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Characterization of Solution-Grown Stilbene for Nonproliferation Applications M.M. Bourne, A. Di Fulvio, S.D. Clarke
Department of Nuclear Engineering & Radiological Sciences,
University of Michigan, Ann Arbor, MI 48109, USA
S.A. Pozzi, pozzisa@umich.edu
Consortium for Verification Technology (CVT)
Motivation New neutron detectors are needed for nonproliferation applications such as NPT verification
Solution-grown stilbene developed by LLNL (Fig. 1)
– Can be grown to diameters of 5 cm or larger
– Excellent pulse-shape discrimination (PSD) capabilities (Fig. 2)
– Solid scintillator has built-in advantages over liquid scintillators in field
Objectives Characterize detector resolution, neutron detection efficiency, and PSD capabilities of solution-grown stilbene detectors
Fig. 1. Photos of stilbene scintillation cells and a detector assembly.
Detector Resolution Stilbene is an organic scintillator, where Compton scattering is the dominant gamma interaction
Backscatter gate technique was developed to isolate energy deposition from Compton scatter [1] (Figs. 3 & 4)
– So far, 137Cs and 54Mn were used, both monoenergetic gamma sources
– Measurement set to record correlated interactions only
– Threshold set to isolate a monoenergetic peak from scattered gamma rays
Neutron Efficiency Important for characterizing stilbene in safeguards applications such as multiplicity counting
Stilbene crystals with length ranging from 1”-4” were used to measure a 252Cf source (Fig. 6)
– 33-keVee threshold
– Room return neglected
2” stilbene crystal has intrinsic neutron efficiency Eff of 27% (Table I)
– 4” crystal improves to 39%
Time-of-Flight Measurement Speed of particle allows for identification independent of PSD and neutron energy
– Allows for characterizing PSD of scintillator
– Can use to measure response function, light output, even resolution (requires long flight path and much longer measurement times)
Measured 252Cf using each stilbene at 1 meter (Fig. 7)
– Time difference be tween start and stop detector gives time-of-flight (Fig. 8)
– 5”x 5” EJ-309 start detector
– 39-keVee measurement threshold
Gamma misclassification rate measured as low as 10-6 for stilbene, an order-of-magnitude lower than EJ-309
Fig. 5. Measured stilbene resolution as function of light output, compared directly to a EJ-309 liquid
scintillator.
Fig. 3. Schematic of measurement for stilbene detector resolution.
Fig. 2. Tail-vs-total integral plot for stilbene measuring 252Cf at a 60-keVee threshold.
Fig. 6. Schematic of measurement for intrinsic neutron efficiency.
Table I. Measured neutron intrinsic efficiency for stilbene as a function of scintillator length.
Fig. 7. Time-of-flight measurement setup.
Fig. 8. Time-of-flight distribution for each stilbene detector.
This work was funded in-part by the Consortium for Verification Technology under Department of Energy National Nuclear Security Administration award number DE-NA0002534
References 1. G. Pausch, K. Roemer, C. Herbach, Y. Kong, R. Lentering, C. Plettner, F. Scherwinski, and J. Stein, Characterization and Calibration of Large-Volume PVT Detectors by Backscatter Gating, transactions of the 2011 IEEE Nuclear Science Symposium.
N. Zaitseva, S. Payne
Source, Angle
Measured Light Output
137Cs, 45° 182 keVee 137Cs, 180° 470 keVee 54Mn, 180° 633 keVee
252Cf
1” stilbene
2” stilbene 3” stilbene
4” stilbene
2”
20 cm
Crystal Photomultiplier tube
Base
Stilb
ene
NaI
Stilb
ene
NaI
Measurement was modeled using MCNPX-PoliMi to determine the geometric uncertainty from this experiment Resolution of stilbene found to be 8.5% at 470 keVee (fig. 5)
Eff =252Cf neutron counts
252Cf incident neutrons
Gamma Region
Neutron Region
Most probable neutron energy ~ 2 MeV
Fig. 4. Correlated pulse height spectrum for stilbene measuring 54Mn scattered gammas.
252Cf
EJ-309 start detector
2.54-cm stilbene stop detector
5.08-cm stilbene stop detector
7.62-cm stilbene stop detector
10.16-cm stilbene stop detector
Stilbene Thickness
Neutron Efficiency
2.54 cm 17.7%
5.08 cm 26.9%
7.62 cm 35.1%
10.16 cm 38.6%
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