ctly steered charged
particle beams in lieu of the use of many discrete ion
chambers. A cone of ionizing radiation emanating from a
point source as a result of incorrect steering intercepts a
portion of 1-5/8" Heliax cable (about 100 meters in
length) filled with Argon gas at 20 psi and induces a
pulsed current which is proportional to the ionizing
charge. 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 the
accelerated primary beam when preset limits are
exceeded. 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 that
might be occupied by people. This paper describes the
design parameters and experience in use in the Final
Focus Test Beam (FFTB)area of the Stanford Linear
Accelerator Center.
1 Introduction
SLAC Long Ion Chambers have been standing guard as
faithful watchdogs protecting the SLAC LINAC and beam
transport lines from potentially dangerous and damaging
errant beam power since the inception of SLAC. Because
of 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 the
Machine 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 of
ionizing radiation. Because SLAC beams are pulsed in
nature, errant beam particles produce pulsed ionization
fields, which in turn induce pulses of current within the
coax cable structure. These pulses are applied to peak
detection discriminators to generate shut-off commands
for 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 the
person 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 in
beam tuneup and guidance.
With the installation of the FFTB (Final Focus Test
Beam) line in the SLAC Research Yard, a new role was
proposed for the LION, that of protecting SLAC personnel
from dangerous radiation resulting from missteered beams
in the Research Yard area. It was at this juncture that the
term LION was coined. This new role introduced the
concept of integrating the pulsed response from ionizing
radiation. The varying D.C. output level produced when
applied 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 the
long shield walls of the FFTB beam line in the Research
Yard area would require a prohibitively large number of
such discrete ICs and was not economically viable. It was
decided 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 be
used for each region along the beam line. The final
arrangement called for three lengths of cable along each
side of the tunnel walls, resulting in six LIONs for the
FFTB (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 greater
than 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 of
1000). It was further estimated that 1 meter of 1-5/8" gas
filled Heliax cable would have the equivalent volume of a
standard discrete Ion Chamber used at SLAC. The length
of the FFTB external tunnel is about 100 meters, and
therefore 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 three
groups of 30 discrete ICs each, thereby providing
distributed IC coverage equivalent to 90 ICs at
considerably lower cost. Background levels of radiation
were estimated to be in the 20-30 mR/hr range (perhaps
as much as 50 mR/hr) during normal FFTB operation.
This corresponds to a background threshold current in the
cable of about 400 picoamps (pA). A rate of 1 R/hr of
radiation induces about 270 pA in a standard IC, and
therefore in 1 meter of cable; thus 50 mR/hr of radiation
induces 0.05(270)(30) or about 405 pA in 30 m of cable.
D.C. housekeeping currents are used to verify the
integrity 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 cable
be less than 0.1 of the set point, or 12 pA. In actual
practice, 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 of
approximately 2.9 nA in the LION segments. This allows
high trip point settings as low as 10 to 15 nA to detect a 1
R/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 2.5
nA, a 100 G-ohm (10
11
) 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 showed
leakage currents of from 3 to 70 nA for unpurged cable.
After purging (i.e., allowing argon gas to flow through the
cable samples for about 1 week) the measured leakage
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 effect
of 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 of
radiation as a function of voltage and gas pressure.
VOLTAGE
ARGON
PRESSURE
INDUCED
CURRENT
250 V
10 psi
53 pA
250 V
20 psi
72 pA
500 V
20 psi
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. With
leakage 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 per
meter. It was suggested by David Burke that the shallow
solid angle of incidence would increase sensitivity by a
factor of 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
Figure 2 Schematic drawing of a single Lion detector
as used for LIONs 5 and 6. bears this out as is depicted by the results plotted in
Figure 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 a
gain of 1
µ
A per volt with a time constant of 0.1s,
utilizing a 1 M resistor shunted by a 0.1
µ
F capacitor.
Changing the op-amp shunt resistor to a value of 10 M in parallel with a 0.01
µ
F capacitor to increase the gain to
100 nA per volt with a time constant of 0.1s resulting in a
system 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