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A Vernier ring-oscillator-based time-to-digital converter (TDC) with a new prelogic is presented. Experimental results show that the proposed architecture achieve a 5.5 ps RMS timing jitter with a 5.1 ps LSB within an area of 0.00151 mm 2 . Thanks to the new prelogic circuit, the power consumption of the circuit was optimised to 22 μ W at a rate of 1 Mevents/s for a dynamic range of 4 ns. The area, timing jitter and power consumption make the TDC suitable for an array of electronic readout in a position emission tomography single photon avalanche diode based detectors.

Author(s): Chambers, C; Walton, T; Fairbank, D; Craycraft, A; Yahne, DR; Todd, J; Iverson, A; Fairbank, W; Alamare, A; Albert, JB; Anton, G; Arnquist, IJ; Badhrees, I; Barbeau, PS; Beck, D; Belov, V; Bhatta, T; Bourque, F; Brodsky, JP; Brown, E; Brunner, T; Burenkov, A; Cao, GF; Cao, L; Cen, WR; Charlebois, SA; Chiu, M; Cleveland, B; Coon, M; Cree, W; Cote, M; Dalmasson, J; Daniels, T; Darroch, L; Daugherty, SJ; Daughhetee, J; Delaquis, S; Mesrobian-Kabakian, A Der; DeVoe, R; Dilling, J; Ding, YY; Dolinski, MJ; Dragone, A; Echevers, J; Fabris, L; Farine, J; Feyzbakhsh, S; Fontaine, R; Fudenberg, D; Giacomini, G; Gornea, R; Gratta, G; Hansen, EV; Heffner, M; Hoppe, EW; Hosl, J; House, A; Hufschmidt, P; Hughes, M; Ito, Y; Jamil, A; Jessiman, C; Jewell, MJ; Jiang, XS; Karelin, A; Kaufman, LJ; Kodroff, D; Koffas, T; Kravitz, S; Krucken, R; Kuchenkov, A; Kumar, KS; Lan, Y; Larson, A; Leonard, DS; Li, G; Li, S; Li, Z; Licciardi, C; Lin, YH; Lv, P; MacLellan, R; Michel, T; Mong, B; Moore, DC | Abstract: The search for neutrinoless double beta decay probes the fundamental properties of neutrinos, including whether or not the neutrino and antineutrino are distinct. Double beta detectors are large and expensive, so background reduction is essential for extracting the highest sensitivity. The identification, or 'tagging', of the $^{136}$Ba daughter atom from double beta decay of $^{136}$Xe provides a technique for eliminating backgrounds in the nEXO neutrinoless double beta decay experiment. The tagging scheme studied in this work utilizes a cryogenic probe to trap the barium atom in solid xenon, where the barium atom is tagged via fluorescence imaging in the solid xenon matrix. Here we demonstrate imaging and counting of individual atoms of barium in solid xenon by scanning a focused laser across a solid xenon matrix deposited on a sapphire window. When the laser sits on an individual atom, the fluorescence persists for $\sim$30~s before dropping abruptly to the background level, a clear confirmation of one-atom imaging. No barium fluorescence persists following evaporation of a barium deposit to a limit of $\leq$0.16\%. This is the first time that single atoms have been imaged in solid noble element. It establishes the basic principle of a barium tagging technique for nEXO.

Silicon photomultipliers are regarded as a very promising technology for next-generation, cutting-edge detectors for low-background experiments in particle physics. This work presents systematic reflectivity studies of Silicon Photomultipliers (SiPM) and other samples in liquid xenon at vacuum ultraviolet (VUV) wavelengths. A dedicated setup at the University of M\"unster has been used that allows to acquire angle-resolved reflection measurements of various samples immersed in liquid xenon with 0.45{\deg} angular resolution. Four samples are investigated in this work: one Hamamatsu VUV4 SiPM, one FBK VUV-HD SiPM, one FBK wafer sample and one Large-Area Avalanche Photodiode (LA-APD) from EXO-200. The reflectivity is determined to be 25-36% at an angle of incidence of 20{\deg} for the four samples and increases to up to 65% at 70{\deg} for the LA-APD and the FBK samples. The Hamamatsu VUV4 SiPM shows a decline with increasing angle of incidence. The reflectivity results will be incorporated in upcoming light response simulations of the nEXO detector., Comment: 18 pages, 11 figures

