20 12 IEEE Nuclear Science Symposiwn and Medical Imaging Conference Record (NSS/MIC)
N1- 1 10
Liquid Scintillator-Based Neutron Detector Development Anthony Lavietes, Senior Member, IEEE, Romano Plenteda, Nicholas Mascarenhas, Member, IEEE, L. Marie Cronholm, Michael Aspinall, Malcolm Joyce, Alice Tomanin, Paolo Peerani Abstract-The IAEA, in collaboration with the Joint Research Center (Ispra, IT) and Hybrid Instruments
(UK),
is developing a
liquid scintillator-based neutron coincidence counting system to address a number of safeguards applications. Interest in this technology is increasing with the advent of high-flashpoint, non hazardous scintillating fluids coupled with significant advances in signal processing electronics. Together, these developments have provided the enabling technologies to allow liquid scintillators to be implemented outside of a laboratory environment. Another important aspect of this detector technology is that it can be used with the current installed infrastructure of safeguards assay instruments and
data acquisition electronics. It is also an 3 excellent candidate for the replacement of He-based systems in 3 many applications. As such, a comparison to an existing He_ based system will be presented to contrast the differences and benefits for several applications. This paper will describe the
experiments
and
associated
modeling
activities
engaged
to
carefully characterize the detection system and refine the models. The latest version of MCNPX-PoliMi Monte Carlo modeling code was
used
to
address
the
specific
requirements
of
liquid
scintillators. Additionally, this development activity has driven the collaborative development with Hybrid Instruments of a high-performance pulse shape discriminator (PSD) unit. Specific applications will be described with particular emphasis on those in which liquid scintillators provide immediate benefit over traditional detection methods.
INTRODUCTION
eutron detection is a fundamental tool used by safeguards
Nto monitor the movement and storage of nuclear material. Passive and active systems have been developed through the years to address numerous applications across the nuclear power fuel cycle. The primary detector technology that is implemented and that constitutes the majority of neutron detectors in use relies on a specific helium gas isotope, heliwn-3 e He). 3He gas has many attractive characteristics that make it highly suitable for use in practically any
safeguards application (e.g., non-hazardous gas, excellent low energy neutron sensitivity, simple detector design, and 3 extremely long lifetime). He has been used for decades as the
default neutron detector technology for safeguards and as such, the IAEA has an extensive inventory of detection systems that span the application space from small handheld devices to large, fixed facility analysis instruments. Fig 1 shows a few examples of 3He neutron detectors used to measure plutonium (pu) characteristics. While the performance and physical aspects of 3He are exceedingly compatible for most safeguards neutron detection applications, the availability and cost of the gas has become a significant obstacle. 3He gas is a by-product of tritium decay, tritium being a hydrogen isotope that is manufactured primarily for use in the production of nuclear weapons. As a combined result of the effects of the end of the Cold War and the cessation of nuclear weapons testing and development, the global production of tritium has essentially ended, resulting in the end of an adequate supply of 3He. What supplies remain have become exceedingly difficult to obtain and exceedingly expensive. This combination of factors has motivated a world wide search for a replacement for 3He -based detector technologies. Noting these developments in their initial stages, an activity began to evaluate any potential candidate technology in the search for an alternative detector material that would also be compatible with the existing safeguards infrastructure. Of the many possibilities, the focus of this activity quickly narrowed to liquid scintillators. Liquid scintillators have been used for decades in high-energy physics research for particle and photon detection, but were typically relegated to the laboratory or large experimental facilities due to the hazardous nature of the scintillating fluids (e.g., corrosive, flammable) and the requirement for extensive support electronics and signal
Manuscript received November 1 9, 20 12. Anthony D. Lavietes is with the International Atomic Energy Agency, Vienna, Austria (telephone: +43 (I) 26002 5 1 32, e-mail:
[email protected]). Romano Plenteda is with the International Atomic Energy Agency, Vienna, Austria. Nicholas Mascahrenas is with the International Atomic Energy Agency, Vienna, Austria. L. Marie Cronholm is with the International Atomic Energy Agency, Vienna, Austria. Michael Aspinal is with Hybrid Instruments, Inc. , Lancaster, United Kingdom. Malcolm Joyce is with Department of Engineering, Lancaster University, Lancaster, United Kingdom. Alice Tomanin is with the Joint Research Center, Ispra, Italy. Paolo Peerani is with the Joint Research Center, Ispra, Italy.
