Answer to Question #989 Submitted to "Ask the Experts"

Q

What is special in a Lithium glass detector?

A

Lithium glass detectors are scintillation-type detectors—that is, they emit light in response to excitation energy received from ionizing radiation. The detectors are silicate-based glasses into which a few weight percent of lithium has been incorporated. They also contain a small percentage of an activator species (necessary to produce the fluorescence effect); this is usually a cerium in the form of an oxide. These detectors have been used most often for neutron detection. In such cases they are often enriched in the isotope ⁶Li. Lithium-6 undergoes a nuclear reaction by absorbing a neutron, usually a low-energy neutron, and releasing an alpha particle and a triton (nucleus of a tritium atom). The alpha particle and triton deposit several million electron volts of energy in the scintillation detector and produce a readily measurable light output toward the blue end of the visible spectrum, usually detected with a photomultiplier tube. The pulses from the photomultiplier tube are fed to a preamplifier, amplifier, and discriminator and an appropriate recording device. The lithium glass detectors for neutron detection do not have very good inherent energy resolution characteristics and are not often used for neutron spectral measurements, although they are commonly used for indirect energy measurements in what is called a time-of-flight measurement. In this process the time is measured from when a neutron is produced until it travels a known distance between the source and a lithium glass detector. The time is correlated with the energy of the neutron. Lithium glass detectors can also be depleted in the isotope ⁶Li and enriched in the second natural isotope of lithium, ⁷Li. Such detectors are quite insensitive to neutrons and have been used to evaluate gamma radiation response in the presence of neutrons. By using two detectors (one enriched in ⁶Li that will respond to both neutrons and gamma rays and the second similar to the first but enriched in ⁷Li) the net response of the first detector to neutrons can be estimated by subtracting the second detector response from that of the first. Lithium glass detectors have the advantage over some other neutron detectors in that the glass can be fabricated into any of several geometries so that detectors of unusual shapes and sizes can be fabricated for specialized applications. They also offer the advantage of being a relatively fast scintillator—that is, the decay time of the fluorescent light pulse is relatively short (about 75 nanoseconds for the cerium-activated glass) so that the detector can handle reasonably high count rates. The glass detectors are also quite rugged and can sustain high temperatures and may have applications in environments unsuitable for some other detector types. Many more details of the properties and applications of glass scintillators can be found in the scientific literature. If you want a quick review and summary with some details and additional references on this topic, see Glen Knoll's book Radiation Detection and Measurement, 3rd ed., John Wiley & Sons, New York, 2000. George Chabot, CHP, Ph.D.