Suppression of Interface-Induced Noise by the Control of Electron-Phonon Interactions

Hammig, Mark D.,Taehoon Kang,Manhee Jeong,Jarrett, Michael IEEE 2013 IEEE transactions on nuclear science Vol.60 No.4

<P>We study the influence of various types of contacting media and contact area on the current-fluctuation level in semiconductors, testing the supposition that the electronic noise is governed, in part, by phonon-leaking dynamics to the environment. Using passivated and gettered silicon PIN diodes as experimental test-beds, the presented data lends credence to the prediction that the phonon-refraction characteristics of the semiconductor-metal interface substantially impacts the current fluctuations in the solid. Specifically, if one implements metallic contacts with lower phonon-reflecting characteristics, such as those composed of silver or palladium, or if one increases the area through which phonons can leak to the surrounding environment, then the leakage current decreases.</P>

Pulse shape discrimination using a stilbene scintillator array coupled to a large-area SiPM array for hand-held dual particle imager applications

부지환,Hammig Mark D.,정만희 한국원자력학회 2023 Nuclear Engineering and Technology Vol.55 No.2

A dual-particle imager (DPI) is configured in a hand-held form factor, then one can efficiently and conveniently deploy the DPI to detect the presence of special nuclear materials (SNM) and identify any isotopic variations that differ from their natural abundances. Here we show that by maximizing the areal coupling between a pixelated scintillator array and the partitioned photosensor readout such as a silicon photomultiplier (SiPM), the information utilization of the gamma-ray and neutron information in the radiation field can be enhanced, thus enabling one to rapidly acquire spatial maps of the distributions on gamma-ray and neutron emitters.

Improving light collection efficiency using partitioned light guide on pixelated scintillator-based -ray imager

현수연,Mark Hammig,정만희 한국원자력학회 2022 Nuclear Engineering and Technology Vol.54 No.5

When gamma-camera sensor modules, which are key components of radiation imagers, are derived fromthe coupling between scintillators and photosensors, the light collection efficiency is an important factorin determining the effectiveness with which the instrument can identify nuclides via their derivedgamma-ray spectra. If the pixel area of the scintillator is larger than the pixel area of the photosensor,light loss and cross-talk between pixels of the photosensor can result in information loss, therebydegrading the precision of the energy estimate and the accuracy of the position-of-interaction determination derived from each active pixel in a coded-aperture based gamma camera. Here we present twomethods to overcome the information loss associated with the loss of photons created by scintillationpixels that are coupled to an associated silicon photomultiplier pixel. Specifically, we detail the use ofeither: (1) light guides, or (2) scintillation pixel areas that match the area of the SiPM pixel. Comparedwith scintillator/SiPM couplings that have slightly mismatched intercept areas, the experimental resultsshow that both methods substantially improve both the energy and spatial resolution by increasing lightcollection efficiency, but in terms of the image sensitivity and image quality, only slight improvementsare accrued.

Development of hand-held coded-aperture gamma ray imaging system based on GAGG(Ce) scintillator coupled with SiPM array

Jeong, Manhee,Hammig, Mark Korean Nuclear Society 2020 Nuclear Engineering and Technology Vol.52 No.11

Emerging gamma ray detection applications that utilize neutron-based interrogation result in the prompt emission of high-energy (>2 MeV) gamma-rays. Rapid imaging is enabled by scintillators that possess high density, high atomic number, and excellent energy resolution. In this paper, we evaluate the bright (50,000 photons/MeV) oxide scintillator, cerium-doped Gd₂Al₂Ga₃O₁₂ (GAGG(Ce)). A silicon photomultiplier (SiPM) array is coupled to a GAGG(Ce) scintillator array (12 × 12 pixels) and integrated into a coded-aperture based gamma-ray imaging system. A resistor-based symmetric charge division circuit was used reduce the multiplicity of the analog outputs from 144 to 4. The developed system exhibits 9.1%, 8.3%, and 8.0% FWHM energy resolutions at 511 keV, 662 keV, and 1173.2 keV, respectively. In addition, a pixel-identification resolution of 602 ㎛ FWHM was obtained from the GAGG(Ce) scintillator array.

Comparison of gamma ray localization using system matrixes obtained by either MCNP simulations or ray-driven calculations for a coded-aperture imaging system

Jeong, Manhee,Hammig, Mark D. Elsevier 2020 Nuclear instruments & methods in physics research. Vol.954 No.-

