable.

**SIGNAL RISE TIME CONSIDERATIONS**

With a 1 msec pulse width, signal rise time is important in the SNS.
Electrons are collected in a few microseconds with either bias polarity,
but the heavier ions take much longer moving to the inner electrode. At
2000 V the ion collection time would be close to 700 μsec. If the losses
were constant the signal would continue to rise over the 1 msec Linac
pulse as the slow ions arrived, followed by a tail for another 700 μsec
after the pulse. While the electron signal would allow rapid beam abort
for large fast losses, the waveform during the pulse would require
unfolding.

Electron and ion charges are generated equally, but will produce equal
voltages only for a parallel plate geometry. For cylindrical geometry,
using the relative capacitance of a line charge to the 2 electrodes,
Shafer[^4] showed that the fraction of current due to electrons at the
inner electrode (anode) is:

$F2\text{>=}\frac{2\pi\int_{b}^{a}{F2(R)\text{RdR}}}{2\pi\int_{b}^{a}\text{RdR}} = \frac{2x^{2}\text{Ln}(x) - (x^{2} - 1)}{2(x^{2} - 1)\text{Ln}(x)}\ \text{where}\ x = \frac{a}{b}$
(1)

For the FNAL ion chambers, a = 0.75" and b= 0.125" and \<F2\>= 0.749 .
That is, the fast electron signal will be 3 times the slow ion current
with the conventional polarity but only 1/3 for the reverse-bias
polarity. This was observed in tests in the RHIC transfer line. Clearly,
the FNAL chambers would be unsuitable for SNS unless the conventional
bias polarity could be used.

The positive ion transit time is given by:

$t = \frac{d^{2}}{\mu_{0}V\left( P_{0}P \right)}$ (2)

where:

$d = \left\lbrack (a^{2} - b^{2})\frac{\text{ln}(ab)}{2} \right\rbrack^{12}$
d = Effective electrode separation \[cm\] for cylindrical geometry[^5]

\(3\)

μ~0~ = Ion mobility at STP \[ cm^2^/(V-sec)\]

V = Applied Voltage \[V\]

P~0~ = Atmospheric pressure

P = Working pressure

The transit time can be reduced by decreasing the electrode gap but with
increased gradient corona may occur. A design was proposed[^6] in which
the radii were closer together and the chamber lengthened to keep the
same volume. Keeping the original outer electrode radius, a = 0 .75" but
increasing the inner radius to b= 0.5", the ion collection time at 3 kV
bias would be reduced from 560 μsec to 72 μsec. The calculated results
are shown in Figure 1. "Negative bias" refers to negative voltage on the
outer electrode. Clearly the "positive bias" case is not suitable for
SNS. The FNAL chamber with negative bias might be acceptable since the
electrons contribute 75% of the signal within a few microseconds. The
new design provides the best response.

![](media/image1.wmf)

**Figure 1.** Calculated signal for SNS Linac pulse on original FNAL and
new BLM.

**COLLECTION EFFICIENCY**

While the equations describing the collection efficiency have been known
since Thomson described them in 1899, they have never been explicitly
solved. Approximate solutions for the simpler parallel plate geometry
can be applied to cylindrical if an "equivalent" gap (eq. 3) is used.
Boag and Wilson[^7] solved the equations assuming the ionization
distributions were second and third order polynomials, and that the
radiation was constant. Their result, which gives a good fit for
efficiency \> 0.7 is given by:

$f = \frac{1}{1 + \frac{\xi^{2}}{6}}$ (4)

where:

$\xi^{2} = \frac{\alpha}{\text{ek}_{1}k_{2}}\frac{P}{P_{0}}\frac{d^{4}q}{V^{2}}$
(5)

e is the electron charge,

$q = \frac{I_{\text{signal}}}{v} \times 3 \times \text{10}^{9}$α is the
first Townsend recombination coefficient,

k~1~ is the electron mobility,

k~2~ is the ion mobility,

q is the ionization change density:

where I~signal~ is the signal current generated in the ion chamber by
the beam loss and

v is the active volume of the ion chamber. For the FNAL IC, the
sensitivity is 70 nA/rad/sec, and the volume is 110 cm^3^ so for the
accepted upper limit of 1 % local loss, the dose rate is 9.2 krad/sec
during the linac pulse.

