Low-background Spectra Research Articles

Influence of baseline subtraction on laser-induced breakdown spectroscopic data

Automatic baseline subtraction significantly improves the quality of high-background spectra but might harm low-background spectra.

Search for neutrinoless double-electron capture ofDy156

Background: Multiple large collaborations are currently searching for neutrinoless double-$\ensuremath{\beta}$ decay, with the ultimate goal of differentiating the Majorana-Dirac nature of the neutrino.Purpose: Investigate the feasibility of resonant neutrinoless double-electron capture, an experimental alternative to neutrinoless double-$\ensuremath{\beta}$ decay.Method: Two clover germanium detectors were operated underground in coincidence to search for the de-excitation $\ensuremath{\gamma}$ rays of $^{156}\mathrm{Gd}$ following the neutrinoless double-electron capture of $^{156}\mathrm{Dy}$. 231.95 d of data were collected at the Kimballton underground research facility with a 231.57 mg enriched $^{156}\mathrm{Dy}$ sample.Results: No counts were seen above background and half-life limits are set at $O({10}^{16}\text{--}{10}^{18})$ yr for the various decay modes of $^{156}\mathrm{Dy}$.Conclusion: Low background spectra were efficiently collected in the search for neutrinoless double-electron capture of $^{156}\mathrm{Dy}$, although the low natural abundance and associated lack of large quantities of enriched samples hinders the experimental reach.

Excitation Mechanism of H, He, C, and F Atoms in Metal-Assisted Atmospheric Helium Gas Plasma Induced by Transversely Excited Atmospheric-Pressure CO2Laser Bombardment

To clarify the excitation mechanism of hydrogen in transversely excited atmospheric-pressure (TEA) CO2 laser-induced helium gas plasma, atomic emission characteristics of H, C, F, and He were studied using a Teflon sheet (thickness of 2 mm) attached to a metal subtarget. The TEA CO2 laser (750 mJ, 200 ns) was focused on the Teflon sheet in the surrounding He gas at 1 atm. Atomic emissions of H, C, F, and He occurred with a long lifetime, a narrow spectrum width, and a low-background spectrum. The correlation emission intensity curves of H–He and F–He indicated a parabolic functions. To explain the emission characteristics, we offered a model in which helium metastable atoms (He*) play an important role in the excitation processes; namely, atoms collide with helium metastable atoms (He*) to be ionized by the Penning effect, and then recombine with electrons to produce excited states, from which atomic emissions occur.

Excitation Mechanism of H, He, C, and F Atoms in Metal-Assisted Atmospheric Helium Gas Plasma Induced by Transversely Excited Atmospheric-Pressure CO2 Laser Bombardment

To clarify the excitation mechanism of hydrogen in transversely excited atmospheric-pressure (TEA) CO2 laser-induced helium gas plasma, atomic emission characteristics of H, C, F, and He were studied using a Teflon sheet (thickness of 2 mm) attached to a metal subtarget. The TEA CO2 laser (750 mJ, 200 ns) was focused on the Teflon sheet in the surrounding He gas at 1 atm. Atomic emissions of H, C, F, and He occurred with a long lifetime, a narrow spectrum width, and a low-background spectrum. The correlation emission intensity curves of H–He and F–He indicated a parabolic functions. To explain the emission characteristics, we offered a model in which helium metastable atoms (He*) play an important role in the excitation processes; namely, atoms collide with helium metastable atoms (He*) to be ionized by the Penning effect, and then recombine with electrons to produce excited states, from which atomic emissions occur.

New Method of Laser Plasma Spectroscopy for Metal Samples Using Metastable He Atoms Induced by Transversely Excited Atmospheric-Pressure CO2 Laser in He Gas at 1 atm

A high-power transversely excited atmospheric-pressure (TEA) CO2 laser (600–1500 mJ) was focused on a metal target in surrounding He gas at 1 atm to produce strong He gas breakdown plasma, while leaving the metal surface undamaged, and a Q-switched Nd:YAG laser (60–90 mJ) was also simultaneously focused on the metal surface to be ablated at a slightly shifted spot position with various delay times (0–150 µs) relative to the TEA CO2 laser. It was concluded that the ablated atoms from the metal sample enter the He gas plasma region and are excited by metastable He atoms, which are produced in very large quantities in the He gas plasma region. Due to the fact that He gas plasma works as an exciting source with a very long lifetime (about 100 µs), the detected atomic emission lines are very narrow in spectral width with a very low background spectrum, and also the atomic emission spectrum profile does not follow the Boltzmann distribution during the long emission life. We call this new method LIPS-He*. This new method has high potential for realizing in situ high-sensitivity analysis.