<P><B>Abstract</B></P> <P>A coded-aperture system based on a large-area silicon photo-multiplier (SiPM) coupled with inorganic scintillator was developed for the gamma ray localization in the field of nuclear safety and security. Monte Carlo simulations of the performance were conducted to verify its performance. For the coded-aperture imaging (CAI) system, the mask was designed with an 11-rank modified uniformly redundant array (MURA), and the SiPM readout consisted of 12 × 12 pixels. A two-centimeter-thick tungsten mask was used to encode the gamma ray field. The 144 pixels are read-out with a resistor-based charge-division circuit that reduces the readout outputs from 144 to four signals per module, from which the deposited energy and interaction position can be extracted. For image localization, maximum-likelihood expectation maximization (MLEM) and compress-sensing (CS) methods are used with either: (1) a system matrix generated both the Monte-Carlo N-Particle (MCNP) code, or (2) a mathematical system-matrix model utilizing a ray-driven method. In this paper, reconstructed images of gamma ray sources with various positions and activities were simulated and measured with the simulation tools and the physical system in order to compare and evaluate the relative strengths of the two different system matrix formulations. Both system matrixes generated by MCNP and ray-driven methods are effective in localizing isolated point sources; however, there are critical differences when several gamma sources with different strengths are in the field of view. Although the MCNP-based system matrix requires more processing time to generate, its more accurate incorporation of the competing gamma-ray interaction processes results in effective localization of multiple point sources. In contrast, the rapidly produced ray-driven reconstruction matrix demonstrates reduced accuracy in localizing extending distributions due to its simplified treatment of the particle transport, the magnitude of which is quantified in the paper.</P>

Comparison of Characteristics of Gamma-Ray Imager Based on Coded Aperture by Varying the Thickness of the BGO Scintillator

Park Seoryeong,Hammig Mark D.,Manhee Jeong 대한방사선방어학회 2022 방사선방어학회지 Vol.47 No.4

Background : The conventional cerium-doped Gd2Al2Ga3O12 (GAGG(Ce)) scintillator-based gamma-ray imager has a bulky detector, which can lead to incorrect positioning of the gamma-ray source if the shielding against background radiation is not appropriately designed . In addition , portability is important in complex environments such as inside nuclear power plants , yet existing gamma-ray imager based on a tungsten mask tends to be weighty and therefore difficult to handle . Motivated by the need to develop a system that is not sensitive to background radiation and is portable , we changed the material of the scintillator and the coded aperture . Materials and Methods : The existing GAGG(Ce) was replaced with Bi4Ge3O12 (BGO) , a scintillator with high gamma-ray detection efficiency but low energy resolution , and replaced the tungsten (W) used in the existing coded aperture with lead (Pb) . Each BGO scintillator is pixelated with 144 elements (12 × 12) , and each pixel has an area of 4 mm × 4 mm and the scintillator thickness ranges from 5 to 20 mm (5 , 10 , and 20 mm) . A coded aperture consisting of Pb with a thickness of 20 mm was applied to the BGO scintillators of all thicknesses . Results and Discussion : Spectroscopic characterization , imaging performance , and image quality evaluation revealed the 10 mm-thick BGO scintillators enabled the portable gamma-ray imager to deliver optimal performance . Although its performance is slightly inferior to that of existing GAGG(Ce)-based gamma-ray imager, the results confirmed that the manufacturing cost and the system’s overall weight can be reduced . Conclusion : Despite the spectral characteristics , imaging system performance , and image quality is slightly lower than that of GAGG(Ce) , the results show that BGO scintillators are preferable for gamma-ray imaging systems in terms of cost and ease of deployment , and the proposed design is well worth applying to systems intended for use in areas that do not require high precision.

Microstrip Transmission-Line Electrodes for Position Sensitive Radiation Detection

Manhee Jeong,Hammig, Mark D. IEEE 2014 IEEE transactions on nuclear science Vol.61 No.6

<P>A serpentine transmission-line in a microstrip configuration is used to enable fine position sensing in semiconductor radiation detectors. Specifically demonstrated with silicon substrates coupled to standard charge-sensitive preamplifiers, a meandering electrode is used to enable lateral position sensing to within an approximately 1 cm length along the electrode. Configured in a rectilinear shape, the lateral resolution is therefore equivalent to the strip pitch, which we have made as small as 4 μm. The position-of-interaction is extracted using either timing or amplitude techniques, both enabled by the slow-wave propagation structure inherent to the standard metal-semiconductor-insulator configuration of the bounding electrodes, the analysis of which is summarized and compared experimentally.</P>

Impact of aperture-thickness on the real-time imaging characteristics of coded-aperture gamma cameras

Park, Seoryeong,Boo, Jiwhan,Hammig, Mark,Jeong, Manhee Korean Nuclear Society 2021 Nuclear Engineering and Technology Vol.53 No.4

The mask parameters of a coded aperture are critical design features when optimizing the performance of a gamma-ray camera. In this paper, experiments and Monte Carlo simulations were performed to derive the minimum detectable activity (MDA) when one seeks a real-time imaging capability. First, the impact of the thickness of the modified uniformly redundant array (MURA) mask on the image quality is quantified, and the imaging of point, line, and surface radiation sources is demonstrated using both cross-correlation (CC) and maximum likelihood expectation maximization (MLEM) methods. Second, the minimum detectable activity is also derived for real-time imaging by altering the factors used in the image quality assessment, consisting of the peak-to-noise ratio (PSNR), the normalized mean square error (NMSE), the spatial resolution (full width at half maximum; FWHM), and the structural similarity (SSIM), all evaluated as a function of energy and mask thickness. Sufficiently sharp images were reconstructed when the mask thickness was approximately 2 cm for a source energy between 30 keV and 1.5 MeV and the minimum detectable activity for real-time imaging was 23.7 MBq at 1 m distance for a 1 s collection time.