  In terms of energy resolution, temporal response to burst events, and thermal
stability, lanthanum bromide doped with Ce is a much better choice than the
traditional NaI(Tl) scintillator for hard X-ray astronomy. We present the test
results of a phoswich detector with a diameter of 101.6 mm consisting of 6 mm
thick LaBr3:Ce and 40 mm thick NaI(Tl), which is the largest one of this type
reported so far. The measured energy resolution is 10.6% at 60 keV, varying
inversely proportional to the square root of the energy, and the energy
nonlinearity is found to be less than 1%, as good as those of smaller
phoswiches. The coupled scintillators and phototube also show excellent
uniformity across the detecting surface, with a deviation of 0.7% on the pulse
amplitude produced by 60 keV gamma-rays. Thanks to the large ratio of light
decay times of NaI(Tl) and LaBr3:Ce, 250 ns vs. 16 ns, pulse shape
discrimination is much easier for this combination than for NaI(Tl)/CsI(Na). As
the light decay time of LaBr3:Ce is about 15 times faster than that of NaI(Tl),
this phoswich is more suitable for detection of bright, transient sources such
as gamma-ray bursts and soft gamma-ray repeaters. The internal activity of
lanthanum produces a count rate of about 6 counts/s at 37.5 keV in the
detector. This peak could be used for in-flight spectral calibration and gain
correction.
