LIONs at the Stanford Linear Accelerator Center
T. N. Constant, R.W. Zdarko, R.H. Simmons, B.M.Bennett
Stanford Linear Accelerator Center, 2575 Sand Hill Road, CA 94025, USAAbstract
The term LION is an acronym for Long Ionization
Chamber. This is a distributed ion chamber which is used
to monitor secondary ionization along the shield walls of abeam line resulting from incorrectly steered charged
particle beams in lieu of the use of many discrete ion
chambers. A cone of ionizing radiation emanating from apoint source as a result of incorrect steering intercepts aportion of 1-5/8" Heliax cable (about 100 meters inlength) filled with Argon gas at 20 psi and induces apulsed current which is proportional to the ionizingcharge. This signal is transmitted via the cable to an
integrator circuit whose output is directed to an electronic
comparator, which in turn is used to turn off theaccelerated primary beam when preset limits areexceeded. This device is used in the Stanford Linear
Accelerator Center (SLAC) Beam Containment System(BCS) to prevent potentially hazardous ionizing radiation
resulting from incorrectly steered beams in areas thatmight be occupied by people. This paper describes thedesign parameters and experience in use in the Final
Focus Test Beam (FFTB)area of the Stanford LinearAccelerator Center.
1 Introduction
SLAC Long Ion Chambers have been standing guard asfaithful watchdogs protecting the SLAC LINAC and beamtransport lines from potentially dangerous and damaging
errant beam power since the inception of SLAC. Becauseof their keen sense of detection they have been used
extensively throughout all of the SLAC beamlines to track
wayward and missteered beam particles and alarm theMachine Protection System (MPS) instantly when errant
beam power strays from its prescribed flight path along
the transport system.
The term LION is an acronym for “Long Ion Chamber”.
The LION at SLAC is a long single length of gas filled
Heliax cable used as an ion chamber for the detection ofionizing radiation. Because SLAC beams are pulsed innature, errant beam particles produce pulsed ionization
fields, which in turn induce pulses of current within thecoax cable structure. These pulses are applied to peakdetection discriminators to generate shut-off commandsfor the MPS, thereby turning off potentially damaging
beam power when pulse heights exceed a preset
threshold. These LIONs, more commonly known as PLICs(Panofsky Long Ion Chamber) [1,2,3], named after theperson who proposed the initial system, have also proved
to be a very sensitive diagnostic tool to indicate where
along the beam path problems arise, and thereby aid inbeam tuneup and guidance.
With the installation of the FFTB (Final Focus TestBeam) line in the SLAC Research Yard, a new role wasproposed for the LION, that of protecting SLAC personnelfrom dangerous radiation resulting from missteered beams
in the Research Yard area. It was at this juncture that theterm “LION” was coined. This new role introduced theconcept of integrating the pulsed response from ionizing
radiation. The varying D.C. output level produced whenapplied to a comparator circuit could develop a shut-off
command to the Beam Containment System (BCS) for
turning off SLAC beams to remove the source of radiation.
The conventional beam containment systems at SLAC use
discrete Ion Chambers (ICs) for the detection of ionizing
radiation. Protecting the exterior environment outside thelong shield walls of the FFTB beam line in the Research
Yard area would require a prohibitively large number ofsuch discrete ICs and was not economically viable. It wasdecided to use a long length of gasfilled 1-5/8" Heliax
cable at beam height to do the job of providing continuous
coverage along the walls; by segmenting the cable along
the length of the beam line, different sensitivities could beused for each region along the beam line. The final
arrangement called for three lengths of cable along eachside of the tunnel walls, resulting in six LIONs for theFFTB (see Figure 1).

Figure 1 Diagram of Beam Containment devices installed in the SLAC FFTB showing the location of the six LIONs.

