Radioactive isotopes leave a unique signature in the form of particles they emit and they are measured by radiation detection meters. These detectors play a key role in areas like nuclear power generation, border security, nuclear medicine, proliferation, decommissioning and decontamination.
Due to the complexity of the detection process and the scarcity of Helium-3 – the preferred choice for neutron detection in various applications – sensitive materials are necessary. And organic and inorganic scintillating crystals are good alternatives.
Crystalline Form
Crystals scintillate when exposed to X-rays and the most common distinction is between inorganic and organic crystals.
Inorganic Crystals
Inorganic crystals were developed for characterization applications and gamma-ray detection owing to their suitability in areas requiring high energy resolution.
Sodium iodide (NaI) is a popular crystal for radiation detection. The alkali halide crystal has a good spectrometric response to gamma-rays. This explains why they find use in geiger counters and other appliances.
To resolve the problem of low light yield in pure NaI crystals, impurities must be introduced to the inorganic crystals. Known as “activators”, they increase the possibility of emitting photons that are detectable via traditional photodetectors.
Tl is one of the most widely used activators. Nal(TI) crystal scintillators boast high luminescence efficiency and are available in various sizes and geometries. They also produce one of the highest signals in a photomultiplier tube per amount of radiation absorbed. Under optimum settings, 1x 104 photoelectrons are produced on average for every MeV gamma ray. NaI(Tl) scintillation crystal are suitable alternatives in radiation detection.
Application in Neutron Detection
The highly used isotopes are 10B, 6Li and 3He due to their high cross-section for low-energy neutron capture. But the scarcity of 3He increases the need for alternatives.
6Li
Organic scintillation detectors utilizing “elastic scattering” of neutrons with hydrogen and other light atoms have been proposed. But detection systems like 6Li that exploit both particle separation and scattering techniques show promising performance in particle identification and source localization.
Of the two remaining isotopes, 6Li is widely adopted in inorganic crystals. These crystal detectors constitute a group of possible candidates for low-energy neutron detection. In ECOTEST’s devices SPRD SPECTRA and PRD CADMIUM high sensitivity LiI (Eu) scintillation crystals are used to register neutron radiation.
Radiation detection meters may also use the high thermal neutron cross-section of 6Li isotope – Ce3+-doped LiCaAlF6 inorganic crystal. The performance of this detector is comparable to commercially available Li-glass scintillators.
Elpasolites
Elpasolites like Cs2LiLa(Br,Cl)6 (CLLBC) and Cs2LiYCl6 (CLYC) are another group of crystals with neutron detection properties. Doped with Ce, the crystals provide excellent n/g separation characteristics along with high energy resolution.
6LiF/ZnS:Ag is another example of an inorganic scintillator for radiation detection. ZnS crystalline powder powers this type of scintillator. It is an efficient thermal neutron detector with low gamma-ray sensitivity.
Organic Crystals
Irrespective of their crystalline form, organic scintillators possess high sensitivity to both gamma-ray photons and fast neutrons. Fast neutrons experience elastic scattering when exposed to a proton. On the other hand, gamma-ray photons come in contact with the scintillant’s atoms through Compton scattering. The outcome is fluorescence, the decay time of which varies as per the rate of the incident particle’s energy loss.
Only two kinds of pure organic crystals are commonly used for radiation detection applications:
- Stilbene
- Anthracene
Anthracene
Anthracene’s popularity stems from its high scintillation efficiency. But stilbene has outstanding n/g separation capabilities. Because of issues connected to the growing of such crystals in bigger dimensions, they did not find popularity for numerous years. But interest has increased in recent years owing to new methods of growing.
Thanks to its high light yield, the material known as anthracene still has the greatest scintillation efficiency and is commonly used for reference during the development of new crystals.
Organic crystals, however, have their share of drawbacks. For example, their anisotropic reaction to incident radiation hampers the performance during detector orientation changes. But this property may be exploited for locating the interaction through the scattered proton’s angle.
Stilbene
Trans-stilbene crystals are traditionally grown via melt growth approach. This growth process, however, is costly and complex. So, the crystal size did not exceed 10 cm.
But another solution growth approach was used with reduced growth time and larger sized samples were grown. It even somewhat addressed the common issue of misclassification between gamma-ray photons and neutrons in low energy areas.
Lightweight stilbene crystals – by virtue of their non-hazardous, non-hygroscopic, solid nature – find used in various applications like portable security gizmos and nuclear decommissioning. While they can now be grown in larger sizes, the cost associated with this is still very expensive. This indicates the usefulness of organic liquids in large-scale radiation detectors.
Summary
Depending on the scintillating crystals presently used in radiation detection, there is no single crystal that accounts for every requirement of a radiation detection meter. Thus, it is necessary to carefully analyze the needs of the detection meter and select the sensitive crystal accordingly.