978- 1-4673-2030-6/ 12/$31.00 ©20 12 IEEE
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�
Fig I. Example 3 He-based detector systems - Left: High Level Neutron Coincidence Counter (HLNCC), Center: Active Well Neutron Coincidence Counter (JCC-5 1 ), Right: Pu Scrap Multiplicity Counter (PSMC) - all manufactured by Canberra Industries, Inc.
processing equipment. What instigated renewed interest in liquid scintillators were the recent developments of non hazardous, non-flammable scintillating fluids and the realization of portable, high-performance signal processing instrumentation. The combination of these enabling developments has provided the necessary capabilities for high speed, high-throughput, real-time neutron detection systems based on liquid scintillators for safeguards applications. Fig 2 shows a few examples of liquid scintillator neutron detectors used in our evaluations. The characteristics of liquid scintillators differ from 3He technology in that scintillators are sensitive to fast (high energy) neutrons, rather than thermal (low energy) neutrons, therefore neutron moderation is not required. Additionally, the signal processing is three orders of magnitude faster than the 3He counterpart, providing significant benefits with respect to reduced accidental neutron coincidence, increased sample analysis speed, and the potential for measuring triple coincidence with low uncertainty. In addition, the neutron energy detection threshold can be adjusted, providing substantial benefits in active interrogation applications by setting the threshold above the neutron energy of the interrogation source. This capability dramatically increases the measurement signal-to-noise ratio, the resulting measurement accuracy, and reduces the sample measurement time to fraction of that required for a typical 3He-based system.
Fig 2. Square and cylindrical liquid scintillator neutron detectors.
discriminator (PSD) units, and a National Instruments Industrial Controller. The controller includes an FPGA-based data acquisition card and data acquisition and analysis software developed with LabVIEW. The PSD units were developed through a collaboration with Hybrid Instruments, Ltd. (UK). This development represents one of two primary enabling technologies that has allowed for the consideration of this technology for safeguards applications, the other being the high flash-point, non-hazardous liquid scintillator fluid. The accompanying LabVIEW data acquisition system and related software analysis implementation was designed and developed within the PET.
Liquid Scintillator Detectors
EXPERIMENTAL SYSTEM
The current prototype system (Fig 3) is comprised of two cylindrical liquid scintillators, two single-channel pulse shape
As the primary use of these systems in safeguards applications is for neutron coincidence counting (NCC), the
I
r I). ,
12.7cm X 7.62cm
Hybrid Instruments
National Instruments
EJ-309 Fluid
Mixed Field Analyzer
3110 Embedded Controller
Fast PMT (2.5ns RT)
-8 MPPS Throughput
PCI-7831 R RIO Card
Real-Time n-r PSD
Xilinx Virtex-II 1 M Gate FPGA
Fig 3 . Prototype liquid scintillator neutron coincidence counter development system, including two cylindrical liquid scintillators, two single-channel PSD units, and an industrial controller with an FPGA-based, high-speed data acquisition card.
23 1
following discussion will be relegated to that application, specifically doubles coincidence (two coincident neutrons in a given time window). There are a number of operational and perfonnance characteristics that need to be considered when using liquid scintillators for neutron detection.
