SUMMARY

Acknowledgment: The funding for this research has been provided by Dr. Massimo Bertino

DETECTING GAMMA-RAYS WITH QUANTUM DOTSEmilio A. Nanni*, Michael Hoffman* and Dr. Massimo Bertino

*Advanced Lab, Physics Department

University of Missouri-Rolla, Rolla, MO 65409

ABSTRACT

The purpose of this experiment is to demonstrate the ability of CdSe quantum dots to detect gamma radiation. Current methods for the detection of gamma radiation require large voltages and possess only semi-portability; moreover, detection of gamma radiation while accurate is plagued by material inefficiencies. Hence, there is need for small portable and efficient gamma-ray detectors. The ability of quantum dots to detect radiation has been predicted but not thoroughly examined in the laboratory. Here we prove that semiconductor quantum dots can be used to detect gamma radiation. Exposure of CdSe quantum dots to gamma rays causes a notable increase in their fluorescence. The increase of luminescence is probably due to photocorrosion of defects, and was found to be linear in the range 20-200 kRad. Quantum dot detectors might represent a simple, rugged, solid state device capable of providing portable and accurate gamma-ray detection.

Photoactivation and Quantum Yield

Quantum Confinement

Gamma Exposure Theory and Results

As an illustrative approximation a quantum dot can be viewed simply as a 3-D cube with V = 0 potential inside and V = . Cartesian coordinates provide a decoupled solution to the Schrodinger Equation below:

The energy levels of an electron inside the box depend on the dimensions of the cube. A smaller box/quantum dot emits light with a smaller wavelength than a larger dot. The size of the particle determines the emission wavelength of the tiny semi-conductor.

Photoluminescence is the process of absorbing a photon, exciting the acceptor, and reemitting a photon leaving the acceptor in the ground state. If every photon we send to the quantum dots is reemitted the quantum yield (QY) is said to be 100%. Ideally we would like the quantum yield to be as high as possible. Unfortunately, newly formed quantum dots have approximately .2% QY. In order to improve to quality of detection the quantum dot’s yield must be improved. This is accomplished by photoactivation with an UV source which initiates a chemical reaction producing a smaller quantum dot core surrounded by an oxidized layer (shown in orange).

It is predicted that exposure of an arbitrarily photoactivated quantum dot solution to gamma rays will result in an initial increase followed by a decrease in the intensity of fluorescence relative to an unexposed base sample. The proposed mechanism for this behavior is through photocorrosion of the surface defects of the quantum dot semi-conductor. The incident gamma ray will initiate the photocorrosion reaction on the surface of the QD; in addition, gamma rays may have enough energy to break bonds near the surface decreasing the overall size of the QD. The new QD is envisioned to be only slightly smaller than the parent quantum dot; therefore, it is still capable of fluorescence. The fluorescence will be enhanced in the daughter QD because the electron is more localized and more defects will have been removed. Eventually, the quantum dot will become to small to emit light and the quantum yield will decrease.

Representative example of increase in fluorescence caused by gamma ray

exposure.

Predicted relative intensity versus exposure curve. The regions highlighted by green arrows are

considered possible regions of the curve suitable for use as a detector.

Simple model schematic of gamma irradiation event. The new QD, following the

arrow, is envisioned to be only slightly smaller than the parent quantum dot;

therefore, it is still capable of fluorescence.

For this experiment quantum dots were only exposed within the first linear region in order to avoid more complicated fluorescence behavior.

Samples were irradiated both with and without UV lamp pre-activation to test the increase in measurement accuracy.

The linear trend predicted in the initial region of the exposure curve was observed.

CdSe quantum dot solutions show great promise as a potential gamma-ray detectors.

The future goals of the project are to create solid state detectors by forming quantum dots in an aero-gel matrix.

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