Physics

The Lanthanum Halide scintillator detectors have been widely used for nuclear spectroscopy experiments because of their excellent energy and time resolutions. Despite having these advantages, the intrinsic alpha and beta contaminations in these scintillators pose a severe limitation in their usage in rare-event detections. In the present work, pulse shape discrimination (PSD) with a fast digitizer has been shown to be an efficient method to separate the effect of alpha contamination from the spectrum. The shape of the beta spectrum has been generated with the help of Monte Carlo based simulation code, and its contribution has been eliminated from the spectrum. The reduction in the background events generated by both intrinsic beta and alpha activities has been demonstrated. The present study will encourage the application of these detectors in low cross-section measurement experiments relevant to nuclear astrophysics.

A photon-counting silicon strip detector with two energy thresholds was investigated for spectral X-ray imaging in a mammography system. Preliminary studies already indicate clinical benefit of the detector, and the purpose of the present study is optimization with respect to energy resolution. Factors relevant for the energy response were measured, simulated, or gathered from previous studies, and used as input parameters to a cascaded detector model. Threshold scans over several X-ray spectra were used to calibrate threshold levels to energy, and to validate the model. The energy resolution of the detector assembly was assessed to range over DeltaE/E = 0.12-0.26 in the mammography region. Electronic noise dominated the peak broadening, followed by charge sharing between adjacent detector strips, and a channel-to-channel threshold spread. The energy resolution may be improved substantially if these effects are reduced to a minimum. Anti-coincidence logic mitigated double counting from charge sharing, but erased the energy resolution of all detected events, and optimization of the logic is desirable. Pile-up was found to be of minor importance at typical mammography rates.

We report measurements of enhanced tensor polarization on solid-state targets. The results here represent an increase in tensor polarization over that previously achieved in high energy and nuclear scattering experiments that focused on the measurement of tensor polarized observables. Enhancement techniques are used which require RF produced close to the Larmor frequency of the target spins and use selective semi-saturation resulting from two sources of irradiation, microwave for the DNP process and the additional RF used to manipulate the population of the energy levels in the target material. The spin dynamics of the solid target are used to align the spins enhancing the ensemble average to improve the figure of merit of the scattering experiment. Target rotation at an optimized rate can lead to additional enhancement by applying selective semi-saturation in polycrystalline materials that possess a Pake doublet in their NMR signal.

Scanning Laser Doppler Vibrometer (SLDV) measurements are affected by sensor head vibrations as if they are vibrations of the target surface itself. This paper presents practical correction schemes to solve this important problem. The study begins with a theoretical analysis, for arbitrary vibration and any scanning configuration, which shows that the only measurement required is of the vibration velocity at the incident point on the final steering mirror in the direction of the outgoing laser beam and this underpins the two correction options investigated. Correction sensor location is critical; the first scheme uses an accelerometer pair located on the SLDV front panel, either side of the emitted laser beam, while the second uses a single accelerometer located along the optical axis behind the final steering mirror. Initial experiments with a vibrating sensor head and stationary target confirmed the sensitivity to sensor head vibration together with the effectiveness of the correction schemes which reduced overall error by 17 dB (accelerometer pair) and 27 dB (single accelerometer). In extensive further tests with both sensor head and target vibration, conducted across a range of scan angles, the correction schemes reduced error by typically 14 dB (accelerometer pair) and 20 dB (single accelerometer). RMS phase error was also up to 30% lower for the single accelerometer option, confirming it as the preferred option. The theory suggests a geometrical weighting of the correction measurements and this provides a small additional improvement. Since the direction of the outgoing laser beam and its incident point on the final steering mirror both change as the mirrors scan the laser beam, the use of fixed axis correction transducers mounted in fixed locations makes the correction imperfect. The associated errors are estimated and expected to be generally small, and the theoretical basis...

Recently, X-ray interferometers with more than one phase grating have been developed for differential phase contrast (DPC) imaging. In this study, a novel framework is developed to predict such interferometers' angular sensitivity responses (the minimum detectable refraction angle). Experiments are performed on the dual and triple phase grating interferometers, separately. Measurements show strong consistency with the predicted sensitivity values. Using this new approach, the DPC imaging performance of X-ray interferometers with multiple phase gratings can be further optimized for future biomedical applications.