Analog and digital SiPMs have revolutionized the field of radiation instrumentation by replacing both avalanche photodiodes and photomultiplier tubes in many applications. However, multiple applications require greater performance than the current SiPMs are capable of, for example timing resolution for time-of-flight positron emission tomography and time-of-flight computed tomography, and mitigation of the large output capacitance of SiPM array for large-scale time projection chambers for liquid argon and liquid xenon experiments. In this contribution, the case will be made that 3D photon-to-digital converters, also known as 3D digital SiPMs, have a potentially superior performance over analog and 2D digital SiPMs. A review of 3D photon-to-digital converters is presented along with various applications where they can make a difference, such as time-of-flight medical imaging systems and low-background experiments in noble liquids. Finally, a review of the key design choices that must be made to obtain an optimized 3D photon-to-digital converter for radiation instrumentation, more specifically the single-photon avalanche diode array, the CMOS technology, the quenching circuit, the time-to-digital converter, the digital signal processing and the system level integration, are discussed in detail.

The nEXO neutrinoless double beta decay experiment is designed to use a time projection chamber and 5000 kg of isotopically enriched liquid xenon to search for the decay in $^{136}$Xe. Progress in the detector design, paired with higher fidelity in its simulation and an advanced data analysis, based on the one used for the final results of EXO-200, produce a sensitivity prediction that exceeds the half-life of $10^{28}$ years. Specifically, improvements have been made in the understanding of production of scintillation photons and charge as well as of their transport and reconstruction in the detector. The more detailed knowledge of the detector construction has been paired with more assays for trace radioactivity in different materials. In particular, the use of custom electroformed copper is now incorporated in the design, leading to a substantial reduction in backgrounds from the intrinsic radioactivity of detector materials. Furthermore, a number of assumptions from previous sensitivity projections have gained further support from interim work validating the nEXO experiment concept. Together these improvements and updates suggest that the nEXO experiment will reach a half-life sensitivity of $1.35\times 10^{28}$ yr at 90% confidence level in 10 years of data taking, covering the parameter space associated with the inverted neutrino mass ordering, along with a significant portion of the parameter space for the normal ordering scenario, for almost all nuclear matrix elements. The effects of backgrounds deviating from the nominal values used for the projections are also illustrated, concluding that the nEXO design is robust against a number of imperfections of the model., Comment: 26 pages, 19 figures, version accepted by Journal of Phys. G

Single photon avalanche diodes (SPAD) are detectors used for applications requiring high timing resolution and single photon sensitivity. Medical imaging modalities such as positron emission tomography benefit from higher timing resolution enabling time-of-flight (ToF) measurements. One of the current challenge in the development of SiPM detectors is combining both high photodetection efficiency (PDE) and high single photon timing resolution (SPTR). Although SiPM have a high fill factor, they are limited for sub-10 ps FWHM SPTR for two reasons: the high output capacitance directly affects the rise time and a timing skew is associated with the SPAD position in the array. The proposed solution to obtain ToF capabilities with both high SPTR and PDE is to make a 3D digital SiPM where every SPAD is individually connected to a quenching circuit (QC) and a time-to-digital converter (TDC). While the 3D integration process is underway, we designed and tested a 2D version of a single channel of the detector composed of a SPAD, a QC and a TDC in TSMC CMOS 65 nm. This paper presents a SPAD and QC with 9 ps FWHM SPTR and a chain composed of a SPAD, a QC and a TDC with 17.5 ps FWHM SPTR.

