Spallation Neutron Source Beam Loss Monitor System[^1]

D. GASSNER\*; R. WITKOVER\*\*; P. CAMERON\*; J. POWER\*\*\*

BNL, Upton, NY 11973,\*\*TechSource, Inc., Santa Fe, NM
87594,\*\*\*LANL, Los Alamos, NM 87545

**Abstract.** The Spallation Neutron Source facility to be built at ORNL
is designed to accumulate 2x10^14^ protons at 1.0 GeV and deliver them
to the experimental target in one bunch at 60 Hz. To achieve this goal
and protect the machine from excessive radiation activation, an
uncontrolled loss criteria of 1 part in 10^4^ (1 W/m) has been
specified. Measured losses will be conditioned to provide machine tuning
data, a beam abort trigger, and logging of loss history. The design of
the distributed loss monitor system utilizing argon-filled glass
ionization chambers and scintillator-photomultipliers will be presented.

# BACKGROUND

The SNS project^1^ is a collaboration of 6 national laboratories to
build an accelerator based Spallation Neutron Source at Oak Ridge
National Laboratory (ORNL). The H-minus ion source, RFQ, LEBT and MEBT
will be provided by Lawrence Berkeley Laboratory (LBL), the Linac will
be designed and built by Los Alamos National Laboratory (LANL) and
Thomas Jefferson National Laboratory (TJNL) and Brookhaven National
Laboratory will design and build the HEBT, accumulator ring and
Ring-to-Target Beam Transport (RTBT) line. The Spallation Target and
conventional facilities will be the responsibility of ORNL and Argonne
National Laboratory. With such a segmented work force it was clear that
a consolidated group should handle all controls. It was also realized
that beam instrumentation should be similarly organized so that the same
detectors and interface hardware should be used, where ever possible, to
measure the same parameters throughout the facility. This should
eliminate questions of differences in measurements due to different
instrumentation design. This consolidation has been done for beam loss,
beam position and beam current monitoring, with BNL having primary
responsibility for the Loss Monitor System.

The SNS (Fig. 1) will consist of an H-minus ion source injecting a beam,
chopped to pre-form the Ring bunch, into a 2.5 MeV RFQ. This beam will
be accelerated to 87 MeV in a Drift Tube Linac (DTL), then to 186 MeV in
a Cavity Coupled Linac (CCL). A Superconducting RF Linac (SRF) raises
the beam energy to 1 GeV, after which it is transported to the Ring via
the HEBT line.

![](media/image1.png){width="4.843055555555556in"
height="1.5402777777777779in"}

**Figure 1.** Drawing of SNS site layout

The Ring accumulates the beam in a single bunch over a 1 msec
macro-pulse duration. It is then ejected into the RTBT and transported
to the liquid mercury neutron target. The beam from the source through
the Linac and Ring will have the same temporal character (1 msec
macro-pulse width, chopped to 695 nsec at the Ring revolution frequency
of 1.05 MHz). Beam current will vary from a peak of 70 mA at the source,
to 56 mA through the Linac, while in the Ring it will increase from the
56 mA first turn to more than 56 A at the end of the accumulation
period, for a total of 2 x 10^14^ protons in the single bunch.

The design average current of 2 mA will produce 2 MW at the target at a
60 Hz rate. This very high average beam power makes it crucial that
losses be kept extremely low to allow normal accelerator maintenance
without high background radiation. This will be done by:

-   Carefully designing the accelerator to prevent halo formation and
    having apertures sufficiently large to reduce scraping.

-   Providing beam dumps in areas where known losses will occur, such
    as,

-   HEBT where stripped H-minus beam from the Linac must be removed

-   Ring injection where H-minus beam which misses the stripping foil
    and beam which is only partially stripped can be safely removed.

-   Installing collimators to clean the beam tails, designed to fully
    contain the resulting radiation.

