PNNL-14490 Progress Report for the Advanced Large-Area Plastic Scintillator (ALPS) Project: FY 2003 Final P. L. Reeder R. C. Craig D.L. Stephens, Jr. B. Geelhood D.V. Jordan November 2003 Prepared for the U.S. Department of Energy under Contract DE-AC06-76RL01830 DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor Battelle Memorial Institute, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or Battelle Memorial Institute. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. PACIFIC NORTHWEST NATIONAL LABORATORY operated by BATTELLE for the UNITED STATES DEPARTMENT OF ENERGY under Contract DE-ACO6-76RLO183O This document was printed on recycled paper. (8/00) PNNL-14490 Progress Report for the Advanced Large-Area Plastic Scintillator (ALPS) Project: FY 2003 Final November 2003 P. L. Reeder D. L. Stephens, Jr. D. V. Jordan R. C. Craig B. Geelhood Prepared for the U.S. Department of Energy under Contract DE-AC06-76RL01830 Pacific Northwest National Laboratory Richland, Washington 99352 Summary Pacific Northwest National Laboratory is investigating possible technological avenues for substantially advancing the state-of-the-art in gamma detection via large-area plastic scintillators. This report describes progress on this project as of the conclusion of FY 2003. The primary focus of the report is on experimental tests conducted with a single large-area plastic scintillator outfitted with a variety of photomultiplier tube configurations. Measurements performed to date include scintillator response under broad-area exposure to various point-like gamma sources, and light-output uniformity mappings obtained by varying the position of a collimated beta-source over the surface of the scintillator. Development of a Monte Carlo program for modeling the response of a large-area scintillator sensor to ionizing radiation, explicitly including resolution-broadening effects of scintillation light generation, propagation, and collection is also described. iii Acknowledgements The authors would like to thank W.K. Pitts and G. Dudder for many helpful discussions over the course of this project. Support for this project by the National Nuclear Security Administration’s Office of Nonproliferation Research and Engineering, NA-22, is gratefully acknowledged. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC06- 76RLO 1830. iv Acronyms ADC analog-to-digital converter ALPS Advanced Large-Area Plastic Scintillators CAMAC Computer Automated Measurement and Control CERN Center for European Nuclear Research CPU central processing unit FWHM full width at half maximum NIM Nuclear Instrument Module PE photoelectron PMT photomultiplier tube TDC Time to Digital (converter) QDC Charge to Digital (converter) QE quantum efficiency ROI region of interest SCSI small computer systems interface v Contents Summary......................................................................................................................................................iii Acknowledgements......................................................................................................................................iv Acronyms......................................................................................................................................................v 1.0 Introduction.......................................................................................................................................1.1 2.0 Experimental: Apparatus..................................................................................................................2.1 2.1 Equipment................................................................................................................................2.1 2.2 Data-Acquisition System.........................................................................................................2.3 2.3 Characteristics of Gamma Sources..........................................................................................2.3 3.0 Results for Single PMT.....................................................................................................................3.1 3.1 Gamma Sources.......................................................................................................................3.1 3.2 Beta Source..............................................................................................................................3.2 4.0 Results for Two PMTs—Opposite Ends...........................................................................................4.1 4.1 Gamma Sources.......................................................................................................................4.1 4.2 Beta Source..............................................................................................................................4.2 5.0 Results for Two PMTs – Same Ends................................................................................................5.1 5.1 Gamma Sources.......................................................................................................................5.1 5.2 Beta Source..............................................................................................................................5.2 6.0 Time Difference Analysis.................................................................................................................6.1 6.1 Calibration of Location Versus Time Difference....................................................................6.1 6.2 Pulse-Height Corrections Based on Location..........................................................................6.2 7.0 Simulation.........................................................................................................................................7.1 8.0 Conclusions and Outlook..................................................................................................................8.1 9.0 References.........................................................................................................................................9.1 Appendix A: Details of the ALPS Simulation..............................................................................................1 vii Appendix B: Measurements on 30-cm × 30-cm Beveled Corner Scintillator.............................................1 Figures 2.1. Design Drawing of the ALPS Project Apparatus Showing Two Scintillator Slabs with Three PMTs on each End of both Slabs............................................................................................2.1 2.2. Photographs of the ALPS Scintillator Sensor as Configured for Laboratory Tests at the Conclusion of FY 2003.....................................................................................................................2.2 3.1. Gamma Pulse Height Spectra for Single PMT Configuration..........................................................3.1 3.2. Beta Pulse Height Spectra from a Single PMT at Three Locations Along