Radiation Protection Utilizing Electronic Berms
John Anderson Jr. and Glenn Federwitz
Fermi National Accelerator Laboratory
Abstract. Excessive radiation losses from particle beams produce personnel, environmental, and component activation
hazards. With the new high intensity fast repetition rate beam lines being constructed at Fermilab, small pulse to pulse
losses are a significant concern. This paper presents the proposed design of a total loss monitoring system for the
MiniBooNE Project.
INTRODUCTION
INPUT DEVICES
The 8 GeV Fixed Target Facility, being built at
Fermilab, is used to transport beam from the Booster
accelerator to a new area dedicated to external beam
physics. MiniBooNE, the first experiment to use the
new facility, has requested 5E20 protons per year.
This equates to more beam per year than has been
accelerated in the history of Fermilab. To minimize
radioactivation of beamline components, groundwater,
and areas outside the overburden; the project has
specified a total beam loss monitoring system termed
an Electronic Berm or E-Berm. This system is
intended to monitor the amount of beam lost between
two toroids and disable the beamline if losses exceed a
predefined level.
Toroid Integrator
The Toroid Integrator receives an event trigger to
open a lOus integrate window. Upon completion of
the integration window, the integrator sends the Eberm comparator module a 16 bit 2's compliment
binary data word and a valid data pulse. There are 4
possible gain settings on the integrator electronics
card; they are +1.875E14, +5.625E13, +1.875E13, and
+5.625E12 protons at full scale. Using the lowest gain
setting and taking into consideration that the signal is
bipolar, the output resolution achieved is 1.717E8
protons per count.
The system described here will use two toroids and
the associated integrating electronics. One toroid will
be located at the upstream end of the 8 GeV beam line
and the other will be located slightly in front of the
target focusing horn. The comparator module will
measure the absolute value of the difference of the two
toroids on a pulse-to-pulse basis. Using the pulse-topulse difference along with summing the error of the
last 10 pulses, both an instantaneous and integrated
loss can be measured.
The integrating electronics card buffers the output
data word and the valid data pulse before being sent to
the E-berm comparator module.
Beam Presence Detector
The beam presence detector utilizes a resistive wall
monitor (beam sensing device) connected to a 53Mhztuned detector to provide a TTL level output pulse for
each beam pulse. This signal is used as a coincidence
check within the E-berm comparator module to verify
the toroid integrator electronics is looking for beam at
the correct time. The beam presence detector will
detect beam signals of approximately 5E10 or higher.
SYSTEM IMPLEMENTATION
Figure 1 shows a block diagram of the E-berm
system. There are four major components to the
system; the toroid and integrator electronics, the
calibration module, the beam presence detector
(resistive wall monitor), and the comparator module.
The focus of this document is on the comparator
module with a brief description of the other three
components.
Toroid Integrator Calibration Module
The Toroid Integrator Calibration Module is a part
of the E-Berm total beam loss monitoring system. The
module provides the ability to monitor the
performance of the toroid and the integrator
electronics, thus greatly improving the reliability of
the beam to the experiment. The module is not part of
the Fermilab Safety Systems, but rather a tool to
improve the systematic performance of the E-Berm
system.
CP642, High Intensity and High Brightness Hadron Beams: 20th ICFA Advanced Beam Dynamics Workshop on
High Intensity and High Brightness Hadron Beams, edited by W. Chou, Y. Mori, D. Neuffer, and J.-F. Ostiguy
© 2002 American Institute of Physics 0-7354-0097-0/02/$ 19.00
125
The intent of the calibration module is to perform a
toroid calibration sequence in approximately 100ms.
To do this, the module generates a family of
calibration pulses sent through the toroid calibration
winding, performs a least squares fit to the data
measured by the toroid integrator module, and outputs
the gain and offset values to the Fermilab control
system in the form of five 8 bit digital data words.
total. Thus generating a sum of the last 10 beam
samples to be used for subsequent comparisons.
Pulse to Pulse Comparison
The absolute value of the difference between
integrator "A" and integrator "B" is compared to a set
of switches located on the comparator board. The
value of the switches for MiniBooNE is set at
approximately (~3E11). This equates to 6% of the
total pulse-to-pulse beam of (~5E12) per pulse. If the
difference value is higher than the switch setting a trip
occurs.
In the normal operating mode, (not performing a
calibration sequence), the module sits idle.
To initiate the calibrate mode, the module receives
a 1 microsecond calibrate trigger from the Fermilab
control system. A calibrate in progress bit is sent to
the E-berm comparator module to trip the output
permits to inhibit beam during a calibrate sequence.
Integrated Loss Comparison
The value of the sum of the last 10 beam pulse
differences is compared to a set of switches again
located on the comparator board. The value of the
switches for MiniBooNE is set at approximately
(-IE 12). This equates to approximately a 20% loss of
beam for the last 10 beam pulses. Looking at this from
a pulse-to-pulse basis yields an average loss of less
than 2%. Again, if the value is higher than the switch
setting, a trip occurs.
COMPARATOR MODULE
The figure 2 shows the interconnecting logic
functions performed by the comparator module. The
logic functions are implemented in Altera™ large scale
EPLD logic to eliminate the need for microprocessors.
The diagram above shows both the "A" and "B" side
comparison circuits. A description of each internal
logic function is described below.
