# Nuclear Security Detection Workshop

Apr 15 – 16, 2019
University of Surrey
Europe/London timezone

## Characterisation of Liquid Suspended Quantum Dots for Radiation Detection in Nuclear Security

Apr 15, 2019, 5:30 PM
1h
University of Surrey

#### University of Surrey

Guildford, UK
Submitted Poster Detector Materials

### Speaker

Mr Callum Grove (University of Surrey)

### Description

Typical radiation detectors used in portal scanning employ plastic scintillators to effectively and inexpensively cover large areas.
To optimise performance the spectral range of scintillation should correspond to the maximum sensitivity of the photodetector.
Wavelength shifters can be employed to enhance this correlation but can be at the expense of scintillation efficiency.
Quantum dots have a tunable emission wavelength which may maintain, or even improve, scintillation efficiency.
This work reports characterisation measurements of four different quantum dot families: CdSe/ZnS, ZnCdSe/ZnS, ZnCdSeS, and ZnCuInS/ZnS.
In each case, the quantum dots were dispersed in liquid toluene with the aim to find the most appropriate for future loading within a plastic scintillator.

Emission and absorption measurements are necessary in order to understand and assess wavelength matching between the quantum dot emission and the coupled photodetector.
Powdered quantum dots, with wavelengths ranging from $440~$nm to $670~$nm, were suspended in a toluene solution.
To excite the entire range of quantum dots investigated a class 3B solid-state $405~$nm laser was used to induce photoluminescence.
A spectrometer coupled to the sample vial via an optical fibre detected the emitted light.
Results showed that the cadmium-based quantum dots had a relatively narrow emission with FWHM of ${\sim}30~$nm compared to the cadmium-free quantum dots with FWHM of ${\sim}100~$nm.
However the brightness, found by integrating the emission peak, did not noticeably vary between each family.
By combining the experimentally observed emission maxima with the Brus equation, estimates of quantum dot diameters showed good agreement with the manufactured values.

Absorption spectra were measured using a broad spectrum UV lamp and monochromator, which transmitted a small wavelength range through each vial containing the different quantum dot dispersions.
A photodiode measured the transmission through each dispersion with an absorption spectra created by rotating the monochromator's diffraction grating to vary the output wavelength band.
Measurements indicated that simple core quantum dots such as CdSe/ZnS have absorption spectra with a more complex structure resulting from different electron transitions compared to the ZnCuInS/ZnS whose alloyed core manifested as a single absorption peak.
The absorption peak maxima were used to calculate Stokes shifts which showed good agreement with manufacturer estimates of ${\sim}20~$nm for cadmium-based and ${\sim}125~$nm for cadmium-free quantum dots.

Alongside optical characterisation, X-ray induced luminescence was applied to investigate the quantum dots response to radiation. Measurements were conducted using an Amptek Mini-X X-ray Tube at $20~$kV with varying current, $0-200~\mu$A, directed towards a vial of dispersed quantum dots.
A photomultiplier tube, placed next to the vial, measured the photons emitted from the quantum dots as a photocurrent.
This paper will report on the radiation-induced response of liquid-suspended quantum dots in advance of solid scintillator studies.

### Primary authors

Mr Callum Grove (University of Surrey) Dr Caroline Shenton-Taylor (University of Surrey)

### Co-authors

Ms Issy Braddock (University of Surrey) Matt Taggart (University of Surrey) Paul Sellin (University of Surrey)