Double Escape Research Articles

Identification of single-site events in germanium detectors by digital pulse shape analysis

A method that discriminates between single- and multiple-site interactions in germanium detectors was developed. For this purpose the output of customary charge-sensitive preamplifiers was differentiated with 10–20 ns sampled with 250 MHz and analyzed off-line. Multiple-site interactions were measured in the 1621 keV photo peak and single-site events in the 1592 keV double escape line of 208Tl. One single parameter describing the broadness of the charge pulse maximum is sufficient to achieve 80% detection efficiencies for both the interaction types, over a wide energy range. As examples, employment of the method in the Heidelberg–Moscow experiment resulted in the consistent half-life of two neutrino double beta decay and the reduction of the background by a factor of three in searches for the neutrinoless double beta decay. Identification of single Compton scattered events improved the minimal detectable activity of a 25% efficiency detector by a factor of two.

Monte Carlo calculations of the response features for NaI detectors

The response features for sodium iodide detector for a monoenergetic gamma-ray source are calculated because of the importance of this type of detector in industry and research. The response features calculated are: full energy peak, single scattered escape, single escape peak, double scattered escape, double escape peak, besides single scatter, double scatter and triple scatter continua. These features are compared for different sizes and geometric efficiencies of the detector. The Monte Carlo simulation provides the weighting factors of the previously mentioned eight features of the response for the Nal detector that are necessary for response function generation for that type of detectors.

Single point mutations may affect the serotype reactivity of serotype G11 porcine rotavirus strains: a widening spectrum?

A panel of single and double neutralization-resistant escape mutants of serotype G11 porcine rotavirus strains A253 and YM, selected with G11 monotype- and serotype-specific neutralizing monoclonal antibodies (MAbs) to VP7, was tested in neutralization assays with hyperimmune sera raised against rotavirus strains of different serotypes. Escape mutants with an amino acid substitution in antigenic region A (amino acids [aa] 87 to 101) resulting in a residue identical or chemically similar to those present at the same positions in serotype G3 strains, at positions 87 for strain A253 and 96 for strain YM, were significantly more sensitive than the parental strains to neutralization with sera against some serotype G3 strains. Also, one YM antigenic variant (YM-5E6.1) acquired reactivity by enzyme-linked immunosorbent assay with MAbs 159, 57/8, and YO-1E2, which react with G3 strains, but not with the serotype G11 parental strain YM. Cross-adsorption studies suggested that the observed cross-neutralization by the G3-specific sera was due to the sera containing antibodies reactive with the parental strain plus antibodies reactive with the epitope(s) on the antigenic variant that mimick the serotype G3 specific one(s). Moreover, antibodies reactive with antigenic region F (aa 235 to 242) of VP7 might also be involved since cross-reactivity to serotype G3 was decreased in double mutants carrying an additional mutation, which creates a potential glycosylation site at position 238. Thus, single point mutations can affect the serotype reactivity of G11 porcine rotavirus strains with both monoclonal and polyclonal antibodies and may explain the origin of rotavirus strains with dual serotype specificity based on sequence divergence of VP7.

Photopeak profile of full energy and escape peaks in neutron capture prompt-gamma ray spectra

This paper describes effect of base profile for precise measurement of photon energy in prompt γ-ray analysis (PGA). The base profile has been examined in the vicinity of full energy (FE), single escape and double escape peaks. Major origins of step-wise base profiles are single and double escape events for triple photon annihilation and single and double Compton escape events for double photon annihilation in the detector, in addition to general events for the FE peak. A formula has been proposed for the quantitative characterization of the base profile.

Gamma ray spectra simulation and optimization in neutron activation analysis

A software package for the simulation of neutron activation spectra has been developed. The simulation of the 14 MeV neutron activation gamma ray spectra has been carried out both for cyclic and conventional modes. The simulation allows the prediction of the optimum parameters for a low level of detection. The parameters are irradiation, counting and decay times. The simulation was based on an accurate calculation of various parameters such as the photoelectric yield, detector efficiency and background estimation. Compton effect, single and double escape are also taken into account. This software was extensively tested using the IAEA certified material SL1 lake sediment and Al(OH) 3 chemical standard. A high agreement between the theoretical simulated spectra and those measured is obtained. The second part of the package implements the optimization of the timing parameters. The Monte Carlo method was used to search within a constrained time space for the optimum condition resulting in the lowest detection limit for a given reaction. In order to illustrate the validity of the optimisation routine, sodium is determined in SL1 using the reaction 23Na(n,α) 20F.