Time-of-flight measurements are becoming essential to the advancement of several fields, such as preclinical positron emission tomography and high energy physics. Recent developments in single photon avalanche diode (SPAD)-based detectors have spawned a great interest in digital silicon photomultipliers (dSiPMs). To overcome the tradeoff between the photosensitive area and the processing capabilities in current 2-D dSiPM, we propose a novel 3-D digital SiPM, where the SPAD, designed for maximal photosensitive area, will be stacked in 3-D over the electronic circuits, designed in a CMOS node technology. All readout circuits will be implemented directly under the SPAD real estate, including quenching circuit, time-to-digital converter (TDC) and digital readout electronics. This paper focusses on the TDC element of this system, designed in TSMC CMOS 65 nm. This ring oscillator-based Vernier TDC requires only 25 $ {\times }$ 50 $ {\mu }\text{m}^{2}$ and 160 $ {\mu }\text{W}$ , and achieves 6.9 ps rms timing accuracy.

nEXO is a proposed tonne-scale neutrinoless double beta decay ($0\nu\beta\beta$) experiment using liquid ${}^{136}Xe$ (LXe) in a Time Projection Chamber (TPC) to read out ionization and scintillation signals. Between the field cage and the LXe vessel, a layer of LXe ("skin" LXe) is present, where no ionization signal is collected. Only scintillation photons are detected, owing to the lack of optical barrier around the field cage. In this work, we show that the light originating in the skin LXe region can be used to improve background discrimination by 5% over previous published estimates. This improvement comes from two elements. First, a fraction of the $\gamma$-ray background is removed by identifying light from interactions with an energy deposition in the skin LXe. Second, background from ${}^{222}Rn$ dissolved in the skin LXe can be efficiently rejected by tagging the $\alpha$ decay in the ${}^{214}Bi-{}^{214}Po$ chain in the skin LXe., Comment: 11 pages, 12 figures

Measurements of electron drift properties in liquid and gaseous xenon are reported. The electrons are generated by the photoelectric effect in a semi-transparent gold photocathode driven in transmission mode with a pulsed ultraviolet laser. The charges drift and diffuse in a small chamber at various electric fields and a fixed drift distance of 2.0 cm. At an electric field of 0.5 kV/cm, the measured drift velocities and corresponding temperature coefficients respectively are 1 . 97 ± 0 . 04 m m ∕ μ s and ( − 0 . 69 ± 0 . 05 ) %/K for liquid xenon, and 1 . 42 ± 0 . 03 m m ∕ μ s and ( + 0 . 11 ± 0 . 01 ) %/K for gaseous xenon at 1.5 bar. In addition, we measure longitudinal diffusion coefficients of 25 . 7 ± 4 . 6 cm 2 /s and 149 ± 23 cm 2 /s, for liquid and gas, respectively. The quantum efficiency of the gold photocathode is studied at the photon energy of 4.73 eV in liquid and gaseous xenon, and vacuum. These charge transport properties and the behavior of photocathodes in a xenon environment are important in designing and calibrating future large scale noble liquid detectors.

The 3D vertical integration of SPAD and readout electronics is a promising avenue to high performance photodetectors. This approach will minimize most limitations of current SiPM and lead to better performances in terms of effective PDE, timing and added functionalities. In this paper, we present a new integrated digital SiPM electronic architecture. This specific architecture aims to replace conventional analog SiPM with the added benefits of a significantly lower and constant output capacitance as well as immunity to SPAD to SPAD gain variation on the single photon resolution while providing an analog-like output signal waveform representing the sum of all triggered SPADs. This electronic architecture also offers options to turn off noisy SPADs, to reduce afterpulsing through a programmable hold-off time and to adjust the dynamic range of the output depending on the photon flux. While the 3D integration process development is underway, we realized a 2D version as a proof of principle of the electronic architecture. The 2D detector presented in this article is composed of an array of 242 pixels for a size of $1.1\times 1.1$ mm2. The output capacitance of the device is 5.6 pF/mm2, the timing jitter associated to the quenching circuit is about 40 ps FWHM and the maximum timing skew within the array is about 70 ps, which could still be optimized.

The characterization of nuisance parameters in digital silicon photomultipliers (SiPMs) is important to their understanding and future development. Methods able to distinguish the types of events are necessary to obtain fair and legitimate measurements. In this work, the zero photon probability (ZPP) method and the time delay (TD) method are used to measure the dark noise of digital SiPMs free from the contribution of correlated noise such as afterpulsing and crosstalk. It highlights the unique features of digital SiPMs such as the holdoff delay, the digital output signal, and the embedded processing (e.g. the selection of the interval sampling width). The two methods correctly separate the correlated and uncorrelated events in digital SiPMs and therefore the determination of a true photon detection efficiency (PDE) is possible. The ZPP method is also implemented inside a digital SiPM using embedded digital signal processing.