Consider a chamber with argon gas:

α = 1 x 10^-6^ \[cm^3^/sec\]

k~1~ = 1.8 \[cm^2^/(V-sec)\]

k~2~ = 1.3 \[cm^2^/(V-sec)\]

V = 3000 \[Volts\] (Negative bias)

P = 725 \[mTorr\]

Then

While the efficiency calculation for values \<0.7 may not be accurate,
clearly the new chamber would be superior to the original, both in ion
transit time and collection efficiency, but, since the electrode gap is
smaller, great care must be taken with the high voltage design. Guard
electrodes would be required to decouple the signal electrode from
leakage. The design would be more difficult to manufacture than the
simple FNAL detector, with higher detector cost. Note that the electron
fraction is 0.567, with 43% of the signal coming from the now ions.

Boag[^8] also found a solution for the pulsed radiation case when the
loss occurs in a time short compared with the ion transit time. For this
case and the above parameters the FNAL chamber efficiency is 21% and the
new design is 60%. This might apply to the first 5-10 mini-pulses for
SNS but the static case solution might better describe the efficiency
late in the pulse.

**THE NEW ION CHAMBER DESIGN**

The design of the new ion chamber is shown in Figure 2. Voltage
gradients have been reduced by rounding the electrode ends. Guard
electrodes divert leakage from the HV electrode to ground and assure the
signal collector is in a uniform electric field region. They also
prevent ions at the chamber ends from contributing to the signal
resulting in longer transit times. Three prototypes units have been
made. The first was limited to 3 kV when filled with argon due to HV
feedthrough breakdown, but when filled with nitrogen was able to reach 5
kV. It was then installed in the transport line between BNL Linac and
Booster. Units 2 and 3 were built with feedthroughs rated for higher
voltage and with improved ceramic design. The radius of curvature of the
ends of the guard rings and signal electrodes were also increased. These
units have been tested to 4.5 kV with argon. Prototype 2 has been
installed in the BNL Booster extraction line for tests with short pulse
beams similar to SNS Ring extraction.

![](media/image2.wmf){width="5.797916666666667in"
height="2.3243055555555556in"}

**Figure 2.** Drawing of new design BLM prototype.

**IMPROVEMENTS TO THE FNAL DETECTOR**

As an alternative to designing a new chamber, the original was studied
to see if modifications could be made to improve its performance. The
difficulties with the conventional (negative) bias polarity in the FNAL
IC were believed to be due to high field points at the tip of the inner
electrode and at the supports for the outer electrode, since these are
where breakdown occurs when the voltage is raised. Changes were made to
the outer electrode to eliminate crimping at the flared end of the outer
electrode at the support pin attachment locations. Figure 3a shows the
original outer electrode crimped inward by the support rods. Figure 3b
shows a modified chamber with the outer electrode slotted to allow the
support to pass the flared end without bending it inward.

![](media/image3.png){width="2.779861111111111in"
height="2.370833333333333in"}![](media/image4.png){width="2.8722222222222222in"
height="2.3777777777777778in"}

**Figure 3a**. Original FNAL Outer electrode **Figure 3b**. Slotted
outer electrode

So far 5 chambers have been modified in this way. Tests indicate that
this has significantly improved the unit to unit response. Figure 4a
shows the cesium source tests of 5 of the first RHIC chambers with
conventional bias polarity. Figure 4b shows results for 5 units with
modified (slotted) outer electrodes. The uniformity is clearly improved
with the slotted electrodes.

It was felt that some of the voltage limitation may have been due to
sharp edges at the open end of the hollow center electrode. Several
chambers were built with the slotted outer and a rounded solid tip
inserted in the end of the electrode. Tests of these units did not show
a significant improvement from the slotted outer electrode alone. While
these modifications seem to have improved the production uniformity, the
ion transit time and high dose rate saturation are still a problem for
SNS. If the design could be improved to allow higher operating voltage
then the ion transit time would decrease and collection efficiency
increase. The improvement is somewhat less than linear with voltage, so
this might not be very productive.

![](media/image5.wmf){width="2.591666666666667in"
height="2.8256944444444443in"}
![](media/image6.wmf){width="2.9506944444444443in"
height="2.8354166666666667in"}

**Figure 4a.** Original FNAL IC test data **Figure 4b.** Modified
(slotted) FNAL data

**TESTS WITH BEAM**

A non-modified FNAL IC and Prototype 1 were installed together in the
BNL Linac-to-Booster (LTB) line, approximated 1 foot from the beam line,
where high dose rate losses from the 200 MeV H-minus beam could be
observed. Two sets of data runs were made. In the first a high
sensitivity multi-channel integrator module of the type used in the
source testing of the chambers was used to condition the signals. They
were modified to have a 470 Ohm input so rise times of several μsec
could be observed on a LeCroy digital scope. The bias voltage was varied
to determine the collection efficiency curves for both bias polarities.
The beam current waveform was also captured. Figure 5 shows the results
of this run. Clearly the positive bias (non-preferred) FNAL response
shows low efficiency, especially at lower bias voltage. The data for the
negative bias shows large excursions, while summation of the beam
current data does not show more than a few percent variation between
data points, implying beam motion. The data for Prototype 1 appears
fairly flat throughout the entire voltage scan. Calculations indicate
that the FNAL chamber should be 77% efficient while Prototype 1 should
be 92% at 3 kV for negative bias at this loss level. Dedicated runs are
planned to study the efficiency at various dose rate levels.

