Quantum Dots May Enable New Radiation Detectors
Daniel S. Burgess
The work of a pair of scientists at Lawrence Livermore National Laboratory in Livermore, Calif., indicates that quantum dots may serve as suitable scintillator materials for the development of high-resolution room-temperature gamma-ray detectors. Such detectors have potential applications in medical imaging, environmental monitoring, and security and defense.
Tzu-Fang Wang, who is collaborating on the project with Sonia E. Létant, said that contemporary approaches to gamma-ray detection carry several drawbacks. Most still employ a photomultiplier tube and single-crystal thallium-activated sodium iodide as a scintillator, he noted, which is limited in size and which offers an energy resolution of only approximately 6 percent at 1 MeV. Alternatively, they may use a semiconductor detector, such as germanium, which offers much better energy resolution than the scintillation approach but which requires cryogenic cooling and is subject to radiation damage.
Quantum dots offer an improvement on scintillator technology in that the size of the phosphorescent material would not be restrained by crystal growth. Rather, the dots would be suspended in a transparent matrix that could be as large as is desired. Moreover, because the output of the quantum dots is a function of their dimensions, they could be produced to emit light at wavelengths suitable for detection by avalanche photodiodes, which offer higher quantum efficiencies than photomultiplier tubes, thus increasing the sensitivity of the scintillation detector.
In proof-of-principle experiments, Wang and Létant used a quantum-dot scintillator to detect alpha radiation from a curium source. They fabricated the composite scintillator using CdSe/ZnS core/shell quantum dots from Evident Technologies Inc. of Troy, N.Y., that they affixed in the pores of 1/16-in.-thick Vycor glass from Advanced Glass and Ceramics of Holden, Mass. Although they detected the green response of the dots with a photomultiplier tube from Hamamatsu Corp. of Bridgewater, N.J., they believe that employing an avalanche photodiode would boost the detection efficiency by a factor of 10.
The results indicate that quantum-dot scintillation detectors should offer an energy resolution of approximately 2 percent, between that of traditional scintillation and cooled semiconductor instruments for gamma-ray detection. Wang said they are using gamma-ray standards to investigate the linearity of the scintillation output in the composite material.
He noted that there are hurdles to overcome before quantum-dot scintillation materials are ready for the detection of gamma rays. For one, the semiconductor dots display a relatively small Stokes shift, so the light they emit in response to stimulation by incoming radiation tends to be absorbed by other dots, resulting in lost output. For another, the low atomic number of the CdSe/ZnS dots translates into a low stopping power. To detect higher-energy gamma rays — in contamination remediation efforts, for example, those emitted by metastable protactinium-234 in the decay chain of depleted uranium — it will be necessary to employ types of dots with a higher atomic number, such as PbS.
Applied Physics Letters, March 6, 2006, 103110.
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