In this paper we report on the characterization of the Hamamatsu VUV4 (S/N: S13370-6152) Vacuum Ultra-Violet (VUV) sensitive Multi-Pixel Photon Counters (MPPC)s as part of the development of a solution for the detection of liquid xenon scintillation light for the nEXO experiment. Various MPPC features, such as: dark noise, gain, correlated avalanches, direct crosstalk and Photon Detection Efficiency (PDE) were measured in a dedicated setup at TRIUMF. MPPCs were characterized in the range 163 K ≤ T ≤ 233 K . At an over voltage of 3 . 1 ± 0 . 2 V and at T = 163 K we report a number of Correlated Avalanches (CAs) per pulse in the 1 μ s interval following the trigger pulse of 0 . 161 ± 0 . 005 . At the same settings the Dark-Noise (DN) rate is 0 . 137 ± 0 . 002 Hz/mm 2 . Both the number of CAs and the DN rate are within nEXO specifications. The PDE of the Hamamatsu VUV4 was measured for two different devices at T = 233 K for a mean wavelength of 189 ± 7 nm . At 3 . 6 ± 0 . 2 V and 3 . 5 ± 0 . 2 V of over voltage we report a PDE of 13 . 4 ± 2 . 6 % and 11 ± 2 % , corresponding to a saturation PDE of 14 . 8 ± 2 . 8 % and 12 . 2 ± 2 . 3 % , respectively. Both values are well below the 24 % saturation PDE advertised by Hamamatsu. More generally, the second device tested at 3 . 5 ± 0 . 2 V of over voltage is below the nEXO PDE requirement. The first one instead yields a PDE that is marginally close to meeting the nEXO specifications. This suggests that with modest improvements the Hamamatsu VUV4 MPPCs could be considered as an alternative to the FBK-LF Silicon Photo-Multipliers for the final design of the nEXO detector.

Characterization of the vacuum ultraviolet (VUV) reflectance of silicon photomultipliers (SiPMs) is important for large-scale SiPM-based photodetector systems. We report the angular dependence of the specular reflectance in a vacuum of SiPMs manufactured by Fondazionc Bruno Kessler (FBK) and Hamamatsu Photonics K.K. (HPK) over wavelengths ranging from 120 nm to 280 nm. Refractive index and extinction coefficient of the thin silicon-dioxide film deposited on the surface of the FBK SiPMs are derived from reflectance data of a FBK silicon wafer with the same deposited oxide film as SiPMs. The diffuse reflectance of SiPMs is also measured at 193 nm. We use the VUV spectral dependence of the optical constants to predict the reflectance of the FBK silicon wafer and FBK SiPMs in liquid xenon.

nEXO is a proposed experiment to search for the neutrino-less double beta decay ($0\nu\beta\beta$) of $^{136}$Xe in a tonne-scale liquid xenon time projection chamber (TPC). The nEXO TPC will be equipped with charge collection tiles to form the anode. In this work, the charge reconstruction performance of this anode design is studied with a dedicated simulation package. A multi-variate method and a deep neural network are developed to distinguish simulated $0\nu\beta\beta$ signals from backgrounds arising from trace levels of natural radioactivity in the detector materials. These simulations indicate that the nEXO TPC with charge-collection tiles shows promising capability to discriminate the $0\nu\beta\beta$ signal from backgrounds. The estimated half-life sensitivity for $0\nu\beta\beta$ decay is improved by $\sim$20$~(32)\%$ with the multi-variate~(deep neural network) methods considered here, relative to the sensitivity estimated in the nEXO pre-conceptual design report.