Improved peptide identification in proteomics by two consecutive stages of mass spectrometric fragmentation.

MS-based proteomics usually involves the fragmentation of tryptic peptides (tandem MS or MS(2)) and their identification by searching protein sequence databases. In ion trap instruments fragments can be further fragmented and analyzed, a process termed MS/MS/MS or MS(3). Here, we report that efficient ion capture in a linear ion trap leads to MS(3) acquisition times and spectra quality similar to those for MS(2) experiments with conventional 3D ion traps. Fragmentation of N- or C-terminal ions resulted in informative and low-background spectra, even at subfemtomol levels of peptide. Typically C-terminal ions are chosen for further fragmentation, and the MS(3) spectrum greatly constrains the C-terminal amino acids of the peptide sequence. MS(3) spectra allow resolution of ambiguities in identification, a crucial problem in proteomics. Because of the sensitivity and rapid scan rates of the linear ion trap, several MS(3) spectra per peptide can be obtained even when sequencing very complex mixtures. We calculate the probability that an experimental MS(3) spectrum originates from fragmentation of a given N- or C-terminal ion of a peptide under consideration. This MS(3) identification score can be combined with the MS(2) scores of the precursor peptide from existing search engines. When MS(3) is performed on the linear ion trap-Fourier transform mass spectrometer combination, accurate peptide masses further increase confidence in peptide identification.

Poly(dimethylsiloxane) electrospray devices fabricated with diamond-like carbon-poly(dimethylsiloxane) coated SU-8 masters.

This study presents coupling of a poly(dimethylsiloxane) (PDMS) micro-chip with electrospray ionization-mass spectrometry (ESI-MS). Stable electrospray is generated directly from a PDMS micro-channel without pressure assistance. Hydrophobic PDMS aids the formation of a small Taylor cone in the ESI process and facilitates straightforward and low-cost batch production of the ESI-MS chips. PDMS chips were replicated with masters fabricated from SU-8 negative photoresist. A novel coating, an amorphous diamond-like carbon-poly(dimethylsiloxane) hybrid, deposited on the masters by the filtered pulsed plasma arc discharge technique, improved significantly the lifetime of the masters in PDMS replications. PDMS chip fabrication conditions were observed to affect the amount of background peaks in the MS spectra. With an optimized fabrication process (PDMS curing agent/silicone elastomer base ratio of 1/8 (w/w), curing at 70 degree C for 48 h) low background spectra were recorded for the analytes. The performance of PDMS devices was examined in the ESI-MS analysis of some pharmaceutical compounds and amino acids.

Monte Carlo simulations of low background detectors

An implementation of the Electron Gamma Shower 4 code (EGS4) has been developed to allow convenient simulation of typical gamma ray measurement systems. Coincidence gamma rays, beta spectra, and angular correlations have been added to adequately simulate a complete nuclear decay and provide corrections to experimentally determined detector efficiencies. This code has been used to strip certain low-background spectra for the purpose of extremely low-level assay. Monte Carlo calculations of this sort can be extremely successful since low background detectors are usually free of significant contributions from poorly localized radiation sources, such as cosmic muons, secondary cosmic neutrons, and radioactive construction or shielding materials. Previously, validation of this code has been obtained from a series of comparisons between measurements and blind calculations. An example of the application of this code to an exceedingly low background spectrum stripping will be presented.

Simulated progress in double-beta decay

A Monte Carlo code has been developed to accurately simulate double-beta decay measurements. Coincident gamma rays, beta spectra, and angular correlations have been added to adequately simulate a complete 100Mo nuclear decay and provide corrections to experimentally determined detector efficiencies. This code has been used to strip certain low-background spectra obtained in the Homestake gold mine in Lead, SD, for the purpose of extremely sensitive materials assay for the construction of new, large, enriched germanium detectors. Assays as low as 9 μBq/g of 210Pb in lead shielding were obtained.

