S.H. Yoo

Ion chamber system for neutron flux measurements

Abstract A helium-filled ion chamber detector for intensity measurements of high-intensity epithermal neutron bursts with instantaneous rates as high as 10 11 Hz is presented. This system consists of an ion chamber to detect a portion of the neutron beam, a current-to-frequency converter and CAMAC scalers to readout the chamber. The chambers and readout electronics have a small temperature sensitivity and have high noise immunity. The statistical precision of the system is measured to be 10 −3 for each neutron beam pulse.

An apparatus and techniques of tests for fundamental symmetries in compound-nucleus scattering with epithermal polarized neutron beams

Abstract The epithermal polarized-neutron beam facility used for tests of fundamental symmetries is described. The initial unpolarized beam is obtained from the spallation source at the Los Alamos Neutron Scattering Center. Characteristics of the polarized and unpolarized beams are described, as well as design and performance of a fast-spin reversal system, neutron flux monitor and target cooler. Experimental results for polarized-neutron transmission experiments are given to illustrate the overall quality of the system.

A High-Rate Detection System to Study Parity Violation with Polarized Epithermal Neutrons at LANSCE

Abstract We describe an apparatus for studies of parity violation in neutron-nucleus scattering. This experiment requires longitudinally polarized neutrons from the Los Alamos Neutron Scattering Center over the energy range from 1 to 1000 eV, the ability to reverse the neutron spin without otherwise affecting the apparatus, the ability to detect neutrons at rates up to 500 MHz and an appropriate data acquisition system. We will discuss the neutron polarizer, fast neutron spin reverser, detector for transmitted neutrons and high rate data acquisition system.

Parity violation at neutron p-wave resonances of238U and232Th and related questions

Parity violation effects have been studied at 40 neutron p-wave resonances of the even-even nuclei238U and232Th. Of these 11 show parity violation effects larger than 2 standard deviations, making parity violation a rather common phenomenon. Parity mixing up to 10% has been found. The root-mean squared matrix elements for parity violation derived from these resonances are M=0.58 (+0.50/-0.25) meV for238U, respectively 1.39 (+0.35/-0.38) meV for232Th.

Measurement of parity violation in compound nuclear resonances using epithermal polarized neutrons

Polarized neutron transmission experiments show remarkable sensitivity to the presence of small symmetry violating forces in nuclear interactions. Parity-nonconserving (PNC) analyzing powers of a percent or more have been measured on p-wave neutron resonances in nuclei ranging from 81Br to 238U. A brief summary is given of the study of PNC effects in neutron resonances. Details of ongoing polarized neutron transmission experiments are discussed and some sample data presented. Several development projects are currently under way, which taken together should improve the sensitivity of these experiments by a factor of ∼ 20. Similar large PNC effects can be observed in (n, γ) resonances with much smaller samples. A design for a near 4π gamma ray detector to be constructed of CsI(pure) is described.

Bulk magnetic structure studies using polarized epithermal neutrons

Abstract Parity violating effects in the nucleus are normally very small, but in certain compound nuclear resonances such effects are enhanced to several percent, as in the 0.734 eV p-wave resonance in 139 La. We have used this enhanced parity violation to measure the depolarization of neutrons traversing a thick single crystal of ferromagnetic holmium. The depolarization was dependent on the relative orientation of the neutron polarization and the holmium c crystalline axis. The effect was explained in terms of neutron precession in an array of highly correlated domains magnetized in the c axis direction. The results show the high degree of correlation of magnetic domains in the single crystal and demonstrate the usefulness of epithermal neutrons in measuring magnetic properties of thick crystals. The use of other parity violating resonances should allow polarization analysis of neutrons over a range of neutron energies.

Parity and Time Reversal Symmetry Violation in Neutron‐Nucleus Scattering

The TRIPLE collaboration has begun a multi year program of study of symmetry violation using the intense pulsed polarized epithermal neutron beam available at LANSCE (Los Alamos Neutron Scattering Center). The parity violation experiments consist of measurement of the helicity dependence of neutron transmission at p‐wave resonances in compound nuclei. A recent success has been the first determination of the spreading width of the weak interaction in 239U.