with 0.5% gadolinium-chloride (GdCl 3 ), contained within a stainless steel tank
(121.9 cm  91.4 cm  119.4 cm). To protect the stainless steel tank from the
corrosive water (due to the chloride content), the tank is coated with a baked-on layer
of Teflon. Eight 10-in. diameter photomultiplier tubes are mounted on the top of the
detector, looking into the detector volume. The inside of the tank is also lined with a
1.0 mm highly reflective (>99% in blue near UV) layer of GORE
® DRP
® material.
Figure 1 shows a 3D model and a photograph of the detector.
The detection mechanism is multi-stepped. A neutron born from a fission event
in the sample well must enter the water volume and thermalize. The thermalization
distance is *35 mm for 1 meV neutrons. Once the neutron has thermalized, it will
capture on a gadolinium nucleus. Gadolinium-157 (15% natural abundance) has the
highest thermal neutron absorption cross-section of any stable isotope (259,000
barns), and natural gadolinium’s thermal neutron absorption cross section is 50,000
barns. Upon capture, the target nucleus enters an 8 meV excited state, then
de-excites by emitting a gamma cascade with a total energy of 8 meV. The gammas
then Compton scatter electrons in the water, ejecting some at high energies.
Electrons that are scattered with a kinetic energy above the Cherenkov threshold
(250 keV [7]) will produce a ring of Cherenkov light, which is then detected by the
photomultiplier tubes.
2 Results
Both gammas and neutrons produce a detector response in the WaND system.
Figure 2 shows the spectral response of a 5.9 microCi cobalt-60 and a 0.82 microCi
californium-252 source. The background spectrum is measured by performing a
data run without sources. Then the background is statistically subtracted from a
source run, leaving only the source contribution.
Fig. 1 A photograph (left) and 3D model created using Sketchup (right) of the WaND system.
Note the person for scale
The Water Neutron Detector
253
(121.9 cm  91.4 cm  119.4 cm). To protect the stainless steel tank from the
corrosive water (due to the chloride content), the tank is coated with a baked-on layer
of Teflon. Eight 10-in. diameter photomultiplier tubes are mounted on the top of the
detector, looking into the detector volume. The inside of the tank is also lined with a
1.0 mm highly reflective (>99% in blue near UV) layer of GORE
® DRP
® material.
Figure 1 shows a 3D model and a photograph of the detector.
The detection mechanism is multi-stepped. A neutron born from a fission event
in the sample well must enter the water volume and thermalize. The thermalization
distance is *35 mm for 1 meV neutrons. Once the neutron has thermalized, it will
capture on a gadolinium nucleus. Gadolinium-157 (15% natural abundance) has the
highest thermal neutron absorption cross-section of any stable isotope (259,000
barns), and natural gadolinium’s thermal neutron absorption cross section is 50,000
barns. Upon capture, the target nucleus enters an 8 meV excited state, then
de-excites by emitting a gamma cascade with a total energy of 8 meV. The gammas
then Compton scatter electrons in the water, ejecting some at high energies.
Electrons that are scattered with a kinetic energy above the Cherenkov threshold
(250 keV [7]) will produce a ring of Cherenkov light, which is then detected by the
photomultiplier tubes.
2 Results
Both gammas and neutrons produce a detector response in the WaND system.
Figure 2 shows the spectral response of a 5.9 microCi cobalt-60 and a 0.82 microCi
californium-252 source. The background spectrum is measured by performing a
data run without sources. Then the background is statistically subtracted from a
source run, leaving only the source contribution.
Fig. 1 A photograph (left) and 3D model created using Sketchup (right) of the WaND system.
Note the person for scale
The Water Neutron Detector
253