Characterization of ZnO, ${\rm BaF}_{2}$ and ${\rm PbWO}_{4}$ Scintillator Detectors for Cargo Inspection Using Transmitted X-ray Spectroscopy

We report on a feasibility study of scintillation detectors with fast decay times which might be used to enable spectroscopy in a high-count-rate, high-energy x-ray environment, such as is used for cargo inspection. Such detectors need to have a small size and high density in order to reliably detect x rays between 1 and 9 MeV with good spatial resolution and need to be suitable for read-out with semiconductor photo-detectors. Three inorganic scintillator detectors with decay constants less than 10 ns were selected: ZnO (in the form of wafers of
$4.5 \times 0.5 \times 20~{\rm mm}$
and single crystals of
$5 \times 5 \times 30~{\rm mm}$
),
${\rm BaF}_{2}~ (5 \times 5 \times 30~{\rm mm})$
and
${\rm PbWO}_{4}~(6 \times 6\times 20~{\rm mm})$
. ZnO has a known problem of self-absorption of the emitted scintillation light. Using the wafers to create stacks of different thicknesses, the effective attenuation length was determined to be
$2.25 \pm 0.25~{\rm mm}$
for one batch of ZnO and
$1.5\pm 0.3~{\rm mm}$
for another batch, making ZnO unlikely for the proposed use. Although the larger ZnO crystal also scintillates, light transmission was so poor that no signal was measured in tests with gamma ray sources. ZnO was thus discarded as a viable candidate for the purposes of this study. The full width at half maximum (FWHM) of the pulses produced by
${\rm BaF}_{2}$
was measured to be
$\sim {2}~{\rm ns}$
, consistent with being dominated by the response of the photomultiplier tube used in the measurement.
${\rm BaF}_{2}$
by itself, however, emits in the UV, which is unsuitable for use with semiconductor readout. In another series of measurements,
${\rm BaF}_{2}$
was coupled to fast wavelength-shifting PVT sheets with Eljen DSB1 dye, and the FWHM of the signal measured this way was
$\sim {4.8}~{\rm ns}$
, with a fast decay constant of
$\sim {11.2}~{\rm ns}$
. The long decay component appears to be suppressed somewhat by the use of the wavelength shifter. The measured FWHM of the pulses produced by
${\rm PbWO}_{4}$
was
$\sim {3}~{\rm ns}$
, with a decay constant of
$\sim {8.3}~{\rm ns}$
and no long component. These results suggest that dense sub-10-ns scintillators exist that can be used for high-count-rate x-ray cargo inspection with good spatial resolution.
${\rm PbWO}_{4}$
is very dense, has no long decay component or self-attenuation and scintillates in the visible spectrum, but due to its low light output, it needs to be read out by a silicon photomultiplier or similar device.
${\rm BaF}_{2}$
is a possible alternative, when used in combination with a fast wavelength shifter.