The X-ray/Gamma-ray Spectrometer on the Near Earth Asteroid Rendezvous Mission

An X-ray/gamma-ray spectrometer has been developed as part of a rendezvous mission with the near-Earth asteroid, 433 Eros, in an effort to answer fundamental questions about the nature and origin of asteroids and comets. During about 10 months of orbital operations commencing in early 1999, the X-ray/Gamma-ray Spectrometer will develop global maps of the elemental composition of the surface of Eros. The instrument remotely senses characteristic X-ray and gamma-ray emissions to determine composition. Solar excited X-ray fluorescence in the 1 to 10 keV range will be used to measure the surface abundances of Mg, Al, Si, Ca, Ti, and Fe with spatial resolutions down to 2 km. Gamma-ray emissions in the 0.1 to 10 MeV range will be used to measure cosmic-ray excited elements O, Si, Fe, H and naturally radioactive elements K, Th, U to surface depths on the order of 10 cm. The X-ray spectrometer consists of three gas-filled proportional counters with a collimated field of view of 5° and an energy resolution of 850 eV @ 5.9 keV. Two sunward looking X-ray detectors monitor the incident solar flux, one of which is the first flight of a new, miniature solid-state detector which achieves 600 eV resolution @ 5.9 keV. The gamma-ray spectrometer consists of a NaI(Tl) scintillator situated within a Bismuth Germanate (BGO) cup, which provides both active and passive shielding to confine the field of view and eliminate the need for a massive and costly boom. New coincidence techniques enable recovery of single and double escape events in the central detector. The NaI(Tl) and BGO detectors achieve energy resolutions of 8.7% and 14%, respectively @ 0.662 MeV. A data processing unit based on an RTX2010 microprocessor provides the spacecraft interface and produces 256-channel spectra for X-ray detectors and 1024-channel spectra for the raw, coincident, and anti-coincident gamma-ray modes. This paper presents a detailed overview of the X-ray/Gamma-ray Spectrometer and describes the science objectives, measurement objectives, instrument design, and shows some results from early in-flight data.

Dynamics and seismotectonics of the West-Alpine arc

In this paper we present a comparative review of structural, seismic and focal mechanism data from the West-Alpine arc. In the Western Alps, seismic activity is concentrated along an external belt, corresponding to the Penninic front, and an internal belt, corresponding to the Austro-Alpine front and its southern extension. These seismic belts are connected in the NE by a seismic lineament, corresponding to the Simplon and Centovalli Line and in the south by the E-W-trending seismic Stura “couloir”, located between the Argentera and Dora Maira massifs. The SimplondashCentovalli-Tonale system and the Stura “couloir” are dextral and sinistral strike-slip systems respectively; this implies a westward translation of the West-Alpine arc and the Po Plain. Based on these observations, a seismotectonic model is proposed in which the frontal Penninic thrust and the basal surface of the accretionary wedge corresponding to the Penninic sole thrust are reactivated. Activity along the frontal thrust increases the arcuate shape of the Western Alps and disengages them from the Central and the Ligurian Alps along the tear faults of the SimplondashCentovalli-Tonale system and the Stura “couloir” respectively. In the wider framework of northern Italy, the sector of the eastern Alps north of the Gailtal Line, is moving in an orogendashparallel direction towards the east. Areas with lateral escape in opposite directions, towards the west for the Po Plain and towards the east for the Eastern Alps, north of the Gailtal Line, are separated by the South Alpine Atesine indenter. A model postulating a double lateral escape and a central indenter matches most of the features of the models of “poinçonnement”. The seismic features of northern Italy agree with the seismicity of the stable sector of northern Europe up to the North Sea. This area is fragmented into blocks bounded by seismic bands, some of which probably reactivate pre-existing structures. The overall stress field of Cratonic Europe is dominated by ndashNW-directed trajectories of maximum horizontal compression. The seismicity of the European Alpine foreland is interpreted as the response of a stable block to the same stress field of northern Italy, which is induced by the ondashgoing NW-SE Africa-Europe convergence. We suggest that the westward translation of the West-Alpine arc continues at present and is the driving mechanism of the observed seismicity.