Versatile apparatus for electron impact ionization experiments at high energies (500 keV)

A new high-energy (500 keV) electron–electron coincidence apparatus is described making possible for the first time triply differential cross-section measurements of inner-shell ionization of atoms of a high atomic number. Moreover, the apparatus has also been designed to deliver doubly differential ionization cross sections of the individual shells by using electron–x-ray coincidences as well as noncoincident low background spectra of inelastically scattered electrons. A special feature of the apparatus is the use of electron spectrum analyzers consisting of a nondispersive magnet and a surface barrier detector. Magnetic electron spectrum analyzers consisting of a sector field magnet and a scintillation detector have been built up. Selected measurements on tantalum (Z=73), silver (Z=47), and aluminum (Z=13) target foils are presented.

An improved method for measuring relative stopping powers of light ions in solids

An improved method is presented for measuring relative stopping powers of light ions in solids by means of Rutherford backscattering. Special care has been taken to obtain low-background spectra, and a procedure (to measure a given sequence of targets again and again) has been used to avoid uncertainties in beam normalization. The target composition can be investigated in situ. Measured stopping-power ratios between copper and silver for 200–2000 keV hydrogen and helium agree with semiempirical tabulations. Results for lanthanum stopping powers are presented to demonstrate the applicability of the method to highly reactive materials.

Measurements of the radiative widths of the first T = 1, 2 + state of 20Ne and their relationship to analog β-decays

The radiative widths for decays of the 20Ne T = 1, 2 + (10.27 MeV) state were measured by resonance α-capture in the reaction 16O(α, γ) 20Ne. A special windowless gas-cell target yielded a low-background spectrum enabling six γ-branches to be observed with a Ge(Li) detector. The six branches correspond to decays from the 10.27 MeV level to the following levels: 2 +(7.83 MeV), 2 +(7.42 MeV), 3 −(5.62 MeV), 2 −(4.97 MeV), 2 +(1.63 MeV) and 0 +(g.s.). The branching ratios and radiative widths Γ γ to these levels are: 7.83 MeV [(0.22 ± 0.06)%, 0.008 ± 0.002 eV], 7.42 MeV [(6.9 ± 0.4)%, 0.31 ± 0.04 eV], 5.62 MeV [(2.1 ± 0.2)%, 0.097 ± 0.014 eV], 4.97 MeV [(1.3 ± 0.1)%, 0.060 ± 0.008 eV], 1.63 MeV [(88.9 ± 0.5)%, 4.08 ± 0.43 eV] and 0.0 MeV [(0.64 ± 0.14)%, 0.029 ± 0.008 eV]. The radiative widths to the 1.63 MeV and 7.42 MeV levels are used to determine the CVC predictions of the weak magnetism form factors and their effects on certain β-decay observables are evaluated.

A study on proton induced X-ray analysis and its application to environmental samples

Proton induced X-ray analysis has been studied by the 5 MV Van de Graaff of Tohoku University emphasizing on backing materials and angular dependence of background. The method has further been applied to analyses of some environmental samples. From experimental results, it was found that 4 μm Mylar foil among Formvar, Mylar and carbon gives sufficiently low background spectrum, high sensitivity and good mechanical strength. The detection limit is much improved at the backward direction with respect to the incident beam comparing with that at 90° which has usually been adopted. The analytical results on soil and aerosol samples were compared with those by the Atomic Absorption Spectrophotometrical method (AAS). Vanadium can easily be analysed by this method, but not by the AAS method. Milk and human milk were also analysed.

Application of proton-induced X-ray emission to quantitative trace element analysis

Abstract Proton-induced X-ray analysis was studied at the 5 MV Van de Graaff accelerator of Tohoku University with emphasis on backing materials and the angular dependence of the background due to bremsstrahlung. From the practical viewpoint, 4 μm Mylar films among Formvar, carbon and Mylar gave sufficiently low background spectra and good mechanical strength. The detection limit was much improved in the backward direction in comparison with that at 90° which has usually been adopted. A detection sensitivity on the subpicogram level was obtained for an extensive range of elements covering a wide range of atomic number using 4 μm Mylar film backing.