In the future ATLAS Inner Tracker, each silicon strip module will be equipped with a switch able to separate the high voltage supply from the sensor in case the latter becomes faulty. The switch, placed in between the HV supply and the sensor, needs to sustain a high voltage in its OFF state, to offer a low resistance path for the sensor leakage current in the ON state, and be radiation hard up to 1.2e15 neq/cm2 along with other requirements. While GaN JFETs have been selected as suitable rad-hard switch, a silicon vertical HV-JFET was developed by Brookhaven National Laboratory as an alternative option. Pre-irradiation results showed the functionality of the device and proved that the silicon HV-JFET satisfied the pre-irradiation requirements for the switch. To assess its suitability after irradiation, a few p-type HV-JFETs have been neutron irradiated at Jozef Stefan Institute (JSI, Ljubljana, Slovenia). This paper reports the static characterization of these irradiated devices and the TCAD numerical simulations used to get an insight of the physics governing the post-irradiation behaviour.

nEXO is a proposed tonne-scale neutrinoless double beta decay ( 0νββ ) 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 γ -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 α decay in the 214 Bi− 214 Po chain in the skin LXe.

Large-volume liquid scintillator detectors with ultra-low background levels have been widely used to study neutrino physics and search for dark matter. Event vertex and event time are not only useful for event selection but also essential for the reconstruction of event energy. In this study, four event vertex and event time reconstruction algorithms using charge and time information collected by photomultiplier tubes were analyzed comprehensively. The effects of photomultiplier tube properties were also investigated. The results indicate that the transit time spread is the main effect degrading the vertex reconstruction, while the effect of dark noise is limited. In addition, when the event is close to the detector boundary, the charge information provides better performance for vertex reconstruction than the time information.

Muon radiography is a promising technique to image the internal density structures upto a few hundred meters scale, such as tunnels, pyramids and volcanos, by measuring the flux attenuation of cosmic ray muons after trvaling through these targets. In this study, we conducted an experimantal cosmic ray muon radiography of the Wudalianchi volcano in northeast China for imaging its internal density structures. The muon detector used in this study is made of plastic scintillator and silicon photomultiplier. After about one and a half month observation for the Laoheishan volcano cone in the Wudalianchi volcano, from September 23rd to November 10th, 2019, more than 3 million muon tracks passing the data selection criteria are obtained. Based on the muon observations and the high-resoluiton topography from aerial photogrammetry by unmanned aerial vehicle, the relative density image of the Laoheishan volcano cone is obtained. The experiment in this study is the first muon radiography of volcano performed in China, and the results suggest the feasibility of radiography technique based on plastic scintillator muon detector. As a new passive geophysical imaging method, cosmic ray muon radiography could become a promising method to obtain the high-resoution 2-D and 3-D density structures for shallow geological targets.

The production of negative ions in cesium sputter ion sources is generally considered to be a pure surface process. It has been recently proposed that ion pair production could explain the higher-than-expected beam currents extracted from these ion sources, therefore opening the door for laser-assisted enhancement of the negative ion yield. We have tested this hypothesis by measuring the effect of various pulsed diode lasers on the O − beam current produced from Al 2 O 3 cathode of a cesium sputter ion source. It is expected that the ion pair production of O − requires populating the 5d electronic states of neutral cesium, thus implying that the process should be provoked only with specific wavelengths. Our experimental results provide evidence for the existence of a wavelength-dependent photo-assisted effect but cast doubt on its alleged resonant nature as the prompt enhancement of beam current can be observed with laser wavelengths exceeding a threshold photon energy. The beam current transients observed during the laser pulses suggest that the magnitude and longevity of the beam current enhancement depends on the cesium balance on the cathode surface. We conclude that the photo-assisted negative ion production could be of practical importance as it can more than double the extracted beam current under certain operational settings of the ion source.