The negative ion of hydrogen

The negative ion of hydrogen continues to be important in atomic physics and astrophysics. Correlations between the two electrons are strong already in the ground state, the only bound state in this three-body system. This state attracted early interest, especially for the description of stellar atmospheres by Chandrasekhar and others. More recently, the rich spectrum of doubly-excited states, and the nature of the double escape above the break-up energy of 14.35 eV, have been central to our understanding of highly-correlated, non-separable problems in quantum physics. This article is a survey of the story of H− as it has developed over the last seventy years.

Gamma-ray escape peak characteristics of radiation-damaged reverse-electrode germanium coaxial detectors

A comparison of the characteristics of full-energy gamma-ray peaks and their corresponding escape peaks when high energy photons interact in radiation damaged reverse-electrode (n-type) germanium coaxial detectors is presented. Coaxial detector geometry is the dominant factor, causing charge collection to be dramatically better for interactions occurring near the outer periphery of the detector as well as increasing of the probability of escape events occurring in this region. It follows that the resolution of escape peaks is better than that of ordinary gamma-ray peaks. This is experimentally verified. A nearly identical but undamaged detector exhibited significant Doppler broadening of single escape peaks. Because double escape events preferentially occur at outer radii, energy shifts of double escape reflect extremely small amounts of charge trapping in undamaged detectors.

Longitudinal study of an epitope-biased serum haemagglutination-inhibition antibody response in rabbits immunized with type A influenza virions

This paper describes a longitudinal study of the antibody specificities generated to the haemagglutinin (HA), the major envelope glycoprotein of type A influenza virus particles, during primary and secondary antibody responses in rabbits. Two New Zealand White rabbits (no. 191, no. 192) and an English Half-Lop rabbit (no. 193) were immunized intravenously with β-propiolactone-inactivated virus at 0, 28 and 56 days. Haemagglutination-inhibition (HI) antibody specificities were measured using neutralizing antibody double escape mutants selected with monoclonal antibodies (mabs) specific for an epitope in antigenic site A, site B and site D. In this regard the epitope reactivity to these mabs was represented as A +B −D −, A −B +D − and A −B −D +, where “+” and “−” represent the non-mutated and mutated epitopes respectively. The HI response was well developed at 7 days after primary immunization and at this time the response was equally divided between all three epitopes. Two rabbits (no. 191 and no. 193), showed a bias to the site B epitope from 14 days onwards, such that about half of the total HI activity was to this epitope and the other half was made up of HI antibody to the other two epitopes in approximately equal proportions. In the other New Zealand White rabbit (no. 192) a non-biased response extended throughout the primary response and for one week after the second immunization. Apart from this, a bias to a single epitope was clearly evident in all rabbits after the second immunization, and this constituted up to 70% of the total HI antibody response. The antibody response did not broaden and remained essentially unchanged even after a third immunizing injection. The observed bias to the site B epitope during the secondary response of HA-specific antibody is in accord with a previous cross-sectional study of nine other rabbits.

Incorporation of Threshold Phenomena in Three-body Coulomb Continuum Wavefunctions

In this work a three-body Coulomb wavefunction for the description of two continuum electrons moving in the field of a nucleus is constructed such that the Wannier threshold law for double escape is reproduced and the asymptotic Coulomb boundary conditions as well as the Kato cusp conditions are satisfied. It is shown that the absolute value of the total cross section, as well as the spin asymmetry, are well described by the present approach. Further, the excess-energy sharing between the two escaping electrons is calculated and analysed in light of the Wannier theory predictions. This is the first time an analytical three-body wavefunction is presented which is asymptotically exact and capable of describing threshold phenomena.

Theoretical premises for experimental verification of the deviations from the wannier threshold behaviour

The Wannier [1] theory describes the escape of two electrons from a charged core in the threshold domain. Its modification [3] is applied to some features of the double escape processes which were not analyzed previously in due extent. The behaviour of the total cross sections for a small, but finite, energy excessE above the threshold is described incorporating the deviations from the well known Wannier power law. The processes with the escape of electron and positron from the core are also considered. The energy dependence of the spin asymmetryA in (e, 2e) collisions is analyzed in connection with the recent experimental data (which seem to manifest some dependenceA(E) in the near-threshold domain contrary to the conventional Wannier theory). The theory suggests the requirements for the accuracy of the experiments intended to verify the Wannier threshold laws and detect the deviations from them.