-   Installing a Beam-in-Gap cleaner in the Ring. This fast rising
    resonant kicker deflects any beam in the gap into the collimators
    over approximately a 20 turn period. This will remove a possible
    source of instability in the Ring and eliminate beam which might
    otherwise be lost on the extraction equipment.

-   Installing a beam loss monitor system to inhibit the beam when high
    loss is observed.

Beam losses can be divided into "Controlled" and Un-Controlled" losses.
Losses in the dumps or collimators which produce no increase in
background are considered Controlled Losses and are acceptable as long
as they do not exceed the design loss rate. The Beam Loss Monitor (BLM)
system will be designed to minimize the Un-Controlled losses by
providing data for tuning the machine and by inhibiting the beam when
excessive losses occur. It will be a crucial tool in reaching the goal
of low losses.

# SYSTEM PHILOSOPHY AND DESIGN

The very high intensity of the SNS will make the BLM system the primary
diagnostic tool for tuning and preventing excessive activation and
equipment damage. As the primary diagnostic tool there must be
sufficient detector coverage in all sections of the SNS. For many years
the AGS accelerator complex has used extended ion chambers made from
hollow large diameter coaxial cables to assure complete spatial
coverage^2^. These \"line\" detectors also offer the advantage of 1/r
spatial dependence rather than the 1/r^2^ of point detectors. However,
their length prevents them from being placed close to the beam line,
lowering their sensitivity, and mechanical considerations preclude high
bias voltage, limiting their linear range. For these reasons smaller
volume sealed glass ion chambers were used in RHIC^3^ and will be used
in SNS. To obtain uniform coverage similar to that of the long coaxial
cable ion chambers, many more detectors are required. In SNS detectors
will be placed at essentially every quadrupole, since that is where the
beta-max will occur, and at other key points. In addition to these fixed
ion chamber locations, there will be other detectors which can be moved
to points requiring more detailed study to reduce losses.

Because the ion chambers will not have the time response to see losses
within the bunch, scintillator-photomultipliers will be installed at
Ring injection and extraction, and other strategic points.

The system must have enough sensitivity to observe low level losses over
many pulses at all significant locations, as well as high local losses
of short duration. This is further complicated by the 3 decade range of
intensity as the Ring accumulates beam. A high level loss can cause
physical damage if allowed to continue for several pulses. Quenching of
the super-conducting cavities in the Linac can also occur from a rapid
local loss. To prevent excessive activation, the Linac, transfer lines
and Ring are all limited to a 1 W/m loss over a 1 second duration. This
is equivalent to a loss of 10^-4^ of the full intensity per pulse summed
over the entire Ring.

These considerations lead to multiple requirements:

-   In all regions there must be a real time limit imposed on local
    losses. This will be implemented in hardware using a programmable
    threshold detector and will shut off the beam at the Front End for
    the remainder of the pulse within 5-10 sec.

-   For each detector a 1-second moving sum will be computed and
    compared against the 1 W/m limit. This will be done in higher level
    code and used to warn the operators.

-   A similar computation will be required for the 10^-4^ criterion in
    the Ring but in this case the sum must include all detectors in the
    array.

The amount of data generated by the BLM system with several hundred
detectors and a pulse rate of 60 Hz is far more than an operator can
accept. It is expected that comfort displays of a subset of the data
will be always presented while other more detailed data will be
available on request. For example, a \"waterfall\" style presentation^4^
of the 1-second sum losses in each section of SNS would at a glance give
the loss levels as color variations with the x-axis the detector
location and the y-axis the history of previous 1-second sums. Another
comfort display might be a pseudo-time chart of the 1-second sum losses
for the various Linac sections, HEBT, Ring and RTBT. Displays on request
might include the output of a single detector at a function of time in
the macro-pulse or within the mini-pulse. Another on-request display
might be a waterfall display of all the detectors in a given section but
with the y-axis now on a pulse-by-pulse basis. This could be useful in
looking for line-frequency dependent loss mechanisms.

