Scintillators, which produce bright light when struck by radiation, are used extensively by the US Government in homeland security as radiation detectors. By observing the amount of light produced, and how quickly, the source of radiation may be identified.

Dr Patrick Feng, who led the Defense Nuclear Nonproliferation project, began to develop new types of scintillators in 2010, in order to “strengthen national security by improving the cost-to-performance ratio of radiation detectors”. To improve this ratio, he had to “bridge the gap” between effective scintillators made from expensive materials, and affordable but far less effective models.

Although there are many types of scintillator available, the best-performing scintillators are made from trans-stilbene. This crystallised form of a molecule allows border security tell the difference between gamma rays, which appear naturally everywhere, and neutrons, which are often associated with threatening materials such as plutonium and uranium, by producing a bright light in response.

The gold standard scintillator material for the past 40 years has been the crystalline form of a molecule called trans-stilbene, despite intense research to develop a replacement. Trans-stilbene is highly effective at differentiating between two types of radiation: gamma rays, which are ubiquitous in the environment, and neutrons, which emanate almost exclusively from controlled threat materials such as plutonium or uranium. Trans-stilbene is very sensitive to these materials, producing a bright light in response to their presence.

These crystals, however, are too fragile and expensive (around S1,000 per cubic inch)  to be used in the field, and instead, security personnel will tend to use plastic-based scintillators, which can be moulded into large shapes but are ineffective at differentiating between different types of radiation or detecting weak sources.

In order to find a good alternative, Dr Feng and his team at Sandia National Laboratories in Livermore, CA, began to experiment with organic glass components, which are capable of discriminating between different types of radiation.

Tests demonstrated that scintillators made with organic glass performed even better than the trans-stilbene scintillators in radiation detection tests.

The researchers were able to improve their design further when they drew a parallel between the behaviour of LEDs, which produce light when electrical energy is applied, and scintillators, which respond to radioactive sources. They found that adding fluorine, which is used in some LEDs, into the scintillator components helped stabilise them. This allowed for the organic glass to be melted down and cast into large blocks without becoming cloudy or crystallising upon cooling.

The result was an indefinitely stable scintillator able to differentiate between non-threatening radioactive sources, such as those used in medical treatments, and those which could constitute threats. The organic glass components are cheap and easy to manufacture, and do not degrade over time.

Next, the researchers will cast a large prototype for field testing, and hopefully demonstrate that the scintillator can withstand environmental wear and tear, for instance, due to the humidity of ports where checks are carried out. They also hope to adjust the scintillator to distinguish between safe sources of gamma radiation, and those which could be used to make “dirty bombs”. Source: Sandia Laboratory