Advanced Large Area Plastic Scintillator Project (ALPS ...

PNNL-17305

Advanced Large-Area Plastic Scintillator Project (ALPS): Final Report

DV Jordan PL Reeder LC Todd GA Warren KR McCormick

DL Stephens, Jr. BD Geelhood JM Alzheimer SL Crowell WA Sliger

July 2007

Prepared for the U.S. Department of Energy under Contract DE-AC05-76RL01830

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-ACO5-76RL01830

PNNL-17305

Advanced Large-Area Plastic Scintillator Project (ALPS): Final Report

Project P.I.: D.V. Jordan Report contributors: P.L. Reeder, D.V. Jordan, L.C. Todd, G.A. Warren Project team: P.L. Reeder, K.R. McCormick, D.L. Stephens, G.A. Warren, B.D. Geelhood, L.C. Todd, J.M. Alzheimer, S.L. Crowell, W.A. Sliger

July 2007

Prepared for the U.S. Department of Energy under Contract DE-AC05-76RL01830

Pacific Northwest National Laboratory Richland, Washington 99352

Executive Summary

The Advanced Large-area Plastic Scintillator (ALPS) project at Pacific Northwest National Laboratory investigated possible technological avenues for substantially advancing the state-of-the-art in gamma-ray detection via large-area plastic scintillators. The three predominant themes of these investigations comprised the following:

? Maximizing light collection efficiency from a single large-area sheet of plastic scintillator, and optimizing hardware event trigger definition to retain detection efficiency while exploiting the power of coincidence to suppress single-PMT "dark current" background;

? Utilizing anti-Compton vetoing and supplementary spectral information from a co-located secondary, or "Back" detector, to both (1) minimize Compton background in the low-energy portion of the "Front" scintillator's pulse-height spectrum, and (2) sharpen the statistical accuracy of the front detector's low-energy response prediction as implemented in suitable energywindowing algorithms; and

? Investigating alternative materials to enhance the intrinsic gamma-ray detection efficiency of plastic-based sensors.

Activities in early phases of the ALPS project [Jordan et al. 2003, Reeder et al. 2003] included (a) Monte Carlo modeling of light collection properties of various configurations of plastic scintillator and photomultiplier tubes (PMTs), and (b) design, fabrication, and testing of a large-area, unwrapped plastic scintillator sensor housed in a light-tight box intended for laboratory experimentation (the so-called "ALPS I"). The main goals of the ALPS I experimental campaign were to quantify the energy deposition resolution improvement afforded by the increase in PMT coverage of a scintillator sheet's edge area, and to understand the variation in pulse height response as a function of the primary ionizing radiation's interaction position within the plastic scintillator sheet. These laboratory experiments indicated roughly a 60% improvement in the Compton-edge energy resolution (for the 835 keV gamma from 54Mn) as the PMT coverage increased from a single PMT to six PMTs, with only a small improvement resulting from the increase from 4 to 6 PMTs (see Figure S1).

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Relative intensity

0.03

0.025 0.02

0.015 0.01

0.005

1 PMT double-ended 2 PMTs, same end 4 PMTs 6 PMTs AND 6 PMTs 3-fold

0 0 20 40 60 80 100 120 140 160 180 200 220 240 Pulse height (channels)

Figure S1. 54Mn gamma pulse-height spectra recorded with the ALPS I sensor. Six PMT configurations are compared: Single PMT; 2 PMTs at opposite ends ("double-ended"); 2 PMTs at same scintillator end; 4 PMTs (two at each end); 6 PMTs with trigger configured to require coincident firing of all 6 ("AND"); and 6 PMTs with readout electronics configured to take the hardware sum of 3 PMTs at each end ("3-fold").

The present report details the design, fabrication and testing of a field-deployable version of the ALPS, referred to herein as the "ALPS II". The ALPS II consists of dual slabs of Bicron/Saint Gobain BC-408 scintillators of dimensions 127 ? 57.15 ? 5.08 cm3 separated by a gap of 13 cm. The two slabs are referred to as the "Front" (F) and "Back" (B) detectors, respectively. In contrast to the ALPS I sensor, which was housed in a light-tight box, the PVT slabs of the ALPS II are mounted vertically in a lighttight, Pb-lined, steel-walled enclosure representative of a field-deployable radiation portal monitor (RPM) form factor. The sensor system and accompanying rack-mounted readout electronics are mounted on a pallet for convenient transport and outdoor deployment. As in the laboratory version of the sensor, each scintillator slab is outfitted with 3 Hamamatsu R1250 127-mm (5-in.) diameter PMTs mounted on each end, for a total of 12 PMTs. The scintillators are mounted vertically and shielded on the bottom, sides, and back by 5.08-cm of lead. A light-tight plastic panel on the front of the steel enclosure door permits entry of gammas to the scintillator detectors through material of low-Z.

A variety of gamma-ray point-source measurements were performed in order to quantify the detection sensitivity of the ALPS II sensor. Two types of instrument response were investigated: (1) the "singlesheet" response, in which the detection sensitivity of the Front detector alone was mapped as a function of PMT coverage; and (2) the "dual-sheet" response, which measured the impact of the co-located Back detector, employed as an anti-coincidence Compton veto and/or a supplementary source of spectral information on the high-energy portion of the incident gamma-ray flux. Table S1 summarizes the variation in single-sheet detection sensitivity as a function of PMT coverage. The minimum detectable activity (MDA) is computed using several algorithms:

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