# EXPECTED LOSSES

The beam loss design criterion has been set at 1 W/m to allow hands-on
accelerator maintenance. This has come about from operating experience
at LAMPF and the PSR, and design studies for the Accelerator Production
of Tritium (APT) project at LANL^5^. Applying an average loss criterion
to a specific local geometry raises questions. Some losses, e.g.,
background gas stripping of the H-minus beam, do occur uniformly.
Shafer^6^ has estimated the gas scattering losses at 0.13 W/m for 10^-7^
Torr vacuum. In the Ring, where the vacuum is much better and the
H-minus beam has been converted to protons, this is not a factor. Most
other losses will take place at specific locations. For example:
off-energy beam will be lost at high dispersion points, while transverse
losses will take place at collimators or quadrupoles where the beta-max
occurs. Simulations and activation measurements at LANL^7^ confirm that
losses are most significant downstream of quadrupoles. In the Ring, if
the same 1 W/m loss is distributed among the number of quads in the Ring
rather than uniformly, the loss would be 4.43 W/quad. This is a
significant difference for the BLM system design.

The SNS CDR^8^ correlates the 1 W/m loss with a 115 Rem/hr dose rate at
28 cm from the beam pipe center. If the detector is mounted on the beam
pipe (13 cm from the pipe center) then the dose rate is 533 Rem/hr which
is equal to 0.15 Rad/sec or 2.5 mRad/pulse. However, this is based on
the 1 W/m average loss. If we mount the detectors downstream of the
quadrupoles then we can apply the geometry factor of 4.43 to get 11
mRad/pulse, or, 11 Rad/sec during the 1 msec pulse. While this provides
an estimate of the losses for the Linac, HEBT, and RTBT, the Ring losses
have a different character since the beam current is continually growing
during accumulation. The 1 W/m loss tolerance corresponds to a loss of
10^-4^ of the total beam injected. But, in the Ring the same particles
may make many turns and the loss rate must be much less. To achieve a
total loss of 1 W/m the loss mid-way during the pulse must be the same
as the average loss in the Linac and HEBT, but the first turn loss must
be about 500 times lower, assuming the loss is linear with intensity. It
is also clear that both instantaneous and integrated losses must be
tested to provide proper beam inhibit protection.

# DETECTORS

## Ion Chambers

The SNS will use about 300 (Linac 94, HEBT 45, Ring 96, RTBT 57) sealed,
argon-filled glass ion chambers as the primary detectors for monitoring
beam losses. Among the alternative detectors considered were long coax
ion chambers, air filled ion chambers and solid state detectors. The
long coax cable ion chamber used at AGS, and ISIS^9^. Although they do
not suffer from the 1/r^2^ response dependency due to their length, they
do have reduced dynamic range due to limited bias voltage potential they
can achieve. Argon filled chambers are used in preference to the simpler
air filled option^10^ to take advantage of their faster (electron
collection) time response, and better saturation characteristics. Solid
state LM's are not used because they utilize PIN photodiodes which have
radiation damage thresholds at about 10^8^ rads. The maximum count rate
of these detectors may limit the response during the 1 msec cycle.

The SNS detector consists of a sealed glass ion chamber housed in an
aluminum cylinder with signal connectors on one end and HV connectors on
the other. The basic ion chamber^11^ was designed by R. Shafer at FNAL
in 1982 and subsequently packaged by various FNAL engineers to the
configuration used at the Tevatron. This design was further developed by
BNL for use in the RHIC. These modifications include improving radiation
hardness by replacing the PTFE insulation in the BNC (signal) and SHV
(HV bias) connectors with Rexolite, breaking a ground loop by using
isolated versions of these connectors, and improvements to the
mechanical supports of the glass chamber within its aluminum case.