Intrinsic Safety Safeguards systems must not only be unobtrusive and highly reliable, but they must not introduce additional hazards. The typical high-performance scintillator fluids unfortunately feature a low flash-point (�26C) and have therefore been deemed unacceptable for use in a nuclear facility. In addition to the low flash-point, these scintillators were also quite
corrosive, generating concern in the event of a leak or rupture of the scintillator cell. These two issues have precluded the use of liquid scintillators from consideration in practically all safeguards applications. In addition and until recently, 3He was relatively cheap and abundant, so there was no motivation to further develop an alternative technology. The recent dramatic change in 3He price and availability has generated a world-wide effort to develop alternative technologies, and liquid scintillators are a leading candidate for many applications. The recent development of high flash (� 144C) point liquid scintillators renewed consideration for the use of this technology in safeguards applications. The follow-on analysis revealing the non-hazardous nature of the fluid further reduced the objections to its use. Liquid scintillators fabricated with low flash-point, non-hazardous fluid (e.g., Eljen EJ-309, Saint Gobain BC-599- 17) have no transportation restrictions and, in the event of a spill, can be disposed in the standard waste stream.
Liquid Scintillator Cell Geometry A number of geometries have been evaluated in this activity and the decisions as to the preferred geometry have been based primarily on empirical results, as an optical modeling
effort has not been executed. Fig. 4 shows drawings of the set of scintillator detectors currently being used in the test campaign. The performance of the scintillators is a combination of the cell geometry and respective optical characteristics, as well as the photomultiplier tube (PMT) performance. Liquid scintillators of interest for safeguards applications tend to be sensitive to both neutrons and gamma radiation, so the ability to distinguish between these two events is critical. Since the light pulse generated from neutron and gamma interactions differ, certain aspects of the pulse shapes can be analyzed (e.g., rise time, fall time, pulse width) to separate neutron events from gamma events. A key contributing factor to accurate, efficient PSD is cell geometry. Large aspect ratios (i.e., excessive length in one dimension) or relatively large volumes have deleterious effects on scintillation light pulse shapes that subsequently result in poor PSD perfonnance.
Multiple Events in a Single Detector Cell With respect to the traditional 3He neutron detector, liquid scintillator detectors are sensitive to fast neutrons and do not require moderation. Neutrons are detected in real-time and as such, must be detected within the event time (approximately 10 to 20ns for fission events). This results in two significant operational characteristics - neutron temporal information is retained and, due to the PMT dead time (�25ns, PMT dependent), two neutrons from the same event that interact within the same detector cannot nonnally be distinguished. Of course, if the difference between two neutron energies is quite large, resulting in a similarly large time-of-flight difference, and the source-detector distance is also large (>�lm), the two events may be separately detected. In most detector implementations, this is not generally the case. One partial solution to address this issue is to pixelate the detection system such that the probability of multiple neutrons from a single event striking the same detector is minimized. This is consistent with the need to keep the detector volume
Fig 4. Current liquid scintillator neutron detector geometries (not to scale). Tn addition to cylindrical and wedge-shaped cells, three square cell geometries have been tested: 1 5 cm x 1 5 cm x 300cm, 1 5 cm x 1 5 cm x 1 5 cm, and IOcm x 1 0cm x 1 0cm (illustrations courtesy of Scion ix, Holland).
232
reasonably small to retain good optical qualities and maintaining the pulse shape. The result is essentially to reduce the solid angle of detection for any individual detector cell to the smallest acceptable value, a critical requirement with respect to cell geometry, as described above.
Dynamic System Efficiency While pixelating the liquid scintillator detection system minimizes the single cell/multiple event probability, it also addresses another critical issue not experienced thermal neutron detection systems: variable system efficiency. For example, an array of n scintillators has an efficiency equivalent to the sum of the individual detector efficiencies:
("accidentals") due to random cosmic and background neutron
nE
detection events. The topic of accidentals is addressed in a later section of this paper.
Where: E = n=
single efficiency of a detector cell. number of detector cells.