Understanding reflective properties of materials and photodetection efficiency (PDE) of photodetectors is important for optimizing energy resolution and sensitivity of the next generation neutrinoless double beta decay, direct detection dark matter, and neutrino oscillation experiments that will use noble liquid gases, such as nEXO, DARWIN, DarkSide-20k, and DUNE. Little information is currently available about reflectivity and PDE in liquid noble gases, because such measurements are difficult to conduct in a cryogenic environment and at short enough wavelengths. Here we report a measurement of specular reflectivity and relative PDE of Hamamatsu VUV4 silicon photomultipliers (SiPMs) with 50 micrometer micro-cells conducted with xenon scintillation light (~175 nm) in liquid xenon. The specular reflectivity at 15 deg. incidence of three samples of VUV4 SiPMs is found to be 30.4+/-1.4%, 28.6+/-1.3%, and 28.0+/-1.3%, respectively. The PDE at normal incidence differs by +/-8% (standard deviation) among the three devices. The angular dependence of the reflectivity and PDE was also measured for one of the SiPMs. Both the reflectivity and PDE decrease as the angle of incidence increases. This is the first measurement of an angular dependence of PDE and reflectivity of a SiPM in liquid xenon., Comment: 21 pages, 15 figures, 6 tables. As accepted by JINST

Including precise time-of-flight measurements in the reconstruction of positron emission tomography (PET) images reduces the required radiotracer dose, scan time and improves the contrast-to-noise ratio. However, major improvements of photodetectors and scintillators are still required to fully exploit the projected 10 ps FWHM coincidence timing resolution (CTR). On the photodetector side, single photon avalanche diode (SPAD) detectors based on digital architectures promise better CTR by timestamping multiple scintillation photons from a single PET event with an array of in-pixel time-to-digital converters (TDCs). The digital nature of these architectures allow the introduction of data processing schemes to optimally combine the produced timestamps for a scintillation event with a time of arrival estimator. This processing is typically performed on an external computing platform, which requires the transfer of large quantities of raw data and leads to a higher dead time owing to the large number of TDCs. To circumvent this issue, we propose a photodetector architecture with in-pixel TDCs that embeds the data processing scheme, reducing the dataset produced by the TDC array to a single timestamp per scintillation event. Hardware-in-the-loop simulations of the fabricated CMOS 65 nm post-processing circuit coupled with simulated SPADs and 1 × 1 × 10 mm3 LYSO scintillators demonstrate a CTR improvement from 160 ps to 126 ps, a bandwidth reduction from 34.7 Mbit/s to 0.5 Mbit/s and a serial transmission dead time reduction from 69 μ s to 1 μ s.

Future tonne-scale liquefied noble gas detectors depend on efficient light detection in the VUV range. In the past years Silicon Photomultipliers (SiPMs) have emerged as a valid alternative to standard photomultiplier tubes or large area avalanche photodiodes. The next generation double beta decay experiment, nEXO, with a 5 tonne liquid xenon time projection chamber, will use SiPMs for detecting the $178\,\text{nm}$ xenon scintillation light, in order to achieve an energy resolution of $\sigma / Q_{\beta\beta} = 1\, \%$. This paper presents recent measurements of the VUV-HD generation SiPMs from Fondazione Bruno Kessler in two complementary setups. It includes measurements of the photon detection efficiency with gaseous xenon scintillation light in a vacuum setup and dark measurements in a dry nitrogen gas setup. We report improved photon detection efficiency at $175\,\text{nm}$ compared to previous generation devices, that would meet the criteria of nEXO. Furthermore, we present the projected nEXO detector light collection and energy resolution that could be achieved by using these SiPMs., Comment: 11 pages, 13 figures, 2 tables

The next-generation Enriched Xenon Observatory (nEXO) is a proposed experiment to search for neutrinoless double beta ($0\nu\beta\beta$) decay in $^{136}$Xe with a target half-life sensitivity of approximately $10^{28}$ years using $5\times10^3$ kg of isotopically enriched liquid-xenon in a time projection chamber. This improvement of two orders of magnitude in sensitivity over current limits is obtained by a significant increase of the $^{136}$Xe mass, the monolithic and homogeneous configuration of the active medium, and the multi-parameter measurements of the interactions enabled by the time projection chamber. The detector concept and anticipated performance are presented based upon demonstrated realizable background rates., Comment: v2 as published

