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%