Ion Chamber Parameters:

-   Net internal volume is 113 cc

-   Filled to 725 Torr with pure Argon gas

-   4" long concentric cylinder electrodes made of nickel

    -   Inner signal electrode diameter 0.25"

    -   Outer HV electrode diameter 1.5"

-   Calculated response is 70 nano-Coulombs per Rad or 19.1 pA/Rad/hr

-   Measured response is 19.6 pA/Rad/hr with 95% within 1.5 pA/Rad/hr

### Detector Assembly

The ion chamber bottle is manufactured by Troy-Onic^12^ (\~ \$120 each,
1999). It is then acceptance tested at BNL and shipped to a local
contractor for the remaining mechanical/electrical assembly (\~\$200-300
each). An RC network built into each ion chamber housing provides
decoupling in the case of a short, noise filtering, and storage
capability to handle large fast losses. The guard ring around the glass
chamber, covering the signal port end of the outer bias cylinder,
reduces leakage currents from the HV to the signal lead as low as 0.1
pA. The double connector configuration on each end allows daisy chain
capability of signal and HV.

![](media/image2.png){width="4.438888888888889in"
height="2.1993055555555556in"}

**FIGURE 2**. Ion Chamber Assembly

The detectors may be mounted directly (electrically isolated) to the
beam pipe, magnet, or moved away from the radiation source to reduce
signal output or integrate losses over a larger area.

![](media/image3.jpeg){width="2.4618055555555554in"
height="1.6444444444444444in"}

**FIGURE 3**. Typical BLM installation in RHIC transfer line

Ion Chamber signals will be transmitted on a special low-tribo-electric
RG59 equivalent cable (similar to Belden 9054). The HV bias will be
carried on red RG-59, which is daisy chained from detector to detector.
Two HV cables will be used in each group of approximately 32 detectors,
picking up alternate BLM's to provide degraded but continued coverage in
the event of a high voltage short.

## Scintillator-Photomultiplier Detectors

Scintillator-photomultipliers (SPMs) offer a wide dynamic range and
nano-second response to resolve losses within a single turn. They will
be located in critical areas around the ring. They will be installed
around injection, extraction, collimators, downstream end of each
straight section, and in the transfer lines. Details of the fast losses
might provide insight about clipping of the bunch ends or losses due to
beam in the gap and losses due to instabilities if they occur. They will
not be used as the detectors in the global loss monitoring system for a
number of reasons:

-   They show significant unit-to-unit variation, thus requiring
    individual calibration over the full dynamic range, which may vary
    over time.

-   The PMTs and the scintillators are subject to radiation damage.

-   The cost per channel is many times greater than for an ion chamber
    based system.

Though the solid scintillators are more efficient, liquid cell
scintillator has the advantage of increased radiation hardness (1
Mega-rad dose to reduce light transmission to 1/e in solids, 5-10
Mega-rad for liquids), this is due to the lack of polymer chain bonds to
break in the liquids. This type of detector, also know as a "paint can",
has been used at several laboratories including Fermilab^13^. However,
in previous home built detectors the leakage of the liquid scintillator
has raised safety concerns. The use of these detectors at Fermilab was
discontinued due to the difficultly and cost of mixed waste disposal.

Typical detector response is on the order of 100 micro-Coulombs per Rad.
Each SPM signal will be buffered and acquired through a standard 60
Msa/s, 14-bit ADC. In addition, all units in a given location will be
selectable through a wideband mux to be read into a fast digitizer of 1
Gsa/s to observe details of losses within the bunch duration.

![](media/image4.jpeg){width="1.9895833333333333in"
height="2.234722222222222in"}

![](media/image5.png){width="1.9625in" height="2.3in"}

**FIGURE 3.** Drawing and photograph of Bicron^TM^ liquid cell
scintillator-photomultiplier assembly.

Bicron ™ offers a commercially available liquid cell scintillator/PMT
assembly with a "Bicrocell" container made of glass or aluminum^14^.
These cells have expansion reservoirs containing oxygen free nitrogen to
maintain stability and light output. The liquid cell is enclosed in an
aluminum cell housing for mechanical strength, and is sealed with a
gasket and to prevent leaks. A liquid scintillator under consideration
is BC-505 (chemical name 1,2,4 -Trimethylbenzene), it offers the highest
light output of any liquid (80% anthracene), excellent light
transmission (2.5 meters minimum 1/e attenuation) and a high flash point
(48^o^ C, 118^o^F).