2 Sensitivity
It was estimated that point source beam losses greaterthan 1 watt in the FFTB shield block tunnel section would
produce ionizing radiation levels of the order of 1R/hr
distributed in a small cone along the inside of the tunnel
shield walls. This would correspond to about 1mR/hr at
the outside of the shield wall in the Research Yard area
(assuming a worst case minimum shielding attenuation of1000). It was further estimated that 1 meter of 1-5/8" gasfilled Heliax cable would have the equivalent volume of astandard discrete Ion Chamber used at SLAC. The lengthof the FFTB external tunnel is about 100 meters, andtherefore three 30-meter lengths of Heliax could be used
to provide coverage along the entire length of the FFTB
tunnel. This would correspond to the equivalent of threegroups of 30 discrete ICs each, thereby providing
distributed IC coverage equivalent to 90 ICs atconsiderably lower cost. Background levels of radiationwere estimated to be in the 20-30 mR/hr range (perhapsas much as 50 mR/hr) during normal FFTB operation.
This corresponds to a background threshold current in thecable of about 400 picoamps (pA). A rate of 1 R/hr ofradiation induces about 270 pA in a standard IC, andtherefore in 1 meter of cable; thus 50 mR/hr of radiationinduces 0.05(270)(30) or about 405 pA in 30 m of cable.
D.C. housekeeping currents are used to verify theintegrity of the Ion Chamber cable plant and, therefore,
using a 50% margin of safety, a housekeeping level of 250
pA was first used with a low threshold trip setting of 120
pA. This requires that leakage currents in the Heliax cablebe less than 0.1 of the set point, or 12 pA. In actualpractice, leakage currents have been maintained below 25
pA. It was decided to set the housekeeping level at 2500
pA (or 2.5 nanoamps [nA]). Adding all of these factors
together yields a resulting background current ofapproximately 2.9 nA in the LION segments. This allows
high trip point settings as low as 10 to 15 nA to detect a 1R/hr radiation field. It has been determined
experimentally that a 10-15 nA level will result from a 1
R/hr flux in the typical LION cable in the FFTB.
In order to establish a D.C. housekeeping level of 2.5nA, a 100 G-ohm (1011) resistor is used as a load at the far
end of the cable in conjunction with a 250 volt isolated
power supply (see Figure 2). Care must be exercised in
the assembly of this system to maintain low leakage
currents. For a schematic drawing of a single LION
detector as used for LIONs 5 and 6 see Figure 2.
3 Leakage
Cable leakage was addressed by measuring the leakage
current on long cable samples (60' and 270') at the FFTB
tunnel site. Initial tests using a Keithley meter, chart
recorder, and a 1KV isolated power supply showedleakage currents of from 3 to 70 nA for unpurged cable.
After purging (i.e., allowing argon gas to flow through thecable samples for about 1 week) the measured leakage

Figure 2 Schematic drawing of a single Lion detector
as used for LIONs 5 and 6.
currents stabilized to values below 50 pA.
Tests with a 500 volt P.S. resulted in leakage currents
less than 20 pA. Normal operation involves the use of 250
volt power supplies, resulting in leakage of about 10 pA.
Maintaining pressure in the cables has kept the leakage
well within specifica-tions and by monitoring gas pressure
with an alarm circuit, system performance is safeguarded.
4 Gas pressure
A one foot section of a 60' length of cable was exposed
to a radiation source set to 1 R/hr to determine the effectof gas pressure on sensitivity. After purging with argon
and stabilization of the 60' length of cable, the test results
obtained are shown in Table 1.
Table 1 Test results of 1' of cable exposed to 1R/hr ofradiation as a function of voltage and gas pressure.
VOLTAGEARGONPRESSURE

INDUCEDCURRENT

250 V250 V
500 V

10 psi20 psi20 psi

53 pA
72 pA
79 pA

Leakage current was less than 20 pA with P.S. voltages
from 250V to 500V. Doubling the gas pressure increased
the induced current by about 35%. Doubling the voltage
had less than 3% effect on the induced current. Withleakage currents of about 20 pA, the net induced current
at 20 psi was about 50 pA (72 pA-20pA) for a 1' length of
exposed cable sample at 250 volts, or about 150 pA permeter. It was suggested by David Burke that the shallow
solid angle of incidence would increase sensitivity by afactor of π in the FFTB tunnel to an estimated integrated
result of about 500 pA per meter for 1 R/hr of ionizing
radiation, as compared to the IC model of 270 pA. This
provides a further margin of safety. Actual experiencebears this out as is depicted by the results plotted inFigure 3. Using an Integrator op-Amp with a throughput
gain of 100 nA/volt resulted in 100-150 mv output for a 1
R/hr radiation field. This corresponds to an input of 10-15
nA respectively which agrees with the predicted value[(30)(500)=15000 pA].