All rabbits immunized with type A influenza virions have a serum haemagglutination-inhibition antibody response biased to a single epitope in antigenic site B.

Nine rabbits were immunized with type A influenza virions and the epitope specificities of the secondary serum haemagglutination-inhibition (HI) antibody response were analysed with a panel of neutralizing monoclonal (MAb) antibody double escape mutants. Each of the latter was made by sequential selection using a MAb directed to an epitope of a discrete antigenic site, site A, site B or site D, of the haemagglutinin (HA). Thus the epitope reactivity of the escape mutants was represented as A+ B- D-, A- B+ D- and A- B- D+. The HI antibody response of all antisera was biased to the site B epitope. In 9/12 antisera, obtained from seven rabbits immunized with whole virions, the site B epitope was predominant, representing 65-82% of the total HI antibody. The restriction of HI antibody was unaffected by strain of rabbit, route of inoculation (intravenous or subcutaneous), use of Freund's adjuvant, and up to four immunizing injections. In 3/7 rabbits immunized with whole virus, there was a HI antibody response to the HC2 (site A) or HC10 (site D) epitope, but not both, of equal magnitude to the site B epitope. The HI antibody response in one of the rabbits (#40) became more biased to the site B epitope between the third and fourth immunizing doses. Two further rabbits were immunized with virions which had been partially digested with bromelain and then purified from free HA. Both of these made equal HI antibody responses to the site B epitope and the site D epitope, possibly because their remaining HA spikes were better exposed. Overall, these data demonstrate an unexpected degree of restriction in the production of biologically relevant antibody, such that some rabbits (e.g. #45) mount an HI antibody response which is essentially epitope-specific. Implications for epitope specificity of HI antibody stimulated by human influenza vaccines, and also for the generation of antigenic drift variants are discussed. The reason for the non-responsiveness of the immune system to the many other HI epitopes of the HA is not known.

Excess energy partition between the electrons in the double electron escape processes

The processes with two electrons in the final continuum state are considered for small energy excess E above the threshold. The distribution of the total energy E between the electrons is calculated within the quantum Wannier theory where the quadratic approximation for the Hamiltonian is employed in the vicinity of the Wannier ridge. The problem is cast in terms of the wavepacket propagation. The Green function formulation is employed and its physical implications are analysed. The final distribution is shown to be uniform and essentially independent of the boundary condition at the border of the inner Wannier zone. The extended model is developed which does not resort to the quadratic approximation but accounts for the attractive Coulomb singularities of the potential. It is solved numerically using the grid method. The manifestations of the competing process are discussed (single ionization with the other electron in a high Rydberg state).

MeV gamma ray detection algorithms for stacked silicon detectors

By making use of the signature of a gamma ray event as it appears in a stack of N=5 to 20 lithium-drifted silicon detectors and applying smart selection algorithms, gamma rays in the energy range of 1 to 8 MeV can be detected with good efficiency and selectivity. Examples of the types of algorithms used for different energy regions include the simple sum mode, the sum-coincidence mode used in segmented detectors, unique variations of sum-coincidence for an N-dimensional vector event, and a new and extremely useful mode for double escape peak spectroscopy at pair-production energies. The latter algorithm yields a spectrum similar to that of the pair spectrometer, but without the need for the dual external segments for double escape coincidence, and without the large loss in efficiency of double escape events. Background events due to Compton scattering are largely suppressed. Monte Carlo calculations were used to model the gamma ray interactions in the silicon in order to enable testing of a wide array of different algorithms on the event N-vectors for a large-N stack. >

Doubly excited ridge states of atoms.

