Exchange reactions with Dick Dalitz

An account is given of the close collaboration of Dick Dalitz with the European K⁻ Collaboration over many decades in many aspects of hypernuclear physics. In particular, emphasis is given to the topics of double hypernuclei and the discovery and resolution of p-wave Λ strangeness exchange states. A brief review of early work on non-mesonic decays of hypernuclei is also given.     

Comparison of phantom materials for use in quality assurance of microbeam radiation therapy

Microbeam radiation therapy (MRT) is a promising radiotherapy modality that uses arrays of spatially fractionated micrometre‐sized beams of synchrotron radiation to irradiate tumours. Routine dosimetry quality assurance (QA) prior to treatment is necessary to identify any changes in beam condition from the treatment plan, and is undertaken using solid homogeneous phantoms. Solid phantoms are designed for, and routinely used in, megavoltage X‐ray beam radiation therapy. These solid phantoms are not necessarily designed to be water‐equivalent at low X‐ray energies, and therefore may not be suitable for MRT QA. This work quantitatively determines the most appropriate solid phantom to use in dosimetric MRT QA. Simulated dose profiles of various phantom materials were compared with those calculated in water under the same conditions. The phantoms under consideration were RMI457 Solid Water (Gammex‐RMI, Middleton, WI, USA), Plastic Water (CIRS, Norfolk, VA, USA), Plastic Water DT (CIRS, Norfolk, VA, USA), PAGAT (CIRS, Norfolk, VA, USA), RW3 Solid Phantom (PTW Freiburg, Freiburg, Germany), PMMA, Virtual Water (Med‐Cal, Verona, WI, USA) and Perspex. RMI457 Solid Water and Virtual Water were found to be the best approximations for water in MRT dosimetry (within ±3% deviation in peak and 6% in valley). RW3 and Plastic Water DT approximate the relative dose distribution in water (within ±3% deviation in the peak and 5% in the valley). PAGAT, PMMA, Perspex and Plastic Water are not recommended to be used as phantoms for MRT QA, due to dosimetric discrepancies greater than 5%.     

Measurement of the parity-violating asymmetry in inclusive electroproduction of π- near the Δ0 resonance

The parity-violating (PV) asymmetry of inclusive π- production in electron scattering from a liquid deuterium target was measured at backward angles. The measurement was conducted as a part of the G0 experiment, at a beam energy of 360 MeV. The physics process dominating pion production for these kinematics is quasifree photoproduction off the neutron via the Δ0 resonance. In the context of heavy-baryon chiral perturbation theory, this asymmetry is related to a low-energy constant d(Δ)- that characterizes the parity-violating γNΔ coupling. Zhu et al. calculated d(Δ)- in a model benchmarked by the large asymmetries seen in hyperon weak radiative decays, and predicted potentially large asymmetries for this process, ranging from A(γ)-=-5.2 to +5.2 ppm. The measurement performed in this work leads to A(γ)-=-0.36±1.06±0.37±0.03 ppm (where sources of statistical, systematic and theoretical uncertainties are included), which would disfavor enchancements considered by Zhu et al. proportional to V(ud)/V(us). The measurement is part of a program of inelastic scattering measurements that were conducted by the G0 experiment, seeking to determine the N-Δ axial transition form factors using PV electron scattering.     

Spectroscopic imaging STM studies of broken electronic symmetries in underdoped cuprates

We use spectroscopic imaging scanning tunneling microscopy (SI-STM) to visualize the spatial symmetries of the electronic states that occur at the pseudogap energy scale in underdoped cuprates. We find evidence for the local intra-unit-cell electronic nematicity—by which we mean the disordered breaking of C_4v symmetry within each CuO₂ unit cell [1]. We also find that the coexisting incommensurate (smectic) electronic modulations couple to the intra-unit-cell nematicity through their 2π$2\mathrm{\pi }$ topological defects [2].     

Longitudinal relaxation of muonium in Ge and GaAs

Longitudinal-field muon-spin depolarization rates in high-purityGe and semiinsulatingGaAs are reported and compared to similar data for intrinsicSi. Depolarization onset temperatures provide a comparison of charge carrier concentrations leading to rapid charge exchange and a shift in the onset, for n-typeGaAs verifies anelectron process. The temperature dependence of the low-field rate constants imply more complicated dynamics inGe than observed earlier inSi. Features near 750K inGaAsTe appear consistent with dissociation of aMu-Te pair.This work was supported by the US National Science Foundation (DMR-8917639 [TLE, BH]), the Science and Engineering Research Council of the UK (EAD, AS, SFJC), the Robert A. Welch Foundation (D-1053 [RLL], and C-1048 [TLE]), and a NATO Collaborative Research Grant (RLL, SFJC, RCD, CS). We wish to thank D.A. Vanderwater of Hewlett Packard for providing theGaAsTe sample.     

Muon spin resonance and repolarization in amorphous silicon monoxide (a-SiO)

Studies of the muonium fractions in the amorphous oxide a-SiO have been carried out by RF resonance at TRIUMF, Canada and LF repolarization techniques at RAL, U.K. The resonance measurements confirm the presence of the interstitial Mu centre in this intermediate oxide of silicon. Analysis of the data gathered at RAL, using a recently-developed fitting technique, reveals that the Mu^* state is present here as well, but with lower relative fractions than in a-Si. However, as in the latter material, but in contrast to c-Si, this bond-centre species appears to be stable up to room temperature.