# ELECTRONICS

Circuitry for the ion chamber BLM system has not yet been designed but
will take advantage of the fast electron signal to provide rapid
interrupt in the case of high beam losses. A potential problem will
arise from the difference in collection times for the electrons and ions
since ions due to early losses will contribute to the signal in the
latter portion of the 1 milli-second pulse. If the signal is sensed as a
current then the electron signal will be much larger than the ion signal
but if integrated over a sufficiently long time, the two should result
in equal charge collection. In the Ring and RTBT at least one gain
change will be required.

While most of the other beam instrumentation in SNS are expected to
interface to a standard 60 Msa/s, 14-bit digitizer, this would seem to
be overkill for such low bandwidth signals. Since there will be a large
number of BLM channels in each of the electronics locations it might be
more reasonable to use an digitization technique more suited to
multi-channel acquisition.

Programmable threshold detectors will be included to provide rapid
sensing of excessive beam loss and allow turn off of the beam within
5-10 sec.

The SPMs will be buffered and viewed by the standard 60 Msa/s digitizers
for general acquisition, allowing 50-60 samples during the micro-bunch.
They will also go to a wideband mux for acquisition on a 1 GSa/s scope
for more detailed viewing.

# ACKNOWLEDGMENTS

The authors would like to R. Shafer for providing valuable insight and
discussions regarding the topics in this work.

**REFERENCES**

. Alonso, J., \"The Spallation Neutron Source Project\", Proceedings of
the 1999 PAC, NY, 1999, p574. For current status see
[http://www.sns.gov](http://www.sns.gov/)

^2^. Witkover. R. L. ,\"Microprocessor Based Beam Loss Monitor System
for the AGS\", IEEE Trans. Nucl. Sci **26** 3313 (1979)

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\. Bai, M., et al., "RHIC Beam Loss Monitor System Commissioning in the
Year 2000 Run",To be published these proceedings.

5\. Hardekopf, R. A., \"Beam Loss and Activation at LANSCE and SNS\",
LA-UR-99-6825, Los Alamos National Laboratory, Los Alamos, NM. Sept.
1999

6\. Shafer, R. A., \"Beam Loss From H^-^ Stripping in the Residual
Gas\", SNS Technical Note LA-UR-99-2497, May 1999

7\. Loc. Cit. Reference 5

8\. NSNS Conceptual Design Review, June 1997, Chapter 3, p97

9\. M. A. Clarke-Gayther, et al. "Global Beam loss Monitoring Using Long
Ionisation Chambers at ISIS" , 4^th^ European Particle Accelerator
Conference, London, England, 1994, p1634

10\. Plum, M., and Brown, D., \"Response of Air-Filled Ion Chambers to
High Intensity Radiation Pulses\", Proceeding of the 1993 PAC, p2181

11\. Shafer, R. E., et al., "The Tevatron Beam Position and Beam Loss
Monitoring Systems", Proc. The 12^th^ Int'l Conf. High Energy Accel.,
p609, 1983

12\. Troy-Ionics Inc., 88 Dell Avenue, P.O. Box 494, Kenvil, NJ., 07847

13\. M. Awschalom, H. Howe, R. Shafer, D. Theriot, "An Inexpensive Beam
Loss Monitor for Use at NAL" NAL-TM-274 (10/30/70)

14\. Bicron, Saint-Gobain Industrial Ceramics, Inc "Organic
Scintillators, Related Materials and Detectors", 12345 Kinsman Road,
Newbury, OH. ([http://www.bicron.com](http://www.bicron.com/))

[^1]: ^^ Work performed under the auspices of the U. S. Dept. of Energy