Figure 3 Calibration data for 2 LION detectors. Radiation
was measured outside the concrete shielding walls
adjacent to the LIONs with neutron & gamma detectors.
The beam was steered around the Protection Collimator(PC8) to achieve different levels of secondary radiation.
5 Integrator
The standard PC card integrator used at SLAC has again of 1 µA per volt with a time constant of 0.1s,utilizing a 1 MΩ resistor shunted by a 0.1 µF capacitor.
Changing the op-amp shunt resistor to a value of 10 MΩ
in parallel with a 0.01 µF capacitor to increase the gain to
100 nA per volt with a time constant of 0.1s resulting in asystem which trips within one pulse at beam rates down to
1 pps. By increasing the time constant to 1.0s, the system
response time is slowed, allowing an operating margin for
1 pps faults, thereby providing beam tune-up capability
without continual beam shut-off. In the case of LIONs 5 &6 the time constant was increased to 3.0s (response time ≅10s) because of the proximity to the beam dump. Thelarger capacitance also provides better filtering for therepitition rate related ripple in the integrator output,
reducing this ripple by a factor of 10 or more. This ripple
is a critical parameter in this circuit arrangement,
especially in view of the low trip threshold of 1.6 nA(corresponding to a 16 mv output level). A low bias
current FET op-amp is used to provide the stability andhigh sensitivity required for this application.
6 Packaging
It has been estimated that a two channel chassis can be
fabricated with power supplies, integrator electronics,
metering, signal distribution, and alarms for about $2500

US and a total system cost, including Heliax cables, gastight connectors, high resistance terminators, and Keithleycalibration source for under $5000 US per channel.
7 Conclusions
After over four years of operational experience with
LIONs at the FFTB, it has been clearly demonstrated thatthey provide an effective “early warning system” topotentially hazardous radiation levels in the SLAC
Research Yard, and have greatly minimized the likelihoodof BSOIC (Beam ShutOff Ion Chamber) hard trips.
BSOICs are the last line of defense in the event ofexcessive external radiation, and when tripped, they shut
down the SLAC LINAC in a way that results in longer
recovery times and a lot of administrative overhead. This
early warning has been a real plus in providing sensitivity
to low beam power losses (of the order of 1 watt) within
the FFTB environment.
In addition, inital concerns of deteriorating cableleakage levels over extended periods of time have provento be without foundation. Careful consideration of systemdesign, isolation and continuous low-level gas purging
have maintained stable low-level cable leakage currents
well within tolerable limits.
SLACs LIONs are cost effective, reliable, and trustworthy, and they continue to act as tireless sentinels, ready to“roar” at the slightest provocation from threatening
secondary ionization.
Acknowledgments
Credits to Dave Burke who originated the concept foruse of D.C. LIONs in the FFTB tunnels for the detectionof lowpower beam losses, and to Rick Iverson for hisenthusiastic support during the initial feasibility studies.
Also our thanks to the SLAC safety office (ADSO), inparticular Bill Kroutil, for his assistance in obtaining thecalibration data with beam.
References[1] W.K.H. Panofsky, “The Use of a Long Coaxial Ion
Chamber Along the Accelerator”, SLAC TN-73-57(1963);
H. de Staebler, “Note on Panofsky’s Long Ion
Chamber”, SLAC TN-63-63 (1963).
[2] M. Fishman and D. Reagan, “The SLAC Long Ion
ChamberforMachineProtection”,
IEEETransactions on Nuclear Science (June 1967).
[3] J. Rolfe, R. Gearhart, R. Jacobsen, T. Jenkins, D.
McComick, R. Nelson, D. Reagan, and M. Ross,“Long Ion Chambers for the SLC”, IEEE Particle
Accelerator Conference (1989: 1531-1533).