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Register a new userTransverse asymmetry of $gamma$ rays from neutron-induced compound states of ${}^{140}{rm La}$
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A correlation term ${{ sigma}_{n} }cdot ({ k_{n}times k_gamma}) $ in the ${}^{139}{rm La}(vec{n},gamma)$ reaction has been studied utilizing epithermal polarized neutrons and germanium detectors. The transverse asymmetry for single $gamma$-ray transition was measured to be $0.60pm0.19$ in the $p$-wave resonance.
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Angular distribution of individual $gamma$-rays, emitted from a neutron-induced compound nuclear state via radiative capture reaction of ${}^{139}$La(n,$gamma$) has been studied as a function of incident neutron energy in the epithermal region by using germanium detectors. An asymmetry $A_{mathrm{LH}}$ was defined as $(N_{mathrm L}-N_{mathrm H})/(N_{mathrm L}+N_{mathrm H})$, where $N_{mathrm L}$ and $N_{mathrm H}$ are integrals of low and high energy region of a neutron resonance respectively, and we found that $A_{mathrm{LH}}$ has the angular dependence of $(Acostheta_gamma+B)$, where $theta_gamma$ is emitted angle of $gamma$-rays, with $A= -0.3881pm0.0236$ and $B=-0.0747pm0.0105$ in 0.74 eV p-wave resonance. This angular distribution was analyzed within the framework of interference between s- and p-wave amplitudes in the entrance channel to the compound nuclear state, and it is interpreted as the value of the partial p-wave neutron width corresponding to the total angular momentum of the incident neutron combined with the weak matrix element, in the context of the mechanism of enhanced parity-violating effects. Additionally we used the result to quantify the possible enhancement of the breaking of the time-reversal invariance in the vicinity of the p-wave resonance.
Neutron energy-dependent angular distributions were observed for individual $gamma$-rays from the 0.74 eV p-wave resonance of $^{139}$La+$n$ to several lower excited states of $^{140}$La. The $gamma$-ray signals were analyzed in a two dimensional histogram of the $gamma$-ray energy, measured with distributed germanium detectors, and neutron energy, determined with the time-of-flight of pulsed neutrons, to identify the neutron energy dependence of the angular distribution for each individual $gamma$-rays. The angular distribution was also found for a photopeak accompanied with a faint p-wave resonance component in the neutron energy spectrum. Our results can be interpreted as interference between s- and p-wave amplitudes which may be used to study discrete symmetries of fundamental interactions.
Annual modulation of $gamma$ rays from ($alpha$, $gamma$) reactions in the Soudan Underground Lab has been observed using a 12-liter scintillation detector. This significant annual modulation, measured over 4 years, can mimic the signature for dark matter and can also generate potential background events for neutrinoless double-$beta$ decay experiments. The measured annual modulation of the event rate from ($alpha$, $gamma$) reactions is strongly correlated with the time-varying radon concentration observed independently in the Lab. The $alpha$ flux from radon decay is simulated starting from the measured radon concentration, and the $gamma$-ray flux is determined using the convolution of the $alpha$ flux and the cross sections for ($alpha$, $gamma$) reactions. The calculated $gamma$-ray flux is sufficient to generate the measured event rate that exhibits an annual modulation.
The UCNA experiment was designed to measure the neutron $beta$-asymmetry parameter $A_0$ using polarized ultracold neutrons (UCN). UCN produced via downscattering in solid deuterium were polarized via transport through a 7 T magnetic field, and then directed to a 1 T solenoidal electron spectrometer, where the decay electrons were detected in electron detector packages located on the two ends of the spectrometer. A value for $A_0$ was then extracted from the asymmetry in the numbers of counts in the two detector packages. We summarize all of the results from the UCNA experiment, obtained during run periods in 2007, 2008--2009, 2010, and 2011--2013, which ultimately culminated in a 0.67% precision result for $A_0$.
Discrimination of the detection of fast neutrons and gamma rays in a liquid scintillator detector has been investigated using digital pulse-processing techniques. An experimental setup with a 252Cf source, a BC-501 liquid scintillator detector, and a BaF2 detector was used to collect waveforms with a 100 Ms/s, 14 bit sampling ADC. Three identical ADCs were combined to increase the sampling frequency to 300 Ms/s. Four different digital pulse-shape analysis algorithms were developed and compared to each other and to data obtained with an analogue neutron-gamma discrimination unit. Two of the digital algorithms were based on the charge comparison method, while the analogue unit and the other two digital algorithms were based on the zero-crossover method. Two different figure-of-merit parameters, which quantify the neutron-gamma discrimination properties, were evaluated for all four digital algorithms and for the analogue data set. All of the digital algorithms gave similar or better figure-of-merit values than what was obtained with the analogue setup. A detailed study of the discrimination properties as a function of sampling frequency and bit resolution of the ADC was performed. It was shown that a sampling ADC with a bit resolution of 12 bits and a sampling frequency of 100 Ms/s is adequate for achieving an optimal neutron-gamma discrimination for pulses having a dynamic range for deposited neutron energies of 0.3-12 MeV. An investigation of the influence of the sampling frequency on the time resolution was made. A FWHM of 1.7 ns was obtained at 100 Ms/s.
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