![](media/image7.wmf){width="3.4256944444444444in"
height="3.102777777777778in"}

**Figure 5.** Measured bias curve (collection efficiency) for 200 MeV
Linac beam. Aproximately 1000 W/m loss.

A second run was made using the prototype analog front end (AFE)
circuitry designed for SNS[^9]. This circuit has a current amplifier
first stage using a Burr-Brown OPA627BM opamp with outputs from a unity
gain (50 kHz BW) buffer, a "leaky integrator" and a filtered (1 kHz)
slow amplifier. Data from the fast output and the beam current
transformer is shown in Figures 6a and 6b.

![](media/image8.wmf){width="2.7284722222222224in"
height="2.3444444444444446in"}![](media/image9.wmf){width="2.622916666666667in"
height="2.359027777777778in"}

**Figure 6a.** FNAL IC Beam Test. **Figure 6b.** Prototype 1 IC beam
test

The signal from the FNAL chamber (Figure 6a) can be seen to have a very
long tail following the pulse, due to the slower ions being collected
after the beam. They are also arriving during the pulse, causing the
gradual rise rather than directly following the beam current (also see
Figure 1). The apparent noise during the pulse is actually modulation of
the beam current to tailor the stacking in the Booster. The signal from
the Prototype ion chamber has a much faster ion collection time, as
expected.

# CONCLUSIONS

Improvements have been made to the original FNAL detector design which
result in more uniform characteristics of the production units. These
changes allow the use of the conventional bias polarity (center
electrode is anode) without conditioning of the chambers. However,
concern over collection efficiency (saturation) resulting in departure
from linearity at high dose rates may preclude its use in SNS. The slow
ion transit times are comparable to the SNS pulse width, requiring
unfolding to obtain the true time behavior.

The new chamber design appears to yield significantly faster ion transit
time and higher collection efficiency at high dose rate, as predicted.
However, higher fabrication costs and the uncertainties of bringing a
new design to production need to be weighed against the benefits to the
SNS application. A firm specification of linearity at high dose rate is
needed before the detector choice can be made.

# Acknowledgments

The authors wish to acknowledge the contributions of C. J. Liaw in the
mechanical design, Dave Kipp is assembling the prototype chambers and
Paul Ziminski in testing the chambers with the cesium source. The
prototype circuit used in the beam tests was built and tested by
Chaofeng Mi. Anthony Curcio (BNL) was invaluable in the installation and
setup of the beam tests.

# References

[^1]: R. E. Shafer, et al., The Tevatron Beam Position and Beam Loss
    Monitoring Systems", Proc. 12^th^ Int'l Conf on High Energy Accel.,
    FNAL (1983) p609

[^2]: Troy-Ionics, Inc., Kenvil, NJ

[^3]: Witkover, R. L., Michnoff, R. J. and Geller, J. M., \"RHIC Beam
    Loss Monitor System Initial Operation\", Proceedings of the 1999
    PAC, NY, 1999, 2247

[^4]: Shafer, R. E., Private communication

[^5]: Boag, J. W. in "Radiation Dosimetry, Vol. II", F. Attix, et al,
    eds.,Academic Press (1966), p22

[^6]: R. Witkover," Proposal for a Quasi-Parallel Plate, Cylindrical
    Geometry Ion Chamber for SNS", November 7, 2001, unpublished

[^7]: Boag, J. W. and Wilson, T., "The saturation curve at high
    radiation intensity", Brit. J. Phys., **3** 222-9, (1952) and Loc.
    Cit (5), p16

[^8]: Boag, J. W., "the Dosimetry of Ionizing Radiation", Vol. 2, Ed. By
    K. R. Kase, B. E. Bjamgard and F. H. Attix, Academic Press, (1987),
    p 191

[^9]: Witkover, R. L. and Gassner, D., "Preliminary Design of the SNS
    Beam Loss Monitoring System", To be published in these proceedings.