Doubly excited ridge states of atoms are states in which two electrons reach high and comparable excitations. Our goal is to calculate sequences for some of these states by using a wave function that treats the pair of electrons as a single entity in solving the two-electron Schr\"odinger equation in hyperspherical coordinates: R, \ensuremath{\alpha}, ${\mathrm{\ensuremath{\theta}}}_{12}$. For double escape of slow electrons, the so-called Wannier theory predicts that the wave function will be concentrated in the region \ensuremath{\alpha}=\ensuremath{\pi}/4, ${\mathrm{\ensuremath{\theta}}}_{12}$=\ensuremath{\pi} (that is, ${\mathbf{r}}_{1}$=-${\mathbf{r}}_{2}$) and we expect something similar for the ridge states. By expanding the Schr\"odinger equation around these points and retaining the first nontrivial quadratic dependences in \ensuremath{\alpha} and ${\mathrm{\ensuremath{\theta}}}_{12}$, we seek a solution in which the form of the wave function in these two variables is analytically determined as in the Wannier theory. The R dependence of this wave function is then handled numerically and differs from the double-escape solution only in the exponential decay appropriate for bound states. For L=0 a polynomial in R is included, giving the proper number of nodes for the solution. Numerical eigenvalues for autoionizing ridge states are calculated and compared with available results.

Energy dependence of the spin asymmetry for electron impact ionization of atomic hydrogen and its implications for the Wannier theory of double escape

The authors extend the formalism introduced by Klar and Schlecht (1976) and more completely developed by Rau and Greene (1982, 1983) to explore the implications of the Wannier threshold theory for the spin dependence of electron impact ionization of atomic hydrogen. By analysing the properties of the contributions to the singlet and triplet cross sections from the multiplicity of (L, S pi ) final states and by combining the results of the investigations of spin-asymmetry measurements of Guo et al. (1990, 1993) and the earlier investigations of spin-averaged cross section measurements of McGowan and Clarke (1968), the authors are able to provide insight into the role played by higher order terms and the possible influence of collisional dynamics. The authors further show that the critical energy above threshold at which the contributions from higher order terms become visible is substantially lower for the singlet and triplet partial cross sections than it is for the spin-averaged cross section.

Large mass bolometric detectors for double-beta decay experiments

TeO/sub 2/ bolometric detectors with masses of 5.5 g, 20.9 g, and 34 g have been developed. The latter have been operating for more than 440 h in the Gran Sasso underground laboratory, and a limit of 1.2*10/sup 20/ y for the half life of the neutrinoless double-beta decay of /sup 130/Te was reached. CaF/sub 2/ detectors for simultaneous detection of scintillating light and phonon signals are under development and will be used to search for double-beta decay of /sup 48/Ca. Scintillation of a CaF/sub 2/ crystal with 0.03% Eu doping was observed at 20 mK. A 2.1 g, 0.01% Eu doped crystal was used as a bolometer, and a resolution of 50 keV for the double escape peak of /sup 232/Th was obtained at 60 mK, showing a negligible contribution of the Eu atoms to the crystal heat capacity. Simultaneous detection of light and phonons will allow an efficient background rejection of the /sup 238/U to /sup 234/Th alpha decay from the /sup 238/U natural series occurring at almost the same energy of the /sup 48/Ca transition. Background rejection is necessary as even U contamination as low as 10/sup -10/ g/g would make the search impossible. >

Angular distribution of slow photoelectrons near the threshold for double escape

A general formula is derived for the asymmetry parameter of single photoelectrons in the energy range just above and just below the Wannier threshold. The treatment uses Herrick's SO(4) analysis of two-electron angular correlations in conjunction with the hyperspherical prediction of dominant radial correlation channels. In each of the rare gases the asymmetry parameter approaches the 'unfavoured value' beta to -1 at the double-escape threshold.

The use of simulation for the optimisation of the signal-to-noise ratio in cyclic activation analysis

It is always useful to predict the result of an experiment without having to carry it out. In cyclic activation analysis where the signal-to-noise ratio for the detection of short-lived nuclides in a given matrix is particularly sensitive to the value of the timing parameters, determination of the best conditions for the detection of a nuclide of interest, without resort to preliminary experiments, is most helpful. Simulation and optimisation of a gamma-ray spectrum is derived for a sample irradiated in cyclic mode. The simulation includes consideration of photopeak, Compton continuum, single and double escape peaks and bremsstrahlung. The simulation output forms the input to the signal-to-noise ratio optimisation routine. Consequently the sensitivities, and detection limits for the isotopes of interest can be deduced from the optimised spectrum. Interference from other gamma-lines is identified, if applicable. In addition, a graphical output of the simulated spectrum and a listing of the optimised activation parameters, detection limits and sensitivites are produced. The programme has been extended to include simulation of conventionally neutron activated samples. Examples for standard reference materials are given as illustrations, together with measured spectra.