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Coincidence timing resolution (CTR) of 10 ps FWHM would drastically increase the contrast of images in pre-clinical and clinical positron emission tomography (PET) due to the capability to localize the annihilation site along the line of response with unprecedented accuracy. Currently, the scintillators and the photodetectors with their electronic readout are the two limiting factors to reach such a timing resolution. On the photodetector side, the single photon timing resolution (SPTR) must be improved below 4 ps RMS to reach a CTR of 10 ps FWHM. To this end, we propose to use a 3D digital silicon photomultiplier (3DdSiPM) where each single photon avalanche diode (SPAD) has its own quenching circuit (QC) and time-to-digital converter (TDC). To reach an SPTR of 4 ps RMS in an array of SPAD requires to correct each readout pixel individually to minimize the impact of TDC least significant bit (LSB) non-uniformities and SPAD-to-SPAD skew. For this purpose, we developed an array of 256 pixels of SPAD readout circuits optimized for high timing precision with embedded digital signal processing. The latter is a code-to-time conversion where each pixel is individually corrected for TDC LSB variation and skew. Measurement results on single pixels (QC and TDC) show timing jitter down to 8 ps RMS. The uncorrected array timing jitter is about 87 ps RMS and is reduced to 18 ps RMS after skew correction and TDC LSB correction.

A new design for the anode of a time projection chamber, consisting of a charge-detecting "tile", is investigated for use in large scale liquid xenon detectors. The tile is produced by depositing 60 orthogonal metal charge-collecting strips, 3~mm wide, on a 10~\si{\cm} $\times$ 10~\si{\cm} fused-silica wafer. These charge tiles may be employed by large detectors, such as the proposed tonne-scale nEXO experiment to search for neutrinoless double-beta decay. Modular by design, an array of tiles can cover a sizable area. The width of each strip is small compared to the size of the tile, so a Frisch grid is not required. A grid-less, tiled anode design is beneficial for an experiment such as nEXO, where a wire tensioning support structure and Frisch grid might contribute radioactive backgrounds and would have to be designed to accommodate cycling to cryogenic temperatures. The segmented anode also reduces some degeneracies in signal reconstruction that arise in large-area crossed-wire time projection chambers. A prototype tile was tested in a cell containing liquid xenon. Very good agreement is achieved between the measured ionization spectrum of a $^{207}$Bi source and simulations that include the microphysics of recombination in xenon and a detailed modeling of the electrostatic field of the detector. An energy resolution $\sigma/E$=5.5\% is observed at 570~\si{keV}, comparable to the best intrinsic ionization-only resolution reported in literature for liquid xenon at 936~V/\si{cm}., Comment: 18 pages, 13 figures, as published

The timing performance of digital silicon photomultipliers (dSiPMs) detectors have placed them as serious candidates for time-of-flight (TOF) measurement in positron emission tomography (PET). Contributors to timing uncertainty in current dSiPM scintillation detectors include: 1) routing skew and array non-uniformities, 2) SPAD dark counts and 3) scintillator statistics. A photodetector with a one-to-one coupling of timeto- digital converters (TDCs) and single photon avalanche diodes (SPADs) is under study as it opens new methods to minimize the impact these contributors. However, the number of TDCs required for this coupling scheme generates a tremendous amount of data. Acknowledging these design considerations, a real-time processing scheme is proposed to minimize the bandwidth while improving the timing resolution by 1) correcting routing skews and array non-uniformities, 2) filtering dark counts and 3) estimating the time of interaction from multiple photoelectron timestamps. Simulations were conducted to evaluate the impact of the contributors to timing uncertainty and the efficiency of the proposed architecture. With a simulated 1 x 1 x 10 mm3 LYSO scintillator, the coincidence timing resolution is improved from 161 ps to 115 ps with the simulated detector.