Self Test Compare
Between beam pulses, the integrator is triggered to
take a sample with no beam present. The absolute
value is then compared to a small positive value to
check if the integrator has drifted away from zero. If
the value gets too high, the module is tripped. This
value is defined inside the EPLD's. At present, the
value is set to (2.7E9) and will be defined during the
commissioning process after the noise floor for the
toroid integrators has been established.
Intensity Compare
The 16-bit signal from the "A" integrator is
compared to switch settings in the "A" side EPLD's of
the module and if the signal from the integrator is
higher, a trip occurs. The same function occurs for the
"B" side with the "B" integrator. This allows setting
of a maximum intensity limit for the beamline. This is
not expected to be used in the MiniBooNE beamline
but is included for future applications.
After completion of the zero test, a current pulse is
sent through the calibrate winding of both toroids.
This is to insure the integrity of all cables and the
associated electronics. The absolute value is then
compared against to a near full scale positive window.
This checks for integrator gain drift and verifies cable
integrity. If the value falls outside the window, the
module is tripped. This value is defined inside the
EPLD's
Difference Engine
This block takes the 16-bit 2's complement data
values from Toroid Integrator "A" and Toroid
Integrator "B", subtracts them and returns the absolute
value of the result. The "A" side EPLD's perform an
|"A-B"| function and the "B" side EPLD's perform an
|"B-A"| function. The inversion of the two functions
between the "A" and "B" sides is done to assist in
preventing common mode errors.
Beam Presence Detector Compare
The Beam Presence Detector compare logic is used
as a beam coincidence check with the toroid integrator
electronics. If either toroid integrator reads more than
1E11 beam, then a pulse indicating beam has passed is
required from the beam presence detector. The
absence of this pulse, when more than IE 11 beam is
Last 10 Sum
This block adds the current value of the difference
engine to a running total and stores the value in a 10
deep by 16-bit shift register. The value that gets
shifted out of the memory is then subtracted from the
126
on the main board. Third, there is a reset in signal
from the safety interlock system.
To prevent
undesired clearing of the shift registers; the reset-in
signal from the safety interlock system is masked
unless the system is in a tripped state.
detected by either of the integrator electronics, will
initiate a system trip.
Timing
The timing block takes the ACNET event strobe
signals and passes the appropriate signal on to each
integrator starting a beam event timing sequence. The
event timing sequence requires that the beam pulse
from the Beam Presence Detector occur between 2.5us
and 8us after the event strobe occurs. Knowing that
the Booster extraction pulse is 1.6us guarantees that
both integrators were looking for the beam pulse at the
correct time. This timing window is defined inside the
EPLD's. Additionally a signal from the Beam
Presence Detector at anytime outside the beam event
will initiate a system trip.
Clock monitor
The comparator module uses two lOMhz clocks for
internal timing. There are two independent clock
watchdog circuits. A failure of either clock will
remove the output permit signals.
Proposed Use
Event strobe signals may not occur at intervals of
less than 40ms. This insures the previous event has
been fully processes and the internal self test has
completed before beginning the next event timing
sequence.
The E-berm system is intended to be used in place
of chipmunk radiation monitor arrays. The system is
not intended to be used beyond allowing the same
reduction in shielding as would be allowed by the use
of chipmunks. The system does not and cannot
prevent the initial beam loss. The major advantages of
the E-berm over the standard chipmunk radiation
monitors are that losses are detected from any
direction. This will help to minimize ground water
activation, component activation, and allow for a
reduction in the required overburden.
Outputs
Design Methodology
The five digital output words connected to an IRM
consist of the buffered 16-bit 2's complement
integrator values, the 16-bit absolute value of the
difference between toroid A and B, the 16-bit sum of
the last 10-beam pulse losses, the zero self test value,
and the test pulse value.
The design methodology used in the proposed
design is consistent with the guidance provided in the
Fermilab Radiological Control Manual (FRCM)
Chapter 10. Two independent monitoring channels are
provided inside the comparator module both capable
of interrupting the output permit loops.
Individual trip indicators are provided to the front
panel for the self test zero and test pulse compare trips,
intensity compare trip, single beam pulse loss trip, sum
of last 10 beam pulse losses trip, strobe A or B errors,
beam presence detector error, event timing errors, and
clock errors.
Failure Modes
Each Integrator Electronics module must provide a
strobe signal indicating valid data is ready within 15us
of an event strobe signal.
During the design process, failure mode reduction
principles utilizing redundant circuits were employed.
Additionally, all signals to and from the comparator
board are buffered. Monitoring two independent
toroid integrators and comparing to a beam presence
detector signal verifies event timing. The "A" side
channel performs an Integrator |A-B| function while
the "B" side channel performs an Integrator |B-A|
function. The output permit signals are active high
signals. Self test functions verify cable integrity. All
data is echoed to the ACNET control system via an
IRM for further analysis.
Two 24vdc active high "A" and "B" output permits
are provided for connection to the safety interlock
system.
Two RS-485 connections for the Safety System
Data Acquisition System are also provided.
Reset
There are three separate reset inputs for the
comparator module. Each reset signal clears the data
shift registers and resets the "A" & "B" output trip
circuits. First, there is an internal power up reset for
the board initialization. Second, there is a manual
reset button located internal to the comparator module
Testing
The calibration module built by Columbia
University will be used to monitor the calibration drift
of each toroid integrator between physical calibrations.
A physical calibration of each toroid integrator will be
127
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128
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FIGURE
129