Mid-Point Axis.............3.3 3.3. Mean Channel Number of Pulse Height Distributions for Beta Source at Various Distances from the Single PMT........................................................................................................................3.4 4.1. Gamma Pulse Height Spectra for Sum of Pulses from each PMT for Configuration with a PMT on Opposite Ends.....................................................................................................................4.1 4.2. Mean Channel Number of Pulse Height Distributions for Beta Source at Various Distances Along the Mid-Point Axis Between Coincident PMTs at Opposite Ends of the Scintillator...........4.2 4.3. Mean Channel Number of Pulse Height Distributions for Beta Source at Various Distances Along the Axis Parallel to the Mid-Point Axis But 17.8 cm away from it.......................................4.3 5.1. Gamma Pulse Height Spectra for Configuration with two PMTs on same End...............................5.1 5.2. Mean Channel Number of Pulse Height Distributions Along Midpoint Axis for Configuration with two PMTs on the same End of the Plastic Scintillator......................................5.2 5.3. Mean Channel Number of Pulse Height Distributions Along Axis Parallel to Midpoint Axis at -17.8 cm for Configuration with two PMTs on the same End of the Plastic Scintillator.............5.3 6.1. Time-Difference Histograms of Two Beta Spectra Collected at 2.54-cm Separation on Scintillator....................................................................................................................................6.2 6.2. Calibration Curve for Time Difference Versus Position for the Configuration with one PMT on each End of the Scintillator.................................................................................................6.3 7.1. Simulation Output for Various Gamma Sources in Single-PMT Configuration..............................7.3 7.2. Effect of Final Pulse-Smearing Event Processing Stage on Simulation Output, 54Mn Gamma Source, One-PMT Configuration......................................................................................................7.3 7.3. Comparison of Simulation to Experimental Data for 54Mn Source in Single-PMT Scintillator Configuration....................................................................................................................................7.4 7.4. Comparison of Simulation to 54Mn Source Data for Configuration with One PMT at each End.....7.5 viii Tables 2.1. Gamma Sources, Energies, Maximum Energy of Compton Scattered Electron, and Gamma Abundances for Sources Used in this Work.....................................................................................2.4 ix 1.0 Introduction The primary goal of the Advanced Large-Area Plastic Scintillator (ALPS) project is to investigate the limits of gamma-ray detection using large area plastic scintillators, with particular emphasis on the use of plastic for detection of radiological threats in vehicle portal monitors. The current generation of portal monitors deployed at international border crossings relies heavily on large-area plastic scintillators for detecting gamma rays emitted by potentially hazardous, clandestinely transported radioactive materials. A large detector area is important for portal-monitor applications to maximize gamma-detection count rate, and thus source-detection sensitivity, because spectrum acquisition times are severely limited by the need to accommodate a reasonable flow of vehicles past the portal. Plastic scintillator is relatively inexpensive per unit area and rugged in comparison to other scintillating materials, e.g., NaI(Tl), and semiconductors, e.g., high-purity germanium, and thus represents an attractive material for constructing a large-area gamma sensor. However, the limited energy-spectrum information available from a plastic scintillator presents challenges to reliably distinguishing radiological threats from naturally occurring and benign radioactive materials. The physical basis of particle detection with a scintillating material is the emission of light when ionizing radiation deposits energy in the material. Plastic scintillators typically emit about 10,000 light photons per MeV of deposited energy, about a factor of 4 fewer than in NaI(Tl). The intrinsic energy resolution of plastic, which scales as the square root of the number of information carriers, or light quanta in the case of scintillating material, is thus about a factor of 2 poorer than NaI(Tl). In addition to this inherent limitation on the number of information carriers generated, only a relatively small fraction of the scintillation light emitted in a typical large-area scintillator actually reaches the photomultiplier tubes (PMTs). In commercially available portal-monitor sensors with one or two 2-in.-diameter PMTs, the light collection efficiency can be as low as 3.5% (one tube) to 7% (two tubes). Thus, the net energy resolution of the scintillator can be as much as a factor of 5 poorer than the scintillator’s intrinsic energy resolution. Light generation and collection limitations are not the only factors contributing to the relatively poor resolution of a plastic scintillator. Gammas interact in plastic primarily by Compton scattering rather than by photoelectric absorption. Thus, the full energy-deposition peak, or photopeak, a familiar feature of spectra in higher-Z materials such as NaI(Tl) and HPGe, is essentially absent in plastic scintillators of thicknesses typical to commercial units. Although energy-deposition information is available from the shape of the Compton scattering continuum in plastic, this information does not necessarily conform to the conventional gamma spectroscopist’s expectation of well-defined, energy-localized peaks that represent a radioisotope’s “fingerprint.” These features of the spectroscopic response of plastic scintillator have conspired to render radionuclide identification in plastic-based, primary portal monitors a challenging problem. An important component of the ALPS project is a systematic study of the dependence of scintillator energy resolution and gamma-detection sensitivity on light collection efficiency. Although the optimum energy resolution of plastic is limited by the number of light quanta emitted per unit of deposited energy, poor light collection in the current generation of portal monitors essentially wastes at least an order of magnitude in light signal. To the extent that the light-collection efficiency can be improved, say by a multiplicative factor, F, the resolution of the energy-deposition spectrum can be expected to improve by a factor of F1/2. The configuration of PMTs on each plastic scintillator sheet of the ALPS sensor will permit light-collection efficiencies of approximately 40%, representing an order of magnitude improvement in light collection over single-tube sensors, and a factor of roughly 6 improvement over existing two-tube 1.1