This paper presents a new quenching circuit (QC) and single photon avalanche diode (SPAD) implemented in TSMC CMOS 65 nm technology. The QC was optimized for single photon timing resolution (SPTR) with a view to an implementation in a 3D digital SiPM. The presented QC has a timing jitter of 4 ps full width at half maximum (FWHM) and the SPAD and QC has a 7.8 ps FWHM SPTR. The QC adjustable threshold allows timing resolution optimization as well as SPAD excess voltage and rise time characterization. The adjustable threshold, hold-off and recharge are essential to optimize the performances of each SPAD. This paper also provides a better understanding of the different contributions to the SPTR. A study of the contribution of the SPAD excess voltage variation combined to the QC time propagation delay variation is presented. The proposed SPAD and QC eliminates the SPAD excess voltage contribution to the SPTR for excess voltage higher than 1 V due to its fixed time propagation delay.

We report on the performance of silicon photomultiplier (SiPM) light sensors operating in electric field strength up to 30 kV/cm and at a temperature of 149K, relative to their performance in the absence of an external electric field. The SiPM devices used in this study show stable gain, photon detection efficiency, and rates of correlated pulses, when exposed to external fields, within the estimated uncertainties. No observable physical damage to the bulk or surface of the devices was caused by the exposure., Comment: 16 pages, 12 figures, 2 tables and two conferences (INPC2016 and TIPP2017)

The aim of our study was to assess the use of orthodontic appliances made in titanium for patients with an allergy to resin and nickel. We aim to highlight the resolution of problems such as stomatitis and peri-labial dermatitis, which generally manifest in patients who use traditional orthodontic appliances in acrylic resin and steel.A total of 120 patients of ages between ten and fifteen undergoing orthodontic treatment requiring a rapid palatal expander or a mobile appliance were evaluated and two patch sensitivity tests were done to assess the presence of allergies.The biocompatibility of a material is directly dependent on its corrosion effect. If a metal does not allow the release of ions it will not have a damaging action or cause destruction on the cellular DNA. The oxidation status of an ion is related to the reactivity of the ion itself and thus may give the latter a carcinogenic effect. Titanium appliances used for this study have obtained excellent results due to their ability to form superficial oxides, which block the oxidation phenomenon and thus corrosion.We have noticed a total regression of the symptoms after the use of titanium appliances in patients who had a sensitivity reaction.

Aim of the present study was to present a case report of a patient suffering from pycnodysostosis and assess how it may manifest and affect the dental/ orthodontic treatment.An 11-year-old patient who was diagnosed with pycnodysostosis at the age of 18 months, attended the orthodontics department requiring treatment for extensive carious lesions, periodontal disease and severe crowding.After an accurate radiographic, clinical and orthodontic assessment of the patient taking into consideration the severity of the condition and the patient's necessities, we have formulated a treatment plan, which was accepted by both the patient and parents. We took into consideration the risks and benefits of the options available and the requests of the patient.We have decided to opt for the extraction of teeth to relieve the crowding as this was impeding good oral hygiene and hence increasing the risk of caries and periodontal disease. Such patients must be placed under an oral hygiene prevention scheme, a treatment plan must be accurately designed and the patient must be constantly motivated.

The aim of this paper was to describe the qualitative and quantitative changes occurring within the oral bacterial flora of several groups of patients following oral prevention protocols during the stages of the dental treatment they required.Three hundred patients were enrolled in the study. The selected patients were categorized into different groups to receive preventive strategies according to their needs and treatment required. Plaque samples from each patient were collected at different intervals according to the treatment they were undergoing. These were then assessed under a microscope to carry out a quantitative and qualitative assessment of the plaque.The Mean Plaque Index Score of most patients generally decreased during the various treatment phases and hence the overall bacterial count. However, we noticed a slight increase in the plaque index in patients undergoing orthodontic surgery after placement of the orthodontic appliance and patients undergoing combined orthodontic-surgical treatment during the intermaxillary fixation phase. We generally found that the coccoidal bacterial form was the most prevalent.Patients who were adequately instructed and motivated through oral hygiene prevention strategies, showed a significant decrease in the plaque levels and in the overall bacterial components between the first visit and the successive sample taking. Even though we noticed a slight increase in the plaque index in patients undergoing the intermaxillary fixation phase, this decreased immediately once the phase ended and the patients managed to return to the routine oral hygiene care. This highlights the importance of constant motivation and oral hygiene instruction reinforcement.