PULSE SHAPE DISCRIMINATOR (PSD) DEVELOPMENT
When the first neutron from a fission event is detected in one of the scintillators, that specific detector is essentially insensitive to further events for the PSD signal processing dead-time (in the case of this work, approximately 125ns). The result is an efficiency reduction of the original system efficiency for any further detections from the same fission event. Since an array of scintillators has a singles efficiency proportional to nE
Where: n= E =
number of cells single efficiency of a detector cell
It follows that proportional to:
the
doubles
coincidence
efficiency
is
n(n - 1)£2 As the number of cells is increased, the percent efficiency loss from the first event decreases, approaching the typical doubles efficiency of n2£2. Fig 5 shows a plot that estimates the doubles efficiency effects as a function of array size. Note that the y axis references generically "electronic dead-time," as the signal processing dead-time is relative to the PSD
� E
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60%�-
Electronic Dead�Time Effect on Efficiency
50%
�co�nCidentNeut� L Efficiency loss J
� 40% u
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30%
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� � .� 10%
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processing speed. While the PMT exhibits a reasonably short dead-time, the signal processing dead-time is significantly larger and is a function of PSD data throughput. For example, if the PSD unit can support 3 x 106 pulses per second (3 MPPS), the processing dead-time is approximately 333ns. For a PSD throughput of 8 MPPS (the current throughput for the PSD system described in this work), this value is reduced to approximately 125ns. While these throughput values far exceed the anticipated single channel event rate due to solid angle and detection efficiency, the effects need to be considered when calculating accidental coincidence
10
12
14
Number of Cells
16
18
ID
22
24
The development of a portable, real-time PSD capability is essential for the successful implementation of liquid scintillator neutron detector technology in the field of advanced safeguards. Previous technologies based on this detector modality were confmed to the laboratory due to the extensive electronics required to capture, integrate, shape, and process the very fast pulses that arise from the PMTs on liquid scintillators. These approaches were exclusively analogue prior to the advent of sufficiently fast ADCs, and the set-up overhead and temperamental nature of the analogue systems, and not least their incompatibility with autonomous computer control, excluded them from safeguards use. However, even the availability of digital waveform analyzers relatively recently did not resolve the challenges with this application and liquid scintillators, because real-time PSD is necessary in order to preserve the coincidence signature of the events, in a self-contained 'plug-and-play' unit. Other techniques in the digital domain had usually relied upon post-processing of the data in order to recover the discriminated neutron and gamma events. This is not tenable in an application where autonomous, rapid and accurate assay is required.
Functional overview The design intent of the Mixed Field Analyzer [ 1] [2] used in this research is to provide a self-contained unit that fulfills all of the power and processing requirements of liquid scintillation detector use. This is shown in Fig. 6. As such, the unit comprises an integrated, stabilized HT power supply unit and dedicated digitizer. The latter is integrated into an associated firmware control system that enables input signals from the detector to be processed in real-time, discriminated on the basis pulse shape and converted into TTL output signals (one for neutron events and one for gamma events). The latter are available via BNC connectors on the back of the instrument and the instrument is available in single- and four channel variants [3]. The instruments can process 8x106 events per second (in single-channel form) and maintains synchronicity with the arrival time of the event to within 6 ns jitter.
Fig 5 . Neutron coincidence efficiency loss versus liquid scintillator array size.
233
immediacy of the data acquISItIOn would be delayed. The systems used in this work make it possible in most cases for the complete assay to be performed in less time than it would take to post-process a single assessment. In a commercial environment where it is rarely possible or desirable to halt fuel-related operation but nonetheless the efficacy of the safeguards assessment is of paramount importance, it is essential that real-time assay is available to provide instant indication of mUltiplicity event rates. LIQUID SCINTILLATOR NEUTRON DETECTION SYSTEM CHARACTERIZATION
Neutron output (BNC)
The performance of a liquid scintillator is first dependent upon the PMT selection to optimize the required gain and timing characteristics. NCC applications require a balance of high gain, low noise, fast rise time, and stable timing
Gamma output (BNC)
Fig 6. Single-channel mixed field analyzer of the type used in this work.
Peak Detect
PSD methodology The analysis methodology is based on a digital variant of pulse-shape discrimination that we pioneered to enable ultra fast processing of scintillation events [4]. This approach exploits the difference in amplitude of samples corresponding to the peak amplitude and a sample in the decay face of the pulse, without the need for integration under the pulse as was often the case in earlier works. This approach offers a superior quality factor in terms of the discrimination that is achieved between neutrons and gamma rays, and is highly compatible with parallelizable firmware systems as used in this work.
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