False Coincidences Research Articles

Covariance mapping and triple coincidence techniques applied to multielectron dissociative ionization

Single versus multiphoton multiple ionization of diatomic and triatomic molecules is discussed. The former experiments use synchrotron radiation, while the latter ones require pico- or femtosecond laser pulses focused to an intensity exceeding 10 14 W/cm 2. A method of improving covariance maps by decorrelating laser fluctuations is presented. Removal of false coincidences in triple coincidence experiments is extended to a varying excitation rate. An explanation of the multielectron dissociative ionization mechanism is sought in terms of bond softening, bond hardening (stabilisation), and ionization enhancement due to Stark-shift excitation. It is expected that this excitation is accompanied by emission of vacuum ultraviolet radiation that differs from high harmonic generation.

Removal of false coincidences in triple coincidence experiments employing continuous excitation

The competing processes in molecular dissociative double ionization can be studied by producing three-dimensional, triple coincidence maps. Even at modest count rates these maps exhibit structured and unstructured background due to false coincidences. A technique for removing these false coincidences is described and applied to the case of the double ionization of CF4 produced by synchrotron radiation at a wavelength of 20 nm.

Multiplying with neurons: Compensation for irregular input spike trains by using time-dependent synaptic efficiencies

A leaky integrate-and-fire (LIF) neurons can act as multipliers by detecting coincidences of input spikes. However, in case of input spike trains with irregular interspike delays, false coincidences are also detected and the operation as a multiplier is degraded. This problem can be solved by using time dependent synaptic weights which are set to zero after each input spike and recover with the same time constant as the decay time of the corresponding excitatory postsynaptic potentials (EPSP). Such a mechanism results in EPSP's with amplitudes independent on the input interspike delays. Neuronal computation is then performed without frequency decoding.

Compton scattering of 320-keV gamma rays by K-shell electrons of holmium and gold.

Compton scattering of 320-keV {gamma} rays through 45{degree} and 115{degree} by {ital K}-shell electrons of gold and holmium has been studied by the well-known coincidence technique. Double-differential cross sections with respect to outgoing photon energy and angle have been determined from 30 keV up to the kinematic limits. Target-{ital Z}-dependent false coincidences arising from bremsstrahlung emitted by photoelectrons were estimated both by theoretical analysis and by measurements carried out with gold targets of thicknesses differing by more than a factor of 4. The analysis of the bremsstrahlung process included the often neglected effects of the angular distribution of photoelectrons and of multiple scattering of electrons during the slowing-down process within the target. The bremsstrahlung generated false coincidences obscure the possible observation of the predicted infrared divergence in the low outgoing photon energy regime and lead to a significant contribution even in the quasi-Compton regime. The single-differential cross sections obtained after a correction for bremsstrahlung are in reasonable agreement with theoretical calculations.

Compton scattering of 279.2-keV gamma rays by K-shell electrons.

Coincidence techniques have been employed in a detailed study of Compton scattering of 279.2keV y rays by K-shell electrons of tin and gold. Double-differential cross sections at four angles have been determined from about 30 keV to the kinematic limit. In the case of gold, clear evidence has been obtained for the infrared divergence effect predicted by Gavrila, and for Cornpton defects towards lower energies. The data obtained with tin do not show significant infrared divergence or Compton defects. The widths of the fully developed quasi-Compton peaks are about 85 and 55 keV in the case of gold and tin, respectively. A detailed investigation has been made of possible false coincidences arising from secondary processes. Energy-integrated single-differential cross sections have also been determined. Comparisons with other experimental data and with results of theoretical calculations have been presented. The experimental cross sections in the regime of low outgoing photon energies are much larger than nonrelativistic and relativistic theoretical predictions. The experimental data above about 100 keV are in much better agreement with the relativistic second-order Smatrix calculations of Whittingham, although at some angles deviations of about 30%%uo are noticeable. The desirability of additional relativistic calculations and measurements with extremely thin targets has been pointed out.

Detection of X-rays and electron energy loss events in time coincidence

Experiments are described in which characteristic X-rays are detected in coincidence with the electron energy losses that are responsible for these X-rays. The possibility to use this technique for improving the detection limits of microanalysis is evaluated. It is concluded that, because of the occurrence of false coincidences, better than the state-of-the-art instrumentation will be required for most practical applications.

Compton scattering of 1.12-MeV gamma rays byK-shell electrons of heavy elements

A study has been made of the Compton scattering of 1.12-MeV gamma rays by $K$-shell electrons of tin, tantalum, gold, lead, and thorium. $K$ x rays of target atoms were detected in coincidence with the scattered gamma rays. A careful investigation of false coincidences has been carried out. The ratio $\frac{d{\ensuremath{\sigma}}_{K}}{d{\ensuremath{\sigma}}_{F}}$ of the $K$-shell electron cross section to the Compton cross section for a free electron at rest was determined at several angles between 25\ifmmode^\circ\else\textdegree\fi{} and 120\ifmmode^\circ\else\textdegree\fi{}. The cross-section ratio in the case of gold and thorium is less than 1 at 25\ifmmode^\circ\else\textdegree\fi{}, larger than 1 in the neighborhood of 90\ifmmode^\circ\else\textdegree\fi{}, and close to 1 at 120\ifmmode^\circ\else\textdegree\fi{}. The experimental results are compared with theoretical calculations based on the incoherent-scattering-function approximation and a relativistic version of the impulse approximation. There is a need for a comprehensive relativistic calculation incorporating the effects of electron binding in intermediate states.

A pulse shape discriminator for an X-ray proportional counter and its application to a coincidence experiment

A pulse shape discrimination system has been developed for an X-ray proportional counter to reduce the background caused by high energy radiations, by the use of a modified time-to-pulse-height converter. Test measurements with argon filled counters have shown that the system is at least 90% effective in reducing the background in the energy range 6–15 keV, while more than 95% of all the X-ray signals can be accepted. Even in the 20–35 keV region, reasonably good results for the background reduction and the X-ray acceptance have also been obtained. The technique has been successfully applied to the coincidence experiment with two proportional counters in order to eliminate false coincidences due to unfavourable high energy radiations passing through both counters.

Angular and energy dependence of the polarization of D-D neutrons

The polarization of D-D neutrons has been measured for a deuteron energy of 375 keV at 10 different reaction angles between 0° and 80° (lab) and for deuteron energies of 275 keV and 450 keV at 50° (lab). The thickness of the targets was 50 keV. The experimental setup used He 4 as analyser and included α-n coincidence measurement and γ-n discrimination. Attention has been paid to the elimination of false coincidences that reduce the polarization. The maximum polarization for 375 keV is found to be −13.5% the absolute value being larger than the value of Pasma (−9%). The differential polarization contains a small sin 4ϑ contribution; Π(mb) = −(0.627 ± 0.016) sin 2 Π + (0.0.7 ± 0.014) sin 4 Π.

Directional Correlations of Gamma Rays in Te124

The directional correlations of the 2.11–0.605 Mev and 1.71–0.605 Mev gamma-gamma cascades in Te 124 have been measured by using a coincidence spectrometer newly designed. Both of the correlation functions are compatible with the sequence 3 (D, Q) 2 (Q) 0, where quadrupole admixture in the first radiation is expected to be very small. The false coincidences occurring in this type of circuit are also described in some details.